ALBERT

Description: On the representation of immersion and condensation freezing in cloud models using different nucleation schemes Atmospheric Chemistry and Physics Discussions, 12, 7167-7209, 2012 Author(s): B. Ervens and G. Feingold Ice nucleation in clouds is often observed at temperatures 〉235 K, pointing to heterogeneous freezing as a predominant mechanism. Many models deterministically predict the number concentration of ice particles as a function of temperature and/or supersaturation. Laboratory experiments at constant temperature and/or supersaturation often report heterogeneous freezing as a stochastic, time-dependent process that follows classical nucleation theory which might appear to contradict singular freezing behavior. We explore the extent to which the choice of nucleation scheme (deterministic/stochastic, single/multiple contact angles θ) affects the prediction of the frozen ice nuclei (IN) fraction and cloud evolution. A box model with constant temperature and supersaturation is used to mimic published laboratory experiments of immersion freezing of kaolinite (~243 K), and the fitness of different nucleation schemes. Sensitivity studies show that agreement of all five schemes is restricted to the narrow parameter range (time, temperature, IN diameter) in the original laboratory studies. The schemes are implemented in an adiabatic parcel model that includes feedbacks of the formation and growth of drops and ice particles on supersaturation during the ascent of an air parcel. Model results show that feedbacks of droplets and ice on supersaturation limit ice nucleation events, often leading to smaller differences in number concentration of ice particles and ice water content (IWC) between stochastic and deterministic approaches than expected from the box model studies. However, the different parameterizations of θ distributions and time-dependencies are highly sensitive to IN size and can lead to great differences in predicted ice number concentrations and IWC between the different schemes. Finally, since the choice of nucleation scheme determines the temperature range over which nucleation occurs, at habit-prone temperatures (~253 K) different onset temperatures of freezing create variability in the initial inherent growth ratio of ice particles, which can lead to amplification or reduction in differences in predicted IWC.

Description: A multi-model assessment of the efficacy of sea spray geoengineering Atmospheric Chemistry and Physics Discussions, 12, 7125-7166, 2012 Author(s): K. J. Pringle, K. S. Carslaw, T. Fan, G.W. Mann, A. Hill, P. Stier, K. Zhang, and H. Tost Artificially increasing the albedo of marine clouds by the mechanical emission of sea spray aerosol has been proposed as a geoengineering technique to slow the warming caused by anthropogenic greenhouse gases. A previous global model study found that only modest increases and sometimes even decreases in cloud drop number (CDN) concentrations would result from plausible emission scenarios. Here we extend that work to examine the conditions under which decreases in CDN can occur, and use three independent global models to quantify maximum achievable CDN changes. We find that decreases in CDN can occur when at least three of the following conditions are met: the injected particle number is 250–300 nm, the background aerosol loading is large (≥150 cm −3 ) and the in-cloud updraught velocity is low (

Description: On teaching styles of water educators and the impact of didactic training Hydrology and Earth System Sciences Discussions, 9, 2959-2986, 2012 Author(s): A. Pathirana, J. H. Koster, E. de Jong, and S. Uhlenbrook Solving today's complex hydrological problems requires originality, creative thinking and trans-disciplinary approaches. Hydrological education that was traditionally teacher centred, where the students look up to the teacher for expertise and information, should change to better prepare hydrologists to develop new knowledge and apply it in new contexts. An important first step towards this goal is to change the concept of education in the educators' minds. The results of an investigation to find out whether didactic training influences the beliefs of hydrology educators about their teaching styles is presented. Faculty of UNESCO-IHE has been offered a didactic certification program named University Teaching Qualification (UTQ). The hypothesis that UTQ training will significantly alter the teaching style of faculty at UNESCO-IHE from expert/formal authority traits towards facilitator/delegator traits was tested. A first survey was conducted among the entire teaching staff (total 101, response rate 58%). The results indicated that there are significantly higher traits of facilitator and delegator teaching styles among UTQ graduates compared to faculty who were not significantly trained in didactics. The second survey which was conducted among UTQ graduates (total 20, response rate 70%), enquiring after their teaching styles before and after UTQ, corroborated these findings.

Description: Observations of glyoxal and formaldehyde as metrics for the anthropogenic impact on rural photochemistry Atmospheric Chemistry and Physics Discussions, 12, 6049-6084, 2012 Author(s): J. P. DiGangi, S. B. Henry, A. Kammrath, E. S. Boyle, L. Kaser, R. Schnitzhofer, M. Graus, A. Turnipseed, J.-H. Park, R. J. Weber, R. S. Hornbrook, C. A. Cantrell, R. L. Maudlin III, S. Kim, Y. Nakashima, G. M. Wolfe, Y. Kajii, E. C. Apel, A. H. Goldstein, A. Guenther, T. Karl, A. Hansel, and F. N. Keutsch We present simultaneous fast, in-situ measurements of formaldehyde and glyoxal from two rural campaigns, BEARPEX 2009 and BEACHON-ROCS, both located in Pinus Ponderosa forests with emissions dominated by biogenic volatile organic compounds (VOCs). Despite considerable variability in the formaldehyde and glyoxal concentrations, the ratio of glyoxal to formaldehyde, R GF , displayed a very regular diurnal cycle over nearly 2 weeks of measurements. The only deviations in R GF were toward higher values and were the result of a biomass burning event during BEARPEX 2009 and very fresh anthropogenic influence during BEACHON-ROCS. Other rapid changes in glyoxal and formaldehyde concentrations have hardly any affect on R GF and could reflect transitions between low and high NO regimes. The trend of increased R GF from both anthropogenic reactive VOC mixtures and biomass burning compared to biogenic reactive VOC mixtures is robust due to the short timescales over which the observed changes in R GF occurred. Satellite retrievals, which suggest higher R GF for biogenic areas, are in contrast to our observed trends. It remains important to address this discrepancy, especially in view of the importance of satellite retrievals and in-situ measurements for model comparison. In addition, we propose that R GF , together with the absolute concentrations of glyoxal and formaldehyde, represents a useful metric for biogenic or anthropogenic reactive VOC mixtures. In particular, R GF yields information about not simply the VOCs in an airmass, but the VOC processing that directly couples ozone and secondary organic aerosol production.

Description: Characterization of aerosol and cloud water at a mountain site during WACS 2010: secondary organic aerosol formation through oxidative cloud processing Atmospheric Chemistry and Physics Discussions, 12, 6019-6047, 2012 Author(s): A. K. Y. Lee, K. L. Hayden, P. Herckes, W. R. Leaitch, J. Liggio, A. M. Macdonald, and J. P. D. Abbatt The water-soluble fractions of aerosol samples and cloud water collected during Whistler Aerosol and Cloud Study (WACS 2010) were analyzed using an Aerodyne aerosol mass spectrometer (AMS). This is the first study to report AMS organic spectra of re-aerosolized cloud water, and to make direct comparison between the AMS spectra of cloud water and aerosol samples collected at the same location. In general, the aerosol and cloud organic spectra were very similar, indicating that the cloud water organics likely originated from secondary organic aerosol (SOA) formed nearby. By using a photochemical reactor to oxidize both aerosol filter extracts and cloud water, we find evidence that fragmentation of aerosol water-soluble organics increases their volatility during oxidation. By contrast, enhancement of AMS-measurable organic mass by up to 30% was observed during aqueous-phase photochemical oxidation of cloud water organics. We propose that additional SOA material was produced by functionalizing dissolved organics via OH oxidation, where these dissolved organics are sufficiently volatile that they are not usually part of the aerosol. This work points out that water-soluble organic compounds of intermediate volatility (IVOC), such as cis -pinonic acid, produced via gas-phase oxidation of monoterpenes, can be important aqueous-phase SOA precursors in a biogenic-rich environment.

Description: Evaluation of chemical transport model predictions of primary organic aerosol for air masses classified by particle-component-based factor analysis Atmospheric Chemistry and Physics Discussions, 12, 5939-6018, 2012 Author(s): C. A. Stroud, M. D. Moran, P. A. Makar, S. Gong, W. Gong, J. Zhang, J. G. Slowik, J. P. D. Abbatt, G. Lu, J. R. Brook, C. Mihele, Q. Li, D. Sills, K. B. Strawbridge, M. L. McGuire, and G. J. Evans Observations from the 2007 Border Air Quality and Meteorology Study (BAQS-Met 2007) in southern Ontario (ON), Canada, were used to evaluate Environment Canada's regional chemical transport model predictions of primary organic aerosol (POA). Environment Canada's operational numerical weather prediction model and the 2006 Canadian and 2005 US national emissions inventories were used as input to the chemical transport model (named AURAMS). Particle-component-based factor analysis was applied to aerosol mass spectrometer measurements made at one urban site (Windsor, ON) and two rural sites (Harrow and Bear Creek, ON) to derive hydrocarbon-like organic aerosol (HOA) factors. Co-located carbon monoxide (CO), PM 2.5 black carbon (BC), and PM 1 SO 4 measurements were also used for evaluation and interpretation, permitting a detailed diagnostic model evaluation. At the urban site, good agreement was observed for the comparison of daytime campaign PM 1 POA and HOA mean values: 1.1 μg m −3 vs. 1.2 μg m −3 , respectively. However, a POA overprediction was evident on calm nights due to an overly-stable model surface layer. Biases in model POA predictions trended from positive to negative with increasing HOA values. This trend has several possible explanations, including (1) underweighting of urban locations in particulate matter (PM) spatial surrogate fields, (2) overly-coarse model grid spacing for resolving urban-scale sources, and (3) lack of a model particle POA evaporation process during dilution of vehicular POA tail-pipe emissions to urban scales. Furthermore, a trend in POA bias was observed at the urban site as a function of the BC/HOA ratio, suggesting a possible association of POA underprediction for diesel combustion sources. For several time periods, POA overprediction was also observed for sulphate-rich plumes, suggesting that our model POA fractions for the PM 2.5 chemical speciation profiles may be too high for these point sources. At the rural Harrow site, significant underpredictions in PM 1 POA concentration were found compared to observed HOA concentration and were associated, based on back-trajectory analysis, with (1) transport from the Detroit/Windsor urban complex, (2) longer-range transport from the US Midwest, and (3) biomass burning. Daytime CO concentrations were significantly overpredicted at Windsor but were unbiased at Harrow. Collectively, these biases provide support for a hypothesis that combines a current underweighting of PM spatial surrogate fields for urban locations with insufficient model vertical mixing for sources close to the urban measurement sites. The magnitude of the area POA emissions sources in the US and Canadian inventories (e.g., food cooking, road and soil dust, waste disposal burning) suggests that more effort should be placed at reducing uncertainties in these sectors, especially spatial and temporal surrogates.

Description: The sudden stratospheric warming of the Arctic winter 2009/2010: comparison to other recent warm winters Atmospheric Chemistry and Physics Discussions, 12, 7243-7271, 2012 Author(s): J. Kuttippurath and G. Nikulin The Arctic winter 2009/10 was moderately cold in December. A minor warming occurred around mid-December due to a wave 2 amplification split the lower stratospheric vortex into two lobes. The vortices merged again and formed a relatively large vortex in a few days. The temperatures began to rise by mid-January and triggered a major sudden stratospheric warming (SSW) by the reversal of westerlies in late (24–26) January, driven by a planetary wave 1 with a peak amplitude of about 100 m 2 s −2 at 60° N/10 hPa. The momentum flux associated with this warming showed the largest value in the recent winters, about 450 m 2 s −2 at 60° N/10 hPa. The associated vortex split confined to altitudes below 10 hPa and hence, the major warming (MW) was a vortex displacement event. Large amounts of Eliassen-Palm (EP) and wave 2 EP fluxes (3.9 ×10 5 kg s −2 ) are found shortly before the MW event at 100 hPa over 45–75° N, suggesting a tropospheric preconditioning of the MW event. We observe an increase in SSWs in the Arctic in recent years, as there were 6 MWs in 6 out of the 7 winters of 2003/04–2009/10, which confirms the conclusions of previous studies on the SSWs in winters prior to 2003/04. Each MW event was unique as far as its evolution and related polar processes were concerned. As compared to the MWs in the recent Arctic winters, the strongest MW was observed in 2008/09 and was initiated by a wave 2 event. A detailed diagnosis of ozone loss during the past fifteen years shows that the loss is inversely proportional to the intensity and timing of SSWs in each winter, where early MWs lead to minimal loss. The ozone loss shows a good correlation with the zonal mean amplitude of zonal winds in January over 60–90° N, suggesting a proxy for MWs in the Arctic winters.

Description: Observing the continental-scale carbon balance: assessment of sampling complementarity and redundancy in a terrestrial assimilation system by means of quantitative network design Atmospheric Chemistry and Physics Discussions, 12, 7211-7242, 2012 Author(s): T. Kaminski, P. J. Rayner, M. Voßbeck, M. Scholze, and E. Koffi This paper investigates the relationship between the heterogeneity of the terrestrial carbon cycle and the optimal design of observing networks to constrain it. We combine the methods of quantitative network design and carbon-cycle data assimilation to a hierarchy of increasingly heterogeneous descriptions of the European terrestrial biosphere as indicated by increasing diversity of plant functional types. We employ three types of observations, flask measurements of CO 2 concentrations, continuous measurements of CO 2 and pointwise measurements of CO 2 flux. We show that flux measurements are extremely efficient for relatively homogeneous situations but not robust against increasing or unknown complexity. Here a hybrid approach is necessary and we recommend its use in the development of integrated carbon observing systems.

Description: River monitoring from satellite radar altimetry in the Zambezi River Basin Hydrology and Earth System Sciences Discussions, 9, 3203-3235, 2012 Author(s): C. I. Michailovsky, S. McEnnis, P. A. M. Berry, R. Smith, and P. Bauer-Gottwein Satellite radar altimetry can be used to monitor surface water levels from space. While current and past altimetry missions were designed to study oceans, retracking the waveforms returned over land allows data to be retrieved for smaller water bodies or narrow rivers. In this study, retracked Envisat altimetry data was extracted over the Zambezi River Basin using a detailed river mask based on Landsat imagery. This allowed for stage measurements to be obtained for rivers down to 80 m wide with an RMSE relative to in situ levels of 0.32 to 0.72 m at different locations. The altimetric levels were then converted to discharge using three different methods adapted to different data-availability scenarios: first with an in situ rating curve available, secondly with one simultaneous field measurement of cross-section and discharge, and finally with only historical discharge data available. For the two locations at which all three methods could be applied the accuracies of the different methods were found to be comparable, with RMSE values ranging from 5.5 to 7.4 % terms of high flow estimation relative to in situ gauge measurements. The precision obtained with the different methods was analyzed by running Monte Carlo simulations and also showed comparable values for the three approaches with standard deviations found between 8.2 and 25.8 % of the high flow estimates.

Description: Water management simulation games and the construction of knowledge Hydrology and Earth System Sciences Discussions, 9, 3063-3085, 2012 Author(s): M. Rusca, J. Heun, and K. Schwartz In recent years simulations have become an important part of teaching activities. The reasons behind the popularity of simulation games are twofold. On the one hand, emerging theories on how people learn have called for an experienced-based learning approach. On the other hand, the demand for water management professionals has changed. Three important developments are having considerable consequences for water management programmes, which educate and train these professionals. These developments are the increasing emphasis on integration in water management, the characteristics and speed of reforms in the public sector and the shifting state-society relations in many countries. In response to these developments, demand from the labour market is oriented toward water professionals who need to have both a specialist in-depth knowledge in their own field, as well as the ability to understand and interact with other disciplines and interests. In this context, skills in negotiating, consensus building and working in teams are considered essential for all professionals. In this paper we argue that simulation games have an important role to play in (actively) educating students and training the new generation of water professionals to respond to the above-mentioned challenges. At the same time, simulations are not a panacea for learners and teachers. Challenges of using simulations games include the demands it places on the teacher. Setting up the simulation game, facilitating the delivery and ensuring that learning objectives are achieved requires considerable knowledge and experience as well as considerable time-inputs of the teacher. Moreover, simulation games usually incorporate a case-based learning model, which may neglect or underemphasize theories and conceptualization. For simulations to be effective they have to be embedded in this larger theoretical and conceptual framework. Simulations, therefore, complement rather than substitute traditional teaching methods.

Description: Evaluation of a complementary based model for mapping land surface evapotranspiration Hydrology and Earth System Sciences Discussions, 9, 3029-3062, 2012 Author(s): Z. Sun, Q. Wang, Z. Ouyang, and Y. Yang A modified Priestley-Taylor (P-T) equation was proposed by Venturini et al. (2008) to map actual evapotranspiration (ET) based solely on satellite remote sensing data, involving a parameter based on a scaled temperature between dew point temperature and surface temperature. In this study, however, theoretical analyses and field experimental evidence show that it is hard to obtain this scaled temperature using dew point temperature and surface temperature. This study also presents a new parameterization method using air temperature, surface temperature, and surface temperature of a reference dry surface. The actual ET estimates obtained by means of our proposed parameterization method are validated at a site scale, and a case study is conducted to map actual ET from Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) images using our proposed method. Results of ground-based validation and a case study of mapping ET using ASTER images indicate that the improvement on the modified P-T equation proposed by Venturini et al. (2008) can contribute to generating reliable actual ET.

Description: Precipitation fields interpolated from gauge stations versus a merged radar-gauge precipitation product: influence on modelled soil moisture at local scale and at SMOS scale Hydrology and Earth System Sciences Discussions, 9, 3385-3413, 2012 Author(s): J. T. dall'Amico, W. Mauser, F. Schlenz, and H. Bach For the validation of coarse resolution soil moisture products from missions such as the Soil Moisture and Ocean Salinity (SMOS) mission, hydrological modelling of soil moisture is an important tool. The spatial distribution of precipitation is among the most crucial input data for such models. Thus, reliable time series of precipitation fields are required, but these often need to be interpolated from data delivered by scarcely distributed gauge station networks. In this study, a commercial precipitation product derived by Meteomedia AG from merging radar and gauge data is introduced as a novel means of adding the promising area-distributed information given by a radar network to the more accurate, but point-like measurements from a gauge station network. This precipitation product is first validated against an independent gauge station network. Further, the novel precipitation product is assimilated into the hydrological land surface model PROMET for the Upper Danube Catchment in southern Germany, one of the major SMOS calibration and validation sites in Europe. The modelled soil moisture fields are compared to those obtained when the operational interpolation from gauge station data is used to force the model. The results suggest that the assimilation of the novel precipitation product can lead to deviations of modelled soil moisture in the order of 0.15 m 3 m −3 on small spatial (∼1 km 2 ) and short temporal resolutions (∼1 day). As expected, after spatial aggregation to the coarser grid on which SMOS data are delivered (~195 km 2 ), these differences are reduced to the order of 0.04 m 3 m −3 , which is the accuracy benchmark for SMOS. The results of both model runs are compared to brightness temperatures measured by the airborne L-band radiometer EMIRAD during the SMOS Validation Campaign 2010. Both comparisons yield equally good correlations, confirming the model's ability to realistically model soil moisture fields in the test site. The fact that the two model runs perform similarly in the comparison is likely associated with the lack of substantial rain events before the days on which EMIRAD was flown.

Description: Advancing data assimilation in operational hydrologic forecasting: progresses, challenges, and emerging opportunities Hydrology and Earth System Sciences Discussions, 9, 3415-3472, 2012 Author(s): Y. Liu, A. H. Weerts, M. Clark, H.-J. Hendricks Franssen, S. Kumar, H. Moradkhani, D.-J. Seo, D. Schwanenberg, P. Smith, A. I. J. M. van Dijk, N. van Velzen, M. He, H. Lee, S. J. Noh, O. Rakovec, and P. Restrepo Data assimilation (DA) holds considerable potential for improving hydrologic predictions as demonstrated in numerous research studies. However, advances in hydrologic DA research have not been adequately or timely implemented into operational forecast systems to improve the skill of forecasts to better inform real-world decision making. This is due in part to a lack of mechanisms to properly quantify the uncertainty in observations and forecast models in real-time forecasting situations and to conduct the merging of data and models in a way that is adequately efficient and transparent to operational forecasters. The need for effective DA of useful hydrologic data into the forecast process has become increasingly recognized in recent years. This motivated a hydrologic DA workshop in Delft, The Netherlands in November 2010, which focused on advancing DA in operational hydrologic forecasting and water resources management. As an outcome of the workshop, this paper reviews, in relevant detail, the current status of DA applications in both hydrologic research and operational practices, and discusses the existing or potential hurdles and challenges in transitioning hydrologic DA research into cost-effective operational forecasting tools, as well as the potential pathways and newly emerging opportunities for overcoming these challenges. Several related aspects are discussed, including (1) theoretical or mathematical considerations in DA algorithms, (2) the estimation of different types of uncertainty, (3) new observations and their objective use in hydrologic DA, (4) the use of DA for real-time control of water resources systems, and (5) the development of community-based, generic DA tools for hydrologic applications. It is recommended that cost-effective transition of hydrologic DA from research to operations should be helped by developing community-based, generic modelling and DA tools or frameworks, and through fostering collaborative efforts among hydrologic modellers, DA developers, and operational forecasters.

