ALBERT

Description: High-altitude meteorological processes in the Himalaya are influenced by complex interactions between the topography and the monsoon and westerly circulation systems. In this study, we use the atmospheric model WRF configured with high-spatial resolution to understand seasonal patterns of near-surface meteorological fields and precipitation processes in the Langtang catchment in the central Himalaya. Using a unique high-altitude observational network, we evaluate a simulation from 17 June 2012 to 16 June 2013 and conclude that, at 1-km horizontal grid spacing, the model captures the main features of observed meteorological variability in the catchment. The finer representation of the complex terrain and explicit simulation of convection at this grid spacing give strong improvements in near-surface air temperature and small improvements in precipitation, in particular in the magnitudes of daytime convective precipitation and at higher elevations. The seasonal differences are noteworthy, including a reversal in the vertical and along-valley distributions of precipitation between the monsoon and winter seasons, with peak values simulated at lower altitudes (~3000 m a.s.l) and in the upper regions (above 5000 m a.s.l.) in each season, respectively. We conclude that there is great potential for improving the local accuracy of climate change impact studies in the Himalaya by using high-resolution atmospheric models to generate the forcing for such studies.

Description: Accurate prediction of total lightning flash rate in thunderstorms is important to improve estimates of nitrogen oxides (NO x ) produced by lightning (LNO x ) from the storm scale to the global scale. In this study, flash rate parameterization schemes from the literature are evaluated against observed total flash rates for a sample of eleven Colorado thunderstorms, including nine storms from the Deep Convective Clouds and Chemistry (DC3) experiment in May-June 2012. Observed flash rates were determined using an automated algorithm that clusters very high frequency (VHF) radiation sources emitted by electrical breakdown in clouds and detected by the northern Colorado lightning mapping array (LMA). Existing schemes were found to inadequately predict flash rates and were updated based on observed relationships between flash rate and simple storm parameters, yielding significant improvement. The most successful updated scheme predicts flash rate based on the radar-derived mixed-phase 35-dBZ echo volume. Parameterizations based on metrics for updraft-intensity were also updated but were found to be less reliable predictors of flash rate for this sample of storms. The 35-dBZ volume scheme was tested on a dataset containing radar reflectivity volume information for thousands of isolated convective cells in different regions of the US. This scheme predicted flash rates to within 5.8% of observed flash rates on average. These results encourage the application of this scheme to larger radar datasets and its possible implementation into cloud-resolving models.

Description: The central United States experiences a wide array of hydrological extremes, with the 1993, 2008, 2013, and 2014 flooding events and the 1988 and 2012 droughts representing some of the most recent extremes, and is an area where water availability is critical for agricultural production. This study aims to evaluate the ability of a set of global impact models (GIMs) from the WaterMIP project to reproduce the regional hydrology of the central United States for the period 1963–2001. Hydrological indices describing annual daily maximum, medium and minimum flow and their timing are extracted from both modeled daily runoff data by nine GIMs and from observed daily streamflow measured at 252 river gauges. We compare trend patterns for these indices, and their ability to capture runoff volume differences for the 1988 drought and 1993 flood. In addition, we use a subset of 128 gauges and corresponding gridcells to perform a detailed evaluation of the models on a gauge-to-gridcell basis. Results indicate that these GIMs capture the overall trends in high, medium, and low flows well. However, the models differ from obervations with respect to the timing of high and medium flows. More specifically, GIMs that only include water balance tend to be closer to the observations than GIMs that also include the energy balance. In general, as it would be expected, the performance of the GIMs is the best when describing medium flows, as opposed to the two ends of the runoff spectrum. With regards to low flows, some of the GIMs having considerably large pools of zeros or low values in their time series, undermining their ability in capturing low flow characteristics and weakening the ensemble's output. Overall, this study provides a valuable examination of the capability of GIMs to reproduce observed regional hydrology over a range of quantities for the central United States.

Description: Atmospheric rivers (ARs) often trigger extreme precipitation events in British Columbia (BC), Canada. Here we analyze how well the autumn AR events with the highest probability for extreme precipitation over BC, henceforth called AR-extreme events, are simulated in five Coupled Model Intercomparison Project Phase 5 (CMIP5) global climate models (GCMs) and how these AR-extreme events are projected to change by the end of the century. We examine the daily synoptic patterns of integrated water vapor transport ( IVT ) over the Pacific Ocean that favor the formation of AR-extreme events. Our analysis and comparison with AR-extreme events in four reanalysis products for the period 1979–2010 reveal that the GCMs more successfully resolve their seasonality and inter-annual variability than their frequencies and amount of precipitation brought to BC. For the CMIP5 scenario's RCP4.5 and RCP8.5, the frequency of AR-extreme events will increase for the period 2070–2100 with the largest increase in December. All models project an increase in total precipitation over BC, due to the increase in frequency and intensity of the AR-extreme events, however, the dominant factor is the increase in frequency, especially of those events with precipitation exceeding 20 mm/day. The path of the ARs during the AR-extreme events is projected to move northward, bringing stronger IVT and more precipitation to the north coast of BC, while the south coast may become drier than at the present-day. The shift in the ARs is driven by the northward shift in the Aleutian Low Pressure System, especially in RCP8.5.

Description: A widely used approach in observational and modeling studies of NO x produced by lightning is to relate NO x production to the number of flashes, without regard for the distribution of lightning flash sizes. Recent studies have begun to consider channel length and flash size, which is now observable with VHF Lightning Mapping Array (LMA) data. This study uses a capacitor model for flash energy based on the flash coverage area, which defines a size scale. This flash area is then filled with channel using a fractal method and compared to other methods that estimate length directly from the VHF source locations. In the presence of instrument measurement errors, area- and fractal-based estimates are shown to be more stable estimators of flash length and than connect-the-dots approaches, and therefore are better suited for comparison to NO x production. A geometric interpretation of using vertical profiles of VHF source density to weight the altitude distribution of total channel length is developed. An example of the time series of moments of the lightning flash size distribution is shown for an example case and some meteorological interpretation is given.

Description: California near-surface air temperatures are influenced by large-scale, regional and local factors. In that sense, a numerical model experiment was carried out to analyze the contribution of large-scale (changes in atmospheric and oceanic conditions) and regional (increased urbanization) factors on the observed California South Coast Air Basin regional summer daily maximum temperature warming pattern from 1950 to 2013. The simulations were performed with past (1950–54) and present (2009–13) land-cover and climate conditions. The past land cover was derived from historical digital maps, the present land cover was updated with high-resolution airborne remote sensing data. Results show that both factors contribute to the total change in daily maximum temperatures. Changes due to large-scale climate conditions dominate in coastal (due to warming SSTs) and non-urban regions, while changes due to urbanization have an impact mainly in urban areas, especially inland where large-scale warming weakens. Increased urbanization has also reduced sea-breeze intensity due to changes in surface roughness. The model was able to reproduce the regional observed warming pattern, as it incorporates urban heat island effects, otherwise underestimated by large-scale climate change only.

Description: During the Stratosphere-Troposphere Analyses of Regional Transport 2008 Experiment (START08) the NCAR/NSF Gulfstream V aircraft observed high concentrations of NO and NO y in the upper troposphere downwind of a weakening squall line in northern Texas, suggesting either convective transport of polluted boundary layer air to the upper troposphere or lightning production of nitrogen oxides in the convection. These hypotheses are tested by computing three-dimensional back trajectories using winds from a high-resolution simulation of the event with the Weather Research and Forecasting (WRF) Model. The WRF model simulation reproduces the storm structure and evolution with good fidelity. The back trajectories reveal two distinct layers of outflow air from different mesoscale convective systems (MCSs). Most air in the upper layer is transported northward from an MCS in southern Texas, while the lower layer is from both the northern squall line and the southern MCS. In both layers inconsistencies between observed concentrations of CO, NO, and O 3 , and predictions from a simple mixing model suggest that there is significant production of NO by lightning in the convective systems. This is consistent with lightning observations from the National Lightning Detection Network (NLDN). Additionally, the model simulation appears to slightly underestimate the depth of vertical transport by the MCS.

Description: This study investigated the air-sea interaction over the Kuroshio in the East China Sea by focusing on the response and feedback of the ocean to typical spring weather events. The Weather Research and Forecasting Model was coupled with the HYbrid Coordinate Ocean Model for use in the study. The study period comprised a sequence of typical weathers in the area: prevailing southwesterly winds, the passage of a cold front and the ensuing cold-air outbreak, and the development of a Taiwan low. The air-sea interaction operated on a diurnal timescale under conditions of moderate wind speeds, high insolation, and a shallow oceanic mixed layer. The sea surface temperature and upper ocean heat content increased progressively prior to the frontal passage. The model reproduced the retreat of Kuroshio in response to the strong wind during the cold-air outbreak. The diurnal cycle vanished at high wind speeds. Wind stirring eroded the upper seasonal thermocline and deepened the oceanic mixed layer. The upper ocean heat content decreased because of entrainment cooling and surface heat losses. Surface restratification was subsequently suppressed in the thick and weakly stratified remnant layer. The consequently insufficient recovery of upper ocean heat content may have preconditioned a stagnation of the Taiwan low. The recovery of upper ocean heat content was discussed to derive the implication for climate simulations.

Description: We present the first coordinated study using two lidars at two separate locations to characterize a 1-h mesoscale gravity wave event in the mesopause region. The simultaneous observations were made with the STAR Na Doppler lidar at Boulder, CO, and the USU Na Doppler lidar and temperature mapper at Logan, UT on 27 November 2013. The high precision possessed by the STAR lidar enabled these waves to be detected in vertical wind. The mean wave amplitudes are ~0.44 m/s in vertical wind and ~1% in relative temperature at altitudes of 82–107 km. Those in the zonal and meridional winds are 6.1 and 5.2 m/s averaged from 84–99 km. The horizontal and vertical wavelengths inferred from the mapper and lidars are ~219±4 and 16.0±0.3 km, respectively. The intrinsic period is ~1.3 h for the airglow layer, Doppler shifted by a mean wind of ~17 m/s. The wave packet propagates from Logan to Boulder with an azimuth angle of ~135° clockwise from North and an elevation angle of ~ 3° from the horizon. The observed phase difference between the two locations can be explained by the travelling time of the 1-h wave from Logan to Boulder, which is about ~2.4 h. The wave polarization relations are examined through the simultaneous quantifications of the three wind components and temperature. This study has developed a systematic methodology for fully characterizing mesoscale gravity waves, inspecting their intrinsic properties, and validating the derivation of horizontal wave structures by applying multiple instruments from coordinated stations.

Description: The climatic impacts of dust on East Asian precipitation, summer monsoon, and sea surface temperature (SST) were investigated by a regional coupled atmosphere–ocean-land model. The regional and annual mean dust clear-sky (all-sky) direct radiative forcings were predicted to be −6.65 W m −2 (−1.78 W m −2 ) at the surface and 3.79 W m −2 (8.65 W m −2 ) at the TOA (top-of-atmosphere). The climatic effects of dust include a cooling effect below 700 hPa and a warming effect above, leading to more stabilized lower troposphere and anomalously cyclonic wind over Japan and surrounding oceans. Sensitivity tests show that the 'surface cooling' effect by dust could reduce evaporation over land and enhance stability of the lower atmosphere, leading to reduced vapor content and precipitation in China in the spring. However, the upward movements either by the 'Elevated Heat Pump' (EHP) effect of dust or by the atmospheric convergence, and the downward movement by secondary circulation dominated Northern, Southern, and Central China, respectively, along with enhanced evaporation and weakened lapse rate over China, leading to increased precipitation in downwind areas of dust source regions and Southern China, and to decreased precipitation in Central China in summer. Results from this simulation also show that dust aerosol tends to weaken the East Asia Summer Monsoon by reducing the land-sea-temperature-contrast. In addition, dust could also perturb SST by the local net heat flux rebalance as well as the northward heat transport from surrounding oceans. The anomalous northward surface wind contributes even larger on SST in JJA.

Description: The effective prediction and estimation of hydro-meteorological variables are important for water resources planning and management. In this study, we propose a multivariate conditional model for streamflow prediction and the refinement of spatial precipitation estimates. This model consists of high-dimensional vine copulas, conditional bivariate copula simulations, and a quantile-copula function. The vine copula is employed because of its flexibility in modeling the high-dimensional joint distribution of multivariate data by building a hierarchy of conditional bivariate copulas. We investigate two cases to evaluate the performance and applicability of the proposed approach. In the first case, we generate one-month-ahead streamflow forecasts that incorporate multiple predictors including antecedent precipitation and streamflow records in a basin located in South China. The prediction accuracy of the vine-based model is compared with that of traditional data-driven models such as the support vector regression (SVR) and the adaptive neural-fuzzy inference system (ANFIS). The results indicate that the proposed model produces more skillful forecasts than SVR and ANFIS. Moreover, this probabilistic model yields additional information concerning the predictive uncertainty. The second case involves refining spatial precipitation estimates derived from the tropical rainfall measuring mission precipitation (TRMM) product for the Yangtze River basin by incorporating remotely sensed soil moisture data and the observed precipitation from meteorological gauges over the basin. The validation results indicate that the proposed model successfully refines the spatial precipitation estimates. Although this model is tested for specific cases, it can be extended to other hydro-meteorological variables for predictions and spatial estimations.

Description: The objective of this research was to test, by means of an experiment in a high-voltage laboratory, the effect of an array of hydrometeors on the processes involved in the streamer-leader formation of lightning. Because the common types of hydrometeors present whenever lightning initiation in thunderstorms occurs are ice particles (graupel, hail, or ice crystals), we used, in this experiment, conductive particles similar to hail in size, with various spacing between them, but all under normal atmospheric pressure and room temperature. The laboratory array was suspended on dielectric threads in a uniform electric field of 1 MV m -1 in the middle of the gap between the high-voltage and ground electrodes. During the first phase of the experiment, we studied the formation of a bidirectional arc discharge from the array, and the effects of the array's size on the electrical characteristics and on the speed of development of the discharge. We continued with the same objectives in the second phase of the experiment, adding high-speed video observations with a recording speed of 10 Mfps, to observe all stages of the streamer-leader formation.

Description: Partition of the energy between sensible heat and latent heat indicates that surface temperatures are affected by soil moisture deficits. Since transpiration by plants is the largest contributor to the land's total latent heat, the coupling of temperature and soil moisture will depend on the response of vegetation to soil moisture deficit and those are influenced by the soil moisture regimes. Utilizing daily precipitation and temperature data from China for a period of 1961–2010, this study computes average annual climatic water balance (AACWB) for defining soil moisture regimes and then quantitatively investigates the summer soil moisture-temperature coupling. With precipitation deficits (indicated by standardized precipitation index with the selected optimal time scale of 3 months) as proxy of soil moisture deficits, results indicate that the relationship between summer precipitation deficits and hot extremes tends to be enhanced when the negative AACWB draws closer towards zero while tends to be weakened with the increase of positive AACWB. For the region with the negative AACWB closing zero, the enhanced relationship should be attributed to the increase of the proportion of latent heat compared to the absorbed total energy. However, the weakened relationship with the increase of positive AACWB may be owing to the different responses of vegetation to precipitation deficit that the transpiration in the region with lower positive AACWB is less when responding to precipitation deficit. However, the physiological mechanisms behind vegetation response to soil moisture deficits still need to be further analyzed. By quantifying relevant biological and hydrological processes and their interaction, it is expected that the uncertainties in future-climate scenarios be reduced, that would then allow the development of early warning and adaptation measures prior to the occurrence of hot extremes. Further, the summer precipitation deficit-temperature coupling is strongest along the strip stretching from southwest to northeast in China.