Description: Impact of climate change on sediment yield in the Mekong River Basin: a case study of the Nam Ou Basin, Lao PDR Hydrology and Earth System Sciences Discussions, 9, 3339-3384, 2012 Author(s): B. Shrestha, M. S. Babel, S. Maskey, A. van Griensven, S. Uhlenbrook, A. Green, and I. Akkharath This paper evaluates the impact of climate change on sediment yield in the Nam Ou Basin located in Northern Laos. The Soil and Water Assessment Tool (SWAT) is used to assess future changes in sediment flux attributable to climate change. Future precipitation and temperature series are constructed through a delta change approach. As per the results, in general, temperature as well as precipitation show increasing trends in both scenarios, A2 and B2. However, monthly precipitation shows both increasing and decreasing trends. The simulation results exhibit that the wet and dry seasonal and annual stream discharges are likely to increase (by up to 15, 17 and 14% under scenario A2; and 11, 5 and 10% under scenario B2 respectively) in the future, which will lead to increased wet and dry seasonal and annual sediment yields (by up to 39, 28 and 36% under scenario A2; and 23, 12 and 22% under scenario B2 respectively). A higher discharge and more sediment flux are expected during the wet seasons, although the changes, percentage-wise, are observed to be higher during the dry months. In conclusion, the sediment yield from the Nam Ou Basin is likely to increase with climate change, which strongly suggests the need for basin-wide sediment management strategies in order to reduce the negative impact of this change.

Description: Correcting the radar rainfall forcing of a hydrological model with data assimilation: application to flood forecasting in the Lez Catchment in Southern France Hydrology and Earth System Sciences Discussions, 9, 3527-3579, 2012 Author(s): E. Harader, V. Borrell Estupina, S. Ricci, M. Coustau, O. Thual, A. Piacentini, and C. Bouvier The present study explores the application of a data assimilation (DA) procedure to correct the radar rainfall inputs of an event-based, distributed, parsimonious hydrological model. A simplified Kalman filter algorithm was built on top of a rainfall-runoff model in order to assimilate discharge observations at the catchment outlet. The study site is the 114 km 2 Lez Catchment near Montpellier, France. This catchment is subject to heavy orographic rainfall and characterized by a karstic geology, leading to flash flooding events. The hydrological model uses a derived version of the SCS method, combined with a Lag and Route transfer function. Because it depends on geographical features and cloud structures, the radar rainfall input to the model is particularily uncertain and results in significant errors in the simulated discharges. The DA analysis was applied to estimate a constant correction to each event hyetogram. The analysis was carried out for 19 events, in two different modes: re-analysis and pseudo-forecast. In both cases, it was shown that the reduction of the uncertainty in the rainfall data leads to a reduction of the error in the simulated discharge. The resulting correction of the radar rainfall data was then compared to the mean field bias (MFB), a corrective coefficient determined using ground rainfall measurements, which are more accurate than radar but have a decreased spatial resolution. It was shown that the radar rainfall corrected using DA leads to improved discharge simulations and Nash criteria compared to the MFB correction.

Description: Numerical modelling of climate change impacts on freshwater lenses on the North Sea Island of Borkum Hydrology and Earth System Sciences Discussions, 9, 3473-3525, 2012 Author(s): H. Sulzbacher, H. Wiederhold, B. Siemon, M. Grinat, J. Igel, T. Burschil, T. Günther, and K. Hinsby A numerical variable-density groundwater model is set up for the North Sea Island of Borkum to estimate climate change impacts on coastal aquifers and especially the situation of barrier islands in the Wadden Sea. The database includes information from boreholes, a seismic survey, a helicopter-borne electromagnetic survey (HEM), monitoring of the freshwater-saltwater boundary by vertical electrode chains in two boreholes, measurements of groundwater table, pumping and slug tests, as well as water samples. Based on a statistical analysis of borehole columns, seismic sections and HEM, a hydrogeological model is set up. The groundwater model is developed using the finite-element programme FEFLOW. The variable-density groundwater model is calibrated on the basis of hydraulic, hydrological and geophysical data, in particular spatial HEM and local monitoring data. Verification runs with the calibrated model show good agreement between measured and computed hydraulic heads. A good agreement is also obtained between measured and computed density or total dissolved solids data for both the entire freshwater lens on a large scale and in the area of the well fields on a small scale. For simulating future changes in this coastal groundwater system until the end of the current century we use the climate scenario A2, specified by the Intergovernmental Panel on Climate Change and in particular the data for the German North Sea coast. Simulation runs show proceeding salinization with time beneath the well fields of the two waterworks Waterdelle and Ostland. The modelling study shows that spreading of well fields is an appropriate protection measure against excessive salinization of the water supply until the end of the current century.

Description: Isoprene emissions in Africa inferred from OMI observations of formaldehyde columns Atmospheric Chemistry and Physics Discussions, 12, 7475-7520, 2012 Author(s): E. A. Marais, D. J. Jacob, T. P. Kurosu, K. Chance, J. G. Murphy, C. Reeves, G. Mills, S. Casadio, D. B. Millet, M. P. Barkley, F. Paulot, and J. Mao We use 2005–2009 satellite observations of formaldehyde (HCHO) columns from OMI to infer biogenic isoprene emissions at monthly 1 × 1° resolution over the African continent. Our work includes new approaches to remove biomass burning influences using OMI absorbing aerosol optical depth data (to account for transport of fire plumes) and anthropogenic influences using AATSR satellite data for persistent small-flame fires (gas flaring). The resulting biogenic HCHO columns (Ω HCHO ) follow closely the distribution of vegetation patterns in Africa. We infer isoprene emission ( E ISOP ) from the local sensitivity S =ΔΩ HCHO /Δ E ISOP derived with the GEOS-Chem chemical transport model using two alternate isoprene oxidation mechanisms, and verify the validity of this approach using AMMA aircraft observations over West Africa and a longitudinal transect across central Africa. Displacement error (smearing) is diagnosed by anomalously high values of S and the corresponding data are removed. We find significant sensitivity of S to NO x under low-NO x conditions that we fit to a linear function of tropospheric column NO 2 from OMI. We estimate a 40% error in our inferred isoprene emissions under high-NO x conditions and 40–90% under low-NO x conditions. Comparison to the state-of-science MEGAN inventory indicates a large overestimate of central African rainforest emissions in that inventory.

Description: Polarization data from SCIAMACHY limb backscatter observations compared to vector radiative transfer model simulations Atmospheric Measurement Techniques Discussions, 5, 2221-2271, 2012 Author(s): P. Liebing, K. Bramstedt, S. Noël, V. Rozanov, H. Bovensmann, and J. P. Burrows SCIAMACHY is a passive imaging spectrometer onboard ENVISAT, designed to obtain trace gas abundances from measured radiances and irradiances in the UV to SWIR range in nadir, limb and occultation viewing modes. Its grating spectrometer introduces a substantial sensitivity to the polarization of the incoming light with nonnegligible effects on the radiometric calibration. To be able to correct for the polarization sensitivity, SCIAMACHY utilizes broadband Polarization Measurement Devices (PMDs). While for the nadir viewing mode the measured atmospheric polarization has been validated against POLDER data (Tilstra and Stammes, 2007, 2010), a similar validation study regarding the limb viewing mode has not yet been performed. This paper aims at an assessment of the quality of the SCIAMACHY limb polarization data. Since limb polarization measurements by other air- or spaceborne instruments in the spectral range of SCIAMACHY are not available, a comparison with radiative transfer simulations by SCIATRAN V3.1(Rozanov et al., 2012) using a wide range of atmospheric parameters is performed. SCIATRAN is a vector radiative transfer model (VRTM) capable of performing calculations of the multiply scattered radiance in a~spherically symmetric atmosphere. The study shows that the limb polarization data exhibit a large systematic bias which is decreasing with wavelength. The most likely reason for this bias is an instrumental phase shift which changes the relative contributions of different Stokes vector components to the PMD signal as compared to on-ground calibration measurements. It is also shown that it is in principle feasible to recalibrate the polarization sensitivity using the in-flight data and the VRTM simulations, enabling also the monitoring of its degradation. Together with an optimization of the algorithm used to calculate the in-flight polarization data an improved polarization correction can increase the radiometric accuracy of SCIAMACHY limb radiance spectra substantially.

Description: Global distribution and climate forcing of marine organic aerosol – Part 2: Effects on cloud properties and radiative forcing Atmospheric Chemistry and Physics Discussions, 12, 7453-7474, 2012 Author(s): B. Gantt, J. Xu, N. Meskhidze, Y. Zhang, A. Nenes, S. J. Ghan, X. Liu, R. Easter, and R. Zaveri In the first part of this paper series (Meskhidze et al., 2011), a treatment of marine organic aerosols (including primary organic aerosol, secondary organic aerosols, and methane sulfonate) was implemented into the Community Atmosphere Model version 5 (CAM5) with a 7-mode Modal Aerosol Module. A series of simulations was conducted to quantify the changes in aerosol and cloud condensation nuclei concentrations in the marine boundary layer. In this study, changes in the cloud microphysical properties and radiative forcing resulting from marine organic aerosols are assessed. Model simulations show that the anthropogenic aerosol indirect forcing (AIF) predicted by CAM5 is decreased in absolute magnitude by up to ~0.10 W m −2 (8%) when marine organic aerosols are included. Changes in the AIF from marine organic aerosols are associated with small global increases in low-level in-cloud droplet number concentration and liquid water path of ~1.3 cm −3 (~1.6%) and 0.2 g m −2 (0.5%), respectively. Areas especially sensitive to changes in cloud properties due to marine organic aerosol include the Southern Ocean, North Pacific Ocean, and North Atlantic Ocean, all of which are characterized by high marine organic emission rates. As climate models are particularly sensitive to the background aerosol concentration, this small but non-negligible change in the AIF due to marine organic aerosols provides a notable link for ocean-ecosystem marine low-level cloud interactions and may be a candidate for consideration in future earth system models.

Description: Ice nuclei in marine air: bioparticles or dust? Atmospheric Chemistry and Physics Discussions, 12, 4373-4416, 2012 Author(s): S. M. Burrows, C. Hoose, U. Pöschl, and M. G. Lawrence Ice nuclei can influence the properties of clouds and precipitation, but their sources and distribution in the atmosphere are still not well known. Particularly little attention has been paid to IN sources in marine environments, although anecdotal evidence suggests that IN populations in remote marine regions may be dominated by biological particles associated with sea spray. In this exploratory model study, we aim to bring attention to this long-neglected topic and identify promising target regions for future field campaigns. We assess the likely global distribution of marine biological ice nuclei using a combination of historical observations, satellite data and model output. By comparing simulated marine biological IN distributions and dust IN distributions, we predict strong regional differences in the importance of marine biological IN relative to dust IN. Our analysis suggests that marine biological IN are most likely to play a dominant role in determining IN concentrations over the Southern Ocean, so future field campaigns aimed at investigating marine biological IN should target that region. Climate-related changes in the abundance and emission of biological marine IN could affect marine cloud properties, thereby introducing previously unconsidered feedbacks that influence the hydrological cycle and the Earth's energy balance. Furthermore, marine biological IN may be an important aspect to consider in proposals for marine cloud brightening by artificial sea spray production.

Description: An observation-based approach to identify local natural dust events from routine aerosol ground monitoring Atmospheric Chemistry and Physics Discussions, 12, 4279-4310, 2012 Author(s): D. Q. Tong, M. Dan, T. Wang, and P. Lee Dust is a major component of atmospheric aerosols in many parts of the world. Although there exist many routine aerosol monitoring networks, it is often difficult to obtain dust records from these networks, because these monitors are either deployed far away from dust active regions (most likely collocated with dense population) or contaminated by anthropogenic sources and other natural sources, such as wildfires and vegetation detritus. Here we propose a new approach to identify local dust events relying solely on aerosol mass and composition from general-purpose aerosol measurements. Through analyzing the chemical and physical characteristics of aerosol observations during satellite-detected dust episodes, we select five indicators to be used to identify local dust records: (1) high PM 10 concentrations; (2) low PM 2.5 /PM 10 ratio; (3) higher concentrations and percentage of crustal elements; (4) lower percentage of anthropogenic pollutants; and (5) low enrichment factors of anthropogenic elements. After establishing these identification criteria, we conduct hierarchical cluster analysis for all validated aerosol measurement data over 68 IMPROVE sites in the Western United States. A total of 182 local dust events were identified over 30 of the 68 locations from 2000 to 2007. These locations are either close to the four US Deserts, namely the Great Basin Desert, the Mojave Desert, the Sonoran Desert, and the Chihuahuan Desert, or in the high wind power region (Colorado). During the eight-year study period, the total number of dust events displays an interesting four-year activity cycle (one in 2000–2003 and the other in 2004–2007). The years of 2003, 2002 and 2007 are the three most active dust periods, with 46, 31 and 24 recorded dust events, respectively, while the years of 2000, 2004 and 2005 are the calmest periods, all with single digit dust records. Among these deserts, the Chihuahua Desert (59 cases) and the Sonoran Desert (62 cases) are by far the most active source regions. In general, the Chihuahua Desert dominates dust activities in the first half of the eight-year period while the Sonoran Desert in the second half. The monthly frequency of dust events shows a peak from March to July and a second peak in autumn from September to November. The large quantity of dust events occurring in summertime also suggests the prevailing impact of windblown dust across the year. This seasonal variation is consistent with previous model simulations over the United States.

Description: Quantifying retrieval uncertainties in the CM-SAF cloud physical property algorithm with simulated SEVIRI observations Atmospheric Chemistry and Physics Discussions, 12, 4311-4340, 2012 Author(s): B. J. Jonkheid, R. A. Roebeling, and E. van Meijgaard The uncertainties in the cloud physical properties derived from satellite observations make it difficult to interpret model evaluation studies. In this paper, the uncertainties in the cloud water path (CWP) retrievals derived with the cloud physical properties retrieval algorithm (CPP) of the climate monitoring satellite application facility (CM-SAF) are investigated. To this end, a numerical simulator of MSG-SEVIRI observations was developed that calculates the reflectances at 0.64 and 1.63 μm for a wide range of cloud parameters, satellite viewing geometries and surface albedos. These reflectances are used as input to CPP, and the retrieved values of CWP are compared to the original input of the simulator. It is shown that the CWP retrievals are very sensitive to the assumptions made in the CPP code. The CWP retrieval errors are generally small for unbroken single-phase clouds with COT 〉10, with retrieval errors of ~3% for liquid water clouds to ~10% for ice clouds. When both liquid water and ice clouds are present in a pixel, the CWP retrieval errors increase dramatically; depending on the cloud, this can lead to uncertainties of 40–80%. CWP retrievals also become more uncertain when the cloud does not cover the entire pixel, leading to errors of ~50% for cloud fractions of 0.75 and even larger errors for smaller cloud fractions. Thus, the satellite retrieval of cloud physical properties of broken clouds and multi-phase clouds is complicated by inherent difficulties, and the proper interpretation of such retrievals requires extra care.

Description: Enhanced cold-season warming in semi-arid regions Atmospheric Chemistry and Physics Discussions, 12, 4627-4653, 2012 Author(s): J. Huang, X. Guan, and F. Ji This study examined surface air temperature trends over global land from 1901–2009. It is found that the warming trend was particularly enhanced, in the boreal cold season (November to March) over semi-arid regions (with precipitation of 200–600 mm yr −1 ), showing a temperature increase of 1.53 °C as compared to the global annual mean temperature increase of 1.13 °C over land. In mid-latitude semi-arid areas of Europe, Asia, and North America, temperatures in the cold season increased by 1.41, 2.42, and 1.5 °C, respectively. The semi-arid regions contribute 44.46% to global annual-mean land-surface temperature trend. The mid-latitude semi-arid regions in the Northern Hemisphere accounting contribute by 27.0% of the total, with the mid-latitude semi-arid areas in Europe, Asia, and North America accounting for 6.29%, 13.81%, and 6.85%, respectively. Such enhanced semi-arid warming (ESAW) may cause these regions to become drier and warmer.

Description: Humidity-dependent phase state of SOA particles from biogenic and anthropogenic precursors Atmospheric Chemistry and Physics Discussions, 12, 4447-4476, 2012 Author(s): E. Saukko, A. T. Lambe, P. Massoli, Koop, T., J. P. Wright, D. R. Croasdale, D. A. Pedernera, T. B. Onasch, A. Laaksonen, P. Davidovits, D. R. Worsnop, and A. Virtanen The physical phase state (solid, semi-solid, or liquid) of secondary organic aerosol (SOA) particles has important implications for a number of atmospheric processes. We report the phase state of SOA particles spanning a wide range of oxygen to carbon ratios (O/C), used here as a surrogate for SOA oxidation level, produced in a flow tube reactor by photo-oxidation of various atmospherically relevant surrogate anthropogenic and biogenic volatile organic compounds (VOCs). The phase state of laboratory-generated SOA was determined by the particle bounce behavior after inertial impaction on a polished steel substrate. The measured bounce fraction was evaluated as a function of relative humidity and SOA oxidation level (O/C) measured by an Aerodyne high resolution time of flight aerosol mass spectrometer (HR-ToF AMS). The main findings of the study are: (1) Biogenic and anthropogenic SOA particles are found to be solid or semi-solid until a relative humidity of at least 50 % RH at impaction is reached. (2) Long-chain alkanes produce liquid SOA particles when generated at low oxidation level of O/C

Description: Carbonaceous aerosol AAE inferred from in-situ aerosol measurements at the Gosan ABC super site, and the implications for brown carbon aerosol Atmospheric Chemistry and Physics Discussions, 12, 4507-4539, 2012 Author(s): C. E. Chung, S.-W. Kim, M. Lee, S.-C. Yoon, and S. Lee Carbon mass of aerosols and its division between EC and OC sources were continuously measured at hourly intervals from October 2009 to June 2010. During this 9-month period, we also measured the aerosol absorption coefficient at 7 wavelengths and obtained PM mass density data at 1-h resolution. The measurement was made at the Gosan ABC super site, which is an ideal location for monitoring long-range transported aerosols from China. We remove the absorption data with significant dust influence using the mass ratio of PM 10 to PM 2.5 . The remaining data shows an Absorption Ångström Exponent (AAE) of about 1.27, which we suggest represent the average carbonaceous aerosol (CA) AAE at Gosan. CA AAE is highest in winter, in which the monthly value is near 1.4. We find a positive correlation between the mass ratio of OC to EC and CA AAE, and successfully increase the correlation by filtering out data associated with weak absorption signal. After the filtering, absorption coefficient is regressed on OC and EC mass densities. Black carbon (BC) and organic aerosol (OA) absorption cross sections per carbon mass are found to be 5.1 (4.2–6.0) and 1.4 (1.1–1.8) m 2 g −1 at 520 nm respectively. From the estimated BC & OA MAC, we find that OA contributes about 45% to CA absorption at 520 nm. OA AAE is found to be 1.7 (1.4–2.1). Compared with a previous estimate of OA MAC and AAE, our estimates at Gosan strongly suggest that the strongly-absorbing so-called brown carbon spheres are either unrelated to biomass burning or absent near the emission source.

Description: Organic molecular markers and signature from wood combustion particles in winter ambient aerosols: aerosol mass spectrometer (AMS) and high time-resolved GC-MS measurements in Augsburg, Germany Atmospheric Chemistry and Physics Discussions, 12, 4831-4866, 2012 Author(s): M. Elsasser, M. Crippa, J. Orasche, P. F. DeCarlo, M. Oster, M. Pitz, T. L. Gustafson, J. B. C. Pettersson, J. Schnelle-Kreis, A. S. H. Prévôt, and R. Zimmermann The impact of wood combustion on ambient aerosols was investigated in Augsburg, Germany during a winter measurement campaign of a six-week period. Special attention was paid to the high time resolution observations of wood combustion with different mass spectrometric methods. Here we present and compare the results from an Aerodyne aerosol mass spectrometer (AMS) and gas chromatographic – mass spectrometric (GC-MS) analysed PM 1 filters on an hourly basis. This includes source apportionment of the AMS derived organic mass by using positive matrix factorisation (PMF) and analysis of levoglucosan as wood combustion marker, respectively. In the measurement period nitrate and organics are the main contributors to the defined submicron particle mass with 28% and 35%, respectively. To the latter wood combustion organic aerosol (WCOA) contributes 23% on average and 27% in the evening and night-time. Conclusively, wood combustion has a strong influence on the organics and overall aerosol composition. Levoglucosan accounts for 14% of WCOA mass with a higher percentage in comparison to other studies. The ratio between the mass of levoglucosan and organic carbon amounts to 0.06. This study is unique in the one-hour time resolution comparison between the wood combustion results of the AMS and the GC-MS analysed filter method at a PM 1 particle size range. This comparison of the concentration courses of the PMF WCOA factor, levoglucosan estimated by the AMS data and the levoglucosan measured by GC-MS is highly correlated, and a detailed discussion on the contributors to the wood combustion marker ion at mass-to-charge ratio 60 will be given. This offers a suitable application possibility for the description of the wood combustion course by the WCOA factor and the levoglucosan concentration estimated by AMS data. However, quantitative description of the levoglucosan concentration estimated by the AMS data is difficult due to the offset of latter compared to measured levoglucosan by the GC-MS.

Description: An isotope view on ionising radiation as a source of sulphuric acid Atmospheric Chemistry and Physics Discussions, 12, 5039-5064, 2012 Author(s): M. B. Enghoff, N. Bork, S. Hattori, C. Meusinger, M. Nakagawa, J. O. P. Pedersen, S. Danielache, Y. Ueno, M. S. Johnson, N. Yoshida, and H. Svensmark Sulphuric acid is an important factor in aerosol nucleation and growth. It has been shown that ions enhance the formation of sulphuric acid aerosols, but the exact mechanism has remained undetermined. Furthermore some studies have found a deficiency in the sulphuric acid budget, suggesting a missing source. In this study the production of sulphuric acid from SO 2 through a number of different pathways is investigated. The production methods are standard gas phase oxidation by OH radicals produced by ozone photolysis with UV light, liquid phase oxidation by ozone, and gas phase oxidation initiated by gamma rays. The distributions of stable sulphur isotopes in the products and substrate were measured using isotope ratio mass spectrometry. All methods produced sulphate enriched in 34 S and we find a δ 34 S value of 8.7 ± 0.4‰ (1 standard deviation) for the UV-initiated OH reaction. Only UV light (Hg emission at 253.65 nm) produced a clear non-mass-dependent excess of 33 S. The pattern of isotopic enrichment produced by gamma rays is similar, but not equal, to that produced by aqueous oxidation of SO 2 by ozone. This, combined with the relative yields of the experiments, suggests a mechanism in which ionising radiation may lead to hydrated ion clusters that serve as nanoreactors for S(IV) to S(VI) conversion.

Description: First intercalibration of column-averaged methane from the Total Carbon Column Observing Network and the Network for the Detection of Atmospheric Composition Change Atmospheric Measurement Techniques Discussions, 5, 1355-1379, 2012 Author(s): F. Forster, R. Sussmann, M. Rettinger, N. M. Deutscher, D. W. T. Griffith, N. Jones, and P. K. Patra We present the intercalibration of dry-air column-averaged mole fractions of methane (XCH 4 ) retrieved from solar FTIR measurements of the Network for the Detection of Atmospheric Composition Change (NDACC) in the mid-infrared (MIR) versus near-infrared (NIR) soundings from the Total Carbon Column Observing Network (TCCON). The study uses multi-annual quasi-coincident MIR and NIR measurements from the stations Garmisch, Germany (47.48° N, 11.06° E, 743 m a.s.l.) and Wollongong, Australia (34.41° S, 150.88° E, 30 m a.s.l.). Direct comparison of the retrieved MIR and NIR time series shows a phase shift in XCH 4 seasonality, i.e. a significant time-dependent bias leading to a standard deviation (stdv) of the difference time series (NIR-MIR) of 8.4 ppb. After eliminating differences in a prioris by using ACTM-simulated profiles as a common prior, the seasonalities of the (corrected) MIR and NIR time series agree within the noise (stdv = 5.2 ppb for the difference time series). The difference time series (NIR-MIR) do not show a significant trend. Therefore it is possible to use a simple scaling factor for the intercalibration without a time-dependent linear or seasonal component. Using the Garmisch and Wollongong data together, we obtain an overall calibration factor MIR/NIR = 0.9926(18). The individual calibration factors per station are 0.9940(14) for Garmisch and 0.9893(40) for Wollongong. They agree within their error bars with the overall calibration factor which can therefore be used for both stations. Our results suggest that after applying the proposed intercalibration concept to all stations performing both NIR and MIR measurements, it should be possible to obtain one refined overall intercalibration factor for the two networks. This would allow to set up a harmonized NDACC and TCCON XCH 4 data set which can be exploited for joint trend studies, satellite validation, or the inverse modeling of sources and sinks.

Description: Moving beyond traditional model calibration or how to better identify realistic model parameters: sub-period calibration Hydrology and Earth System Sciences Discussions, 9, 1885-1918, 2012 Author(s): S. Gharari, M. Hrachowitz, F. Fenicia, and H. H. G. Savenije Conceptual hydrological models often rely on calibration for the identification of their parameters. As these models are typically designed to reflect real catchment processes, a key objective of an appropriate calibration strategy is the determination of parameter sets that reflect a "realistic" model behavior. Previous studies have shown that parameter estimates for different calibration periods can be significantly different. This questions model transposability in time, which is one of the key conditions for the set-up of a "realistic" model. This paper presents a new approach that selects parameter sets that provide a consistent model performance in time. The approach consists of confronting model performance in different periods, and selecting parameter sets that are as close as possible to the optimum of each individual sub-period. While aiding model calibration, the approach is also useful as a diagnostic tool, illustrating tradeoffs in the identification of time consistent parameter sets. The approach is demonstrated in a case study where we illustrate the multi-objective calibration of the HyMod hydrological model to a Luxembourgish catchment.

Description: Characterization and airborne deployment of a new counterflow virtual impactor inlet Atmospheric Measurement Techniques Discussions, 5, 1515-1541, 2012 Author(s): T. Shingler, S. Dey, A. Sorooshian, F. J. Brechtel, Z. Wang, A. Metcalf, M. Coggon, J. Mülmenstädt, L. M. Russell, H. H. Jonsson, and J. H. Seinfeld A new counterflow virtual impactor (CVI) inlet is introduced with details of its design, laboratory characterization tests, and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i) negligible organic contamination; (ii) a significant sample flow rate to downstream instruments (~15 l min −1 ) that reduces the need for dilution; and (iii) a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterized the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7–13.1 μm. The percentage error between cut size measurements and predictions from aerodynamic drag theory are less than 13%. The CVI was deployed on the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California between July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter.

Description: Improved cloud screening in MAIAC aerosol retrievals using spectral and spatial analysis Atmospheric Measurement Techniques Discussions, 5, 1575-1595, 2012 Author(s): A. Lyapustin, Y. Wang, I. Laszlo, and S. Korkin An improved cloud/snow screening technique in the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm is described. It is implemented as part of MAIAC aerosol retrievals based on analysis of spectral residuals and spatial variability. Comparisons with AERONET aerosol measurements and a large-scale MODIS data analysis show strong suppression of aerosol optical depth outliers due to unresolved clouds and snow. At the same time, the developed filter does not reduce the aerosol retrieval capability at high 1 km resolution in strongly inhomogeneous environments, such as near centers of the active fires. Despite significant improvement, the optical depth outliers in high spatial resolution data are and will remain the problem to be addressed by the application-dependent specialized filtering techniques.

Description: Impacts of inhomogeneous landscapes in oasis interior on the oasis self-maintaining mechanism by integrating numerical model with satellite data Hydrology and Earth System Sciences Discussions, 9, 1979-2004, 2012 Author(s): X. Meng, S. Lu, T. Zhang, Y. Ao, S. Li, Y. Bao, L. Wen, and S. Luo Mesoscale meteorological modeling is an important tool to help understand the energy budget of the oasis. While basic dynamic and thermodynamic processes for oasis self-maintaining in the desert environment is well investigated, influence of heterogeneous landscapes of oasis interior on the processes are still important and remain to be investigated. In this study, two simulations are designed for investigating the influence of inhomogeneity. In the first case, land surface parameters including land-use types, vegetation cover fraction, and surface layer soil moisture are derived by satellite remote sensing data from EOS/MODIS, and then be used specify the respective options in the MM5 model, to describe a real inhomogeneity for the oasis interior. In the other run, land use types are set to MM5 default, in which landscapes in the oasis interior is relative uniform, and then surface layer soil moisture and vegetation fraction is set to be averages of the first case for the respective oasis and desert surface lying, to represent a relative homogeneity. Results show that the inhomogeneity leads to a weaker oasis "cold-wet island" effect and a stronger turbulence over the oasis interior, both of which will reduce the oasis-desert secondary circulation and increase the evaporation over the oasis, resulting in a negative impact on the oasis self-protecting mechanism. The simulation of homogeneity indicates that the oasis may be more stable even with relative lower soil moisture if landscapes in the oasis interior are comparatively uniform.

Description: Measuring variations of δ 18 O and δ 2 H in atmospheric water vapour using laser spectroscopy: an instrument characterisation study Atmospheric Measurement Techniques Discussions, 5, 1597-1655, 2012 Author(s): F. Aemisegger, P. Sturm, P. Graf, H. Sodemann, S. Pfahl, A. Knohl, and H. Wernli Variations of stable water isotopes in water vapour have become measurable at a measurement frequency of about 1 Hz in recent years using novel laser spectroscopic techniques. This enables us to perform continuous measurements for process-based investigations of the atmospheric water cycle at the time scales relevant for synoptic meteorology. An important prerequisite for the interpretation of data from automated field measurements lasting for several weeks or months is a detailed knowledge about instrument properties and the sources of measurement uncertainty. We present here a comprehensive characterisation and comparison study of two commercial laser spectroscopic systems based on cavity ring-down spectroscopy (Picarro) and off-axis integrated cavity output spectroscopy (Los Gatos Research). The uncertainty components of the measurements were first assessed in laboratory experiments, focussing on the effects of (i) water vapour mixing ratio, (ii) measurement stability, (iii) uncertainties due to calibration and (iv) response times of the isotope measurements due to adsorption-desorption processes on the tubing and measurement cavity walls. Based on the experience from our laboratory experiments we set up a one-week field campaign for comparing measurements of the ambient isotope signals of the two laser spectroscopic systems. The optimal calibration strategy determined for both instruments was applied as well as the correction functions for water vapour mixing ratio effects. The root mean square difference between the isotope signals from the two instruments during the field deployment was 2.3‰ for δ 2 H, 0.5‰ for δ 18 O and 3.1‰ for deuterium excess. These uncertainty estimates from field measurements compare well to those found in the laboratory experiments. The present quality of measurements from laser spectroscopic instruments combined with a calibration system opens new possibilities for investigating the atmospheric water cycle and the land-atmosphere moisture fluxes.

Description: Calibration and evaluation of a semi-distributed watershed model of sub-Saharan Africa using GRACE data Hydrology and Earth System Sciences Discussions, 9, 2071-2120, 2012 Author(s): H. Xie, L. Longuevergne, C. Ringler, and B. Scanlon Irrigation development is rapidly expanding in mostly rainfed Sub-Saharan Africa. This expansion underscores the need for a more comprehensive understanding of water resources beyond surface water. Gravity Recovery and Climate Experiment (GRACE) satellites provide valuable information on spatio-temporal variability of water storage. The objective of this study was to calibrate and evaluate a semi-distributed regional-scale hydrological model, or a large-scale application of the Soil and Water Assessment Tool (SWAT) model, for basins in Sub-Saharan Africa using seven-year (2002–2009) 10-day GRACE data. Multi-site river discharge data were used as well, and the analysis was conducted in a multi-criteria framework. In spite of the uncertainty arising from the tradeoff in optimizing model parameters with respect to two non-commensurable criteria defined for two fluxes, it is concluded that SWAT can perform well in simulating total water storage variability in most areas of Sub-Saharan Africa, which have semi-arid and sub-humid climates, and that among various water storages represented in SWAT, the water storage variations from soil, the vadose zone, and groundwater are dominant. On the other hand, the study also showed that the simulated total water storage variations tend to have less agreement with the GRACE data in arid and equatorial humid regions, and the model-based partition of total water storage variations into different water storage compartments could be highly uncertain. Thus, future work will be needed for model enhancement in these areas with inferior model fit and for uncertainty reduction in component-wise estimation of water storage variations.

Description: Quantification of diesel exhaust gas phase organics by a thermal desorption proton transfer reaction mass spectrometer Atmospheric Chemistry and Physics Discussions, 12, 5389-5423, 2012 Author(s): M. H. Erickson, H. W. Wallace, and B. T. Jobson A new approach was developed to measure the total abundance of long chain alkanes (C 12 and above) in urban air using thermal desorption with a proton transfer reaction mass spectrometer (PTR-MS). These species are emitted in diesel exhaust and may be important precursors to secondary organic aerosol production in urban areas. Long chain alkanes undergo dissociative proton transfer reactions forming a series of fragment ions with formula C n H 2n+1 . The yield of the fragment ions is a function of drift conditions. At a drift field strength of 80 Townsends, the most abundant ion fragments from C 10 to C 16 n -alkanes were m/z 57, 71 and 85. The PTR-MS is insensitive to n -alkanes less than C 8 but displays an increasing sensitivity for larger alkanes. Higher drift field strengths yield greater normalized sensitivity implying that the proton affinity of the long chain n -alkanes is less than H 2 O. Analysis of diesel fuel shows the mass spectrum was dominated by alkanes (C n H 2 n +1 ), monocyclic aromatics, and an ion group with formula C n H 2 n −1 ( m/z 97, 111, 125, 139). The PTR-MS was deployed in Sacramento, CA during the Carbonaceous Aerosols and Radiative Effects Study field experiment in June 2010. The ratio of the m/z 97 to 85 ion intensities in ambient air matched that found in diesel fuel. Total diesel exhaust alkane concentrations calculated from the measured abundance of m/z 85 ranged from the method detection limit of ~1 μg m −3 to 100 μg m −3 in several air pollution episodes. The total diesel exhaust alkane concentration determined by this method was on average a factor of 10 greater than the sum of alkylbenzenes associated with spark ignition vehicle exhaust.

Description: Arctic climate response to forcing from light-absorbing particles in snow and sea ice in CESM Atmospheric Chemistry and Physics Discussions, 12, 5341-5388, 2012 Author(s): N. Goldenson, S. J. Doherty, C. M. Bitz, M. M. Holland, B. Light, and A. J. Conley The presence of light-absorbing aerosol particles deposited on arctic snow and sea ice influences the surface albedo, causing greater shortwave absorption, warming, and loss of snow and sea ice, lowering the albedo further. The Community Earth System Model version 1 (CESM1) now includes the radiative effects of light-absorbing particles in snow on land and sea ice and in sea ice itself. We investigate the model response to the deposition of black carbon and dust to both snow and sea ice. For these purposes we employ a slab ocean version of CESM1, using the Community Atmosphere Model version 4 (CAM4), run to equilibrium for year 2000 levels of CO 2 and fixed aerosol deposition. We construct experiments with and without aerosol deposition, with dust or black carbon deposition alone, and with varying quantities of black carbon and dust to approximate year 1850 and 2000 deposition fluxes. The year 2000 deposition fluxes of both dust and black carbon cause 1–2 °C of surface warming over large areas of the Arctic Ocean and sub-Arctic seas in autumn and winter and in patches of Northern land in every season. Atmospheric circulation changes are a key component of the surface-warming pattern. Arctic sea ice thins by on average about 30 cm. Simulations with year 1850 aerosol deposition are not substantially different from those with year 2000 deposition, given constant levels of CO 2 . The climatic impact of particulate impurities deposited over land exceeds that of particles deposited over sea ice. Even the surface warming over the sea ice and sea ice thinning depends more upon light-absorbing particles deposited over land. For CO 2 doubled relative to year 2000 levels, the climate impact of particulate impurities in snow and sea ice is substantially lower than for the year 2000 equilibrium simulation.

Description: Consistency between Fourier transform and small-volume few-wave decomposition for spectral and spatial variability of gravity waves above a typhoon Atmospheric Measurement Techniques Discussions, 5, 1763-1793, 2012 Author(s): C. I. Lehmann, Y.-H. Kim, P. Preusse, H.-Y. Chun, M. Ern, and S.-Y. Kim Convective gravity wave (GW) sources are spatially localized and emit at the same time waves with a wide spectrum of phase speeds. Any wave analysis therefore compromises between spectral and spatial resolution. Future satellite borne limb imagers will for a first time provide real 3d volumes of observations. These volumes will be however limited which will impose further constraints on the analysis technique. In this study a three dimensional few-wave appoach fitting sinusoidal waves to limited 3-D volumes is introduced. The method is applied to simulated GWs above typhoon Ewiniar and GW momentum flux is estimated from temperature fluctuations. Phase speed spectra as well as average profiles of positive, negative and net momentum fluxes are compared to momentum flux estimated by Fourier transform as well as spatial averaging of wind fluctuations. The results agree within 10–20%. The few-wave method can also reveal the spatial orientation of the GWs with respect to the source. The relevance of the results for different types of measurements as well as its applicability to model data is discussed.

Description: Modelling of organic aerosols over Europe (2002–2007) using a volatility basis set (VBS) framework with application of different assumptions regarding the formation of secondary organic aerosol Atmospheric Chemistry and Physics Discussions, 12, 5425-5485, 2012 Author(s): R. Bergström, H. A. C. Denier van der Gon, A. S. H. Prévôt, K. E. Yttri, and D. Simpson A new organic aerosol (OA) module has been implemented into the EMEP chemical transport model. Four different volatility basis set (VBS) schemes have been tested in long-term simulations for Europe, covering the six years 2002–2007. Different assumptions regarding partitioning of primary OA (POA) and aging of POA and secondary OA (SOA), have been explored. Model results are compared to filter measurements, AMS-data and source-apportionment studies, as well as to other model studies. The present study indicates that many different sources contribute significantly to OA in Europe. Fossil POA and oxidised POA, biogenic and anthropogenic SOA (BSOA and ASOA), residential burning of biomass fuels and wildfire emissions may all contribute more than 10% each over substantial parts of Europe. Simple VBS based OA models can give reasonably good results for summer OA but more observational studies are needed to constrain the VBS parameterisations and to help improve emission inventories. The volatility distribution of primary emissions is an important issue for further work. This study shows smaller contributions from BSOA to OA in Europe than earlier work, but relatively greater ASOA. BVOC emissions are highly uncertain and need further validation. We can not reproduce winter levels of OA in Europe, and there are many indications that the present emission inventories substantially underestimate emissions from residential wood burning in large parts of Europe.

Description: Elusive drought: uncertainty in observed trends and short- and long-term CMIP5 projections Hydrology and Earth System Sciences Discussions, 9, 13773-13803, 2012 Author(s): B. Orlowsky and S. I. Seneviratne Recent years have seen a number of severe droughts in different regions around the world, causing agricultural and economic losses, famines and migration. Despite their devastating consequences, the Standardised Precipitation Index (SPI) of these events lies within the range of internal climate variability, which we estimate from simulations from the 5th phase of the Coupled Model Intercomparison Project (CMIP5). In terms of drought magnitude, regional trends of SPI over the last decades remain mostly inconclusive in observations and CMIP5 simulations, although Soil Moisture Anomalies (SMAs) in CMIP5 simulations hint at increased drought in a few regions (e.g. the Mediterranean, Central America/Mexico, the Amazon, North-East Brazil and South Africa). Also for the future, projections of meteorological (SPI) and agricultural (SMA) drought in CMIP5 display large uncertainties over all time frames, generally impeding trend detection. Analogue analyses of the frequencies rather than magnitudes of future drought display, however, more robust signal-to-noise ratios with detectable trends towards more frequent drought until the end of the 21st century in the Mediterranean, South Africa and Central America/Mexico. Other present-day hot spots are projected to become less drought-prone, or to display unsignificant changes in drought occurrence. A separation of different sources of uncertainty in drought projections reveals that for the near term, internal climate variability is the dominant source, while the formulation of Global Climate Models (GCMs) generally becomes the dominant source of uncertainty by the end of the 21st century, especially for agricultural (soil moisture) drought. In comparison, the uncertainty in Green-House Gas (GHG) concentrations scenarios is negligible for most regions. These findings stand in contrast to respective analyses for a heat wave indicator, for which GHG concentrations scenarios constitute the main source of uncertainty. Our results highlight the inherent difficulty of drought quantification and the uncertainty of drought projections. However, high uncertainty should not be equated with low drought risk, since potential scenarios include large drought increases in key agricultural and ecosystem regions.