Description: Climate model simulations are routinely compared to observational data sets for evaluation purposes. The resulting differences can be large and induce artifacts if propagated through impact models. They are usually termed ‘model biases’, suggesting that they exclusively stem from systematic models errors. Here we explore for Switzerland the contribution of two other components of this mismatch, which are usually overlooked: interpolation errors and natural variability. Precipitation and temperature simulations from the RCM COSMO-CLM were compared to two observational data sets, for which estimates of interpolation errors were derived. Natural variability on the multi-decadal time scale was estimated using three approaches relying on homogenized time series, multiple runs of the same climate model and bootstrapping of 30-year meteorological records. We find that although these methods yield different estimates, the contribution of the natural variability to RCM-observation differences in 30-year means is usually small. In contrast, uncertainties in observational data sets induced by interpolation errors can explain a substantial proportion of the mismatch of 30-year means. In those cases, we argue that the model biases can hardly be distinguished from interpolation errors, making the characterization and reduction of model biases particularly delicate. In other regions, RCM biases clearly exceed the estimated contribution of natural variability and interpolation errors, enabling bias characterization and robust model evaluation. Overall, we argue that bias correction of climate simulations needs to account for observational uncertainties and natural variability. We particularly stress the need for reliable error estimates to accompany observational datasets.

Description: Seasonal changes in near-surface aerosol mass loading and their chemical characteristics over the marine environment of Bay of Bengal (BoB) have been investigated, based on the ship-based experiments carried out as part of three major field campaigns in pre-monsoon, winter and monsoon seasons. These spatio-temporal properties of in-situ measured near-surface aerosols were compared with the columnar aerosol optical depth (AOD), Angstrom exponent, effective radius, mass concentration, fine mode AOD and large mode AOD retrieved by MODIS. The spatial heterogeneity in the chemical nature of aerosols for the three contrasting seasons has been addressed. In all the seasons, BoB is found to be polluted by anthropogenic aerosols as revealed by their chemical composition. North BoB exhibits highest near-surface aerosol loading, columnar AOD and aerosol mass concentration irrespective of the season. While the high AOD in winter is due to fine mode anthropogenic aerosols, that in monsoon is due to large-sized sea-salt aerosols. The fine mode AOD over BoB contributes 45% in monsoon, 69% in pre-monsoon and 73% in winter. The column AOD in winter and pre-monsoon is ~50% of its monsoonal value. The e-fold scale distances of total aerosol mass loading and those of various chemical species over BoB are also investigated. The column mass concentration has been estimated from the in-situ measured near-surface aerosol mass loading and compared with that retrieved from MODIS. Regions where low near-surface aerosol mass loading was observed, the satellite-retrieved columnar loading was found to be over-estimated suggesting seasonal and regional differences in scale height.

Description: In contrast to the impacts of anthropogenic aerosols and greenhouse gases, little is known about the impact of urban land-surface forcing (ULSF) on large-scale atmospheric circulation. This study explores atmospheric responses to idealized ULSF in eastern China during the boreal spring using the Community Atmosphere Model version 5.1 coupled with the Community Land Model version 4. Results show that the ULSF leads to an increased air temperature in northern China both near the surface and in the lower troposphere. Related to a strong thermal feedback loop, a mid-upper tropospheric cooling is found in eastern China while a relatively strong warming occurs in the mid-high latitudes, which acts to enhance the meridional temperature gradient to the north of the source region and then shifts the East Asian subtropical jet stream (EASJ) southward. A weakened southwesterly in the lower troposphere in southern China slows down moisture transportation to northern China, and the southward-shifted EASJ induces strong anomalous sinking motion to the north of the Yangtze River Valley (YRV). The associated changes in moisture and vertical airflow result in moisture divergence along the YRV and convergence in southern China. Thus, the spring rain belt is shifted southward, as characterized by below-normal rainfall extending from the Huai River Valley to South Korea and above-normal rainfall from southern China to the south coast of Japan. In addition, analysis of the upper tropospheric wave activity signifies that large-scale atmospheric responses due to the ULSF also exert an important influence on local climate.

Description: Using ERA-Interim and MERRA re-analysis datasets, we investigated the effects of the central pacific (CP) El Niño on the Southern Hemispheric (SH) stratosphere particularly during the austral spring. SH stratosphere warming is at a maximum in September rather than in November and December, as suggested by previous studies. SH stratospheric temperature anomalies become significant beginning in July and reach a peak of approximately 4K in September, reflecting a weakened SH vortex and a strengthened SH stratospheric Brewer-Dobson circulation. The anomalous Eliassen-Palm flux and its divergence in the SH mid-latitudes are most significantly enhanced in August, leading to the SH maximum stratospheric temperature anomalies approximately one month later. In the middle latitudes of the SH, the poleward and upward propagation of enhanced planetary waves (PWs) during the austral winter (July-September) causes anomalous SH polar warming and tropical cooling in the stratosphere. The wavenumber 1 (WN1) pattern is responsible for PW anomalies in August, whereas the WN2 pattern is responsible those in September. Eddy heat flux during CP El Niño is also anomalously enhanced in extra-tropical SH stratosphere in both August and September and subsequently weaken during the following months.

Description: Terra MODerate Resolution Imaging Spectroradiometer (MODIS) is one of the key sensors in the NASA's Earth Observing System (EOS), which has successfully completed fifteen years of on-orbit operation. Terra MODIS continues to collect valuable information of the Earth's energy radiation from visible to thermal infrared wavelengths. The instrument has been well characterized over its lifetime using On Board Calibrators (OBC) whose calibration references are traceable to the NIST standards. In this paper, we focus on the electronic crosstalk effect of Terra MODIS band 29, a Thermal Emissive Band (TEB) whose center wavelength is 8.55 µm. Previous works have established the mechanism to describe the effect of the electronic crosstalk in the TEB channels of Terra MODIS. This work utilizes the established methodology to apply to band 29. The electronic crosstalk is identified and characterized using the regularly scheduled lunar observations. The moon being a near pulse-like source allowed easy detection of extraneous signals around the actual moon surface. First, the crosstalk transmitting bands are identified along with their amplitudes. The crosstalk effect then is characterized using a moving average mechanism that allows a high fidelity of the magnitude to be corrected. The lunar based analysis unambiguously shows that the crosstalk contamination is becoming more severe in recent years and should be corrected in order to maintain calibration quality for the affected spectral bands. Finally, two radiometrically well characterized sites, Pacific Ocean and Libya 1 desert, are used to assess the impact of crosstalk effect. It is shown that the crosstalk contamination induces a long-term upward drift of 1.5 K in band 29 brightness temperature of MODIS Collection 6 L1B, which could significantly impact the science products. The crosstalk effect also induces strong detector-to-detector differences, which result in severe stripping in the Earth view images. With crosstalk correction applied, both the long-term drift and detector differences are significantly reduced.

Description: Atmospheric profiles of temperature (T), vapor density (ρ v ), and relative humidity (RH) retrieved from ground-based microwave radiometer (MWR) measurements are compared with radiosonde soundings at Wuhan, China. The MWR retrievals were averaged in the ±30-min period centered at sounding times of 00 and 12 UTC. A total of 403 and 760 profiles under clear and cloudy skies were selected. Based on the comparisons, temperature profiles have better consistency than the ρ v and RH profiles, lower levels are better than upper levels, and the cloudy are better than the clear-sky profiles. Three cloud types (low, middle, and high) were identified by matching the infrared radiation thermometer (IRT) detected cloud-base temperature to the MWR retrieved temperature-height profiles. Temperature profile under high cloud has the highest correlation coefficient (R) and the lowest bias and RMS, but under low cloud is in the opposite direction. The ρ v profile under middle cloud has the highest R and the lowest bias, but under high cloud has the lowest R, the largest bias and RMS. Based on the radiosonde soundings, both clear and cloudy wind speeds and drifting distances increase with height, but increases much faster under clear than cloudy above 4 km. The increased wind speeds and drifting distances with height have resulted in decreased correlation coefficient and increased temperature biases and RMSs with height for both clear and cloudy skies. The differences in R, bias and RMS between clear and cloudy skies are primarily resulted from their wind speeds and drifting distances.

Description: Solar Radiation Management (SRM) has been proposed as a means to partly counteract global warming. The Geoengineering Model Intercomparison Project (GeoMIP) has simulated the climate consequences of a number of SRM techniques. Thus far, the effects on vegetation have not yet been thoroughly analyzed. Here, the vegetation response to the idealized GeoMIP G1 experiment from eight fully coupled earth system models (ESMs) is analyzed, in which a reduction of the solar constant counterbalances the radiative effects of quadrupled atmospheric CO 2 concentrations (abrupt4xCO2). For most models and regions, changes in net primary productivity (NPP) are dominated by the increase in CO 2 , via the CO 2 fertilization effect. As SRM will reduce temperatures relative to abrupt4xCO2, in high latitudes this will offset increases in NPP. In low latitudes, this cooling relative to the abrupt4xCO2 simulation decreases plant respiration while having little effect on gross primary productivity, thus increasing NPP. In Central America and the Mediterranean, generally dry regions which are expected to experience increased water stress with global warming, NPP is highest in the G1 experiment for all models due to the easing of water limitations from increased water-use efficiency at high-CO 2 concentrations and the reduced evaporative demand in a geoengineered climate. The largest differences in the vegetation response are between models with and without a nitrogen-cycle, with a much smaller CO 2 fertilization effect for the former. These results suggest that until key vegetation processes are integrated into ESM predictions, the vegetation response to SRM will remain highly uncertain.

Description: We present a simple chemical definition to demark the edge of the mesospheric polar vortices. Because this vortex definition does not rely on the wind field it is useful in the mesosphere where wind observations are sparse and reanalysis winds are unreliable. The chemical definition is also insensitive to double jets that complicate vortex identification in the mesosphere. The algorithm is based on horizontal gradients of carbon monoxide (CO) and mirrors the widely used vortex edge definition in the stratosphere based on potential vorticity (PV) gradients. Here, the approach is used to identify the Arctic vortex in the mesosphere during a 10-year (2004–2014) record of Microwave Limb Sounder data. Vortex size and shape comparisons are made where the CO and PV methods overlap in the upper stratosphere. A case study is presented during the NH 2008–2009 winter that demonstrates the fidelity of the CO gradient method on individual days and emphasizes the impact of double jets on methods to identify the polar vortex. We recommend transitioning from a PV or stream function-based vortex definition in the stratosphere to using a CO gradient definition above 0.1 hPa (~60 km). The CO gradient method identifies a coherent region of high CO at 80 km that is confined to mid-to-high latitudes 99.8% of the time during Arctic winter. Taking advantage of the CO gradient method to identify the polar vortex adds ~20 km of reliable vortex information (from 60–80 km) in a region of the atmosphere where reanalyses are most suspect.

Description: The severe 2013/14 cold winter has been examined in the context of the previous 55 winters using the NCEP reanalysis data for the period 1960–2014. North America is dominated by pronounced cold anomalies over the Great Plains and Great Lakes in December 2013 and February 2014, but exhibits an east–west contrast pattern with warm anomalies over most of the North American West in January 2014. A relevant temperature index, defined as land surface temperature anomalies averaged over (40°-60°N, 105°-80°W), reveals a warming trend as well as interannual variability with a significant power peak of 6.0 years. While 2013/14 was the second coldest winter during 1960–2014, it is the coldest one in the linearly detrended series, with a negative anomaly of 2.63 standard deviations. This indicates that the long-term warming has made the 2013/14 winter less severe than it could have been. The temperature and circulation variability in association with the zonally symmetric variability of the polar vortex projects weakly on the corresponding anomalies in the 2013/14 winter, whereas the variability associated with the principal mode of North American surface temperature projects strongly on the corresponding anomalies in the winter. This mode is associated with a sea surface temperature (SST) pattern of significant anomalies over the North Pacific and North Atlantic mid-high latitudes. The anomalous atmospheric circulation shows an anticyclonic anomaly over the Gulf of Alaska-Bering Sea and a cyclonic anomaly downstream over North America. It bears resemblance to the North Pacific Oscillation/Western Pacific (NPO/WP) pattern, and drives the SST in the North Pacific. Over western-central Canada and the northern US, below-average heights are associated with above-normal precipitation, implying enhanced upward vertical motion and variation of local cloud forcing, leading to a variation of the surface energy budget dominated by surface longwave radiation anomalies. Over North America, there is less downwelling longwave radiation at the surface when the atmosphere is cold, which is offset by the corresponding reduction in outgoing longwave radiation.

Description: An accurate understanding of the instantaneous, dynamic land surface emissivity is necessary for a physically based, multi-channel passive microwave precipitation retrieval scheme over land. In an effort to assess the feasibility of the physical approach for land surfaces, a semi-empirical emissivity model is applied for calculation of the surface component in a test area of the US Southern Great Plains (SGP). A physical emissivity model, using land surface model data as input, is used to calculate emissivity at the 10 GHz frequency, combining contributions from the underlying soil and vegetation layers, including the dielectric and roughness effects of each medium. An empirical technique is then applied, based upon a robust set of observed channel covariances, extending the emissivity calculations to all channels. For calculation of the hydrometeor contribution, reflectivity profiles from the Tropical Rainfall Measurement Mission Precipitation Radar (TRMM-PR) are utilized along with coincident brightness temperatures (Tbs) from the TRMM radiometer (TMI), and cloud resolving model profiles. Ice profiles are modified to be consistent with the higher frequency microwave Tbs. Resulting modeled top of the atmosphere (TOA) Tbs show correlations to observations of 0.9, biases of 1K or less, RMS errors on the order of 5K, and improved agreement over the use of climatological emissivity values. The synthesis of these models and datasets leads to creation of a simple prototype Tb database that includes both dynamic surface and atmospheric information physically consistent with the LSM, emissivity model, and atmospheric information.

Description: Forecast uncertainties and physical mechanisms of a quasi-linear extreme-rain-producing mesoscale convective system (MCS) along the Mei-yu front in East China, during the midnight-to-morning hours of 8 July 2007, are studied using ensembles of 24-h convection-permitting simulations with a nested grid spacing of 1.11 km. The simulations reveal a strong sensitivity to uncertainties in the initial state despite the synoptic environment being favorable for heavy rainfall production. Linear changes of a less skillful member's initial state toward that of a skillful member lead to a monotonic improvement in the precipitation simulation, with the most significant contribution arising from changes in the moisture field. Sensitivity to physics parameterizations representing sub-grid-scale processes fail to account for the larger simulation errors (missing the MCS) with the physics variation examined, but could result in a large spread in the location and amount of accumulative rainfall. A robust feature of the best-performing members that reasonably simulate the MCS-associated heavy rainfall is the presence of a cold dome ahead of the Mei-yu front generated by previous convection. The cold dome promotes nocturnal convective initiation by lifting high equivalent potential temperature air in the southwesterly flow to its level of free convection. The skillful members reproduce the convective backbuilding and echo-band training processes that are observed during this event and many other heavy rainfall events over China. In contrast, the less skillful members that miss the development of the MCS either do not simulate the previous convection or produce a cold dome that is too shallow to initiate the MCS.

Description: This study aims to investigate some characteristics of the moist processes of the Madden-Julian oscillation (MJO), by making use of joint HDO (or δ D) and H 2 O vapor measurements. The MJO is the main intraseasonal mode of the tropical climate, but is hard to properly simulate in global atmospheric models. The joint use of δ D-H 2 O diagnostics yields additional information compared to sole humidity measurements. We use mid-tropospheric Infrared Atmospheric Sounding Interferometer (IASI) satellite δ D and H 2 O measurements to determine the mean MJO humidity and δ D evolution. Moreover, by making use of high temporal resolution data, we determine the variability in this evolution during about eight MJO events from 2010 to 2012 (including those monitored during the CINDY/DYNAMO campaign). This data has a higher spatio-temporal coverage than previous δ D measurements, enabling the sampling of individual MJO events. IASI measurements over the Indian Ocean confirm earlier findings that the moistening before the precipitation peak of an MJO event is due to water vapor slightly enriched in HDO. There is then a HDO depletion around the precipitation peak that also corresponds to the moister environment. Most inter-event variability determined in the current study occurs 5 to 10 days after the MJO event. In 75% of the events, humidity decreases while the atmosphere remains depleted. In a quarter of the events, humidity increases simultaneously with an increase in δ D. After this, the advection of relatively dry and enriched air brings back the state to the mean. Over the maritime continent, δ D-H 2 O cycles are more variable on timescales shorter than the MJO and the inter-event variability is larger than over the Indian Ocean. The sequence of moistening and drying processes as revealed by the q- δ D cycles can be used as a benchmark to evaluate the representation of moist processes in models. This is done here by comparing observations to simulations of the isotope enabled LMDZ GCM nudged with reanalysis wind fields. These simulations also give informations to investigate possible physical origins of the observed q- δ D cycles.

Description: This study investigates the seasonality in anthropogenic aerosol optical depth (AOD) distributions and their effects on clouds and precipitation in East Asia with the Community Atmospheric Model version 5. The differences between the model experiments with and without anthropogenic emissions exhibit a northward shift of the maximal AOD change in East Asia from March to July and then a southward withdrawal from September to November, which are induced by East Asian monsoon circulation. Associated with the shift, the direct and semi-direct effects of the anthropogenic aerosols are the most pronounced in spring and summer, with a maximum center in North China during summer and a secondary center in South China during spring. The cloud liquid water path and shortwave cloud forcing changes, however, are the weakest in North China during summer. The indirect effect is the strongest in South China during spring, which is related to the large amount of middle-low level clouds in cold seasons in East China. A positive feedback between aerosol induced surface cooling and low-level cloud increase is identified in East China, which acts to enforce the aerosol indirect effect in spring. Accordingly, the climate response to the anthropogenic aerosols is also characterized by a northward shift of reduced precipitation from spring to summer, leading to a spring drought in South China and a summer drought in North China. The spring drought is attributed to both direct and indirect effects of the anthropogenic aerosols, while the summer drought is primarily determined by the aerosols' direct effect.

Description: This study describes the large-scale atmospheric flow patterns that favor mixing events within western Long Island Sound (wLIS), and how inter-annual and inter-decadal variations in surface winds relate to bottom dissolved oxygen (DOb) variability. DOb data from the wLIS Coastal Observing System (LISCOS) buoy were used in conjunction with the surface winds at La Guardia Airport (LGA) and National Buoy Data Center (NBDC) buoy to identify criteria for water column ventilation and mixing from June-September (JJAS). It is shown that mixing for a 36 hour period after onset is favored when a majority of the surface wind observations for a day (starting at 00 UTC) are from 30°-110° (NE to ESE) and 〉= 4 m s −1 . This criterion was used to develop a synoptic climatology and the trend in potential mixing events from 1950–2009. These mixing events were categorized based on three synoptic patterns: high pressure, low pressure, and a hybrid high and low. High pressure patterns, which include a hybrid system with a high building from the north/west and low to the south, result in the largest percentage of potential mixing events (76.9%). The number of potential mixing events increases from the 1950s to 1990s (full season and July-August) primarily from an increasing number of high pressure systems; however, the seasonal DOb decreased during this period. There was a slight decrease in the number of July-August potential mixing events from 1990–2009, mainly from a decrease in the number of low pressure systems.

Description: A recent empirical study of Stanhill et al. [2014], which was based on the Angstrom-Prescott relationship between global radiation and sunshine duration, was evaluated. The parameters of this relationship seemed to be rather stable across the dimming and brightening periods. Thus, the authors concluded that the variation in global radiation is more influenced by changes in cloud cover and sunshine duration than by the direct aerosol effects. In our study, done for the Potsdam station (one of six globally distributed stations, the source of one of the longest observational records and closely located to former hotspots of aerosol emission), we tested and rejected the hypothesis that the dimming of global radiation directly caused by aerosols is negligible. The residuals of the Angstrom-Prescott regression reveal a statistically significant positive temporal trend and a temporal level segmentation. The latter was consistent with the temporal emission patterns around Potsdam. The trend in the residuals only disappeared when the model intercept varied according to the temporal level segmentation. The magnitude of the direct aerosol effect on the level changes in global radiation derived from the modified Angstrom-Prescott relationship was in the range indicated in previous studies. Thus from here, a specific request cannot be made for a revision of current climate models state-of-the-art representation of both the cooling effect directly caused by aerosols and the temperature sensitivity to the increase of greenhouse gases.

Description: In this study, we investigated the impacts of the triggering function of the deep convection scheme on diurnal rainfall variation in the middle latitudes by using the single-column version of the Community Atmospheric Model (SCAM). Using the climate statistics of a long-term ensemble analysis of SCAM simulations, we quantified and validated the diurnal rainfall climatological regimes at the Atmospheric Radiation Measurement Southern Great Plains (SGP) site. The results showed that the averaged diurnal rainfall cycle simulated using the default Zhang-Mcfarlane (ZM) scheme of the SCAM peaks near noon, which is far earlier than the observed nighttime peak phase. This bias was due to the ZM scheme, which produced spurious daytime rainfall, even during days in which only light rainfall was observed. By contrast, using a weather-focused scheme, the Simplified Arakawa-Schubert (SAS) scheme, we successfully simulated the nocturnal peak of the diurnal cycle. Experiments conducted on the ZM and SAS schemes featuring different triggering functions revealed that, the relaxation of launching parcels above the planetary boundary layer (PBL) and the inclusion of convective inhibition (CIN) were crucial designs for the model to capture the nocturnal rainfall events of the SGP. The inclusion of CIN reduces spurious weak convective events, and the allowance of launching parcels being above the PBL better captures convective cloud base. The results of this study highlight the modulatory effect of low-level inhomogeneity on the diurnal variation of convection over mid-latitudes and the importance of the triggering function of the deep convection scheme in capturing those variations.

Description: To determine the complex dependencies of currents and electric fields within the Global Electric Circuit (GEC) on the underlying physics of the atmosphere, a new modeling framework of the GEC has been developed for use within global circulation models. Specifically, the Community Earth System Modeling (CESM) framework has been utilized. A formulation of atmospheric conductivity based on ion production and loss mechanisms (including galactic cosmic rays, radon, clouds and aerosols), conduction current sources, and ionospheric potential changes due to the influence of external current systems are included. This paper presents a full description of the calculation of the electric fields and currents within the model, which now includes several advancements to GEC modeling as it incorporates many processes calculated individually in previous articles into a consistent modeling framework. This framework uniquely incorporates effects from the troposphere up to the ionosphere within a single GEC model. The incorporation of a magnetospheric potential, which is generated by a separate magnetospheric current system, acts to modulate or enhance the surface level electric fields at high latitude locations. This produces a distinct phasing signature with the GEC potential that is shown to depend on the observation location around the globe. Lastly, the model output for Vostok and Concordia, two high latitude locations, is shown to agree with the observational data obtained at these sites over the same time period.

Description: Climate change in response to a change in external forcing can be understood in terms of fast response to the imposed forcing and slow feedback associated with surface temperature change. Previous studies have investigated the characteristics of fast response and slow feedback for different forcing agents. Here, we examine to what extent fast response and slow feedback derived from time-mean results of climate model simulations can be used to infer total climate change. To achieve this goal, we develop a multivariate regression model of climate change in which the change in a climate variable is represented by a linear combination of its sensitivity to CO 2 forcing, solar forcing, and change in global mean surface temperature. We derive the parameters of the regression model using time-mean results from a set of HadCM3L climate model step-forcing simulations, and then use the regression model to emulate HadCM3L-simulated transient climate change. Our results show that the regression model emulates well HadCM3L-simulated temporal evolution and spatial distribution of climate change, including surface temperature, precipitation, runoff, soil moisture, cloudiness, and radiative fluxes under transient CO 2 and/or solar forcing scenarios. Our findings suggest that temporal and spatial patterns of total change for the climate variables considered here can be represented well by the sum of fast response and slow feedback. Furthermore, by using a simple 1-D heat-diffusion climate model, we show that the temporal and spatial characteristics of climate change under transient forcing scenarios can be emulated well using information from step-forcing simulations alone.

Description: The cloud detection algorithm for passive sensors is usually based on a fuzzy logic system with thresholds determined from previous observations. In recent years, haze and high aerosol concentrations with high AOD occur frequently in China and may critically impact the accuracy of the MODIS cloud detection. Thus, we comprehensively explore this impact by comparing the results from MODIS/Aqua (passive sensor), CALIOP/CALIPSO (lidar sensor), and CPR/CloudSat (microwave sensor) of the A-Train suite of instruments using an averaged AOD as an index for an aerosol concentration value. Case studies concerning the comparison of the three sensors indicate that MODIS cloud detection is reduced during haze events. In addition, statistical studies show that an increase in AOD creates an increase in the percentage of uncertain flags and a decrease in hit rate, a consistency index between consecutive sets of cloud retrievals. On average, AOD values lower than 0.1 give hit rate values up to 80.0% and uncertainty values lower than 16.8%, while AOD values greater than 1.0 reduce the hit rate below to 66.6% and increase the percentage of uncertain flags up to 46.6%. Therefore, we can conclude that the ability of MODIS cloud detection is weakened by large concentrations of aerosols. This suggests that use of the MODIS cloud mask, and derived higher level products, in situations with haze requires caution. Further improvement of this retrieval algorithm, is desired as haze studies based on MODIS products are of great interest in a number of related fields.

Description: Multi-year applications of an online-coupled meteorology-chemistry model allow an assessment of the variation trends in simulated meteorology, air quality, and their interactions to changes in emissions and meteorology, as well as the impacts of initial and boundary conditions (ICONs/BCONs) on simulated aerosol-cloud-radiation interactions over a period of time. In this work, the Weather Research and Forecasting model with Chemistry version 3.4.1 (WRF/Chem v. 3.4.1) with the 2005 Carbon Bond mechanism coupled with the Volatility Basis Set module for secondary organic aerosol formation (WRF/Chem-CB05-VBS) is applied for multiple years (2001, 2006, and 2010) over continental U.S. This work also examines the changes in simulated air quality and meteorology due to changes in emissions and meteorology and the model's capability in reproducing the observed variation trends in species concentrations from 2001 to 2010. In addition, the impacts of the chemical ICONs/BCONs on model predictions are analyzed. ICONs/BCONs are downscaled from two global models, the modified Community Earth System Model/Community Atmosphere model version 5.1 (CESM/CAM) v5.1 and the Monitoring Atmospheric Composition and Climate model (MACC). The evaluation of WRF/Chem-CB05-VBS simulations with the CESM ICONs/BCONs for 2001, 2006, and 2010 shows that temperature at 2-m (T2) is underpredicted for all three years likely due to inaccuracies in soil moisture and soil temperature, resulting in biases in surface relative humidity, wind speed, and precipitation. With the exception of cloud fraction, other aerosol-cloud variables including aerosol optical depth, cloud droplet number concentration, and cloud optical thickness are underpredicted for all three years, resulting in overpredictions of radiation variables. The model performs well for O 3 and PM 2.5 for all three years comparable to other studies from literature. The model is able to reproduce observed annual average trends in O 3 and PM 2.5 concentrations from 2001 to 2006 and from 2006 to 2010, but is less skillful in simulating their observed seasonal trends. The 2006 and 2010 results using CESM and MACC ICONs/BCONs are compared to analyze the impact of ICONs/BCONs on model performance and their feedbacks to aerosol, clouds, and radiation. Comparing to the simulations with MACC ICONs/BCONs, the simulations with the CESM ICONs/BCONs improve the performance of O 3 mixing ratios (e.g., the normalized mean bias for maximum 8-hr O 3 is reduced from −17% to −1% in 2010), PM 2.5 in 2010, and sulfate in 2006 (despite a slightly larger NMB for PM 2.5 in 2006). The impacts of different ICONs/BCONs on simulated aerosol-cloud-radiation variables are not negligible, with larger impacts in 2006 compared to 2010.

Description: We examined high-speed video and electric field records of 23 subsequent strokes following the previous stroke channel in 3 natural negative lightning flashes, which were obtained at the Lightning Observatory in Gainesville (LOG), Florida. Five strokes exhibited faintly luminous formations (FLFs) occurring in a single, pre-return-stroke frame and ranging from 130 to 908 m in length between the lower end of downward leader and the prospective strike point. The FLFs were inferred to be not streamers (as in first strokes), but manifestations of an increase in conduction current in the defunct channel between the leader and ground in response to the increasing electric field produced by the descending leader. Further, in eight strokes, we observed residual channel luminosity persisting over many frames (for 4.7 to 18 ms), through the pre-return-stroke frame. The residual luminosity was apparently associated with a stronger channel heating and a larger channel radius (and, hence, a lower temperature decay rate), both associated with the relatively long preceding continuing current. Presence of either FLF or residual channel luminosity did not appear to significantly influence the mode and velocity of propagation of the descending leader.

Description: Observational constraints on the change in the radiative energy budget caused by the presence of aerosols, i.e. the aerosol direct radiative effect (DRE), have recently been made using observations from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite (CALIPSO). CALIPSO observations have the potential to provide improved global estimates of aerosol DRE compared to passive sensor-derived estimates due to CALIPSO's ability to perform vertically-resolved aerosol retrievals over all surface types and over cloud. In this study, uncertainties in CALIPSO-inferred aerosol DRE are estimated using multiple years of observations from the Atmospheric Radiation Measurement (ARM) program's Raman lidars (RL) at midlatitude and tropical sites. We find that CALIPSO is unable to detect all radiatively-significant aerosol, resulting in an underestimate in the magnitude of the aerosol DRE by 30–50% at the two ARM sites. The undetected aerosol is likely the consequence of random noise in CALIPSO measurements and therefore will affect global observations as well. This suggests that the global aerosol DRE inferred from CALIPSO observations are likely too weak. Also examined is the impact of the ratio of extinction-to-backscatter (i.e. the lidar ratio) whose value CALIPSO retrievals must assume to obtain the aerosol extinction profile. It is shown that if CALIPSO can reproduce the climatological value of the lidar ratio at a given location, then the aerosol DRE there can be accurately calculated (within about 3%).

Description: Decadal hemispheric WRF-CMAQ simulations from 1990-2010 were conducted to examine the meteorology and air quality responses to the aerosol direct radiative effects. The model's performance for the simulation of hourly surface temperature, relative humidity, wind speed and direction was evaluated through comparison with observations from NOAA's National Climatic Data Center (NCDC) Integrated Surface Data. The inclusion of aerosol direct radiative effects improves the model's ability to reproduce the trend in daytime temperature range (DayTR) which over the past two decades was increasing in eastern China but decreasing in eastern U.S. and Europe. Trends, spatial, and diurnal variations of the surface-level gaseous and particle concentrations to the aerosol direct effect were analyzed. The inclusion of aerosol direct radiative effects was found to increase the surface-level concentration of SO 2 , NO 2 , O 3 , SO 4 2- , NO 3 - and PM 2.5 in eastern China, eastern U.S. and Europe by 1.5-2.1%, 1-1.5%, 0.1-0.3%, 1.6-2.3%, 3.5-10.0%, 2.2-3.2% respectively on average over the entire 21-yr period. However, greater impacts are noted during polluted days with increases of 7.6-10.6%, 6.2-6.7%, 2.0-3.0%, 7.8-9.5%, 11.1-18.6% and 7.2-10.1% respectively. Due to the aerosol direct radiative effects, stabilizing of the atmosphere associated with reduced PBL height and ventilation leads to an enhancement of pollution. Consequently, the continual increase of AOD in eastern China leads to an increasing trend in the air quality feedback which exacerbates air pollution, while emission reductions in eastern US and Europe result in a declining trend in both AODs and feedbacks which make the air pollution control strategies more effective.