Description: Dissolved organic carbon (DOC) and select aldehydes in cloud and fog water: the role of the aqueous phase in impacting trace gas budgets Atmospheric Chemistry and Physics Discussions, 12, 33083-33125, 2012 Author(s): B. Ervens, Y. Wang, J. Eagar, W. R. Leaitch, A. M. Macdonald, K. T. Valsaraj, and P. Herckes Cloud and fog droplets efficiently scavenge and process water-soluble compounds and thus modify the chemical composition of the gas and particle phases. The concentrations of dissolved organic carbon (DOC) in the aqueous phase reach concentrations on the order of ~10 mg C L −1 which is typically on the same order of magnitude as the sum of inorganic anions. Aldehydes and carboxylic acids typically comprise a large fraction of DOC because of their high solubility. The dissolution of species in the aqueous phase can lead to (i) the removal of species from the gas phase preventing their processing by gas phase reactions (e.g. photolysis of aldehydes) and (ii) the formation of unique products that do not have any efficient gas phase sources (e.g. dicarboxylic acids). We present measurements of DOC and select aldehydes in fog water at high elevation and intercepted clouds in a biogenically-impacted location (Whistler, Canada) and in fog water in a more polluted area (Davis, CA). Concentrations of formaldehyde, glyoxal and methylglyoxal were in the micromolar range and comprised ≤2% each individually of the DOC. Comparison of the DOC and aldehyde concentrations to those at other locations shows good agreement and reveals highest levels for both in anthropogenically impacted regions. Based on this overview, we conclude that the fraction of organic carbon (dissolved and insoluble inclusions) in the aqueous phase comprises 1–~40% of total organic carbon. Higher values are observed to be associated with aged air masses where organics are expected to be more highly oxidized and thus more soluble. Accordingly, the aqueous/gas partitioning ratio expressed here as an effective Henry's law constant for DOC ( K H *DOC ) increases by an order of magnitude from 7×10 3 M atm −1 to 7×10 4 M atm −1 during the ageing of air masses. The measurements are accompanied by photochemical box model simulations. They suggest that the scavenging of aldehydes by the aqueous phase can reduce HO 2 gas phase levels by two orders of magnitude due to a weaker net source of HO 2 production from aldehyde photolysis in the gas phase. Despite the high solubility of dialdehydes (glyoxal, methylglyoxal), their impact on the HO 2 budget by scavenging is

Description: Limitations of wind extraction from 4-D-Var assimilation of trace gases Atmospheric Chemistry and Physics Discussions, 12, 32985-33023, 2012 Author(s): D. R. Allen, K. W. Hoppel, G. E. Nedoluha, D. D. Kuhl, N. L. Baker, L. Xu, and T. E. Rosmond Time-dependent variational data assimilation allows the possibility of extracting wind information from observations of long-lived trace gases. Since trace gas observations are not available at sufficient resolution for deriving feature-track winds, they must be combined with model background information to produce an analysis. If done with time-dependent variational assimilation, wind information may be extracted via the adjoint of the linearized tracer continuity equation. This paper presents idealized experiments that illustrate the mechanics of tracer-wind extraction and demonstrate some of the limitations of this procedure. We first examine tracer-wind extraction using a simple one-dimensional advection equation. The analytic solution for a single trace gas observation is discussed along with numerical solutions for multiple observations. The limitations of tracer-wind extraction are then explored using highly idealized ozone experiments performed with a development version of the Navy Global Environmental Model (NAVGEM) in which stratospheric globally-distributed hourly stratospheric ozone profiles are assimilated in a single 6-h update cycle in January 2009. Starting with perfect background ozone conditions, but imperfect dynamical conditions, ozone errors develop over the 6-h background window. Wind increments are introduced in the analysis in order to reduce the differences between background ozone and ozone observations. For "perfect" observations (unbiased and no random error), this results in root mean square (RMS) vector wind error reductions of up to ∼ 3 m s −1 in the winter hemisphere and tropics. Wind extraction is more difficult in the summer hemisphere due to weak ozone gradients and smaller background wind errors. The limitations of wind extraction are also explored for observations with imposed random errors and for limited sampling patterns. As expected, the amount of wind information extracted degrades as observation errors or data voids increase. In the case of poorly specified observation error covariances, assimilation of ozone data with imposed errors may result in erroneous wind increments, since the assimilation is constrained too tightly to the noisy observations.

Description: Mass deposition fluxes of Saharan mineral dust to the tropical northeast Atlantic Ocean: an intercomparison of methods Atmospheric Chemistry and Physics Discussions, 12, 33025-33081, 2012 Author(s): N. Niedermeier, A. Held, T. Müller, B. Heinold, K. Schepanski, I. Tegen, K. Kandler, M. Ebert, S. Weinbruch, K. Read, J. Lee, K. W. Fomba, K. Müller, H. Herrmann, and A. Wiedensohler The aim of this study is to determine the mass deposition flux of mineral dust to the tropical northeast Atlantic Ocean at the Cape Verde Atmospheric Observatory (CVAO) on the island Sao Vicente for January 2009. Five different methods were applied to estimate the deposition flux, using different meteorological and microphysical measurements, remote sensing, and regional dust transport simulations. The set of observations comprises micrometeorological measurements with an ultra-sonic anemometer and profile measurements using 2-D anemometers at two different heights, and microphysical measurements of the size-resolved mass concentrations of mineral dust. In addition, the total mass concentration of mineral dust was derived from absorption photometer observations and passive sampling. The regional dust model COSMO-MUSCAT was used for simulations of dust emission and transport, including dry and wet deposition processes. The four observation-based methods yield a monthly average deposition flux of mineral dust of 12–29 ng m −2 s −1 . The simulation results come close to the upper range of the measurements with an average value of 47 ng m −2 s −1 . It is shown, that the mass deposition flux of mineral dust obtained by the combination of micrometeorological (ultra-sonic anemometer) and microphysical measurements (particle mass size distribution of mineral dust) is within 5% to modeled mass deposition fluxes when the mineral dust is relatively homogenously distributed over the investigated area.

Description: Intercomparison of shortwave radiative transfer schemes in global aerosol modeling: results from the AeroCom Radiative Transfer Experiment Atmospheric Chemistry and Physics Discussions, 12, 32631-32706, 2012 Author(s): C. A. Randles, S. Kinne, G. Myhre, M. Schulz, P. Stier, J. Fischer, L. Doppler, E. Highwood, C. Ryder, B. Harris, J. Huttunen, Y. Ma, R. T. Pinker, B. Mayer, D. Neubauer, R. Hitzenberger, L. Oreopoulos, D. Lee, G. Pitari, G. Di Genova, J. Quaas, Fred G. Rose, S. Kato, S. T. Rumbold, I. Vardavas, N. Hatzianastassiou, C. Matsoukas, H. Yu, F. Zhang, H. Zhang, and P. Lu In this study we examine the performance of 31 global model radiative transfer schemes in cloud-free conditions with prescribed gaseous absorbers and no aerosols (Rayleigh atmosphere), with prescribed scattering-only aerosols, and with more absorbing aerosols. Results are compared to benchmark results from high-resolution, multi-angular line-by-line radiation models. For purely scattering aerosols, model bias relative to the line-by-line models in the top-of-the atmosphere aerosol radiative forcing ranges from roughly −10 to 20%, with over- and underestimates of radiative cooling at higher and lower sun elevation, respectively. Inter-model diversity (relative standard deviation) increases from ~10 to 15% as sun elevation increases. Inter-model diversity in atmospheric and surface forcing decreases with increased aerosol absorption, indicating that the treatment of multiple-scattering is more variable than aerosol absorption in the models considered. Aerosol radiative forcing results from multi-stream models are generally in better agreement with the line-by-line results than the simpler two-stream schemes. Considering radiative fluxes, model performance is generally the same or slightly better than results from previous radiation scheme intercomparisons. However, the inter-model diversity in aerosol radiative forcing remains large, primarily as a result of the treatment of multiple-scattering. Results indicate that global models that estimate aerosol radiative forcing with two-stream radiation schemes may be subject to persistent biases introduced by these schemes, particularly for regional aerosol forcing.

Description: Immersion freezing of birch pollen washing water Atmospheric Chemistry and Physics Discussions, 12, 32911-32943, 2012 Author(s): S. Augustin, S. Hartmann, B. Pummer, H. Grothe, D. Niedermeier, T. Clauss, J. Voigtländer, L. Tomsche, H. Wex, and F. Stratmann In the present study, the immersion freezing behavior of birch pollen, i.e. its ice nucleating active (INA) macromolecules, was investigated at the Leipzig Aerosol Cloud Interaction Simulator (LACIS). For that, washing water of two different birch pollen samples with different regional origin (Northern birch and Southern birch) were used. The immersion freezing of droplets generated from the pollen washing water was already observed at temperatures higher than −20 °C, for both samples. Main differences between the Northern birch pollen and the Southern birch pollen were obvious in a temperature range, between −18 °C and −24 °C, where the ice fraction increased with decreasing temperature. There, the Northern birch pollen washing water featured two different slopes, with one being steeper and one being similar to the slope of the Southern birch pollen washing water. As we assume single INA macromolecules being the reason for the ice nucleation, we concluded that the Northern birch pollen are able to produce at least two different types of INA macromolecules. We were able to determine the heterogeneous nucleation rates for both INA macromolecule types and so could explain the ice nucleation behavior of both, the Southern and the Northern birch pollen washing water.

Description: Effect of sampling variation on error of rainfall variables measured by optical disdrometer Atmospheric Measurement Techniques Discussions, 5, 8895-8924, 2012 Author(s): X. C. Liu, T. C. Gao, and L. Liu During the sampling process of precipitation particles by optical disdrometers, the randomness of particles and sampling variability has great impact on the accuracy of precipitation variables. Based on a marked point model of raindrop size distribution, the effect of sampling variation on drop size distribution and velocity distribution measurement using optical disdrometers are analyzed by Monte Carlo simulation. The results show that the samples number, rain rate, drop size distribution, and sampling size have different influences on the accuracy of rainfall variables. The relative errors of rainfall variables caused by sampling variation in a descending order as: water concentration, mean diameter, mass weighed mean diameter, mean volume diameter, radar reflectivity factor, and number density, which are independent with samples number basically; the relative error of rain variables are positively correlated with the margin probability, which is also positively correlated with the rain rate and the mean diameter of raindrops; the sampling size is one of the main factors that influence the margin probability, with the decreasing of sampling area, especially the decreasing of short side of sample size, the probability of margin raindrops is getting greater, hence the error of rain variables are getting greater, and the variables of median size raindrops have the maximum error. To ensure the relative error of rainfall variables measured by optical disdrometer less than 1%, the width of light beam should be at least 40 mm.

Description: Evidence and quantitation of aromatic organosulfates in ambient aerosols in Lahore, Pakistan Atmospheric Chemistry and Physics Discussions, 12, 32795-32823, 2012 Author(s): S. Kundu, T. A. Quraishi, G. Yu, C. Suarez, F. N. Keutsch, and E. A. Stone Organosulfates are important components of atmospheric organic aerosols, yet their structures, abundances, sources and formation processes are not adequately understood. This study presents the identification and quantitation of benzyl sulfate in atmospheric aerosols, which is the first reported atmospheric organosulfate with aromatic carbon backbone. Benzyl sulfate was identified and quantified in fine particulate matter (PM 2.5 ) collected in Lahore, Pakistan during 2007–2008. An authentic standard of benzyl sulfate was synthesized, standardized, and identified in atmospheric aerosols using ultra-performance liquid chromatography (UPLC) coupled with quadrupole time-of-flight (Q-ToF) mass spectrometry (MS). Benzyl sulfate was quantified in aerosol samples using UPLC coupled to negative electrospray ionization triple quadrupole (TQ) MS. The highest benzyl sulfate concentrations were recorded in November and January 2007 (0.50 ± 0.11 ng m −3 ) whereas the lowest concentration was observed in July (0.05 ± 0.02 ng m −3 ). To evaluate matrix effects, benzyl sulfate concentrations were determined using external calibration and the method of standard addition; comparable concentrations were detected by the two methods, which ruled out significant matrix effects in benzyl sulfate quantitation. Three additional organosulfates with m / z 187, 201 and 215 were qualitatively identified as aromatic organosulfates with additional methyl substituents by high-resolution mass measurements and tandem MS. The observed aromatic organosulfates form a homologous series analogous to toluene, xylene, and trimethylbenzene, which are abundant anthropogenic volatile organic compounds (VOC), suggesting that aromatic organosulfates may be formed by secondary reactions. Further studies are needed to elucidate the sources and formation pathways of aromatic organosulfates in the atmosphere.

Description: Simulation of hydrological processes in the Zhalong Wetland within a river basin, Northeast China Hydrology and Earth System Sciences Discussions, 9, 14035-14063, 2012 Author(s): X. Q. Feng, G. X. Zhang, and Y. Jun Xu Zhalong National Nature Preserve is a large wetland reserve on the Songnen Plain in Northeast China. Wetlands in the preserve play a key role in maintaining regional ecosystem function and integrity. Global climate change and intensified anthropogenic activities in the region have raised great concerns over the change of natural flow regime, wetland degradation and losses. In this study, two key hydrologic components in the preserve, open water area and storage, as well as their variations during the period 1985–2006 were investigated with a spatially-distributed hydrologic modeling system, SWAT. A wetland module was incorporated into the SWAT model to represent hydrological linkages between the wetland and adjacent upland areas. The modified modeling system was calibrated with streamflow measurements from 1987 to 1989, in a Nash efficiency coefficient ( E ns ) of 0.86, and was validated for the period 2005–2006, in an E ns of 0.66. In the past 20 yr, open water area in the Zhalong Wetland fluctuated from approximately 200 km 2 to 1145 km 2 with a rapid decreasing trend through the early 2000s. Consequently, open water storage in the preserve decreased largely, especially in the dry seasons. The situation changed following the implementation of a river diversion in 2001. Overall, the modeling yielded plausible estimates of hydrologic changes in this large wetland reserve, building a foundation for assessing ecological water requirements and developing strategies and plans for water resources management within the river basin.

Description: OH and HO 2 radical chemistry during PROPHET 2008 and CABINEX 2009 – Part 1: Measurements and model comparison Atmospheric Chemistry and Physics Discussions, 12, 33165-33218, 2012 Author(s): S. M. Griffith, R. F. Hansen, S. Dusanter, P. S. Stevens, M. Alaghmand, S. B. Bertman, M. A. Carroll, M. Erickson, M. Galloway, N. Grossberg, J. Hottle, J. Hou, B. T. Jobson, A. Kammrath, F. N. Keutsch, B. L. Lefer, L. H. Mielke, A. O'Brien, P. B. Shepson, M. Thurlow, W. Wallace, N. Zhang, and X. L. Zhou Hydroxyl (OH) and hydroperoxyl (HO 2 ) radicals are key species driving the oxidation of volatile organic compounds that can lead to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments with high isoprene, low NO x conditions have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and peroxy radical concentrations were measured using a laser-induced fluorescence instrument as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere-Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NO x (NO + NO 2 ) and other inorganic species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include the Mainz Isoprene Mechanism (RACM-MIM). The CABINEX model OH predictions were in good agreement with the measured OH concentrations, with an observed-to-modeled ratio near one (0.70 ± 0.31) for isoprene mixing ratios between 1–2 ppb on average. The measured peroxy radical concentrations, reflecting the sum of HO 2 and isoprene-based peroxy radicals, were generally lower than predicted by the box model in both years.

Description: Estimation of volatile organic compound emissions for Europe using data assimilation Atmospheric Chemistry and Physics Discussions, 12, 33219-33263, 2012 Author(s): M. R. Koohkan, M. Bocquet, Y. Roustan, Y. Kim, and C. Seigneur The emission of volatile organic compounds (VOCs) over western Europe for the year 2005 are estimated via inverse modelling, by assimilation of in situ observations of concentration and compared to a standard emission inventory. The study focuses on fifteen VOC species: five aromatics, six alkanes, two alkenes, one alkyne and one biogenic diene. The inversion relies on a validated fast adjoint of the chemistry transport model used to simulate the fate and transport of these VOCs. The assimilated ground-based measurements over Europe are provided by the European Monitoring and Evaluation Programme (EMEP) network. The background emissions errors and the prior observational errors are estimated by maximum likelihood approaches. The positivity assumptions on the VOC emission fluxes is pivotal for a successful inversion and this maximum likelihood approach consistently accounts for the positivity of the fluxes. For most species, the retrieval leads to a significant reduction of the bias, which underlines the misfit between the standard inventories and the observed concentrations. The results are validated through a forecast test and a cross-validation test. It is shown that the statistically consistent non-Gaussian approach based on a reliable estimation of the errors offers the best performance. The efficiency in correcting the inventory depends on the lifetime of the VOCs. In particular, it is shown that the use of in-situ observations using a sparse monitoring network to estimate emissions of isoprene is inadequate because its short chemical lifetime significantly limits the spatial radius of influence of the monitoring data. For species with longer lifetime (a few days), successful, albeit partial, emission corrections can reach regions hundreds of kilometres away from the stations. Domainwide corrections of the emissions inventories of some VOCs are significant, with underestimations on order of a factor of two of propane, ethane, ethylene and acetylene.

Description: On the use of spring baseflow recession for a more accurate parameterization of aquifer transit time distribution functions Hydrology and Earth System Sciences Discussions, 9, 14109-14128, 2012 Author(s): J. Farlin and P. Maloszewski Baseflow recession analysis and groundwater dating have up to now developed as two distinct branches of hydrogeology and were used to solve entirely different problems. We show that by combining two classical models, namely Boussinesq's Equation describing spring baseflow recession and the exponential piston-flow model used in groundwater dating studies, the parameters describing the transit time distribution of an aquifer can be in some cases estimated to a far more accurate degree than with the latter alone. Under the assumption that the aquifer basis is sub-horizontal, the mean residence time of water in the saturated zone can be estimated from spring baseflow recession. This provides an independent estimate of groundwater residence time that can refine those obtained from tritium measurements. This approach is demonstrated in a case study predicting atrazine concentration trend in a series of springs draining the fractured-rock aquifer known as the Luxembourg Sandstone. A transport model calibrated on tritium measurements alone predicted different times to trend reversal following the nationwide ban on atrazine in 2005 with different rates of decrease. For some of the springs, the best agreement between observed and predicted time of trend reversal was reached for the model calibrated using both tritium measurements and the recession of spring discharge during the dry season. The agreement between predicted and observed values was however poorer for the springs displaying the most gentle recessions, possibly indicating the stronger influence of continuous groundwater recharge during the dry period.

Description: Development of a method of robust rain gauge network optimization based on intensity-duration-frequency results Hydrology and Earth System Sciences Discussions, 9, 14205-14230, 2012 Author(s): A. Chebbi, Z. K. Bargaoui, and M. da Conceição Cunha Based on rainfall intensity-duration-frequency (IDF) curves, a robust optimization approach is proposed to identify the best locations to install new rain gauges. The advantage of robust optimization is that the resulting design solutions yield networks which behave acceptably under hydrological variability. Robust optimisation can overcome the problem of selecting representative rainfall events when building the optimization process. This paper reports an original approach based on Montana IDF model parameters. The latter are assumed to be geostatistical variables and their spatial interdependence is taken into account through the adoption of cross-variograms in the kriging process. The problem of optimally locating a fixed number of new monitoring stations based on an existing rain gauge network is addressed. The objective function is based on the mean spatial kriging variance and rainfall variogram structure using a variance-reduction method. Hydrological variability was taken into account by considering and implementing several return periods to define the robust objective function. Variance minimization is performed using a simulated annealing algorithm. In addition, knowledge of the time horizon is needed for the computation of the robust objective function. A short and a long term horizon were studied, and optimal networks are identified for each. The method developed is applied to north Tunisia (area = 21 000 km 2 ). Data inputs for the variogram analysis were IDF curves provided by the hydrological bureau and available for 14 tipping bucket type rain gauges. The recording period was from 1962 to 2001, depending on the station. The study concerns an imaginary network augmentation based on the network configuration in 1973, which is a very significant year in Tunisia because there was an exceptional regional flood event in March 1973. This network consisted of 13 stations and did not meet World Meteorological Organization (WMO) recommendations for the minimum spatial density. So, it is proposed to virtually augment it by 25, 50, 100 and 160% which is the rate that would meet WMO requirements. Results suggest that for a given augmentation robust networks remain stable overall for the two time horizons.