Description: The CloudSat 2C-ICE data product is derived from a synergetic ice cloud retrieval algorithm that takes as input a combination of CloudSat radar reflectivity ( Z e ) and CALIPSO lidar attenuated backscatter profiles. The algorithm uses a variational method for retrieving profiles of visible extinction coefficient, ice water content and ice particle effective radius in ice or mixed-phase clouds. Because of the nature of the measurements and to maintain consistency in the algorithm numerics, we choose to parameterize (with appropriately large specification of uncertainty) Z e and lidar attenuated backscatter in the regions of a cirrus layer where only the lidar provides data and where only the radar provides data, respectively. To improve the Z e parameterization in the lidar-only region, the relations among Z e , extinction, and temperature have been more thoroughly investigated using Atmospheric Radiation Measurement (ARM) long-term millimeter cloud radar (MMCR) and Raman lidar measurements. This Z e parameterization provides a first order estimation of Z e as a function extinction and temperature in the lidar-only regions of cirrus layers. The effects of this new parameterization have been evaluated for consistency using radiation closure methods where the radiative fluxes derived from retrieved cirrus profiles compare favorably with CERES measurements. Results will be made publicly available for the entire CloudSat record (since 2006) in the most recent product release known as R05.

Description: Atmospheric gravity waves (GWs) significantly influence global circulation. Deep convection, particularly that associated with typhoons, is believed to be an important source of gravity waves. Stratospheric gravity waves induced by Typhoon Mindulle (2004) were detected by the Atmospheric Infrared Sounder (AIRS). Semicircular GWs with horizontal wavelengths of 100–400 km were found over Taiwan through an inspection of AIRS radiances at 4.3 μ m. Characteristics of the stratospheric gravity waves generated by Typhoon Mindulle were investigated using the Weather Research and Forecasting (WRF) model. The initial and boundary data were determined by the high-resolution European Center for Medium-Range Weather Forecasts (ECMWF) re-analysis data. The WRF simulation reproduces the main features of Typhoon Mindulle and the significant GWs. The simulated GWs with horizontal wavelengths of 100–400 km match the AIRS observations: they propagate upward and eastward, and the westward components are mostly filtered in the stratosphere. By comparing the measured waves with a WRF simulation in the absent of orography (WRF-FLAT), we find that the orographic gravity waves (OGWs) generated by the flow of Typhoon Mindulle over the Central Mountain Range (CMR) in Taiwan account for approximately 50% of the total wave momentum flux in the troposphere. The dominant orientation of the OGW wave fronts is parallel to the CMR rideline. When entering into the stratosphere, OGW propagation is determined by the position of the typhoon center relative to the CMR.

Description: A new parameterization for quantifying the mixing state of aerosol populations has been applied for the first time to samples of ambient particles analyzed using spectro-microscopy techniques. Scanning transmission x-ray microscopy/near edge x-ray absorption fine structure (STXM/NEXAFS) and computer controlled scanning electron microscopy/energy dispersive x-ray spectroscopy (CCSEM/EDX) were used to probe the composition of the organic and inorganic fraction of individual particles collected on June 27 th and 28 th during the 2010 Carbonaceous Aerosols and Radiative Effects (CARES) study in the Central Valley, California. The first field site, T0, was located in downtown Sacramento, while T1 was located near the Sierra Nevada Mountains. Mass estimates of the aerosol particle components were used to calculate mixing state metrics, such as the particle-specific diversity, bulk population diversity, and mixing state index, for each sample. The STXM data showed evidence of changes in the mixing state associated with a build-up of organic matter confirmed by collocated measurements and the largest impact on the mixing state was due to an increase in soot dominant particles during this build-up. The mixing state from STXM was similar between T0 and T1 indicating that the increased organic fraction at T1 had a small effect on the mixing state of the population. The CCSEM/EDX analysis showed the presence of two types of particle populations; the first was dominated by aged sea salt particles and had a higher mixing state index (indicating a more homogeneous population), the second was dominated by carbonaceous particles and had a lower mixing state index.

Description: A detailed examination is made in both observations and the Community Earth System Model (CESM) of relationships among top-of-atmosphere (TOA) radiation, water vapor, temperatures and precipitation for 2000-2014 to assess the origins of radiative perturbations and climate feedbacks empirically. The 30-member large ensemble coupled runs are analyzed along with one run with specified sea surface temperatures for 1994 to 2005 (to avoid volcanic eruptions). The vertical structure of the CESM temperature profile tends to be top-heavy in the model, with too much deep convection and not enough lower stratospheric cooling as part of the response to tropospheric heating. There is too much absorbed solar radiation (ASR) over the southern oceans and not enough in the tropics, and ENSO is too large in amplitude in this version of the model. However, the co-variability of monthly mean anomalies produces remarkably good replication of most of the observed relationships. There is a lot more high frequency variability in radiative fluxes than in temperature, highlighting the role of clouds and transient weather systems in the radiation statistics. Over the Warm Pool in the tropical western Pacific and Indian oceans, where non-local effects from the Walker circulation driven by the ENSO events are important, several related biases emerge: in response to high SST anomalies there is more precipitation, water vapor and cloud, and less ASR and Outgoing Longwave Radiation (OLR) in the model than observed. Different model global mean trends are evident, however, and possibly hinting at too much positive cloud feedback in the model.

Description: The geographical shift of global anthropogenic aerosols from the developed countries to the Asian continent since the 1980s could potentially perturb the regional and global climate due to aerosol-cloud-radiation interactions. We use an atmospheric general circulation model with different aerosol scenarios to investigate the radiative and microphysical effects of anthropogenic aerosols from different regions on the radiation budget, precipitation, and large-scale circulations. An experiment contrasting anthropogenic aerosol scenarios in 1970 and 2010 shows that the altered cloud reflectivity and solar extinction by aerosols results in regional surface temperature cooling in East and South Asia, and warming in US and Europe respectively. These aerosol induced temperature changes are consistent with the relative temperature trends from 1980 to 2010 over different regions in the reanalysis data. A reduced meridional streamfunction and zonal winds over the tropics as well as a poleward shift of the jet stream suggest weakened and expanded tropical circulations, which are induced by the redistributed aerosols through a relaxing of the meridional temperature gradient. Consequently, precipitation is suppressed in the deep tropics and enhanced in the sub-tropics. Our assessments of the aerosol effects over the different regions suggest that the increasing of Asian pollution accounts for the weakening of the tropics circulation, while the decreasing of pollution in Europe and US tends to shift the circulation systems southward. Moreover, the aerosol indirect forcing is predominant over the total aerosol forcing in magnitude, while aerosol radiative and microphysical effects jointly shape the meridional energy distributions and modulate the circulation systems.

Description: One of the challenges in evaluating and applying regional climate models (RCMs) is the non-linear behavior of atmospheric processes, which is still poorly understood. The non-linearities induce chaos which leads to an internal variability in the model. Therefore, an ensemble of RCM simulations has been run and a budget study for potential temperature has been applied to investigate the internally generated variability. Hence, the physical processes associated with diabatic and dynamical terms inducing the inter-member variability have been analyzed. The study is applied over the Arctic on an ensemble of 20 members, differing in their initial conditions, simulated with the RCM HIRHAM5 during summer 2012. This time period is of particular importance because of the melting sea ice and its influence on atmospheric circulation and the resulting effect on the inter-member variability. The amplitude of the inter-member variability of the simulations fluctuates strongly both temporally and spatially. During the beginning of August 2012 the inter-member variability is strongest and coincides with the great Arctic cyclone event. The most important contributions for the inter-member variability tendency are the horizontal and vertical ‘baroclinic’ terms. Both terms have largest absolute values along the coastlines of the Arctic Ocean which are associated with the Arctic frontal zone leading to the cyclone maximum over the Arctic Ocean during summer.

Description: Reconstruction of unknown atmospheric releases using measured concentrations is an ill-posed inverse problem. Due to insufficient measurements and dispersion model uncertainties, reliable interpretation of a retrieved source is limited by lack of resolution, non-uniqueness and instability in the inverse solution. The study presents an optimality analysis, in terms of resolution, stability and reliability, of an inverse solution given by a recently proposed inversion technique, called as Renormalization . The inversion technique is based on an adjoint source-receptor framework and construction of a weight function which interprets a priori information about the unknown release apparent to the monitoring network. The properties of weight function provides a perfect data resolution, maximum model resolution and minimum variance (or stability) for the retrieved source. The reliability of the retrieved source is interpreted in view of the information derived from the geometry of the monitoring network. The inversion technique and resolution features are evaluated for a point source reconstruction using measurements from a recent dispersion experiment (Fusion Field Trials 2007) conducted at Dugway Proving Ground, Utah. With the real measurements, the point release is reconstructed within an average distance of 23 m from the true release where the average distance of the nearest receptor from the true source was 32 m. In all the trials, the point release is retrieved within 3–60 m euclidean distance from their true location. The source strength is retrieved within a factor of 1.5 to the true release mass. The posterior uncertainty in the release parameters is observed to be within 20% of their mean value. The source localization features are resolved to its maximum extent feasible with the design of the monitoring network. The sensitivity studies are conducted to highlight the importance of receptors reporting zero concentration measurements and variations in the resolution features of the source retrieval with respect to the various arrangements of the receptors.

Description: Due to the harshness and inaccessibility of desert regions, the uncertainties concerning the processes of dust mobilization at the surface, airborne transport, and sedimentation are still considerable, limiting the ability to perform model simulations. In June 2011 a comprehensive data set of ground-based and airborne in-situ measurements and remote sensing observations was acquired within the Fennec/LADUNEX field campaign in the western Sahara region. Here, we evaluate the ability of the state-of-the-art Lagrangian particle dispersion model FLEXPART, newly fitted with a dust mobilization capability, to simulate dust transport in this region. We investigate a case where a large Mesoscale Convective System (MCS) triggered dust emissions in central Mali, which subsequently moved as a large cold-pool dust front towards northern Mauritania. Specifying dust mobilization for this case is shown to be an important obstacle to simulating dust transport during this event, since neither the MCS nor the associated cold pool causing dust emission are represented in the meteorological analysis. Obtaining a realistic dust transport simulation for this case therefore requires an inversion approach using a manual specification of the dust sources supported by satellite imagery. When compared to in-situ and remote-sensing data from two aircraft, the Lagrangian dust transport simulations represent the overall shape and evolution of the dust plume well. While accumulation and coarse mode dust are well represented in the simulation, giant mode particles are considerably underestimated. Our results re-emphasize that dust emission associated with deep moist convection remains a key issue for reliable dust model simulations in northern Africa.

Description: We examined atmospheric responses to 35,000+ oceanic eddies in the Kuroshio Extension (KE) region during the period of 2006–2009. Using satellite data, we showed that cold (warm) eddies cause surface winds to decelerate (accelerate), and reduce (increase) latent and sensible heat fluxes, cloud liquid water, water vapor content, and rain rate; all of these changes are quantified. Both the linear correlation between wind divergence and downwind SST gradient and the correspondence between vorticity and crosswind SST gradient support the vertical momentum mixing mechanism, which indicates that SST perturbations modify surface winds by changing the vertical turbulent mixing in the marine atmospheric boundary layer (MABL). High-resolution NCEP Climate Forecast System Reanalysis (CFSR) data can reproduce the atmospheric responses to the oceanic eddies in the MABL albeit with some differences in intensity. In addition, the CFSR data reveal that the atmospheric responses to these oceanic eddies are not confined in the MABL. MABL deepens (shoals) over the warm (cold) eddies; enhanced (reduced) vertical transport of transient zonal momentum occurs over the warm (cold) eddies from the sea surface to about 850-hPa level; vertical velocity anomalies over oceanic eddies penetrate beyond the MABL into free atmosphere; there exists a positive correlated relationship between SST and convective rain rate anomalies, indicative of ocean eddies' impact on the free troposphere. However, the composites of cloud liquid water and rain rate are different from the results based on the satellite data.

Description: The impact of size distribution of mineral dust aerosol on radiative transfer was investigated using the AERONET retrieved aerosol size distributions. Three methods for determining the aerosol optical properties using size distributions were discussed. The first is referred to as a bin method in which the aerosol optical properties are determined for each bin of the size distribution; The second is named as an assembly mean method in which the aerosol optical properties are determined with an integration of the aerosol optical parameters over the observed size distribution; The third is a normal parameterization method based on an assumed size distribution. The bin method was used to generate the benchmark results in the radiation calculations against the methods of the assembly mean and parameterizations based on two size distribution functions, namely log-normal and Gamma were examined. It is seen that the assembly mean method can produce aerosol radiative forcing with accuracy of better than 1%. The accuracies of the parameterizations based on log-normal and Gamma size distributions are about 25% and 5%, respectively. Both the log-normal and Gamma size distribution can be determined by two parameters, the effective radius and effective variance. The better results from the Gamma size distribution can be explained by a third parameter of skewness which is found to be useful for judging how close the assumed distribution is to the observation result. The parameterizations based on the two assumed size distributions are also evaluated in a climate model. The results show that the reflected solar fluxes over the desert areas determined by the scheme based on the Gamma size distribution are about 1 W m −2 less than those from the scheme based on the log-normal size distribution, bringing the model results closer to the observations.

Description: In this paper, we evaluate the impact of mineral dust (MD) on snow radiative properties in the European Alps at ground, aerial and satellite scale. A field survey was conducted to acquire snow spectral reflectance measurements with an ASD Field-spec Pro spectroradiometer. Surface snow samples were analyzed to determine the concentration and size distribution of MD in each sample. An overflight of a four-rotor Unmanned Aerial Vehicle (UAV) equipped with an RGB digital camera sensor was carried out during the field operations. Finally, Landsat 8 Operational Land Imager (OLI) data covering the Central European Alps were analyzed. Observed reflectance evidenced that MD strongly reduced the spectral reflectance of snow, in particular from 350 to 600 nm. Reflectance was compared with that simulated by parameterizing the Snow, Ice and Aerosol Radiation (SNICAR) radiative transfer model. We defined a novel spectral index, the Snow Darkening Index (SDI), that combines different wavelengths showing nonlinear correlation with measured MD concentrations (R 2 =0.87, RMSE=0.037). We also estimated a positive instantaneous radiative forcing that reaches values up to 153 W/m 2 for the most concentrated sampling area. SDI maps at local scale were produced using the UAV data, while regional SDI maps were generated with OLI data. These maps show the spatial distribution of MD in snow after a natural deposition from the Saharan desert. Such post-depositional experimental data are fundamental for validating radiative transfer models and Global Climate Models (GCM) that simulate the impact of MD on snow radiative properties.

Description: Future changes in aridity, defined as the ratio of annual precipitation to potential evapotranspiration (PET), are analyzed using simulations from the Community Earth System Model (CESM) Large Ensemble (LE) and the phase 5 of the Coupled Model Intercomparison Project (CMIP5) during the period 1980–2080. Both CESM and CMIP5 ensembles can reproduce the observed temporal and spatial variability of aridity. On the interannual time scale, annual average PET is sensitive to the variability of relative humidity, net surface energy flux, and surface air temperature while the precipitation variability is the dominant component of annual average aridity sensitivity. For the long-term trends, differences between the two ensembles illustrate that the impact of the internal variability is smaller than that of the model structural uncertainty with the trends from the CMIP5 ensemble of models having a much larger spread than those from the single model CESM-LE. The annual mean aridity averaged over global land increases (becomes drier) by 6.4% in 2055–2080 relative to 1980–2005. Aridity trends differ by region in the ensemble mean. In the future, increasing precipitation leads to decreasing aridity over northwest China and central (or tropical) Africa, while decreasing precipitation leads to drying (increasing aridity) in the sub-tropics, northern and southern Africa and the Amazon. Increases in PET can lead to increasing aridity even in regions with increasing precipitation.

Description: Marine stratocumulus clouds (MSC) cover large areas over the oceans and possess super sensitivity of their cloud radiative effect to changes in aerosol concentrations. Aerosols can cause transitions between regimes of fully cloudy closed cells and open cells. The possible role of aerosols in cloud cover has a big impact on the amount of reflected solar radiation from the clouds, thus potentially constitutes very large aerosol indirect radiative effect, which can exceed 100 Wm −2 . It is hypothesized that continentally-polluted clouds remain in closed cells regime for longer time from leaving continent and hence for longer distance away from land, thus occupying larger ocean areas with full cloud cover. Attributing this to anthropogenic aerosols would imply a very large negative radiative forcing with a significant climate impact. This possibility is confirmed by analyzing a detailed case study based on geostationary and polar-orbiting satellite observations of the microphysical and dynamical evolution of MSC. We show that large area of closed cells was formed over the northeast Atlantic Ocean downwind of Europe in a continentally polluted air mass. The closed cells undergo cleansing process that was tracked for 3.5 days that resulted with a rapid transition from closed to open cells once the clouds started drizzling heavily. The mechanism leading to the eventual breakup of the clouds due to both meteorological and aerosol considerations are elucidated. We termed this cleansing and cloud breakup process maritimization. Further study is needed to assess the climatological significance of such situations.

Description: Drought properties and the socio-economic impact it makes, are expected to increase in the coming years due to climate change. Here, we review the possible impacts of changes in climate variability on the properties of different drought types. The downscaled and bias-corrected data from five General Circulation Models (GCMs) was used to produce an ensemble of precipitation, temperature, and wind speed, through a relative entropy approach and was used for drought analysis. A novel multivariate drought index known as Multivariate Drought Index (MDI) was then employed for an integrated quantification of all physical forms of drought. We studied the spatial patterns of drought properties, and performed multivariate frequency analysis for each planning region in Texas to recognize the distribution of potential drought hazard areas under climate change impact by formulating a Drought Hazard Index (DHI). A drought vulnerability assessment was also carried out by taking into consideration various socio-economic factors, leading to the development of socio-economic Drought Vulnerability Index (DVI). A set of composite drought risk maps that combines hazard and vulnerability analysis were developed. This study also explored the cause-effect relationship between the drought events and several hydro climatic triggers. A transfer entropy measure was used to quantify the causal relationships, thus indicating the predominant future drought triggers. Overall, the findings are expected to help achieve an effective drought mitigation strategy for the state of Texas.

Description: We estimate the extent of upper tropospheric aerosol layers (UT ALs) surrounding mesoscale convective systems (MCSs) and explore the relationships between UT AL extent and the morphology, location, and developmental stage of collocated MCSs in thetropics. Our analysis is based on satellite data collected over equatorial Africa, South Asia, and the Amazon basin between June 2006 and June 2008. We identify substantial variations in the relationships between convective properties and aerosoltransport by region and stage of convective development. The most extensive UT ALs over equatorial Africa are associated with mature MCSs, while the most extensive UT ALs over South Asia and the Amazon are associated with growing MCSs. Convective aerosol transport over the Amazon is weaker than that observed over the other two regions despite similar transport frequencies, likely due to the smaller sizes and shorter mean lifetimes of MCSs over the Amazon. Variations in UT ALs in the vicinity oftropical MCSs are primarily explained by variations in the horizontal sizes of the associated MCSs, and are largely unrelated to aerosol loading in the lower troposphere. We also identify potentially important relationships with the number of convective cores, vertical wind shear, and convective fraction during the growing and mature stages of MCS development. Relationships between convective properties and aerosol transport are relatively weak during the decaying stage of convective development. Our results provide an interpretive framework for devising and evaluating numerical model experiments that examine relationships between convective properties and ALs in the upper troposphere.

Description: Surface ozone (O 3 ) air pollution in populated regions has been attributed to emissions of nitrogen oxides (NO + NO 2 = NO x ) and reactive volatile organic compounds (VOCs). These constituents react with hydrogen oxide radicals (OH + HO 2 = HO x ) in the presence of sunlight and heat to produce O 3 . The question of whether to reduce NO x emissions, VOC emissions, or both, is complicated by spatially and temporally heterogeneous ozone-NO x -VOC sensitivity. This study characterizes spatial and temporal variation in O 3 sensitivity by analyzing the ratio of formaldehyde (HCHO, a marker of VOCs) to nitrogen dioxide (NO 2 ), a metric known as the Formaldehyde Nitrogen Ratio (FNR). Level-3 gridded retrievals from the Ozone Monitoring Instrument (OMI) aboard the NASA Aura satellite were used to calculate FNR, with our analysis focusing on China. Based on previous studies, we take FNR〈1.0 as indicating VOC-limited regimes, FNR〉2.0 as indicating NO x -limited regime, and FNR between 1.0 and 2.0 as indicating transitional regime (where either NO x reductions or VOC reductions would be expected to reduce O 3 ). We find that the transitional regime is widespread over the North China Plain (NCP), the Yangtze River Delta (YRD) and the Pearl River Delta (PRD) during the ozone season (defined as having near surface air temperatures 〉 20° C at the early-afternoon OMI overpass time). Outside of these regions, the NO x -limited regime is dominant. Because HCHO and NO 2 have distinct seasonal patterns, FNR also has a pronounced seasonality, consistent with the seasonal cycle of surface O 3 . Examining trends from 2005 to 2013 indicates rapid growth in NO 2 , especially over less-developed areas where O 3 photochemistry is NO x -limited. Over this time period, HCHO decreased in southern China where VOC emissions are dominated by biogenic sources, but increased slightly over the NCP where VOC emissions are dominated by anthropogenic sources. A linear regression approach suggests that most of China (70% of grid cells) will be characterized by a transitional regime during the O 3 season by 2030. However, in megacities such as Guangzhou, Shanghai and Beijing, NO 2 has decreased such that the chemical regime has shifted from VOC-limited in 2005 to transitional in 2013.

Description: A 60-hour case study of continental boundary layer cumulus clouds is examined using two large-eddy simulation (LES) models. The case is based on observations obtained during the RACORO Campaign (Routine Atmospheric Radiation Measurement [ARM] Aerial Facility [AAF] Clouds with Low Optical Water Depths [CLOWD] Optical Radiative Observations) at the ARM Climate Research Facility's Southern Great Plains site. The LES models are driven by continuous large-scale and surface forcings, and are constrained by multi-modal and temporally varying aerosol number size distribution profiles derived from aircraft observations. We compare simulated cloud macrophysical and microphysical properties with ground-based remote sensing and aircraft observations. The LES simulations capture the observed transitions of the evolving cumulus-topped boundary layers during the three daytime periods, and generally reproduce variations of droplet number concentration with liquid water content (LWC), corresponding to the gradient between the cloud centers and cloud edges at given heights. The observed LWC values fall within the range of simulated values; the observed droplet number concentrations are commonly higher than simulated, but differences remain on par with potential estimation errors in the aircraft measurements. Sensitivity studies examine the influences of bin microphysics versus bulk microphysics, aerosol advection, supersaturation treatment, and aerosol hygroscopicity. Simulated macrophysical cloud properties are found to be insensitive in this non-precipitating case, but microphysical properties are especially sensitive to bulk microphysics supersaturation treatment and aerosol hygroscopicity.

Description: Generally, only clear infrared spectral radiances (not affected by clouds) are assimilated in weather analysis systems. This is due to difficulties in modeling cloudy radiances as well as in observing their vertical structure from space. To take full advantage of the thermodynamic information in advanced infrared (IR) sounder observations requires assimilating radiances from cloud contaminated regions. An optimal imager/sounder cloud-clearing technique has been developed by the Cooperative Institute for Meteorological Satellite Studies (CIMSS) at the University of Wisconsin-Madison. This technique can be used to retrieve clear column radiances through combining collocated multi-band imager IR clear radiances and the sounder cloudy radiances; no background information is needed in this method. The imager/sounder cloudclearing technique is similar to that of the microwave/IR cloud-clearing in the derivation of the clear-sky equivalent radiances. However, it retains the original IR sounder resolution, which is critical for regional NWP applications. In this study, we have investigated the assimilation of cloud-cleared IR sounder radiances using AIRS/MODIS for three hurricanes, Sandy (2012), Irene (2011) and Ike (2008). Results show that assimilating additional cloud-cleared AIRS radiances reduces the 48- and 72-hour temperature forecast RMSE by 0.1 - 0.3 K between 300 and 850 hPa. Substantial improvement in reducing track forecasts errors in the range of 10 km to 50 km was achieved.

Description: A cloud pattern matching technique is applied to polar mesospheric cloud (PMC) images taken by the Cloud Imaging and Particle Size instrument (CIPS) to infer the wind velocities in the mesopause region. CIPS measurements are analyzed to detect patterns that repeat from one orbit to the next, but are displaced in location; the displacement provides a measure of the wind velocity. Pattern matching is achieved by re-sampling the CIPS data to longitude and latitude grids with the grid-box size forced at ~5km in both directions. The correlated patterns are searched within a geographic region referred to as a " frame" of ~500 km in longitude × 400 km in latitude. The histograms of the derived velocities indicate that easterly winds prevail, with a mean zonal wind of −20 ~ −15 m/s. Mean meridional winds are overall small, but in late summer the histogram indicated a poleward wind of ~20-30m/s. The variability of CIPS cloud albedo on consecutive orbits is also examined at fixed geolocations. The statistical results suggest that ~86% of pairs underwent mean cloud albedo variation of 〈 50% on consecutive orbits, suggesting a moderate change. It is also found that the correlation of the cloud structures between two consecutive orbits at a fixed location is generally poor. These findings suggest that cloud patterns are subject to wind advection, but the cloud patches are more extended in size than the movement that occurs. Cloud voids are found to be more likely to remain at the same geolocations.

Description: The lifetime of nitrous oxide, the third-most-important human-emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calculates a semi-empirical lifetime based on: Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone and temperature; laboratory cross sections data for ozone and molecular oxygen plus kinetics for O( 1 D); the observed solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 yr. The observed monthly-to-biennial variations in lifetime and tropical abundance are well matched by four independent chemistry-transport models driven by reanalysis meteorological fields for the period of observation (2005–2010), but all these models overestimate the lifetime due to lower abundances in the critical loss region near 32 km in the tropics. These models plus a chemistry-climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N 2 O perturbations an effective residence time of 109 yr. Combining this new empirical lifetime with model estimates of residence time and pre-industrial lifetime (123 yr) adjusts our best estimates of the human-natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.

Description: The sensitivity of tropical tropospheric composition to the source strength of nitrogen oxides (NO x ) produced by lightning (LNO x ) is analyzed for September through November 2007 using the NASA GEOS-Replay Model with full GMI Combo chemistry and an LNO x algorithm that is appropriate for use in a climate modeling setting; satellite retrievals from OMI, TES and OMI/MLS; and in situ measurements from SHADOZ ozonesondes. Global mean LNO x production rates of 0 to 492 mol NO flash −1 and the subsequent responses of NO x , ozone (O 3 ), hydroxyl radical (OH), nitric acid (HNO 3 ), peroxyacetyl nitrate (PAN) and NO y (NO x + HNO 3 + PAN) are investigated. The radiative implications associated with LNO x -induced changes in tropospheric O 3 are assessed. Increasing the LNO x production rate by a factor of 4 (from 123 to 492 mol flash −1 ) leads to tropical upper-tropospheric enhancements of greater than 100 % in NO x , OH, HNO 3 and PAN. This increase in LNO x also leads to O 3 enhancements of up to 60 %, which subsequently yields a factor-of-three increase in the mean net radiative flux at the tropopause. An LNO x source of 246 mol flash −1 agrees reasonably well with measurements, with an approximate factor-of-two uncertainty due to the short length of the study, inconsistencies in the observational data sets, and systematic biases in modeled LNO x production. Further research into the regional dependencies of lightning flash rates and LNO x production per flash, as well as improvements in satellite retrievals, should help resolve the discrepancies that currently exist between the model and observations.

Description: Recently published studies of triple frequency radar observations of snowfall have demonstrated that naturally occurring snowflakes exhibit scattering signatures that are in some cases consistent with spheroidal particle models, and in others can only be explained by complex aggregates. Until recently, no in-situ observations have been available to investigate links between microphysical snowfall properties and their scattering properties. In this study, we investigate for the first time relations between collocated ground-based triple frequency observations with in-situ measurements of snowfall at the ground. The three analyzed snowfall cases obtained during a recent field campaign in Finland, cover light to moderate snowfall rates with transitions from heavily rimed snow to open-structured, low density snowflakes. The observed triple frequency signatures agree well with the previously published findings from airborne radar observations. A rich spatio-temporal structure of triple frequency observations throughout the cloud is observed during the three cases, which often seems to be related to riming and aggregation zones within the cloud. The comparison of triple frequency signatures from the lowest altitudes with the ground-based in-situ measurements reveals that in the presence of large (〉5 mm) snow aggregates, a bending-away in the triple frequency space from the curve of classical spheroid scattering models is always observed. Rimed particles appear along an almost horizontal line in the triple frequency space, which was not observed before. Overall, the three case studies indicate a close connection of triple frequency signatures and snow particle structure, bulk snowfall density, and characteristic size of the PSD.

Description: Tibetan glaciers are substantially influenced by smoke aerosols derived from intensive biomass burning (BB) emissions in surrounding regions. However, knowledge regarding the impact of smoke aerosols on Tibetan glaciers is limited. Here we present levoglucosan records extracted from two southeastern Tibetan (SET) glaciers. We found that Zuoqiupu (ZQP) Glacier, situated on the windward side of the mountains, is more strongly affected by BB aerosols when compared with Cuopugou (CPG) Glacier on the leeward side. On ZQP Glacier, the highest levoglucosan concentration was detected at an elevation near the equilibrium line altitude (ELA). The injection height of smoke plumes and the actions of post-depositional processes on the glacier surface determined the distribution patterns of levoglucosan concentrations at different altitudes. Spatiotemporal variability in levoglucosan and black carbon (BC) distributions after deposition may be caused by the different source characteristics and by different post-depositional geochemical behaviors on the glacier surface. Intense wildfires can lead to extremely high concentrations (higher than 25 ng mL -1 ) of black carbon in ice near the surface of SET glaciers, and can therefore play an important role in glacial melt during the pre-monsoon season.