Description: Technical Note: An open source library for processing weather radar data ( wradlib ) Hydrology and Earth System Sciences Discussions, 9, 12333-12356, 2012 Author(s): M. Heistermann, S. Jacobi, and T. Pfaff The potential of weather radar observations for hydrological and meteorological research and applications is undisputed, particularly with increasing world-wide radar coverage. However, several barriers impede the use of weather radar data. These barriers are of both scientific and technical nature. The former refers to inherent measurement errors and artefacts, the latter to aspects such as reading specific data formats, geo-referencing, visualisation. The radar processing library wradlib is intended to lower these barriers by providing a free and open source tool for the most important steps in processing weather radar data for hydro-meteorological and hydrological applications. Moreover, the community-based development approach of wradlib allows scientists to share their knowledge about efficient processing algorithms and to make this knowledge available to the weather radar community in a transparent, structured and well-documented way.

Description: GloFAS – global ensemble streamflow forecasting and flood early warning Hydrology and Earth System Sciences Discussions, 9, 12293-12332, 2012 Author(s): L. Alfieri, P. Burek, E. Dutra, B. Krzeminski, D. Muraro, J. Thielen, and F. Pappenberger Anticipation and preparedness for large-scale flood events have a key role in mitigating their impact and optimizing the strategic planning of water resources. Although several developed countries have well-established systems for river monitoring and flood early warning, figures of population affected every year by floods in developing countries are unsettling. This paper presents the Global Flood Awareness System, which has been set up to provide an overview on upcoming floods in large world river basins. The Global Flood Awareness System is based on distributed hydrological simulation of numerical ensemble weather predictions with global coverage. Streamflow forecasts are compared statistically to climatological simulations to detect probabilistic exceedance of warning thresholds. In this article, the system setup is described, together with an evaluation of its performance over a two-year test period and a qualitative analysis of a case study for the Pakistan flood, in summer 2010. It is shown that hazardous events in large river basins can be skilfully detected with a forecast horizon of up to 1 month. In addition, results suggest that an accurate simulation of initial model conditions and an improved parameterization of the hydrological model are key components to reproduce accurately the streamflow variability in the many different runoff regimes of the Earth.

Description: On the effect of moisture on the detection of tropospheric turbulence from in situ measurements Atmospheric Measurement Techniques Discussions, 5, 8223-8240, 2012 Author(s): R. Wilson, H. Luce, H. Hashiguchi, M. Shiotani, and F. Dalaudier The present note addresses the detection of turbulence based on the Thorpe (1977) method applied to an atmosphere where saturation of water vapor occurs. The detection method proposed by Thorpe relies on the reordering in ascending order of a measured profile of a variable conserved through adiabatic processes (e.g. potential temperature). For saturated air, the reordering should be applied to a moist-conservative potential temperature, θ m , which is analogous to potential temperature for a dry (subsaturated) atmosphere. Here, θ m is estimated from the Brunt-Väisälä frequency derived by Lalas and Einaudi (1974) in a saturated atmosphere. The application to balloon data shows that the effective turbulent fraction of the troposphere can dramatically increase when saturation is taken into account. Preliminary results of comparisons with data simultaneously collected from the VHF Middle and Upper atmosphere radar (MUR, Japan) seem to give credence to the proposed approach.

Description: Flood discharge measurement of mountain rivers Hydrology and Earth System Sciences Discussions, 9, 12655-12690, 2012 Author(s): Y.-C. Chen An efficient method that accounts for personal safety, accuracy and reliability for measuring flood discharge of mountain rivers is proposed. It is composed of new measurement method, tools, and techniques. Measuring flood discharge from mountain rivers by using conventional method is costly, time-consuming, and dangerous. Thus previous discharge measurements for mountainous areas were typically based on estimated precipitation, which alone cannot generate accurate measurements. This study applies a novel flood discharge measurement system composed of an Acoustic Doppler Profiler and crane system to accurately and quickly measure velocity distributions and water depths. Moreover a novel and efficient method for measuring discharge, which is based on the relationship between mean and maximum velocities and the relationship between cross-sectional area and gauge height is applied to estimate flood discharge. Flood discharge from mountain rivers can be estimated easily and rapidly by measuring maximum velocity in the river crosssection and the gauge height. The measured flood discharges can be utilized to create a reliable stage-discharge relationship for continuous estimations of discharge using records of water stage. The proposed method was applied to the Nanshih River, Taiwan. Results of measured discharges and estimated discharges only slightly differed from each other, demonstrating the efficiency and accuracy of the proposed method.

Description: New climate change scenarios reveal uncertain future for Central Asian glaciers Hydrology and Earth System Sciences Discussions, 9, 12691-12727, 2012 Author(s): A. F. Lutz, W. W. Immerzeel, A. Gobiet, F. Pellicciotti, and M. F. P. Bierkens Central Asian water resources largely depend on (glacier) melt water generated in the Pamir and Tien Shan mountain ranges, located in the basins of the Amu and Syr Darya rivers, important life lines in Central Asia and the prominent water source of the Aral Sea. To estimate future water availability in the region, it is thus necessary to project the future glacier extent and volume in the Amu and Syr Darya river basins. The aim of this study is to quantify the impact of uncertainty in climate change projections on the future glacier extent in the Amu and Syr Darya river basins. The latest climate change projections provided by the fifth Coupled Model Intercomparison Project (CMIP5) generated for the upcoming fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC) are used to model future glacier extent in the Central Asian region for the two large river basins. The outcomes are compared to model results obtained with the climate change projections used for the fourth IPCC assessment (CMIP3). We use a regionalized glacier mass balance model to estimate changes in glacier extent as a function of glacier size and projections of temperature and precipitation. The model is developed for implementation in (large scale) hydrological models, when the spatial model resolution does not allow for modelling of individual glaciers and data scarcity is an issue. Both CMIP3 and CMIP5 model simulations point towards a strong decline in glacier extent in Central Asia. However, compared to the CMIP3 projections, the CMIP5 projections of future glacier extent in Central Asia provide a wider range of outcomes, mostly owing to greater variability in precipitation projections among the latest suite of climate models. These findings have great impact on projections of the timing and quantity of water availability in glacier melt dominated rivers in the region. Uncertainty about the size of the decline in glacier extent remains large, making estimates of future Central Asian glacier extent and downstream water availability uncertain.

Description: Basin-wide water accounting using remote sensing data: the case of transboundary Indus Basin Hydrology and Earth System Sciences Discussions, 9, 12921-12958, 2012 Author(s): P. Karimi, W. G. M. Bastiaanssen, D. Molden, and M. J. M. Cheema The paper describes the application of a new Water Accounting Plus (WA+) framework to produce spatial information on water flows, sinks, uses, storages and assets, in the Indus Basin, South Asia. It demonstrates how satellite-derived estimates of land use, land cover, rainfall, evaporation ( E ), transpiration ( T ), interception ( I ) and biomass production can be used in the context of WA+. The results for one selected year showed that total annual water depletion in the basin (502 km 3 ) plus outflows (21 km 3 ) exceeded total precipitation (482 km 3 ). The deficit in supply was augmented through abstractions beyond actual capacity, mainly from groundwater storage (30 km 3 ). The "landscape ET" (depletion directly from rainfall) was 344 km 3 (69% of total consumption). "Blue water" depletion ("utilized flow") was 158 km 3 (31%). Agriculture was the biggest water consumer and accounted for 59% of the total depletion (297 km 3 ), of which 85% (254 km 3 ) was through irrigated agriculture and the remaining 15% (44 km 3 ) through rainfed systems. While the estimated basin irrigation efficiency was 0.84, due to excessive evaporative losses in agricultural areas, half of all water consumption in the basin was non-beneficial. Average rainfed crop yields were 0.9 t ha −1 and 7.8 t ha −1 for two irrigated crop growing seasons combined. Water productivity was low due to a lack of proper agronomical practices and poor farm water management. The paper concludes that the opportunity for a food-secured and sustainable future for the Indus Basin lies in focusing on reducing soil evaporation. Results of future scenario analyses suggest that by implementing techniques to convert soil evaporation to crop transpiration will not only increase production but can also result in significant water savings that would ease the pressure on the fast declining storage.

Description: Water Accounting Plus (WA+) – a water accounting procedure for complex river basins based on satellite measurements Hydrology and Earth System Sciences Discussions, 9, 12879-12919, 2012 Author(s): P. Karimi, W. G. M. Bastiaanssen, and D. Molden Coping with the issue of water scarcity and growing competition for water among different sectors requires proper water management strategies and decision processes. A pre-requisite is a clear understanding of the basin hydrological processes, manageable and unmanageable water flows, the interaction with land use and opportunities to mitigate the negative effects and increase the benefits of water depletion on society. Currently, water professionals do not have a common framework that links hydrological flows to user groups of water and their benefits. The absence of a standard hydrological and water management summary is causing confusion and wrong decisions. The non-availability of water flow data is one of the underpinning reasons for not having operational water accounting systems for river basins in place. In this paper we introduce Water Accounting Plus (WA+), which is a new framework designed to provide explicit spatial information on water depletion and net withdrawal processes in complex river basins. The influence of land use on the water cycle is described explicitly by defining land use groups with common characteristics. Analogous to financial accounting, WA+ presents four sheets including (i) a resource base sheet , (ii) a consumption sheet , (iii) a productivity sheet , and (iv) a withdrawal sheet . Every sheet encompasses a set of indicators that summarize the overall water resources situation. The impact of external (e.g. climate change) and internal influences (e.g. infrastructure building) can be estimated by studying the changes in these WA+ indicators. Satellite measurements can be used for 3 out of the 4 sheets, but is not a precondition for implementing WA+ framework. Data from hydrological models and water allocation models can also be used as inputs to WA+.

Description: Multi-variable evaluation of hydrological model predictions for a headwater basin in the Canadian Rocky Mountains Hydrology and Earth System Sciences Discussions, 9, 12825-12877, 2012 Author(s): X. Fang, J. W. Pomeroy, C. R. Ellis, M. K. MacDonald, C. M. DeBeer, and T. Brown One of the purposes of the Cold Regions Hydrological Modelling platform (CRHM) is to diagnose inadequacies in the understanding of the hydrological cycle and its simulation. A physically based hydrological model including a full suite of snow and cold regions hydrology processes as well as warm season, hillslope and groundwater hydrology was developed in CRHM for application in the Marmot Creek Research Basin (~ 9.4km 2 ), located in the Front Ranges of Canadian Rocky Mountains. Parameters were selected from digital elevation model, forest, soil and geological maps, and from the results of many cold regions hydrology studies in the region and elsewhere. Non-calibrated simulations were conducted for six hydrological years during 2005–2011 and were compared with detailed field observations of several hydrological cycle components. Results showed good model performance for snow accumulation and snowmelt compared to the field observations for four seasons during 2007–2011, with a small bias and normalized root mean square difference (NRMSD) ranging from 40 to 42% for the subalpine conifer forests and from 31 to 67% for the alpine tundra and tree-line larch forest environments. Overestimation or underestimation of the peak SWE ranged from 1.6 to 29%. Simulations matched well with the observed unfrozen moisture fluctuation in the top soil layer at a lodgepole pine site during 2006–2011, with a NRMSD ranging from 17% to 39%, but with consistent overestimation of 7 to 34%. Evaluations of seasonal streamflow during 2006–2011 revealed the model generally predicted well compared to observations at the basin scale, with a NRMSD of 77% and small model bias (6%), but at the sub-basin scale NRMSD were larger, ranging from 86 to 106%; though overestimation or underestimation for the cumulative seasonal discharge was within 24%. Timing of discharge was better predicted at the Marmot Creek basin outlet having a Nash-Sutcliffe efficiency (NSE) of 0.31 compared to the outlets of the sub-basins where NSE ranged from −0.03 to −0.76. The Pearson product-moment correlation coefficient of 0.12 and 0.17 for comparisons between the simulated groundwater storage and observed groundwater level fluctuation at two wells indicate weak but positive correlations. The model results are encouraging for uncalibrated prediction and indicate research priorities to improve simulations of snow accumulation at treeline, groundwater dynamics and small-scale runoff generation processes in this environment. The study shows that improved hydrological cycle model prediction can be derived from improved hydrological understanding and therefore is a model that can be applied for prediction in ungauged basins.

Description: A new source of oxygenated organic aerosol and oligomers Atmospheric Chemistry and Physics Discussions, 12, 29069-29098, 2012 Author(s): J. Liggio and S.-M. Li A large oxygenated organic uptake to aerosols was observed when exposing ambient urban air to inorganic acidic and non-acidic sulfate seed aerosol. For non-acidic seed aerosol the uptake was attributed to the direct condensation of primary vehicle exhaust gases, and was correlated to the initial seed sulfate mass. The uptake of primary oxygenated organic gases to aerosols in this study represents a significant amount of organic aerosol (OA) when compared to that reported for primary organic aerosol (POA), but is considerably more oxygenated (O : C ~ 0.3) than traditional POA. Consequently, a fraction of measured ambient oxygenated OA, which correlate with secondary sulfate, may in fact be of a primary, rather than secondary source. These results represent a new source of oxygenated OA on neutral aerosol and imply that the uptake of primary organic gases will occur in the ambient atmosphere, under dilute conditions, and in the presence of pre-existing SO 4 aerosols. Under acidic seed aerosol conditions, oligomer formation was observed with the uptake of organics being enhanced by a factor of three or more compared to neutral aerosols, and in less than 2 min. This resulted in a trajectory in Van Krevelen space towards higher O : C (slope ~ −1.5), despite a lack of continual gas-phase oxidation in this closed system. The results demonstrate that high molecular weight species will form on acidic aerosols at the ambient level and mixture of organic gases, but are otherwise unaffected by subsequent aerosol neutralization, and that aerosol acidity will affect the organic O : C via aerosol-phase reactions. These new processes under both neutral and acidic conditions can contribute to ambient OA mass and the evolution of ambient aerosol O : C ratios and may be important for properly representing organic aerosol O : C ratios in air quality and climate models.

Description: Characterization of photochemical pollution at different elevations in mountainous areas in Hong Kong Atmospheric Chemistry and Physics Discussions, 12, 29025-29067, 2012 Author(s): H. Guo, Z. H. Ling, K. Cheung, F. Jiang, D. W. Wang, I. J. Simpson, T. J. Wang, X. M. Wang, S. M. Saunders, and D. R. Blake To advance our understanding on the factors that affect photochemical pollution at different elevations in mountainous areas, concurrent systematic field measurements (September to November 2010) were conducted at a mountain site and at an urban site at the foot of the mountain in Hong Kong. The mixing ratios of air pollutants were greater at the foot of the mountain (i.e. Tsuen Wan urban site, TW) than near the summit (i.e. Tai Mao Shan mountain site, TMS), except for ozone. In total, only 1 O 3 episode day was observed at TW, whereas 21 O 3 episode days were observed at TMS. The discrepancy of O 3 at the two sites was attributed to the mixed effects of NO titration, vertical meteorological conditions, regional transport and mesoscale circulations. The lower NO levels at TMS and the smaller differences of "oxidant" O x (O 3 + NO 2 ) than O 3 between the two sites suggested that variations of O 3 at the two sites were partly attributed to different degree of NO titration. In addition, analysis of vertical structure of meteorological variables revealed that the inversion layer at the range of altitudes of 500–1000 m might be another factor that caused the high O 3 levels at TMS. Furthermore, analyses of the wind fields and the levels of air pollutants in different air flows indicated that high O 3 concentrations at TMS were somewhat influenced by regional air masses from the highly polluted Pearl River Delta (PRD) region. In particular, the analysis of diurnal profiles and correlations of gaseous pollutants suggested influence of mesoscale circulations which was further confirmed using the Master Chemical Mechanism moving box model (Mbox) and the Weather Research and Forecasting (WRF) model. By investigating the correlations of observed O 3 and NO x *, as well as the ratios of VOC/NO x , it was concluded that photochemical O 3 formation was VOC-sensitive or both NO x and VOC-sensitive at TMS, while it was VOC-sensitive at TW.

Description: Is bias correction of Regional Climate Model (RCM) simulations possible for non-stationary conditions? Hydrology and Earth System Sciences Discussions, 9, 12765-12795, 2012 Author(s): C. Teutschbein and J. Seibert In hydrological climate-change impact studies, Regional Climate Models (RCMs) are commonly used to transfer large-scale Global Climate Model (GCM) data to smaller scales and to provide more detailed regional information. However, there are often considerable biases in RCM simulations, which have led to the development of a number of bias correction approaches to provide more realistic climate simulations for impact studies. Bias correction procedures rely on the assumption that RCM biases do not change over time, because correction algorithms and their parameterizations are derived for current climate conditions and assumed to apply also for future climate conditions. This underlying assumption of bias stationarity is the main concern when using bias correction procedures. It is in principle not possible to test whether this assumption is actually fulfilled for future climate conditions. In this study, however, we demonstrate that it is possible to evaluate how well bias correction methods perform for conditions different from those used for calibration. For five Swedish catchments, several time series of RCM simulated precipitation and temperature were obtained from the ENSEMBLES data base and different commonly-used bias correction methods were applied. We then performed a differential split-sample test by dividing the data series into cold and warm respective dry and wet years. This enabled us to evaluate the performance of different bias correction procedures under systematically varying climate conditions. The differential split-sample test resulted in a large spread and a clear bias for some of the correction methods during validation years. More advanced correction methods such as distribution mapping performed relatively well even in the validation period, whereas simpler approaches resulted in the largest deviations and least reliable corrections for changed conditions. Therefore, we question the use of simple bias correction methods such as the widely used delta-change approach and linear scaling for RCM-based climate-change impact studies and recommend using higher-skill bias correction methods.

Description: Estimation of debris flow critical rainfall thresholds by a physically-based model Hydrology and Earth System Sciences Discussions, 9, 12797-12824, 2012 Author(s): M. N. Papa, V. Medina, F. Ciervo, and A. Bateman Real time assessment of debris flow hazard is fundamental for setting up warning systems that can mitigate its risk. A convenient method to assess the possible occurrence of a debris flow is the comparison of measured and forecasted rainfall with rainfall threshold curves (RTC). Empirical derivation of the RTC from the analysis of rainfall characteristics of past events is not possible when the database of observed debris flows is poor or when the environment changes with time. For landslides triggered debris flows, the above limitations may be overcome through the methodology here presented, based on the derivation of RTC from a physically based model. The critical RTC are derived from mathematical and numerical simulations based on the infinite-slope stability model in which land instability is governed by the increase in groundwater pressure due to rainfall. The effect of rainfall infiltration on landside occurrence is modelled trough a reduced form of the Richards equation. The simulations are performed in a virtual basin, representative of the studied basin, taking into account the uncertainties linked with the definition of the characteristics of the soil. A large number of calculations are performed combining different values of the rainfall characteristics (intensity and duration of event rainfall and intensity of antecedent rainfall). For each combination of rainfall characteristics, the percentage of the basin that is unstable is computed. The obtained database is opportunely elaborated to derive RTC curves. The methodology is implemented and tested on a small basin of the Amalfi Coast (South Italy).