Description: Tropospheric O 3 has been decreasing across much of the eastern U.S. but has remained steady or even increased in some western regions. Recent increases in VOC and NO x emissions associated with the production of oil and natural gas (O&NG) may contribute to this trend in some areas. The Northern Front Range of Colorado has regularly exceeded O 3 air quality standards during summertime in recent years. This region has VOC emissions from a rapidly developing O&NG basin and low concentrations of biogenic VOC in close proximity to urban-Denver NO x emissions. Here, VOC OH reactivity (OHR), O 3 production efficiency (OPE), and an observationally constrained box model are used to quantify the influence of O&NG emissions on regional summertime O 3 production. Analyses are based on measurements acquired over two summers at a central location within the Northern Front Range that lies between major regional O&NG and urban emission sectors. Observational analyses suggest that mixing obscures any OPE differences in air primarily influenced by O&NG or urban emissions sectors. The box model confirms relatively modest OPE differences that are within the uncertainties of the field observations. Box model results also indicate that maximum O 3 at the measurement location is sensitive to changes in NO x mixing ratio but also responsive to O&NG VOC reductions. Combined, these analyses show that O&NG alkanes contribute over 80% to the observed carbon mixing ratio, roughly 50% to the regional VOC OHR, and approximately 20% to regional photochemical O 3 production.

Description: Temporal variability in the vertical distribution of aerosol optical thickness (AOT) derived from the 0.532 µm aerosol extinction coefficient is described using Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) observations over eight and a half years (June 2006 – December 2014). Temporal variability of CALIOP column-integrated AOT is largely consistent with total-column AOT trends from several passive satellite sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS), Multi-angle Imaging Spectroradiometer (MISR), and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Globally, a 0.0002 AOT per year positive trend in deseasonalized CALIOP total-column AOT for daytime conditions is attributed to corresponding changes in near-surface (i.e., 0.0-0.5 km or 0.5-1.0 km above ground level [AGL]) aerosol particle loading, while a -0.0006 AOT per year trend during nighttime is attributed to elevated (i.e., 1.0-2.0 km or 〉 2.0 km AGL) aerosols. Regionally, increasing daytime CALIOP AOTs are found over Southern Africa and India, mostly due to changes in aerosol loading at the 1.0-2.0 km and 0.0-0.5 km AGL layers, respectively. Decreasing daytime CALIOP AOTs are observed over Northern Africa, Eastern U.S., and South America (due mostly to elevated aerosol loading), while the negative CALIOP AOT trends found over Eastern China, Europe, and Western U.S. are due mostly to aerosol layers nearer the surface. To our knowledge, this study is the first to provide both a globally comprehensive estimation of the temporal variation in aerosol vertical distribution and insight into passive sensor column AOT trends in the vertical domain.

Description: A large-scale precipitation tracking (LPT) method is developed to track convection and precipitation associated with the MJO using the TRMM 3B42 rainfall data from October-March 1998-2015. LPT uses spatially smoothed 3-day rainfall accumulation to identify and track precipitation features in time with a minimum size of 300, 000 km 2 and time continuity at least 10 days. While not all LPT systems (LPTs) are attributable to the MJO, among the 199 LPTs, there were 42 with a mean eastward propagation of at least 2 m s -1 , which are considered to be MJO convective initiation events. These LPTs capture the diversity of the MJO convection, which is not well depicted by the Realtime Multivariate MJO (RMM) index or the OLR MJO Index (OMI). During the 17 years, there were 17 instances out of 45 with a MJO signature in the RMM without eastward propagating LPTs. Among the 42 eastward propagating LPTs, 24 propagated across the Maritime Continent (MC), which confirms the MC barrier effect. Among the cases that crossed the MC from the Indian Ocean to the west Pacific (MC-crossing), 18 (75%) had a significant MJO signature in the RMM index. In contrast, only 6 (33%) of the non-MC-crossing cases occurred with a RMM MJO signal. There is a significant seasonal and interannual variability with MC-crossing LPTs occurring in December more commonly than other months. More MC-crossing events were observed during La Niña than El Niño, which is consistent with observations of stronger and more frequent MJO events identified by RMM during La Niña years.

Description: Here we assess estimates of atmospheric evaporative demand over China in twelve state-of-the-art Global Climate Models (GCMs) against observed D20 pan evaporation ( E pan ) over the period 1961–2000. To do that, we use an energy-relevant and physical-based approach, namely PenPan model, to comprehensively evaluate GCM performance with respect to their ability to simulate annual, seasonal and monthly statistics of E pan (and its radiative and aerodynamic components, E p , R and E p , A ). The results indicated that most GCMs generally captured the spatial pattern and seasonal cycle of E pan , E p , R and E p , A . However, regional means of annual and monthly E pan , E p , R and E p , A were underestimated by most GCMs mainly due to negatively biased surface air temperature ( T a ) and vapour pressure deficit ( vpd ) outputted/simulated by the GCMs. Overall, the discrepancies among GCMs in estimating the regional statistics (regional means and seasonal cycles) of E p , A were relatively larger than that ofE p , R , which indicates considerable uncertainties in the calculation of the aerodynamic component of evaporation based on the GCM outputs. Moreover, a few GCMs captured negative trends of regional mean annual and seasonal E pan , E p , R and E p , A well over the period 1961– 2000, but most showed positive trends. The underestimation of net radiation ( R n ) and overestimation of wind speed at 2 meters ( u 2 ) in most GCMs may, to some extent, accentuate/compensate the negative biases in GCM-estimated annual and seasonal E pan , E p , R and E p , A . The results demonstrate the importance of incorporating observation of pan evaporation and well validated PenPan model to evaluate GCMs performance on atmospheric evaporative demand that is relevant to projections of future drought and regional water-energy budgets.

Description: A meteorological model requires accurate initial conditions and boundary conditions to obtain realistic numerical weather predictions. The land surface controls the surface heat and moisture exchanges, which can be determined by the physical properties of the soil and soil state variables, subsequently exerting an effect on the boundary layer meteorology. The initial and boundary conditions of soil moisture are currently obtained via NCEP FNL (Final) Operational Global Analysis data, which are collected operationally in 1 degree by 1 degree resolutions every six hours. Another input to the model is the soil map generated by the FAO-UNESCO soil database, which combines several soil surveys from around the world. Both soil moisture from the FNL analysis data and the default soil map lack accuracy and feature coarse resolutions, particularly for certain areas of China. In this study, we update the global soil map with data from Beijing Normal University in 1 km by 1 km grids and propose an alternative method of soil moisture initialization. Simulations of the Weather Research and Forecasting Model show that spinning-up the soil moisture improves near-surface temperature and relative humidity prediction using different types of soil moisture initialization. Explanations of that improvement and improvement of the planetary boundary layer height in performing process analysis are provided.

Description: No abstract is available for this article.

Description: We use a single-particle aerosol mass spectrometer (SPAMS) coupled with a ground-based counterflow virtual impactor (GCVI) to measure the chemical compositions of individual submicron fog droplet residues. This is the first report of single-particle mass spectrometry measurements of fog droplet residual particles at ground level in an urban area. We show that most of the fog droplet residues were composed of elemental carbon (EC) (67.7%), followed by K-rich (19.2%) and mineral dust/metal (12.3%) particles. The predominance of EC-containing particles demonstrated that these particles could be effective fog nuclei and highlights the important influence of anthropogenic emissions on regional climate system. Compared with interstitial and ambient aerosols, nitrate was enhanced, sulfate was depressed, and ammonium- and organics-containing particles were hardly found in the fog droplet residues during fog events, suggesting that dust and metal particles containing nitrate may be preferentially activated and that ammonium and organics may not play important roles in fog formation in Guangzhou. We also present direct observational evidence that trimethylamine (TMA) and hydroxymethanesulfonate (HMS) are not found within fog droplet residues, although we previously observed enhanced gas-to-particle partitioning of these compounds by fog processing. Additionally, higher fraction or intensities of [K] + , [Fe] + and [SiO 3 ] - were found in fog droplet residues than in ambient and interstitial particles.

Description: This paper presents an analysis of the relationship between land surface temperatures (LST) and screen-level air temperatures (T2m) using in situ observations from 19 Atmospheric Radiation Measurement (ARM) deployments located in a range of geographical regimes. The diurnal cycle is resolved using one-minute observations: a particular focus of the study is on the relationship between daily extremes of LST (LSTmax, LSTmin) and T2m (Tmax, Tmin). Temperature differences are analysed with respect to cloud, wind speed and snow cover. Under cloud-free, low wind speed conditions, day time LST is often several degrees Celsius (°C) higher than T2m at low-to-mid latitudes, and at high latitudes during the summer months. In contrast, LST and T2m are often close (e.g. within 2 °C) under cloudy and/or moderate-to-high wind speed conditions, or when solar insolation is low or absent. LSTmin and Tmin are generally well correlated (r 〉0.8, often r 〉0.9), while seasonal correlations between LSTmax and Tmax are weaker (r 〉0.6, often r 〉0.8). At high latitudes, LST and T2m are well coupled in Spring/Autumn/Winter; the relationship between LST and T2m tends to weaken with decreasing latitude. The timing of daily extremes is also investigated and it is found that LSTmin and Tmin typically occur close to sunrise, with Tmin occurring slightly after LSTmin. LSTmax occurs close to solar noon, with Tmax typically occurring 1-3 hours later. This study will inform temperature data users on differences between LST and T2m, and aid development of methods to estimate T2m using satellite LSTs.

Description: Satellite remote sensing of Precipitable Water Vapor (PWV) is essential for monitoring moisture in real-time for weather applications, as well as tracking the long-term changes in PWV for climate change trend detection. This study assesses the accuracies of the current satellite observing system, specifically the National Aeronautics and Space Administration (NASA) Atmospheric Infrared Sounder (AIRS) v6 PWV product and the European Organization for the Exploitation of Meteorological Satellite Studies (EUMETSAT) Infrared Atmospheric Sounding Interferometer (IASI) v6 PWV product, using Ground-Based SuomiNet Global Positioning System (GPS) network as truth. Elevation-corrected collocated matchups to each SuomiNet GPS station in North America and around the world was created and results were broken down by station, ARM-region, climate zone, and latitude zone. The greatest difference, exceeding 5%, between IASI and AIRS retrievals occurred in the tropics. Generally, IASI and AIRS fall within a 5% error in the PWV range of 20-40 mm (a mean bias less than 2 mm), with a wet bias for extremely low PWV values (less than 5 mm) and a dry bias for extremely high PWV values (greater than 50 mm). The operational IR satellite products are able to capture the mean PWV but degrade in the extreme dry and wet regimes.

Description: We use data from the Aeronomy of Ice in the Mesosphere (AIM) mission and simulations using the Whole Atmosphere Community Climate Model (WACCM) to determine the impact of the January 23-30, 2012 solar proton event (SPE) on polar mesospheric clouds (PMCs) and mesospheric water vapor. We see a small heating and loss of ice mass on 26 January that is consistent with prior results, but is not statistically significant. We also find a previously unreported but statistically significant ~10% increase in ice mass and in water vapor in the sublimation area in the model that occurs in the 7 to 14 days following the start of the event. The magnitude of the response to the January 2012 SPE is small compared to other sources of variability like gravity waves and planetary waves; however, sensitivity tests suggest that with larger SPEs this delayed increase in ice mass will increase, while there is little change in the loss of ice mass early in the event. The PMC response to SPEs in models is dependent on the gravity wave parameterization, and temperature anomalies from SPEs may be useful in evaluating and tuning gravity wave parameterizations.

Description: Extreme precipitation events, commonly associated with 'Atmospheric Rivers', are projected to increase in frequency and severity in western North America, however, the intensity and landfall position are difficult to forecast accurately. As the isotopic signature of precipitation has been widely utilized as a tracer of the hydrologic cycle and could potentially provide information about key physical processes, we utilize both climate and precipitation isotope data to investigate these events in California from 2001–2011. Although individual events have extreme isotopic signatures linked to associated circulation anomalies, the composite across all events unexpectedly resembles the weighted mean for the entire study period, reflecting diverse moisture trajectories and associated teleconnection phases. We document that 90% of events reaching this location occurred during the negative Arctic Oscillation, suggesting a possible link with higher latitude warming. We also utilize precipitation data of extreme precipitation events across the entire Western US to investigate the relationships between key tropical and Arctic climate modes known to influence precipitation in this region. Results indicate the wettest conditions occur when the negative Arctic Oscillation, negative Pacific North American pattern, and positive Southern Oscillation are in sync and that precipitation has increased in the Southwestern US and decreased in the Northwestern US relative to this phase combination's 1979–2011 climatology. Furthermore, the type of El Niño Southern Oscillation event, Central Pacific or Eastern Pacific, influences the occurrence, landfall location, and isotopic composition of precipitation.

Description: The snow-albedo feedback (SAF) strongly influences climate over mid-latitude mountainous regions. However, over these regions the skill of regional climate models (RCMs) at simulating properties such as snow cover and surface albedo is poorly characterized. These properties are evaluated in a pair of 7-year long high-resolution RCM simulations with the Weather Research and Forecasting (WRF) model over the central Rocky Mountains. Key differences between the simulations include the computational domain (regional vs. continental) and land surface model used (Noah vs. Noah-MP). Simulations are evaluated against high-resolution satellite estimates of snow cover and albedo from the Moderate Resolution Imaging Spectroradiometer (MODIS). Both simulations generally reproduce the observed seasonal and spatial variability of snow cover, but also exhibit important biases. One simulation substantially over-predicts sub-pixel fractional snow cover over snowy pixels (by up to 0.4) causing large positive biases in surface albedo, likely due in part to inadequate representation of canopy effects. The other simulation exhibits a negative bias in areal snow extent (as much as 19% of the analysis domain). Surface measurements reveal large positive biases in snow albedo (exceeding 0.2) during late spring caused by neglecting radiative effects of impurities deposited onto snow. Semi-idealized climate change experiments show substantially different magnitudes of SAF-enhanced warming in the two simulations that can be tied to the differences in snow cover in their control climates. More-confident projections of regional climate change over mountains will require further work to evaluate and improve representation of snow cover and albedo in RCMs.

Description: Using a set of fully-coupled climate model simulations, the response to partial deforestation over the Amazon due to agricultural expansion has been analyzed. Three variations of 50% deforestation (all of western half, all of eastern half, and half of each grid box) were compared with total deforestation to determine the degree and character of non-linearity of the climate response to partial deforestation. A metric is developed to quantify the degree and distribution of non-linearity in the response, applicable to any variable. The metric also quantifies whether the response is saturating or accelerating, meaning either significantly more or less than 50% of the simulated response to total deforestation is attained at 50% deforestation. The spatial structure of the atmospheric response to Amazon deforestation reveals large areas across the tropics that exhibit a significant non-linear component, particularly for temperature and geopotential height. Over the domain between 45°S-45°N across all longitudes, 50% deforestation generally provides less than half of the total response to deforestation over oceans, indicating the marine portion of climate system is somewhat resilient to progressive deforestation. However, over continents there are both accelerating and saturating responses to 50% Amazon deforestation, and the response is different depending on whether the eastern or western half of Amazonia is deforested, or half of the forest is removed uniformly across the region.

Description: This study expands upon the results of Bang and Zipser [2015], which demonstrated that oceanic precipitation features with lightning tended to be over ten times larger and more stratiform than those over continents, and suggested the hypothesis that some form of external forcing is acting in ways that lead to both larger features and stronger updrafts over tropical oceans. In this work, we evaluate this hypothesis by using reanalysis data to compare large-scale vertical motion and thermodynamic data for RPFs in archetypal land and ocean regimes in the Congo and Central Pacific. We then expand our study to the entire TRMM domain (35°S to 35°N) over all seasons. Over the ocean, there is a tendency for stronger large-scale upward motion, linear organization of the convective region, and larger precipitation areas in systems with lightning. By comparison, over land, features with lightning tend to be smaller, in environments of higher instability, with little difference in large-scale vertical motion. Expanding our analysis to 21 different regions, we highlight those in which seasonal synoptic patterns and water temperature gradients lead to distributions that are exceptions to the findings that ocean storms with lightning tend to be larger and more mature than land storms with lightning. These findings support the hypothesis that most land storms have updrafts sufficiently strong to develop lightning in their early growth stages, while ocean storms require large-scale ascent and growth into mesoscale convective systems before convective scale updrafts become strong enough to provide the necessary conditions for lightning.

Description: The temporal and spatial slopes of water isotope-temperature relations are studied for the last 21,000 years over the middle and high latitudes using a series of snapshot simulations of global climate and water isotopes in the isotope-enabled atmospheric model isoCAM3. Our model simulation suggests that both the temporal slope and spatial slope remain largely stable throughout the last deglaciation. Furthermore, the temporal slope can vary substantially across regions. Nevertheless, on average, and most likely, the temporal slope is about 0.3‰∙ °C − 1 , and is about half of the spatial slope. Finally, the relation between temporal and spatial slopes is understood using a semi-empirical equation that is derived based on both the Rayleigh distillation and a fixed spatial slope. The slope equation quantifies the Boyle's mechanism and suggests that the temporal slope is usually smaller than the spatial slope in the extratropics mainly because of the polar amplification feature in global climate change, such that the response in local temperature at mid- and high-latitudes is usually greater than that in the total equivalent source temperature.

Description: Using a coastal ice core collected from Prince of Wales (POW) Icefield on Ellesmere Island, we investigate source regions of sea ice-modulated chemical species (methanesulfonic acid (MSA) and chloride (Cl - )) to POW Icefield, and the influence of large-scale atmospheric variability on the transport of these marine aerosols (1979–2001). Our key findings are: (1) MSA in the POW Icefield core is derived primarily from productivity in the sea-ice zone of Baffin Bay and the Labrador Sea, with influence from waters within the North Water (NOW) polynya, (2) sea-ice formation processes within the NOW polynya may be a significant source of sea-salt aerosols to the POW core site, in addition to offshore open water source regions primarily in Hudson Bay, and (3) the tropical Pacific influences the source and transport of marine aerosols to POW Icefield through its remote control on regional winds and sea-ice variability. Regression analyses during times of MSA deposition reveal sea level pressure (SLP) anomalies favorable for opening of the NOW polynya, and subsequent oceanic dimethyl sulfide production. Regression analyses during times of Cl - deposition reveal SLP anomalies that indicate a broader oceanic region of sea-salt sources to the core site. These results are supported by SMMR- and SSM/I-based sea-ice reconstructions and air mass transport density analyses, and suggest the marine biogenic record may capture local polynya variability, while sea-salt transport to the site from larger offshore source regions in Baffin Bay is likely. Regression analyses show a link to tropical dynamics via an atmospheric Rossby wave.

Description: The effects of two types of ENSO events on tropical ozone (O 3 ) variations from 1982 to 2010, and the mechanisms underlying these effects, were analyzed using observations and model simulations. Tropospheric column O 3 anomalies (TCOA) during canonical El Niño were different from El Niño Modoki. Absolute TCOA values are larger during canonical El Niño than during El Niño Modoki in most regions. La Niña events were not separated into the different types because of their similarity in terms of sea surface temperature patterns. TCOA in La Niña showed a reversed dipole from canonical El Niño. During canonical El Niño, anomalous downward motion together with suppressed convection weakened O 3 outflow from the troposphere, causing an increase in tropospheric O 3 over western Pacific. Over central and eastern Pacific, decreased O 3 concentrations resulted primarily from a change in net chemical production of O 3 . The change in net O 3 chemical production relates to increased levels of HOx under wetter condition. During El Niño Modoki, transport and chemical fluxes were similar but weaker than during canonical El Niño. During La Niña, O 3 anomalies and transport fluxes were the opposite of those during the El Niño Modoki. Stratospheric O 3 played a key role in the development of O 3 anomaly above 250 hPa during ENSO events, contributing 〉30% to the O 3 anomalies. The change in free tropospheric O 3 affected the O 3 anomaly from 850 hPa to 200 hPa (60% of O 3 anomaly). The contribution of O 3 from planetary boundary layer was concentrated at the surface, with a contribution of 〈15%.

Description: The performance of Beijing Climate Center Climate system model (BCC_CSM) with 2 horizontal resolutions in simulating the precipitation seasonal variation over Eastern China has been evaluated. The possible reasons related to the differences in the simulations of BCC_CSM1.1 m model with fine resolution and BCC_CSM1.1 model with coarse resolution also have been addressed and discussed. Results show that the the improved simulation of the timing and amount of precipitation in dry seasons except for larger biases during rainy seasons can be noted in BCC_CSM1.1 m model relative to BCC_CSM1.1 model. The occurrence time of the precipitation annual peaks in BCC_CSM1.1 m model shows better agreements with the observation compared to BCC_CSM1.1 model. Mechanism analysis indicates that BCC_CSM1.1 produced earlier East Asian summer monsoon (EASM) onset and northward jump of Western Pacific subtropical high (WPSH), leading to the earlier start of the rainy seasons and occurrence time of the precipitation annual peaks over Eastern China comparing with the observation and BCC_CSM1.1 m simulation. The improved EASM onset and northward jump of WPSH in BCC_CSM1.1 m model resulted in better simulation of precipitation seasonal transition and occurrence time of the precipitation annual peaks. However, compared to BCC_CSM1.1 model, the much more underestimated summer precipitation over most Eastern China in BCC_CSM1.1 m model is mainly due to the weakly simulated northeastward water vapor transport which is resulted from the much stronger WPSH with farther northwest location and weaker land sea thermal contrast (LSTC).

Description: The emission of mineral dust aerosols in arid and semi-arid regions is a complex process whose representation in atmospheric models remains crude, due to insufficient knowledge about the aerosol lifting process itself, the lack of global data on soil characteristics, and the impossibility for the models to resolve the fine-scale variability in the wind field that drives some of the dust events. As a result, there are large uncertainties in the total emission flux of mineral dust, its natural variability at various timescales, and the possible contribution from anthropogenic land use changes. This work aims for estimating dust emissions and reduce their uncertainty over the Sahara Desert and the Arabian Peninsula –the largest dust source region of the globe. We use a data assimilation approach to constrain dust emission fluxes at a monthly resolution for 18 sub-regions. The MODIS satellite-derived aerosol optical depth is assimilated in a regional configuration of a general circulation model coupled to an aerosol model. We describe this data assimilation system and apply it for one year, resulting in a total mineral dust emissions flux estimate of 2900 Tg year −1 over the Sahara Desert and the Arabian Peninsula for the year 2006. The analysis field of aerosol optical depth shows an improved fit relative to independent AERONET measurements as compared to the model prior field.

Description: The marine boundary layer of the northeastern U.S. is studied with focus on wind speed, atmospheric stability, and turbulent kinetic energy (TKE), the three most relevant properties in the context of offshore wind power development. Two long-term observational datasets are analyzed. The first one consists of multi-level meteorological variables measured during 2003-2011 at the offshore Cape Wind tower up to 60 m, located near the center of the Nantucket Sound. The second dataset comes from the 2013-2014 IMPOWR campaign (Improving the Modeling and Prediction of Offshore Wind Resources), in which wind and wave data were collected with new instruments on the Cape Wind platform, in addition to meteorological data measured during 19 flight missions offshore of New York, Connecticut, Rhode Island, and Massachusetts. It is found that in this region: 1) the offshore wind resource is remarkable, with monthly-average wind speeds at 60 m exceeding 7 m s -1 all year round, highest winds in winter (10.1 m s -1 ) and lowest in summer (7.1 m s -1 ), and a relatively weak diurnal modulation on average; 2) the marine boundary layer is predominantly unstable, meaning that mixing is strong, heat fluxes are positive, and the wind speed profile is often non-logarithmic (~40% of the time); and 3) the shape of the wind speed profile (log versus non-log) is an effective qualitative proxy for atmospheric stability, whereas TKE alone is not.

Description: Hydroclimatic process in the Yarlung Tsangpo River (YTR) basin, a sensitive area to climate change, is obviously changing during recent years, but there has limited understanding about it. In this study, we investigated the spatiotemporal variation of precipitation over last four decades in the basin, and the impact thereon of the changing Indian summer monsoon at inter-annual and decadal time scales. All the precipitation series have similar scaling behavior, reflecting similar climatic regime throughout the basin. However, the effect of the Indian monsoon strengthens from the downstream to upstream, causing spatial variability in the seasonal distribution of precipitation, and on this basis the YTR basin is roughly divided into three regions: east, mid and west. Both the occurrence times and magnitude of precipitation extremes, ranging 25-50 mm/day, are exhibiting downward trends over the last four decades, which bodes well for water disaster controls in the basin. The Indian summer monsoon index, as an intensity indicator for the Indian summer monsoon, shows a positive relationship with the summer precipitation in the YTR basin. Periodic variability of the Indian monsoon determines the inter-annual nonstationary fluctuations of precipitation. Especially, the weakening effect of the Indian summer monsoon has caused an obvious decrease in precipitation over the rainy season after 1998. If the Indian summer monsoon keeps weakening, the precipitation would decrease and potentially water shortage would become more severe in the basin. Effective adaptation strategy should therefore be developed proactively to handle the unfavorable water situation, which is likely to occur in the future.

Description: The light absorbing properties of water-soluble brown carbon (WS-BrC) and methanol-soluble brown carbon (MeS-BrC) were studied in PM 10 aerosols collected at the “Nepal Climate Observatory-Pyramid” (NCO-P) station (5079 m a.s.l.) during the period 2013-2014. The light absorption coefficients of WS-BrC and MeS-BrC were the highest during the pre-monsoon season and the lowest during monsoon. MeS-BrC absorbs about 2 times higher at 365 nm and about 3 times more at 550 nm compared to WS-BrC. The mass absorption cross-section (MAC) of WS-BrC measured at 365 nm is similar to that observed previously at South Asian low-altitude sites. Fractional solar radiation absorption by BrC compared to BC considering the full solar spectrum showed that WS-BrC absorbs 4 ± 1% and MeS-BrC absorbs 9 ± 2% compared to BC at NCO-P. Such ratios become 8 ± 1% (for WS-BrC respect to BC) and 17 ± 5% (for MeS-BrC respect to BC) when accounting for correction factors proposed by previous studies to convert absorption coefficients in bulk solutions into light absorption by accumulation mode aerosol particles. These results confirm the importance of BrC in contributing to light-absorbing aerosols in this region of the world. However, the BrC absorption at 550 nm appears small compared to that of BC (1-4%, or 3 – 8% with conversion factors), and it is lower compared to global model estimates constrained by AERONET observations. Finally, our study provides no clear evidence of a change in the fractional contribution of BrC with respect to BC to light absorption in the middle troposphere respect to the Indo-Gangetic plain boundary layer.

Description: Clouds in the presence of absorbing aerosols results in their apparent darkening, observed at the Top of Atmosphere (TOA), which is associated with the radiative effects of aerosol absorption. Owing to the large warming radiative effect and potential impacts on regional climate, above-cloud aerosols have recently been characterized in multiple satellite-based studies. While satellite data are particularly useful in demonstrating the radiative impact of above-cloud aerosols at the TOA, there remains uncertainties in the in-depth understanding of aerosol-cloud radiative interactions and climate effects. Furthermore, recent literature indicates large uncertainties in satellite retrievals of above-cloud Aerosol Optical Depth (AOD) and Single Scattering Albedo (SSA), which are among the most important parameters in the assessment of associated radiative effects. In this study, we analyze radiative characteristics of clouds in the presence of wildfire smoke using airborne data primarily from NASA's Cloud Absorption Radiometer, collected during the ARCTAS campaign in Canada during the 2008 summer season. We found a strong positive reflectance ( R ) gradient in the UV-VIS-NIR spectrum for clouds embedded in dense smoke, as opposed to an (expected) negative gradient for cloud-free smoke and a flat spectrum for smoke-free cloud cover. Several cases of clouds embedded in thick smoke were found, when the aircraft made circular/spiral measurements, which not only allowed the complete characterization of angular distribution of smoke scattering, but also provided the vertical distribution of smoke and clouds (within 0.5 – 5 km). Specifically, the largest darkening by smoke was found in the UV/VIS, with R 0.34μm reducing to 0.2 (or 20%), in contrast to 0.8 observed at NIR wavelengths (e.g. 1.27 µm). The observed darkening was found to be associated with large AODs (0.5 – 3.0) and moderately low SSA (0.85 – 0.93 at 0.53 µm), resulting in a significantly large instantaneous aerosol forcing efficiency of 254 ± 47 Wm -2 τ -1 . Our observations of smoke-cloud radiative interactions were found to be physically consistent with theoretical plane-parallel 1D and Monte Carlo 3D radiative transfer calculations, capturing the observed gradient across UV-VIS-NIR. Results from this study offer insights into aerosol-cloud radiative interactions, and may help in better constraining satellite-retrieval algorithms.

Description: Variations in oxygen isotope compositions (δ 18 O) provide insight into modern climate and past changes in climate and topography. In addition, in regions such as Tibet, geologic archives of isotope ratios record climate change driven by plateau uplift and therefore also provide information about the surface uplift history. A good understanding of modern day controls on δ 18 O is crucial for interpreting geologic δ 18 O in this context. We use the ECHAM5-wiso global atmospheric general circulation model to calculate δ 18 O in precipitation (δ 18 O p ) for the present-day climate. In the region of the Tibetan Plateau, spatial variations of monthly means of δ 18 O p are statistically related to spatial variations of 2 m-air temperature and precipitation rate, as well as to topography. The size and location of investigated regions are varied in our study to capture regional differences in these relationships and the processes governing the modern δ 18 O p . In addition to correlation analyses, a cross-validated stepwise multiple regression is carried out using δ 18 O p as the predictand and topography and atmospheric variables (temperature, precipitation amount) as predictors. 2 m-air temperature and topography yield the highest spatial correlation coefficients of 〉0.9 and 〈 -0.9 respectively, throughout most of the year. Particularly high correlation coefficients are calculated for the region along the Himalayan orogeny and parts of Western China. The predictors explain 〉90% of the δ 18 O p spatial variance in the same regions. 2 m-air temperature is the dominant predictor and contributes 93.6% to the total explained spatial variance on average. The results demonstrate that most of the δ 18 O p pattern on and around the Tibetan Plateau can be explained by variation in 2 m-air temperature and altitude. Correlation of the dependent predictors indicate that in low-altitude regions where topography does not determine temperature variability, the high correlation of temperature and δ 18 O p may partially be explained by variations in precipitation rates.

Description: Light-absorbing atmospheric aerosols such as carbonaceous particles influence the climate through absorbing sunlight. The mixing states of these aerosol particles affect their optical properties. This study examines the changes in the mixing states and abundance of strongly light-absorbing carbonaceous particles by using transmission electron microscopy (TEM) and single-particle soot photometer (SP2), as well as of iron-oxide particles, in Tokyo, Japan. TEM and SP2 use fundamentally different detection techniques for the same light-absorbing particles. TEM allows characterization of the morphological, chemical, and structural features of individual particles, whereas SP2 optically measures the number, size, and mixing states of black carbon (BC). A comparison of the results obtained using these two techniques indicates that the peaks of high soot (nanosphere soot (ns-soot)) concentration periods agree with those of the BC concentrations determined by SP2 and that the high Fe-bearing particle fraction periods measured by TEM agree with that of high number concentrations of iron-oxide particles measured using SP2 during the first half of the observation campaign. The results also show that the changes in the ns-soot/BC mixing states primarily correlate with the air mass sources, wind speed, precipitation, and photochemical processes. Nano-sized, aggregated, iron-oxide particles mixed with other particles were commonly observed by using TEM during the high-iron-oxide particle periods. We conclude that, although further quantitative comparison between TEM and SP2 data will be needed, the morphologically and optically defined ns-soot and BC, respectively, are essentially the same substance and that their mixing states are generally consistent across the techniques.