Description: Cloud-resolving modeling of aerosol indirect effects in idealized radiative-convective equilibrium with interactive and fixed sea surface temperature Atmospheric Chemistry and Physics Discussions, 12, 29099-29127, 2012 Author(s): M. F. Khairoutdinov and C.-E. Yang The study attempts to evaluate the aerosol indirect effects over tropical oceans in regions of deep convection applying a three-dimensional cloud-resolving model run over a doubly-periodic domain. The Tropics are modeled using a radiative-convective equilibrium idealization when the radiation, turbulence, cloud microphysics, and surface fluxes are explicitly represented while the effects of large-scale circulation are ignored. The aerosol effects are modeled by varying the number concentration of cloud condensation nuclei (CCN) at 1% supersaturation, which serves as a proxy for the aerosol amount in the environment, over a wide range, starting from pristine maritime (50 cm −3 ) to polluted (1000 cm −3 ) conditions. No direct effects of aerosol on radiation are included. Two sets of simulations have been run to equilibrium: fixed (non-interactive) sea surface temperature (SST) and interactive SST as predicted by a simple slab-ocean model responding to the surface radiative fluxes and surface enthalpy flux. Both sets of experiments agree on the tendency to make the shortwave cloud forcing more negative and reduce the longwave cloud forcing in response to increasing CCN concentration. These, in turn, tend to cool the SST in interactive-SST case. It is interesting that the absolute change of the SST and most other bulk quantities depends only on relative change of CCN concentration; that is, same SST change can be the result of doubling CCN concentration regardless of clean or polluted conditions. It is found that the 10-fold increase of CCN concentration can cool the SST by as much as 1.5 K. This is quite comparable to 2 K warming obtained in a simulation for clean maritime conditions, but doubled CO 2 concentration. Qualitative differences between the interactive and fixed SST cases have been found in sensitivity of the hydrological cycle to the increase in CCN concentration; namely, the precipitation rate shows some tendency to increase in fixed SST case, but robust tendency to decrease in interactive SST case.

Description: Calibration and validation of the advanced E-Region Wind Interferometer Atmospheric Measurement Techniques Discussions, 5, 8271-8311, 2012 Author(s): S. K. Kristoffersen, W. E. Ward, S. Brown, and J. R. Drummond The advanced E-Region Wind Interferometer (ERWIN II) combines the imaging capabilities of a CCD detector with the wide field associated with field widened Michelson interferometry. This instrument is capable of simultaneous multi-directional wind observations for three different airglow emissions (oxygen green line (O( 1 S)), the P Q(7) and P P(7) emission lines in the O 2 (0–1) atmospheric band and P 1 (3) emission line in the (6,2) hydroxyl Meinel band) on a three minute cadence. In each direction, for 45 s measurements for typical airglow brightness the instrument is capable of line-of-sight wind precisions of ~ 1 m s −1 for hydroxyl and O( 1 S) and ~ 4 m s −1 for O 2 . This precision is achieved using a new data analysis algorithm which takes advantage of the imaging capabilities of the CCD detector along with knowledge of the instrument phase variation as a function of pixel location across the detector. This instrument is currently located in Eureka, Nunavut as part of the Polar Environment Atmospheric Research Laboratory (PEARL). The details of the physical configuration, the data analysis algorithm, the measurement calibration and validation of the observations are described. Field measurements which demonstrate the capabilities of this instrument are presented. To our knowledge, the wind determinations with this instrument are the most accurate and have the highest observational cadence for airglow wind observations of this region of the atmosphere and match the capabilities of other wind measuring techniques.

Description: Formation of organic aerosol in the Paris region during the MEGAPOLI summer campaign: evaluation of the Volatility-Basis-Set approach within the CHIMERE model Atmospheric Chemistry and Physics Discussions, 12, 29475-29533, 2012 Author(s): Q. J. Zhang, M. Beekmann, F. Drewnick, F. Freutel, J. Schneider, M. Crippa, A. S. H. Prévôt, U. Baltensperger, L. Poulain, A. Wiedensohler, J. Sciare, V. Gros, A. Borbon, A. Colomb, V. Michoud, J.-F. Doussin, H. A. C. Denier van der Gon, M. Haeffelin, J.-C. Dupont, G. Siour, H. Petetin, B. Bessagnet, S. N. Pandis, A. Hodzic, O. Sanchez, C. Honoré, and O. Perrussel Results of the chemistry transport model CHIMERE are compared with the measurements performed during the MEGAPOLI summer campaign in the Greater Paris Region in July, 2009. The Volatility-Basis-Set approach (VBS) is implemented into this model, taking into account the volatility of primary organic aerosol (POA) and the chemical aging of semi-volatile organic species. Organic aerosol is the main focus and is simulated with three different configurations related to the volatility of POA and the scheme of secondary organic aerosol (SOA) formation. In addition, two types of emission inventories are used as model input in order to test the uncertainty related to the emissions. Predictions of basic meteorological parameters and primary and secondary pollutant concentrations are evaluated and four pollution regimes according to the air mass origin are defined. Primary pollutants are generally overestimated, while ozone is consistent with observations. Sulfate is generally overestimated, while ammonium and nitrate levels are well simulated with the refined emission data set. As expected, the simulation with non-volatile POA and a single-step SOA formation mechanism largely overestimates POA and underestimates SOA. Simulation of organic aerosol with the VBS approach taking into account the aging of semi-volatile organic compounds (SVOC) shows the best correlation with measurements. All observed high concentration events are reproduced by the model mostly after long range transport, indicating that long range transport of SOA to Paris is well reproduced. Depending on the emission inventory used, simulated POA levels are either reasonable or underestimated, while SOA levels tend to be overestimated. Several uncertainties related to the VBS scheme (POA volatility, SOA yields, the aging parameterization), to emission input data, and to simulated OH levels can be responsible for this behavior. Despite these uncertainties, the implementation of the VBS scheme into the CHIMERE model allowed for much more realistic organic aerosol simulations for Paris during summer time. The advection of SOA from outside Paris is mostly responsible for the highest OA concentration levels. During advection of polluted air masses from north-east (Benelux and Central Europe), simulations indicate high levels of both anthropogenic and biogenic SOA fractions, while biogenic SOA dominates during days with advection from Southern France and Spain.

Description: Ice water content of arctic, midlatitude, and tropical cirrus – Part 2: Extension of the database and new statistical analysis Atmospheric Chemistry and Physics Discussions, 12, 29443-29474, 2012 Author(s): A. E. Luebke, L. M. Avallone, C. Schiller, C. Rolf, and M. Krämer Ice clouds are known to be major contributors to radiative forcing in the Earth's atmosphere, yet describing their microphysical properties in climate models remains challenging. Among these properties, the ice water content (IWC) of cirrus clouds is of particular interest both because it is measurable and because it can be directly related to a number of other radiatively important variables such as extinction and effective radius. This study expands upon the work of Schiller et al. (2008), extending a climatology of IWC by combining datasets from several European and US airborne campaigns and ground-based lidar measurements over Jülich, Germany. The relationship between IWC and temperature is further investigated using the new merged dataset and probability distribution functions (PDFs). A PDF-based formulation allows for representation of not only the mean values of IWC, but also the variability of IWC within a temperature band. The IWC-PDFs are found to be bimodal over the whole cirrus temperature range, which might be attributed to different cirrus formation mechanisms such as heterogeneous and homogeneous freezing. The PDFs of IWC are further compared to distributions of cirrus ice crystal number and mass mean radius, which show that the general relationship between IWC and temperature appears to be influenced much more by particle number than by particle size.

Description: Systematic investigation of bromine monoxide in volcanic plumes from space by using the GOME-2 instrument Atmospheric Chemistry and Physics Discussions, 12, 29325-29389, 2012 Author(s): C. Hörmann, H. Sihler, N. Bobrowski, S. Beirle, M. Penning de Vries, U. Platt, and T. Wagner During recent years, volcanic emissions turned out to be a natural source of bromine compounds in the atmosphere. While the inital formation process of bromine monoxide (BrO) has been successfully studied in local ground-based measurements at quiescent degassing volcanoes worldwide, literature on the chemical evolution of BrO on large spatial and temporal scales is sparse. The first space-based observation of a volcanic BrO plume following the Kasatochi eruption in 2008 demonstrated the capability of satellite instruments to monitor such events on a global scale. In this study, we systematically examined GOME-2 observations from January 2007 until June 2011 for significantly enhanced BrO slant column densities (SCDs) in the vicinity of volcanic plumes. In total, 772 plumes from at least 37 volcanoes have been found by using sulphur dioxide (SO 2 ) as a tracer for a volcanic plume. All captured SO 2 plumes were subsequently analysed for a simultaneous enhancement of BrO and the data were checked for a possible spatial correlation between the two species. Additionally, the mean BrO/SO 2 ratios for all volcanic plumes have been calculated by the application of a bivariate linear fit. A total number of 64 volcanic plumes from at least 11 different volcanoes showed clear evidence for BrO of volcanic origin, revealing large differences in the BrO/SO 2 ratios (ranging from some 10 −5 to several 10 −4 ) and the spatial distribution of both species. A close correlation between SO 2 and BrO occurred only for some of the observed eruptions or just in certain parts of the examined plumes. For other cases, only a rough spatial relationship was found. We discuss possible explanations for the occurrence of the different spatial SO 2 and BrO distributions in aged volcanic plumes.

Description: A one-year comprehensive chemical characterisation of fine aerosol (PM 2.5 ) at urban, suburban and rural background sites in the region of Paris (France) Atmospheric Chemistry and Physics Discussions, 12, 29391-29442, 2012 Author(s): M. Bressi, J. Sciare, V. Ghersi, N. Bonnaire, J. B. Nicolas, J.-E. Petit, S. Moukhtar, A. Rosso, N. Mihalopoulos, and A. Féron Studies describing the chemical composition of fine aerosol (PM 2.5 ) in urban areas are often conducted during few weeks only, and at one sole site, giving thus a narrow view of their temporal and spatial characteristics. This paper presents a one-year (11 September 2009–10 September 2010) survey of the daily chemical composition of PM 2.5 in the region of Paris, which is the second most populated "Larger Urban Zone" in Europe. Five sampling sites representative of suburban (SUB), urban (URB), northeast (NER), northwest (NWR) and south (SOR) rural backgrounds were implemented. The major chemical components of PM 2.5 were determined including elemental carbon (EC), organic carbon (OC), and the major ions. OC was converted to organic matter (OM) using the chemical mass closure methodology, which leads to conversion factors of 1.95 for the SUB and URB sites, and 2.05 for the three rural ones. On average, gravimetrically determined PM 2.5 annual mass concentrations are 15.2, 14.8, 12.6, 11.7 and 10.8 μg m −3 for SUB, URB, NER, NWR and SOR sites, respectively. The chemical composition of fine aerosol is very homogeneous at the five sites and is composed of OM (38–47%), nitrate (17–22%), non-sea-salt sulfate (13–16%), ammonium (10–12%), EC (4–10%), mineral dust (2–5%) and sea salt (3–4%). This chemical composition is in agreement with those reported in the literature for most European environments. On the annual scale, Paris (URB and SUB sites) exhibits its highest PM 2.5 concentrations during late autumn, winter and early spring (higher than 15 μg m −3 on average, from December to April), intermediates during late spring and early autumn (between 10 and 15 μg m −3 during May, June, September, October, and November) and the lowest during summer (below 10 μg m −3 during July and August). PM levels are mostly homogeneous at the regional scale, on the whole duration of the project (e.g. for URB plotted against NER sites: slope = 1.06, r 2 = 0.84, n = 330), suggesting the importance of mid- or long-range transport, and regional instead of local scale phenomena. During this one-year project, two third of the days exceeding the PM 2.5 2015 EU annual limit value of 25 μg m −3 were due to continental import from countries located northeast, east of France. This result questions the efficiency of local, regional and even national abatement strategies during pollution episodes, pointing the need for a wider collaborative work with the neighbourhood countries on these topics. Nevertheless, emissions of local anthropogenic sources lead to higher levels at the URB and SUB sites compared to the others (e.g. 26% higher on average at the URB than at the NWR site for PM 2.5 , during the whole campaign), which can even be emphasised by specific meteorological conditions such as low boundary layer heights. OM and secondary inorganic species (nitrate, non-sea-salt sulfate and ammonium, noted SIA) are mainly imported by mid- or long-range transport (e.g. for NWR plotted against URB sites: slope = 0.79, r 2 = 0.72, n = 335 for OM, and slope = 0.91, r 2 = 0.89, n = 335 for SIA) whereas EC is primarily locally emitted (e.g. for SOR plotted against URB sites: slope = 0.27; r 2 = 0.03; n = 335). This database will serve deepest investigations of carbonaceous aerosols, metals as well as the main sources and geographical origins of PM in the region of Paris.

Description: Elucidating multipollutant exposure across a complex metropolitan area by systematic deployment of a mobile laboratory Atmospheric Chemistry and Physics Discussions, 12, 31585-31627, 2012 Author(s): I. Levy, C. Mihele, G. Lu, J. Narayan, N. Hilker, and J. R. Brook In urban areas, air quality is the outcome of multiple emission sources, each emitting a different combination of air pollutants. The result is a complex mixture of pollutants with a different spatiotemporal variability for each constituent. Studies exploring average spatial patterns across urban areas typically rely on air quality monitoring networks of a few sites, short multi-site saturation monitoring campaigns measuring a limited number of pollutants and/or air quality models. Each of these options has limitations. This study elucidates the main complexities of urban air quality with respect to small scale spatial differences for multiple pollutants so as to gain a better understanding of the variability in exposure estimates in urban areas. Mobile measurements of 23 air pollutants were taken at high resolution in Montreal, Quebec, Canada, and examined with respect to space, time and their interrelationships. The same route was systematically followed on 34 measurement days spread over different seasons and measurements were compared to adjacent air quality monitoring network stations. This approach allowed linkage of the mobile measurements to the network observations and to generate average maps that provide reliable information on the typical, annual average spatial pattern. Sharp differences in the spatial distribution were found to exist between different pollutants on the sub-urban scale, i.e. the neighbourhood to street scales, even for pollutants usually associated with the same specific sources. Nearby microenvironments may have a wide range in average pollution levels varying by up to 300%, which may cause large misclassification errors in estimating chronic exposures in epidemiological studies. For example, NO 2 measurements next to a main road microenvironment are shown to be 210–265% higher than levels measured at a nearby urban background monitoring site, while black carbon is higher by 180–200% and ultrafine particles are 300% higher. For some pollutants (e.g. SO 2 and benzene), there is good correspondence on a large scale due to similar emission sources, but differences on a small scale in proximity to these sources. Moreover. hotspots of different pollutants were identified and quantified. These results demonstrate the ability of an independent heavily instrumented mobile laboratory survey to quantify the representativeness of the monitoring sites to unmonitored locations, reveal the complex relationships between pollutants and understand chronic multi-pollutant exposure patterns associated with outdoor concentrations in an urban environment.

Description: Exceptional emissions of NH 3 and HCOOH in the 2010 Russian wildfires Atmospheric Chemistry and Physics Discussions, 12, 31561-31584, 2012 Author(s): Y. R'Honi, L. Clarisse, C. Clerbaux, D. Hurtmans, V. Duflot, S. Turquety, Y. Ngadi, and P.-F. Coheur In July 2010, several hundred forest and peat fires broke out across Central Russia during its hottest summer on record. Here, we analyze these wildfires using observations of the Infrared Atmospheric Sounding Interferometer (IASI). Carbon monoxide (CO), ammonia (NH 3 ) and formic acid (HCOOH) total columns are presented for the year 2010. Maximum total columns have been observed reaching over 40 (for CO and HCOOH) and 200 (for NH 3 ) times higher than typical background values. The temporal evolution of NH 3 and HCOOH enhancement ratios relative to CO are presented. Strong evidence of secondary formation of HCOOH is found, with enhancement ratios exceeding 10 times reported emission ratios in fresh plumes. We estimate the total emitted masses for the period July–August 2010 over the center of Western Russia; they are 19–33 Tg (CO), 0.7–2.6 Tg (NH 3 ) and 0.9–3.9 Tg (HCOOH). For NH 3 andHCOOH, these quantities are comparable to what is emitted in the course of a whole year by all extratropical forest fires.

Description: Climatology of middle atmospheric water vapour above the ALOMAR observatory in northern Norway Atmospheric Chemistry and Physics Discussions, 12, 31531-31560, 2012 Author(s): K. Hallgren, P. Hartogh, and C. Jarchow We have been observing the water vapour line at 22.235 GHz above ALOMAR in northern Norway (69° N, 16° E) since early 1996 with ground-based microwave spectrometers (WASPAM and cWASPAM) and will here describe a climatology based on these observations. Maintenance, different spectrometers and upgrades of the hardware have slightly changed the instruments. Therefore great care has been taken to make sure the different datasets are compatible with each other. In order to maximise the sensitivity at high altitude for the older instrument a long integration time (168 h) was chosen. The complete dataset was thereafter recompiled into a climatology which describes the yearly variation of water vapour at polar latitudes on a weekly basis. The atmosphere is divided into 16 layers between 40–80 km, each 2.5 km thick. The dataset, spanning 15 yr from 1996 to 2010, enabled us to investigate the long-term behaviour of water vapour at these latitudes. By comparing the measurements from every year to the climatological mean we were also able to look for indications of trends in the dataset at different altitudes during the time period of our observations. In general there is a weak negative trend which differs slightly at different altitudes. There are however no drifts in the annual variation of water vapour from the point of view of onset of summer and winter. We compare our climatology to the reference water vapour profiles from AFGL, a free and easy accessible reference atmosphere. There are strong deviations between our observations and the reference profile, therefore we publish our climatological dataset in a table in the paper.

Description: Observationally-constrained estimates of global small-mode AOD Atmospheric Chemistry and Physics Discussions, 12, 31663-31698, 2012 Author(s): K. Lee and C. E. Chung Small aerosols are mostly anthropogenic, and an area average of the small-mode aerosol optical depth (sAOD) is a powerful and independent measure of anthropogenic aerosol emission. We estimate AOD and sAOD globally on a monthly time scale from 2001 to 2010 by integrating satellite-based (MODIS and MISR) and ground-based (AERONET) observations. For sAOD, three integration methods were developed to maximize the influence of AERONET data and ensure consistency between MODIS, MISR and AERONET sAOD data. We evaluated each method by applying the technique with fewer AERONET data and comparing its output with the unused AERONET data. The best performing method gives an overall error of 13 ± 2%, compared with an overall error of 62% in simply using MISR sAOD, and this method takes advantage of an empirical relationship between the Ångström exponent (AE) and fine mode fraction (FMF). This relationship is obtained by analyzing AERONET data. Using our integrated data, we find that the global 2001–2010 average of 500 nm AOD and sAOD is 0.17 and 0.094, respectively. sAOD over eastern China is several times as large as the global average. The linear trend from 2001 to 2010 is found to be slightly negative in global AOD or global sAOD. In India and eastern China combined, however, sAOD increased by more than 4% against a backdrop of decreasing AOD and large-mode AOD. On the contrary to India and China, the west (Western Europe and US/Canada combined) is found to have a sAOD reduction of −20%. These results quantify the overall anthropogenic aerosol emission reduction in the west, and rapidly deteriorating conditions in Asia. Moreover, our results in the west are consistent with the so-called surface brightening phenomenon in the recent decades.

Description: ACE-FTS observations of pyrogenic trace species in boreal biomass burning plumes during BORTAS Atmospheric Chemistry and Physics Discussions, 12, 31629-31661, 2012 Author(s): K. A. Tereszchuk, G. González Abad, C. Clerbaux, J. Hadji-Lazaro, D. Hurtmans, P.-F. Coheur, and P. F. Bernath To further our understanding of the effects of biomass burning emissions on atmospheric composition, the Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS) campaign was conducted on 12 July to 3 August 2011 during the Boreal forest fire season in Canada. The simultaneous aerial, ground and satellite measurement campaign sought to record instances of Boreal biomass burning to measure the tropospheric volume mixing ratios (VMRs) of short- and long-lived trace molecular species from biomass burning emissions. The goal was to investigate the connection between the composition and the distribution of these pyrogenic outflows and their resulting perturbation to atmospheric chemistry, with particular focus on oxidant species to determine the overall impact on the oxidizing capacity of the free troposphere. Measurements of pyrogenic trace species in Boreal biomass burning plumes were made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) onboard the Canadian Space Agency (CSA) SCISAT-1 satellite during the BORTAS campaign. Even though most biomass burning smoke is typically confined to the boundary layer, emissions are often injected directly into the upper troposphere via fire-related convective processes, thus allowing space-borne instruments to measure these pyrogenic outflows. An extensive set of 15 molecules, CH 3 OH, CH 4 , C 2 H 2 , C 2 H 6 , C 3 H 6 O, CO, HCN, HCOOH, HNO 3 , H 2 CO, NO, NO 2 , OCS, O 3 and PAN have been analyzed. Included in this analysis is the calculation of age-dependent sets of enhancement ratios for each of the species.

Description: Expected changes in future temperature extremes and their elevation dependency over the Yellow River source region Hydrology and Earth System Sciences Discussions, 9, 13609-13634, 2012 Author(s): Y. Hu, S. Maskey, and S. Uhlenbrook Using the Statistical DownScaling Model (SDSM) and the outputs from two global climate models we investigate possible changes in mean and extreme temperature indices and their elevation dependency over the Yellow River source region for the period 2081–2100 under the IPCC SRES A2, A1B and B1 emission scenarios. Changes in interannual variability of mean and extreme temperature indices are also analyzed. The validation results show that SDSM performs better in reproducing the maximum temperature-related indices than the minimum temperature-related indices. The projections show that by the end of the 21st century all parts of the study region may experience increases in both mean and extreme temperature in all seasons, along with an increase in the frequency of hot days and warm nights and with a decrease in frost days. Interannual variability increases in all seasons for the frequency of hot days and warm nights and in spring for frost days while it decreases for frost days in summer. Autumn demonstrates pronounced elevation-dependent changes in which six out of eight indices show significant increasing changes with elevation.

Description: Do probabilistic forecasts lead to better decisions? Hydrology and Earth System Sciences Discussions, 9, 13569-13607, 2012 Author(s): M. H. Ramos, S. J. van Andel, and F. Pappenberger The last decade has seen growing research in producing probabilistic hydro-meteorological forecasts and increasing their reliability. This followed the promise that, supplied with information about uncertainty, people would take better risk-based decisions. In recent years, therefore, research and operational developments have also start putting attention to ways of communicating the probabilistic forecasts to decision makers. Communicating probabilistic forecasts includes preparing tools and products for visualization, but also requires understanding how decision makers perceive and use uncertainty information in real-time. At the EGU General Assembly 2012, we conducted a laboratory-style experiment in which several cases of flood forecasts and a choice of actions to take were presented as part of a game to participants, who acted as decision makers. Answers were collected and analyzed. In this paper, we present the results of this exercise and discuss if indeed we make better decisions on the basis of probabilistic forecasts.

Description: Validation of an hourly resolved global aerosol model in answer to solar electricity generation information needs Atmospheric Chemistry and Physics Discussions, 12, 31917-31953, 2012 Author(s): M. Schroedter-Homscheidt and A. Oumbe Solar energy applications need global aerosol optical depth (AOD) information to derive historic surface solar irradiance databases from geostationary meteorological satellites reaching back to the 1980's. This paper validates the MATCH/DLR model originating in the climate community against AERONET ground measurements. Hourly or daily mean AOD model output is evaluated individually for all stations in Europe, Africa and the Middle East – an area highly interesting for solar energy applications being partly dominated by high aerosol loads. Overall, a bias of 0.02 and a root mean square error of 0.23 are found for daily mean AOD values, while the RMSE increases to 0.28 for hourly mean AOD values. Large differences between various regions and stations are found providing a feedback loop for the aerosol modelling community. The difference in using daily means versus hourly resolved modelling with respect to hourly resolved observations is evaluated. Nowadays state of the art in solar resource assessment relies on monthly turbidity or AOD climatologies while at least hourly resolved irradiance time series are needed by the solar sector. Therefore, the contribution of higher temporally modelled AOD is evaluated.

Description: Classifying organic materials by oxygen-to-carbon elemental ratio to predict the activation regime of cloud condensation nuclei (CCN) Atmospheric Chemistry and Physics Discussions, 12, 31829-31870, 2012 Author(s): M. Kuwata, W. Shao, R. Lebouteiller, and S. T. Martin The governing highly soluble, slightly soluble, or insoluble activation regime of organic compounds as cloud condensation nuclei (CCN) was examined as a function of oxygen-to-carbon elemental ratio (O : C). New data were collected for adipic, pimelic, suberic, azelaic and pinonic acids. Secondary organic materials (SOMs) produced by α-pinene ozonolysis and isoprene photo-oxidation were also included in the analysis. The saturation concentrations C of the organic compounds in aqueous solutions served as the key parameter for delineating regimes of CCN activation, and the values of C were tightly correlated to the O : C ratios. The highly soluble, slightly soluble, and insoluble regimes of CCN activation were found to correspond to ranges of [O : C] 〉 0.6, 0.2 〈 [O : C] 〈 0.6, and [O : C] 〈 0.2, respectively. These classifications were evaluated against CCN activation data of isoprene-derived SOM (O : C = 0.69–0.72) and α-pinene-derived SOM (O : C = 0.38–0.48). Isoprene-derived SOM had highly soluble activation behavior, consistent with its high O : C ratio. For α-pinene-derived SOM, although CCN activation can be modeled as a highly soluble mechanism, this behavior was not predicted by the O : C ratio, for which a slightly soluble mechanism was anticipated. Complexity in chemical composition, resulting in continuous water uptake and the absence of a deliquescence transition that can thermodynamically limit CCN activation, might explain the differences of α-pinene-derived SOM compared to the behavior of pure organic compounds. The present results suggest that atmospheric particles dominated by hydrocarbon-like organic components do not activate (i.e. insoluble regime) whereas those dominated by oxygenated organic components activate (i.e. highly soluble regime).

Description: Secondary organic aerosol formation from gasoline passenger vehicle emissions investigated in a smog chamber Atmospheric Chemistry and Physics Discussions, 12, 31725-31765, 2012 Author(s): E. Z. Nordin, A. C. Eriksson, P. Roldin, P. T. Nilsson, J. E. Carlsson, M. K. Kajos, H. Hellén, C. Wittbom, J. Rissler, J. Löndahl, E. Swietlicki, B. Svenningsson, M. Bohgard, M. Kulmala, M. Hallquist, and J. Pagels Gasoline vehicles have elevated emissions of volatile organic compounds during cold starts and idling and have recently been pointed out as potentially the main source of anthropogenic secondary organic aerosol (SOA) in megacities. However, there is a lack of laboratory studies to systematically investigate SOA formation in real-world exhaust. In this study, SOA formation from pure aromatic precursors, idling and cold start gasoline exhaust from one Euro II, one Euro III and one Euro IV passenger vehicles were investigated using photo-oxidation experiments in a 6 m 3 smog chamber. The experiments were carried out at atmospherically relevant organic aerosol mass concentrations. The characterization methods included a high resolution aerosol mass spectrometer and a proton transfer mass spectrometer. It was found that gasoline exhaust readily forms SOA with a signature aerosol mass spectrum similar to the oxidized organic aerosol that commonly dominates the organic aerosol mass spectra downwind urban areas. After 4 h aging the formed SOA was 1–2 orders of magnitude higher than the Primary OA emissions. The SOA mass spectrum from a relevant mixture of traditional light aromatic precursors gave f 43 (mass fraction at m/z = 4 3) approximately two times higher than to the gasoline SOA. However O : C and H : C ratios were similar for the two cases. Classical C 6 –C 9 light aromatic precursors were responsible for up to 60% of the formed SOA, which is significantly higher than for diesel exhaust. Important candidates for additional precursors are higher order aromatic compounds such as C 10 , C 11 light aromatics, naphthalene and methyl-naphthalenes.

Description: Long-term changes of tropospheric NO 2 over megacities derived from multiple satellite instruments Atmospheric Chemistry and Physics Discussions, 12, 31767-31828, 2012 Author(s): A. Hilboll, A. Richter, and J. P. Burrows Tropospheric NO 2 , a key pollutant in particular in cities, has been measured from space since the mid-1990s by the GOME, SCIAMACHY, OMI, and GOME-2 instruments. These data provide a unique global long-term data set of tropospheric pollution. However, the measurements differ in spatial resolution, local time of measurement, and measurement geometry. All these factors can severely impact the retrieved NO 2 columns, which is why they need to be taken into account when analysing time series spanning more than one instrument. In this study, we present several ways to explicitly account for the instrumental differences in trend analyses of the NO 2 columns derived from satellite measurements, while preserving their high spatial resolution. Both a physical method, based on spatial averaging of the measured earthshine spectra and extraction of a resolution pattern, and statistical methods, including instrument-dependent offsets in the fitted trend function, are developed. These methods are applied to data from GOME and SCIAMACHY separately, to the combined time series and to an extended data set comprising also GOME-2 and OMI measurements. All approaches show consistent trends of tropospheric NO 2 for a selection of areas on both regional and city scales, for the first time allowing consistent trend analysis of the full time series at high spatial resolution and significantly reducing the uncertainties of the retrieved trend estimates compared to previous studies. We show that measured tropospheric NO 2 columns have been strongly increasing over China, the Middle East, and India, with values over East Central China triplicating from 1996 to 2011. All parts of the developed world, including Western Europe, the United States, and Japan, show significantly decreasing NO 2 amounts in the same time period. On a megacity level, individual trends can be as large as +27 ± 3.7% yr −1 and +20 ± 1.9% yr −1 in Dhaka and Baghdad, respectively, while Los Angeles shows a very strong decrease of −6.0 ± 0.37% yr −1 . Most megacities in China, India, and the Middle East show increasing NO 2 columns of +5–10% yr −1 , leading to a doubling to triplication within the observed period. While linear trends derived with the different methods are consistent, comparison of the GOME and SCIAMACHY time series as well as inspection of time series over individual areas shows clear indication of non-linear changes in NO 2 columns in response to rapid changes in technology used and the economical situation.

Description: Measurements of natural deposition ice nuclei in Córdoba, Argentina Atmospheric Chemistry and Physics Discussions, 12, 31699-31723, 2012 Author(s): M. L. López and E. E. Ávila Ice nucleation in the atmosphere is of practical and fundamental importance since ice crystals influence the release of snow, rain and hail. Suspended aerosols in the atmosphere can initiate freezing at temperatures below −15 °C. In this work we describe an experimental device designed to measure the concentration of natural ice nuclei under controlled temperature and supersaturation conditions. The measurements were performed at Córdoba City, for temperatures between −15 °C and −30 °C and the sampled air was supersaturated with respect to ice and subsaturated with respect to liquid water; under these conditions the deposition ice nuclei were quantified. There are few studies reported in the literature regarding measurements of deposition ice nuclei concentration and, to our knowledge, there are no previous laboratory data of this kind of ice nuclei for T 〈 −20 °C. The results show that the number of deposition ice nuclei increases at colder temperatures and higher supersaturations. These results are in general in good agreement with results previously reported by other authors. A fitting function which depends on temperature and supersaturation is proposed to parameterize the results obtained in the present work.

Description: Wintertime peroxyacetyl nitrate (PAN) in the megacity Beijing: the role of photochemical and meteorological processes Atmospheric Chemistry and Physics Discussions, 12, 31871-31916, 2012 Author(s): H. Zhang, X. Xu, W. Lin, and Y. Wang Peroxyacetyl nitrate (PAN) is one of the key photochemical pollutants and acts as an important reservoir for the peroxyacetyl (PA) radical and nitrogen oxides (NO x ) over cold and less polluted regions. Previous measurements of PAN in Asian megacities were scarce and mainly conducted for relatively short periods in summer. In this study, we present and analyze the measurements of PAN, O 3 , NO x , CO, and some meteorological variables, made at an urban site (CMA) in Beijing from 25 January to 22 March 2010. During the observations, the hourly concentration of PAN varied from 0.23 to 3.51 ppb, with an average of 0.70 ppb. Both PAN and O 3 showed small but significant diurnal cycle, with PAN peaking around 17:00 LT, three hours later than O 3 . The observed concentration of PAN is well correlated with that of NO x but not O 3 . These phenomena indicate that the variations of the winter concentrations of PAN and O 3 in urban Beijing are decoupled with each other. Wind conditions and transport of air masses exert very significant impacts on O 3 , PAN, and other species. The strong WNW-N winds caused elevated concentrations of surface O 3 and lower concentrations of PAN, NO x , and CO. Weak winds from the other directions led to enhanced levels of PAN, NO x , and CO and decreased level of O 3 . Air masses arriving at our site originated either from the boundary layer over the highly polluted N-S-W sector or from the free troposphere over the W-N sector. The descending free-tropospheric air was rich in O 3 , with an average PAN/O 3 ratio smaller than 0.031, while the boundary layer air over the polluted sector contained higher levels of PAN and primary pollutants, with an average PAN/O 3 ratio of 0.11. These facts related with meteorological conditions, specifically the air transport conditions, can well explain the observed PAN-O 3 decoupling. The impact of meso-scale transport is demonstrated using a case during 21–22 February 2010. In addition to transport, photochemical production is important to PAN in the winter boundary layer over Beijing. The PA concentration is estimated from the measurements of PAN and related variables. The estimated PA concentration for three days with stable atmospheric condition, 7 February, 23 February, and 11 March, are in the range of 0–0.012, 0–0.036, and 0–0.040 ppt, respectively. We found that both the formation reaction and thermal decomposition contributed significantly to PAN's variation. The results here suggest that even in the colder period, both photochemical production and thermal decomposition of PAN in the polluted boundary layer over Beijing are not negligible, with the production exceeding the decomposition.

Description: A case study of the highly time-resolved evolution of aerosol chemical and optical properties in urban Shanghai, China Atmospheric Chemistry and Physics Discussions, 12, 31955-31990, 2012 Author(s): Y. Huang, L. Li, J. Li, X. Wang, H. Chen, J. Chen, X. Yang, D. S. Gross, H. Wang, L. Qiao, and C. Chen Characteristics of the chemical and optical properties of aerosols in urban Shanghai and their relationship were studied over a three-day period in October 2011. A suite of real-time instruments, including an Aerosol Time-Of-Flight Mass Spectrometer (ATOFMS), a Monitor for AeRosols and GAses (MARGA), a Cavity Ring Down Spectrometer (CRDS), a nephelometer and a Scanning Mobility Particle Sizer (SMPS), was employed to follow the quick changes of the aerosol properties within the 72-h sampling period. The origin of the air mass arriving in Shanghai during this period shifted from the East China Sea to the northwest area of China, offering a unique opportunity to observe the evolution of aerosols influenced by regional transport from the most polluted areas in China. According to the meteorological conditions and temporal characterizations of the chemical and optical properties, the sampling period was divided into three periods. During Period 1 (00:00–23:00, 13 October), the aerosols in urban Shanghai were mainly fresh and the single scattering albedo varied negatively with the emission of elemental carbon, indicating that local sources dominated. Period 2 (23:00 on 13 October to 10:00 on 15 October) was impacted by regionally transported pollutants and had the highest particulate matter (PM) mass loading and the lowest particle acidity, characterized by large fractions of aged particles and high secondary ion (nitrate, sulfate and ammonium) mass concentrations. Two sub-periods were identified in Period 2 based on the scattering efficiency of PM 1 mass. The comparison of these sub-periods highlights the influence of particle mixing state on aerosol optical properties. Period 3 (from 10:00 on 15 October to 00:00 on 16 October) had a low PM 1 /PM 10 ratio and a new particle formation event. We directly observed the influence of regionally transported pollutants on local aerosol properties and demonstrate that the PM mass extinction efficiency is largely determined by the chemical components and mixing states of the aerosol.

Description: An opportunity of application of excess factor in hydrology Hydrology and Earth System Sciences Discussions, 9, 13635-13649, 2012 Author(s): V. Kovalenko, E. Gaidukova, and A. Kachalova In last few years in hydrology an interest to excess factor has appeared as a reaction to unsuccessful attempts to simulate and predict evolving hydrological processes, which attributive property is statistical instability. The article shows, that the latter has a place at strong relative multiplicative noises of probabilistic stochastic model of a river flow formation, phenomenological display of which are "the thick tails" and polymodality, for which the excess factor "answers", by being ignored by a modern hydrology in connection to the large error of its calculation because of insufficient duration of lines of observation over a flow. However, it is found out, that the duration of observation of several decades practically stabilizes variability of the excess factor, the error of which definition appears commensurable with an error of other calculated characteristics used in engineering hydrology.

Description: Spatial distribution of solute leaching with snowmelt and irrigation: measurements and simulations Hydrology and Earth System Sciences Discussions, 9, 13451-13490, 2012 Author(s): D. Schotanus, M. J. van der Ploeg, and S. E. A. T. M. van der Zee Transport of a tracer and a degradable solute in a heterogeneous soil was measured in the field, and simulated with several transient and steady state infiltration rates. Leaching surfaces were used to investigate the solute leaching in space and time simultaneously. In the simulations, a random field for the scaling factor in the retention curve was used for the heterogeneous soil, which was based on the spatial distribution of drainage in an experiment with a multi-compartment sampler. As a criterion to compare the results from simulations and observations, the sorted and cumulative total drainage in a cell was used. The effect of the ratio of the infiltration rate over the degradation rate on leaching of degradable solutes was investigated. Furthermore, the spatial distribution of the leaching of degradable and non-degradable solutes was compared. The infiltration rate determines the amount of leaching of the degradable solute. This can be partly explained by a decreasing travel time with an increasing infiltration rate. The spatial distribution of the leaching also depends on the infiltration rate. When the infiltration rate is high compared to the degradation rate, the leaching of the degradable solute is similar as for the tracer. The fraction of the soil that contributes to solute leaching increases with an increasing infiltration rate. This fraction is similar for a tracer and a degradable solute. With increasing depth, the leaching becomes more homogeneous, as a result of dispersion. The spatial distribution of the solute leaching is different under different transient infiltration rates, therefore also the amount of leaching is different. With independent stream tube approaches, this effect would be ignored.

Description: Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 2: Application to the Research Scanning Polarimeter Atmospheric Chemistry and Physics Discussions, 12, 32063-32107, 2012 Author(s): B. van Diedenhoven, B. Cairns, A. M. Fridlind, A. S. Ackerman, and T. J. Garrett A new method to retrieve ice cloud asymmetry parameters from multi-directional polarized reflectance measurements is applied to measurements of the airborne Research Scanning Polarimeter (RSP) obtained during the CRYSTAL-FACE campaign in 2002. The method assumes individual hexagonal ice columns and plates serve as proxies for more complex shapes and aggregates. The closest fit is searched in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with a virtually continuous selection of aspect ratios and distortion. The asymmetry parameter, aspect ratio and distortion of the hexagonal particle that leads to the best fit with the measurements are considered the retrieved values. Two cases of thick convective clouds and two cases of thinner anvil cloud layers are analyzed. Median asymmetry parameters retrieved by the RSP range from 0.76 to 0.78, and are generally smaller that those currently assumed in most climate models and satellite retrievals. In all cases the measurements indicate roughened ice crystals, which is consistent with previous findings. Retrieved aspect ratios in three of the cases range from 0.9 to 1.6, indicating compact particles dominate the cloud-top shortwave radiation. Retrievals for the remaining case indicate plate-like ice crystals with aspect ratios around 0.3. The RSP retrievals are qualitatively consistent with the CPI images obtained in the same cloud layers. Retrieved asymmetry parameters are compared to those determined in situ by the Cloud Integrating Nephelometer (CIN). For two cases, the median values of asymmetry parameter retrieved by CIN and RSP agree within 0.01, while for the two other cases RSP asymmetry parameters are about 0.03–0.05 greater than those obtained by the CIN. Part of this bias might be explained by vertical variation of the asymmetry parameter.

Description: Characterisation of GOME-2 formaldehyde retrieval sensitivity Atmospheric Measurement Techniques Discussions, 5, 7095-7139, 2012 Author(s): W. Hewson, H. Bösch, M. P. Barkley, and I. De Smedt Formaldehyde (HCHO) is an important tracer of tropospheric photochemistry, whose slant column abundance can be retrieved from satellite measurements of solar backscattered UV radiation, using differential absorption retrieval techniques. In this work a spectral fitting sensitivity analysis is conducted on HCHO slant columns retrieved from the Global Ozone Monitoring Experiment 2 (GOME-2) instrument. Despite quite different spectral fitting approaches, the retrieved HCHO slant columns have geographic distributions that generally match expected HCHO sources, though the slant column magnitudes and corresponding uncertainties are particularly sensitive to the retrieval set-up. The choice of spectral fitting window, polynomial order, I 0 correction, and inclusion of minor absorbers tend to have the largest impact on the fit residuals. However, application of a reference sector correction using observations over the remote Pacific Ocean, is shown to largely homogenise the resulting HCHO vertical columns, thereby largely reducing any systematic erroneous spectral fitting.

Description: Feasibility study of using a "travelling" CO 2 and CH 4 instrument to validate continuous in-situ measurement stations Atmospheric Measurement Techniques Discussions, 5, 7141-7185, 2012 Author(s): S. Hammer, G. Konrad, A. T. Vermeulen, O. Laurent, M. Delmotte, A. Jordan, L. Hazan, S. Conil, and I. Levin In the course of the ICOS (Integrated Carbon Observation System)Demo Experiment a feasibility study on the usefulness of a Travelling Comparison Instrument (TCI) was conducted in order to evaluate continuous atmospheric CO 2 and CH 4 measurements at two European stations. The aim of the TCI is to independently measure ambient air in parallel to the standard station instrumentation, thus providing a comprehensive comparison that includes the sample intake system, the instrument itself as well as its calibration and data evaluation. Observed differences between the TCI and the Heidelberg gas chromatographic system, which acted as a reference for the TCI, were −0.02 ± 0.08 μmol mol −1 for CO 2 and −0.3 ± 2.3 nmol mol −1 for CH 4 . Over a period of two weeks each, the continuous CO 2 and CH 4 measurements at two ICOS field stations, Cabauw and OPE, were compared to co-located TCI measurements. At Cabauw mean differences of 0.21 ± 0.06 μmol mol −1 for CO 2 and 0.41 ± 0.50 nmol mol −1 for CH 4 were found. For OPE the mean differences were 0.13 ± 0.07 μmol mol −1 for CO 2 and 0.44 ± 0.36 nmol mol −1 for CH 4 . Potential causes of these observed differences are leakages or contaminations in the intake lines and/or there flushing pumps. At Cabauw station an additional error contribution originates from insufficient flushing of standard gases. Offsets arising from differences in the working standard calibrations or leakages/contaminations in the drying systems are too small to explain the observed differences. Finally a comprehensive quality management strategy for atmospheric monitoring networks is proposed.

Description: Biogenic volatile organic compound emissions during BEARPEX 2009 measured by eddy covariance and flux-gradient similarity methods Atmospheric Chemistry and Physics Discussions, 12, 25081-25120, 2012 Author(s): J.-H. Park, S. Fares, R. Weber, and A. H. Goldstein The Biosphere Effects on AeRosols and Photochemistry EXperiment (BEARPEX) took place in Blodgett Forest, a Ponderosa pine forest in the Sierra Nevada Mountains of California, during summer 2009. We deployed a Proton Transfer Reaction – Mass Spectrometer (PTR-MS) to measure fluxes and concentrations of biogenic volatile organic compounds (BVOCs). Eighteen ion species including the major BVOC expected at the site were measured sequentially at 5 heights to observe their vertical gradient from the forest floor to above the canopy. Fluxes of the 3 dominant BVOCs methanol, 2-Methyl-3-butene-2-ol (MBO), and monoterpenes, were measured above the canopy by the eddy covariance method. Canopy scale fluxes were also determined by the flux-gradient similarity method ( K -theory). A universal K ( K univ ) was determined as the mean of individual K 's calculated from the measured fluxes divided by vertical gradients for methanol, MBO, and monoterpenes. This K univ was then multiplied by the gradients of each observed ion species to compute their fluxes. The flux-gradient similarity method showed very good agreement with the Eddy Covariance method. Fluxes are presented for all measured species and compared to historical measurements from the same site, and used to test emission algorithms used to model fluxes at the regional scale. MBO was the dominant emission observed followed by methanol, monoterpenes, acetone, and acetaldehyde. The flux-gradient similarity method is shown to be a useful, and we recommend its use especially in experimental conditions when fast measurement of BVOC species is not available.

Description: Black carbon physical properties and mixing state in the European megacity Paris Atmospheric Chemistry and Physics Discussions, 12, 25121-25180, 2012 Author(s): M. Laborde, M. Crippa, T. Tritscher, Z. Jurányi, P. F. DeCarlo, B. Temime-Roussel, N. Marchand, S. Eckhardt, A. Stohl, U. Baltensperger, A. S. H. Prévôt, E. Weingartner, and M. Gysel Aerosol hygroscopicity and black carbon (BC) properties were characterised during wintertime in Paris, one of the biggest European megacities. Hygroscopic growth factor (GF) distributions, characterised by distinct modes of more-hygroscopic background aerosol and non- or slightly hygroscopic aerosol of local (or regional) origin, revealed an increase of the relative contribution of the local sources compared to the background aerosol with decreasing particle size. BC particles in Paris were mainly originating from fresh traffic emissions, whereas biomass burning was only a minor contribution. The mass size distribution of the BC cores peaked on average at a BC core mass equivalent diameter of D MEV ≈150 nm. The BC particles were moderately coated (Δ coat ≈30 nm on average for BC cores with D MEV =160–260 nm) and an average mass absorption coefficient (MAC) of ~8.6 m 2 g −1 at the wavelength λ = 880 nm was observed. Different time periods were selected to investigate the properties of BC particles as a function of source and air mass type. The traffic emissions were found to be non-hygroscopic (GF ≈ 1.0), and essentially all particles with a dry mobility diameter larger than D 0 = 110 nm contained a BC core. BC from traffic emissions was further characterised by literally no coating (Δ coat ≈2 nm), the smallest maximum of the BC core mass size distribution ( D MEV ≈100 nm) and the smallest MAC (~7.3 m 2 g −1 at λ = 880 nm). The biomass burning aerosol was slightly more-hygroscopic than the traffic emissions (with a distinct slightly hygroscopic mode peaking at GF≈1.1–1.2). Furthermore, only a minor fraction (⩽10%) of the slightly hygroscopic particles with GF⩾1.1 (and D 0 = 265 nm) contained a detectable BC core. The BC particles from biomass burning were found to have a medium coating thickness as well as slightly larger mean BC core sizes and MAC values compared to traffic emissions. The aerosol observed under the influence of aged air masses and air masses from Eastern Continental Europe was dominated by a more-hygroscopic mode peaking at GF≈1.6. Most particles (95%) with a D 0 = 265 nm, in this mode, did not contain a detectable BC core. A significant fraction of the BC particles had a substantial coating with non-refractory aerosol components. MAC values of ~8.8 m 2 g −1 and ~8.3 m 2 g −1 at λ = 880 nm and mass mean BC core diameters of 150 nm and 200 nm were observed for the aged and continental air mass types, respectively. The reason for the larger BC core sizes compared to the fresh emissions – transport effects or a different BC source – remains unclear. The dominant fraction of the BC-containing particles was found to have no or very little coating with non-refractory matter. The lack of coatings is consistent with the observation that the BC particles are non- or slightly hygroscopic, which makes them poor cloud condensation nuclei.

Description: Linkages between ozone depleting substances, tropospheric oxidation and aerosols Atmospheric Chemistry and Physics Discussions, 12, 25551-25572, 2012 Author(s): A. Voulgarakis, D. T. Shindell, and G. Faluvegi Coupling between the stratosphere and the troposphere allows changes in stratospheric ozone abundances to affect tropospheric chemistry. Large-scale effects from such changes on chemically produced tropospheric aerosols have not been systematically examined in past studies. We use a composition-climate model to investigate potential past and future impacts of changes in stratospheric Ozone Depleting Substances (ODS) on tropospheric oxidants and sulfate aerosols. In most experiments, we find significant responses in tropospheric photolysis and oxidants, with small but significant effects on methane radiative forcing. The response of sulfate aerosols is sizeable when examining the effect of increasing future nitrous oxide (N 2 O) emissions. We also find that without the regulation of chlorofluorocarbons (CFCs) through the Montreal Protocol, sulfate aerosols could have increased by 2050 by a comparable amount to the decreases predicted due to relatively stringent sulfur emissions controls. The historical radiative forcing of CFCs through their indirect effects on methane (−22.6 mW m −2 ) and sulfate aerosols (−3.0 mW m −2 ) discussed here is non-negligible when compared to known historical CFC forcing. Our results stress the importance of accounting for stratosphere-troposphere, gas-aerosol and composition-climate interactions when investigating the effects of changing emissions on atmospheric composition and climate.

Description: CLARA-SAL: a global 28-yr timeseries of Earth's black-sky surface albedo Atmospheric Chemistry and Physics Discussions, 12, 25573-25615, 2012 Author(s): A. Riihelä, T. Manninen, V. Laine, K. Andersson, and F. Kaspar We present a novel 28-yr dataset of Earth's black-sky surface albedo, derived from AVHRR instruments. The dataset is created using algorithms to separately derive the surface albedo for different land use areas globally. Snow, sea ice, open water and vegetation are all treated independently. The product features corrections for the atmospheric effect in satellite-observed surface radiances, a BRDF correction for the anisotropic reflectance properties of natural surfaces, and a novel topography correction of geolocation and radiometric accuracy of surface reflectance observations over mountainous areas. The dataset is based on a homogenized AVHRR radiance timeseries. The product is validated against quality-controlled in situ observations of clear-sky surface albedo at various BSRN sites around the world. Snow and ice albedo retrieval validation is given particular attention using BSRN sites over Antarctica, Greenland Climate Network stations on the Greenland Ice Sheet (GrIS), as well as sea ice albedo data from the SHEBA and Tara expeditions. The product quality is found to be comparable to other previous long-term surface albedo datasets from AVHRR.

Description: On selection of the optimal data time interval for real-time hydrological forecasting Hydrology and Earth System Sciences Discussions, 9, 10829-10875, 2012 Author(s): J. Liu and D. Han With the advancement in modern telemetry and communication technologies, hydrological data can be collected with an increasingly higher sampling rate. An important issue deserving attention from the hydrological community is what suitable time interval of the model input data should be chosen in hydrological forecasting. Such a problem has long been recognised in the control engineering community but is a largely ignored topic in operational applications of hydrological forecasting. In this study, the intrinsic properties of rainfall-runoff data with different time intervals are first investigated from the perspectives of the sampling theorem and the information loss using the discrete wavelet decomposition tool. It is found that rainfall signals with very high sampling rates may not always improve the accuracy of rainfall-runoff modelling due to the catchment low-pass filtering effect. To further investigate the impact of data time interval in real-time forecasting, a real-time forecasting system is constructed by incorporating the Probability Distributed Model (PDM) with a real-time updating scheme, the autoregressive-moving average (ARMA) model. Case studies are then carried out on four UK catchments with different concentration times for real-time flow forecasting using data with different time intervals of 15 min, 30 min, 45 min, 60 min, 90 min and 120 min. A positive relation is found between the forecast lead time and the optimal choice of the data time interval, which is also highly dependent on the catchment concentration time. Finally, based on the conclusions from the case studies, a hypothetical pattern is proposed in three-dimensional coordinates to describe the general impact of the data time interval and to provide implications on the selection of the optimal time interval in real-time hydrological forecasting. Although nowadays most operational hydrological systems still have low data sampling rates (daily or hourly), the trend in the future is that higher sampling rates will become widespread and there is an urgent need for both academic and practising hydrologists to realise the significance of the data time interval issue. It is important that more case studies in different catchments with various hydrological forecasting models should be explored in the future to further verify and improve the proposed hypothetical pattern.

Description: Catchment classification based on characterisation of streamflow and precipitation time-series Hydrology and Earth System Sciences Discussions, 9, 10805-10828, 2012 Author(s): E. Toth The formulation of objective procedures for the delineation of homogeneous groups of catchments is a fundamental issue in both operational and research hydrology. For assessing catchment similarity, a variety of hydrological information may be considered; in this paper, gauged sites are characterised by a set of streamflow signatures that include a representation, albeit simplified, of the properties of fine time-scale flow series and in particular of the dynamic components of the data, in order to keep into account the sequential order and the stochastic nature of the streamflow process. The streamflow signatures are provided in input to a clustering algorithm based on unsupervised SOM neural networks, providing an overall reasonable grouping of catchments on the basis of their hydrological response. In order to assign ungauged sites to such groups, the catchments are represented through a parsimonious set of morphometric and pluviometric variables, including also indexes that attempt to synthesize the variability and correlation properties of the precipitation time-series, thus providing information on the type of weather forcing that is specific to each basin. Following a principal components analysis, needed for synthesizing and better understanding the morpho-pluviometric catchment properties, a discriminant analysis finally classifies the ungauged catchments, through a leave-one-out cross-validation, to one of the above identified hydrologic response classes. The approach delivers quite satisfactory results for ungauged catchments, since the comparison of the two cluster sets shows an acceptable overlap. Overall results indicate that the inclusion of information on the properties of the fine time-scale streamflow and rainfall time-series may be a promising way for better representing the hydrologic and climatic character of the study catchments.

Description: Modeling postfire water erosion mitigation strategies Hydrology and Earth System Sciences Discussions, 9, 10877-10916, 2012 Author(s): M. C. Rulli, L. Offeddu, and M. Santini Severe wildfires are often followed by significant increase in runoff and erosion, due to vegetation damages and changes in physical and chemical soil properties. Peak flows and sediment yields can increase up to two orders of magnitude becoming dangerous for human lives and ecosystem, especially in the wildland-urban interface. Watershed post fire rehabilitation measures are usually used to mitigate the effects of fire on runoff and erosion, by protecting soil from splash and shear stress detachment and enhancing its infiltration capacity. Modeling post fire erosion and erosion mitigation strategies can be useful in selecting the effectiveness of rehabilitation method. In this paper a distributed model based on Revised Universal Soil Loss Equation (RUSLE), properly parameterized for a Mediterranean basin located in Sardinia, is used to determine soil losses for six different scenarios describing both natural and post-fire basin condition, the last accounting also for the single and combined effect of different erosion mitigation measures. Fire effect on vegetation and soil properties have been mimed by changing soil drainage capacity and organic matter content, and RUSLE factors related to soil cover and protection measures. Model results show for the analyzed rehabilitation treatments their effect in reducing the amount of soil losses with the peculiar characteristics of the spatial distribution of such changes.

Description: Seasonal forecasts of drought indices in African basins Hydrology and Earth System Sciences Discussions, 9, 11093-11129, 2012 Author(s): E. Dutra, F. Di Giuseppe, F. Wetterhall, and F. Pappenberger Vast parts of Africa rely on the rainy season for livestock and agriculture. Droughts can have a severe impact in these areas which often have a very low resilience and limited capabilities to mitigate their effects. This paper tries to assess the predictive capabilities of an integrated drought monitoring and forecasting system based on the Standard precipitation index (SPI). The system is firstly constructed by temporally extending near real-time precipitation fields (ECMWF ERA-Interim reanalysis and the Climate Anomaly Monitoring System-Outgoing Longwave Radiation Precipitation Index, CAMS-OPI) with forecasted fields as provided by the ECMWF seasonal forecasting system and then is evaluated over four basins in Africa: the Blue Nile, Limpopo, Upper Niger, and Upper Zambezi. There are significant differences in the quality of the precipitation between the datasets depending on the catchments, and a general statement regarding the best product is difficult to make. All the datasets show similar patterns in the South and North West Africa, while there is a low correlation in the tropical region which makes it difficult to define ground truth and choose an adequate product for monitoring. The Seasonal forecasts have a higher reliability and skill in the Blue Nile, Limpopo and Upper Niger in comparison with the Zambezi. This skill and reliability depends strongly on the SPI time-scale, and more skill is observed at larger time-scales. The ECMWF seasonal forecasts have predictive skill which is higher than using climatology for most regions. In regions where no reliable near real-time data is available, the seasonal forecast can be used for monitoring (first month of forecast). Furthermore, poor quality precipitation monitoring products can reduce the potential skill of SPI seasonal forecasts in two to four months lead time.

Description: Note on the application of planar-fit rotation for non-omnidirectional sonic anemometers Atmospheric Measurement Techniques Discussions, 5, 7323-7340, 2012 Author(s): M. Li, W. Babel, K. Tanaka, and T. Foken For non-omnidirectional sonic anenometers like the Kaijo-Denki DAT 600 TR61A probe, it is shown that separate planar fit rotations must be used for the undisturbed (open part of the sonic anemometer) and the disturbed sector. This increases the friction velocity while no effect on the scalar fluxes was found. In the disturbed sector, irregular values of − u ′ w ′ 〈 0 were detected for low wind velocities. This study was done for data sets from the Naqu-BJ site on the Tibetan Plateau.

Description: A unified approach to aerosol remote sensing and type specification in the infrared Atmospheric Chemistry and Physics Discussions, 12, 26871-26928, 2012 Author(s): L. Clarisse, P.-F. Coheur, F. Prata, J. Hadji-Lazaro, D. Hurtmans, and C. Clerbaux Atmospheric aerosols impact air quality and global climate. Space based measurements are the best way to observe their spatial and temporal distributions, and can also be used to gain better understanding of their chemical, physical and optical properties. Aerosol composition is the key parameter affecting the refractive index, which determines how much radiation is scattered and absorbed. Composition of aerosols is unfortunately not measured by state of the art satellite remote sounders. Here we use high resolution infrared measurements for aerosol type differentiation, exploiting, in that part of spectrum, the dependency of their refractive index on wavelength. We review existing detection methods and present a unified detection method based on linear discrimination analysis. We demonstrate this method on measurements of the Infrared Atmospheric Sounding Interferometer (IASI) and six different aerosol types, namely volcanic ash, windblown sand, ice crystals, sulfuric acid droplets, ammonium sulfate and smoke particles. The detection of the last three types is unprecedented in the infrared in nadir mode, but is very promising, especially for sulfuric acid droplets which are detected in the lower troposphere and up to 6 months after injection in the upper troposphere/lower stratosphere.

Description: Multi-satellite rainfall sampling error estimates – a comparative study Hydrology and Earth System Sciences Discussions, 9, 11677-11706, 2012 Author(s): M. Itkin and A. Loew This study focus is set on quantifying sampling related uncertainty in the satellite rainfall estimates. We conduct observing system simulation experiment to estimate sampling error for various constellations of Low-Earth orbiting and geostationary satellites. There are two types of microwave instruments currently available: cross track sounders and conical scanners. We evaluate the differences in sampling uncertainty for various satellite constellations that carry instruments of the common type as well as in combination with geostationary observations. A precise orbital model is used to simulate realistic satellite overpasses with orbital shifts taken into account. With this model we resampled rain gauge timeseries to simulate satellites rainfall estimates free of retrieval and calibration errors. We concentrate on two regions, Germany and Benin, areas with different precipitation regimes. Our results show that sampling uncertainty for all satellite constellations does not differ greatly depending on the area despite the differences in local precipitation patterns. Addition of 3 hourly geostationary observations provides equal performance improvement in Germany and Benin, reducing rainfall undersampling by 20–25% of the total rainfall amount. Authors do not find a significant difference in rainfall sampling between conical imager and cross-track sounders.

Description: Soil moisture controls on patterns of grass green-up in Inner Mongolia: an index based approach Hydrology and Earth System Sciences Discussions, 9, 11641-11675, 2012 Author(s): H. Liu, F. Tian, H. Hu, H. Hu, and M. Sivapalan Water availability is one of the most important environmental controls on vegetation phenology, especially in semi-arid regions, and is often represented in terms of soil moisture in small-scale studies whereas it tends to be represented by precipitation in large-scale (e.g. regional) studies. Clearly, soil moisture is the more appropriate indicator for root water uptake and vegetation growth/phenology and therefore its potential advantage and applicability needs to be demonstrated at regional scales. This paper represents a data-based regional study of the effectiveness of alternative indices based on water and energy availability on space-time patterns of spring vegetation green-up onset dates estimated from Normalized Difference Vegetation Index (NDVI) datasets in the grasslands of Inner Mongolia, China. The macro-scale hydrological model, VIC, is employed to generate a soil moisture database across the region. In addition to standard index based on temperature, two potential hydrology based indices for prediction of spring onset dates are defined based on the simulated soil moisture data as well as on observed precipitation data. Results indicate that the correspondence between the NDVI-derived green-up onset date and the soil moisture derived potential onset date exhibits a significantly better correlation as a function of increasing aridity, compared to that based on precipitation. In this way the soil moisture based index is demonstrated to be superior to the precipitation based index in terms of capturing grassland spring phenology. The results also showed that both of the hydrological (water based) indices were superior to the thermal (temperature based) index in determining the patterns of grass green-up in the Inner Mongolia region, indicating water availability to be the dominant control, on average. The understanding about the relative controls on grassland phenology, and the effectiveness of alternative indices to capture these controls, are important for future studies and predictions of vegetation phenology change under climate change.