Description: No abstract is available for this article.

Description: The present study aims to examine the new understanding of cyclogenesis by analyzing the genesis sequence of formation of a very severe cyclonic storm Madi (6-13 December 2013) occurred over the Bay of Bengal (BoB). We have generated high resolution (18 km, 6 km, and 2 km) analysis using three-dimensional variational (3DVAR) data assimilation technique and Weather Research and Forecasting (WRF) model. The genesis sequence of Madi cyclone is analyzed using the concepts in the marsupial theory and other theories of tropical cyclone formation. Major results are as follows, the developed analysis is found useful for tracking the movement of westward moving parent disturbance from 15 days prior to the genesis; identifying developed pouch region in the Lagrangian frame of reference; understanding the evolution of the pouch and convection within the pouch region and for the study of intensification inside the pouch region. Also, large scale priming of environment concurs with the hypotheses of the marsupial theory of tropical cyclogenesis. The analysis of dynamical and thermodynamical processes within the pouch region showed gradual moistening, uplifting of moisture, diabatic heating causing buoyant convection in the vorticity rich environment followed by vortex tube stretching, development of convection, heavy precipitation, strengthening of lower level convergence and hence spinup during a day or two preceding the genesis of Madi cyclone. In general, it is concluded that intensification within pouch region during the cyclogenesis phase followed the marsupial paradigm and bottom-up mechanism.

Description: The coupled global climate model fidelity in representing upper ocean salinity including near-surface bulk salinity (SSS) is evaluated in this study, with a focus on the Pacific Ocean. The systematic biases in ocean surface evaporation (E) minus precipitation (P) and SSS are found to be fairly similar in the 20th century simulations of the Coupled Model Intercomparison Phase 3 (CMIP3) and Phase 5 (CMIP5) relative to the observations. One of the potential causes of the CMIP model biases is the missing representation of the radiative effects of precipitating hydrometeors (i.e., snow) in most CMIP models. To examine the radiative effect of cloud snow on SSS, sensitivity experiments with and without such effect are conducted by the NCAR coupled Community Earth System Model (CESM). This study investigates the difference in SSS between sensitivity experiments and its relationship with atmospheric circulation, E-P and air-sea heat fluxes. It is found that the exclusion of the cloud snow radiative effect in CESM produces weaker Pacific trade winds, resulting in enhanced precipitation, reduced evaporation, and a reduction of the upper-ocean salinity in the tropical and sub-tropical Pacific. The latter results in an improved comparison with climatological upper-ocean bulk salinity. The introduction of cloud snow also altered the budget terms that maintain the time-mean salinity in the mixed layer.

Description: The El Niño–Southern Oscillation (ENSO) has significant effects on the extratropical stratosphere. This study explores the nonlinearity and the asymmetry of these influences by distinguishing the different effects of four types of ENSO: “moderate El Niño”, “strong El Niño”, “moderate La Niña”, and “strong La Niña”. It is revealed that the moderate El Niño and the strong La Niña are much more efficient than the strong El Niño and the moderate La Niña, respectively, in modulating the northern winter stratospheric variability, resulting in significant nonlinearity and asymmetry. The tropical rainfall anomalies induced by a moderate El Niño or a strong La Niña are centered over the central equatorial Pacific region near the dateline, while the convection responses to a strong El Niño or a moderate La Niña are centered farther eastward. Accordingly, the anomalous Pacific–North America wave train pattern is modulated by ENSO in a nonlinear and asymmetric way, which leads the large nonlinear and asymmetric components of the vertical Eliassen-Palm (E–P) flux responses to ENSO. The increase of planetary wave activity in the extratropical stratosphere in moderate El Niño winters is thus greater than in strong El Niño winters, whereas a strong La Niña gives a larger decrease in propagation than a moderate La Niña. The relatively strong (weak) E–P flux responses to moderate (strong) El Niño and strong (moderate) La Niña explain the remarkable nonlinearity and asymmetry in the response of the extratropical stratosphere to ENSO.

Description: Microphysical characteristics of pre-monsoon and monsoon deep cumuli over India observed by an instrumented aircraft are contrasted focusing on influences of environmental conditions and entrainment-mixing processes. Differences in the lower-tropospheric temperature and moisture profiles lead to contrasting undiluted cloud buoyancy profiles around the cloud base, larger in the pre-monsoon case. It is argued that this affects the variation of the mean and maximum cloud droplet number concentrations and the droplet radius within the lowest several hundred meters above the cloud base. The conserved-variable thermodynamic diagram analysis suggests that entrained parcels originate from levels close to the observational level. Mixing processes and their impact on the droplet size distribution (DSD) are investigated contrasting 1 Hz and 10 Hz observations. Inhomogeneous-type mixing, likely because of unresolved small-scale structures associated with active turbulent stirring, is noted at cloud edge volumes where dilution is significant and DSDs shift towards smaller sizes with reduced droplet number concentrations due to complete evaporation of smaller droplets and partial evaporation of larger droplets. DSDs within cloud core volumes suggest that the largest droplets are formed in the least diluted volumes where raindrops can form at higher levels; no super-adiabatic droplet growth is observed. The typical diluted parcel size is approximately 100–200 m for cloud edge volumes, and it is much smaller, 10–20 m, for cloud core volumes. Time scale analysis indicates the possibility of inhomogeneous type mixing within the diluted cloud edge volumes at spatial scales of a 100 m or more.

Description: Soil moisture is at the heart of many processes connected to water cycle, climate, ecosystem and societal conditions. This paper investigates the ability of a relatively simple analytical soil-moisture model to reproduce temporal variability dynamics in long-term data series for: (i) remotely sensed large-scale water storage change in twenty-five large catchments around the world, and (ii) measured soil water content and groundwater level in individual stations within ten smaller catchments across the United States. The model-data comparison for large-scale water storage change (i) shows good model ability to reproduce the observed temporal variability around long-term average conditions in most of the large study catchments. Also the model comparison with locally measured data for soil water content and groundwater level in the smaller U.S. catchments (ii) shows good representation of relative seasonal and longer-term fluctuations and their timings and frequencies. Overall, the model results tend to underestimate rather than exaggerate the range of temporal soil moisture fluctuations and storage changes. The model synthesis of large-scale hydro-climatic data is based on fundamental catchment-scale water balance and is as such useful for identifying flux imbalance biases in the hydro-climatic data series that are used as model inputs.

Description: Thinning of Arctic sea ice gives rise to ice fracturing and leads (areas of open water surrounded by sea ice) that are a potential source of sea salt aerosol. Atmospheric particle inorganic ion concentrations, local sea ice conditions, and meteorology at Barrow, AK from 2006-2009 were combined to investigate the dependence of submicron (aerodynamic diameter 〈 1 µm) and supermicron (aerodynamic diameter 1-10 µm) sea salt mass concentrations on sea ice coverage and wind speed. Consistent with a wind-dependent source, supermicron sea salt mass concentrations increased in the presence of nearby leads and wind speeds greater than 4 m s -1 . Increased supermicron and submicron sea salt chloride depletion was observed for periods of low winds or a lack of nearby open water, consistent with transported sea salt influence. Sea salt aerosol produced from leads has the potential to alter cloud formation, as well as the chemical composition of the Arctic atmosphere and snowpack.

Description: Long-term simulations from the Community Earth System Model with the Whole Atmosphere Community Climate Model (CESM-WACCM) as its atmospheric component are used to investigate the asymmetry and the nonlinearity of the influences of El Niño–Southern Oscillation (ENSO) on the northern winter stratosphere. As in Part I of this study, four different types of ENSO are considered. The composite CESM-WACCM results first confirm the conclusions drawn from the observations, that the stratospheric polar jet responses to “moderate El Niño” and “strong La Niña” are stronger than those to “strong El Niño” and “moderate La Niña”. In association with the ENSO sea surface temperature (SST) patterns that are reproduced well in the model, the tropical rainfall response centers exhibit an asymmetric east–west shift between El Niño and La Niña, which directly leads to the nonlinear and asymmetric PNA responses in the extratropics. Accordingly, the strengthening (weakening) planetary wave response in the stratosphere during warm (cold) ENSO also exhibits nonlinear and asymmetric features. When the ENSO SST forcing is prescribed to be linear and symmetric in WACCM, the nonlinearity and asymmetry of the stratospheric responses to moderate ENSO reveal the dominant role of the inherent properties of the atmosphere. However, the absence of asymmetry and nonlinearity in the stratospheric responses to strong ENSO in our sensitivity experiments indicates that the asymmetry in SST forcing between strong El Niño and La Niña still plays an important role in the asymmetric and nonlinear influences of ENSO on the extratropics.

Description: Shortwave Infrared (SWIR) imaging spectroscopy enables accurate remote mapping of cloud thermodynamic phase at high spatial resolution. We describe a measurement strategy to exploit signatures of liquid and ice absorption in cloud top apparent reflectance spectra from 1.4-1.8  μ m. This signal is generally insensitive to confounding factors such as solar angles, view angles, and surface albedo. We first evaluate the approach in simulation, and then apply it to airborne data acquired in the Calwater-2 / ACAPEX campaign of Winter 2015. Here, NASA's “Classic” Airborne Visible Infrared Imaging Spectrometer (AVIRIS-C) remotely observed diverse cloud formations while the US Department of Energy ARM Aerial Facility (AAF) G-1 aircraft measured cloud integral and microphysical properties in situ . The coincident measurements demonstrate good separation of the thermodynamic phases for relatively homogeneous clouds.

Description: Surface based temperature inversions (SBIs) occur frequently over Antarctica and play an important role in climate and weather. Antarctic SBIs are examined during Austral spring, 2010 using measurements from dropsondes, ERA-Interim Atmospheric Reanalysis Model, and the recently released version 6 of the Infrared Atmospheric Sounding Interferometer (IASI) level 2 product. A SBI detection algorithm is applied to temperature profiles from these datasets. The results will be used to determine if satellite and reanalysis products can accurately characterize SBIs and if so, then they may be used to study SBIs outside of the spring 2010 study period. From the dropsonde data, SBIs occurred in 20% of profiles over sea ice and 54% of profiles over land. IASI and ERA-Interim surface air temperatures are found to be significantly warmer than dropsonde observations at high plateau regions, while IASI surface air temperature is colder over sea ice. IASI and ERA-Interim have a cold bias at nearly all levels above the surface when compared to the dropsonde. SBIs are characterized by their frequency, depth, and intensity. It is found that SBIs are more prevalent, deeper, and more intense over the continent than over sea ice, especially at higher surface elevations. Using IASI and ERA-Interim data the detection algorithm has a high probability of detection of SBIs but is found to severely overestimate the depth and underestimate the intensity for both data sets. These over- and underestimations are primarily due to the existence of extremely shallow inversion layers that neither satellite nor reanalysis products can resolve.

Description: Recent warming of the Antarctic Peninsula during austral autumn, winter, and spring has been linked to sea surface temperature (SST) trends in the tropical Pacific and tropical Atlantic, while warming of the northeast Peninsula during summer has been linked to a strengthening of westerly winds traversing the Peninsula associated with a positive trend in the Southern Annular Mode (SAM). Here we demonstrate that circulation changes associated with the SAM dominate interannual temperature variability across the entire Antarctic Peninsula during both summer and autumn, while relationships with tropical Pacific SST variability associated with the El Niño-Southern Oscillation (ENSO) are strongest and statistically significant primarily during winter and spring only. We find the ENSO-Peninsula temperature relationship during autumn to be weak on interannual timescales, and regional circulation anomalies associated with the SAM more important for interannual temperature variability across the Peninsula during autumn. Consistent with previous studies, western Peninsula temperatures during autumn, winter, and spring are closely tied to changes in the Amundsen Sea Low (ASL) and associated meridional wind anomalies. The interannual variability of ASL depth is most strongly correlated with the SAM index during autumn, while the ENSO relationship is strongest during winter and spring. Investigation of western and northeast Peninsula temperatures separately reveals that interannual variability of northeast Peninsula temperatures is primarily sensitive to zonal wind anomalies crossing the Peninsula and resultant lee-side adiabatic warming rather than to meridional wind anomalies, which is closely tied to variability in the zonal portion of the SAM pattern.

Description: We use a 0-D photochemical box model and a 3-D global chemistry-climate model, combined with observations from the NOAA Southeast Nexus (SENEX) aircraft campaign, to understand the sources and sinks of glyoxal over the Southeast United States. Box model simulations suggest a large difference in glyoxal production among three isoprene oxidation mechanisms (AM3ST, AM3B, and MCM v3.3.1). These mechanisms are then implemented into a 3-D global chemistry-climate model. Comparison with field observations shows that the average vertical profile of glyoxal is best reproduced by AM3ST with an effective reactive uptake coefficient γ glyx of 2 × 10 -3 , and AM3B without heterogeneous loss of glyoxal. The two mechanisms lead to 0-0.8 µg m -3 secondary organic aerosol (SOA) from glyoxal in the boundary layer of the Southeast U.S. in summer. We consider this to be the lower limit for the contribution of glyoxal to SOA, as other sources of glyoxal other than isoprene are not included in our model. In addition, we find that AM3B shows better agreement on both formaldehyde and the correlation between glyoxal and formaldehyde ( R GF  = [GLYX]/[HCHO]), resulting from the suppression of δ-isoprene peroxy radicals (δ-ISOPO 2 ). We also find that MCM v3.3.1 may underestimate glyoxal production from isoprene oxidation, in part due to an underestimated yield from the reaction of IEPOX peroxy radicals (IEPOXOO) with HO 2 . Our work highlights that the gas-phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.

Description: Based on the results from eleven flux sites during the third Tibetan Plateau (TP) Experiment (TIPEX III), land surface parameters and the turbulence characteristics of the atmospheric surface layer over the TP and surrounding region are analyzed. Monin-Obukhov similarity theory has been used to calculate the aerodynamic roughness length z 0m and the excess resistance to heat transfer kB − 1  = ln( z 0 m / z 0 h ), and the factors that cause variations of z 0m and kB − 1 are investigated. The main sdrivers for the diurnal variations of surface albedo( α ) at different sites are solar elevation, solar radiation, and soil moisture. The eddy correlation method is utilized to inversely calculate bulk transfer coefficients for momentum ( C D ) and heat ( C H ) at different sites. The relationships between C D and C H , and the wind speed at 10 m follow a power law for unstable stratification. For stable stratification, both C D and C H increase with increasing wind speed when wind speed is less than 5 m/s. Diurnal variations of turbulent fluxes are compared at different sites, and the relationships between turbulent fluxes and other variables are analyzed. Wind speed variance normalized by the friction velocity ( σ u / u * ,  σ v / u * ,  σ w / u * ) for neutral stratification ( C u1 , C v1 , C w1 ), and temperature and humidity variance normalized by a temperature and humidity scale ( σ T / T * ,  σ q / q * ) under free convection ( z/L  〈 -0.1) ( C T , C q ) are fitted with similarity relations. The differences in similarity constants ( C u1 , C v1 , C w1 , C T , C q ) at different sites are discussed. For stable stratification, cases are divided into weakly stable conditions and intermittent turbulence, and the critical values for these two states are determined. Shear and buoyancy terms in the turbulence kinetic energy (TKE) equation for different stratifications are analyzed.

Description: Contrail cirrus is the largest known component contributing to the radiative forcing associated with aviation. Despite major advances simulating contrail cirrus, their microphysical and optical properties and the associated radiative forcing remain largely uncertain. We use a contrail cirrus parameterization in a global climate model which was extended to include a microphysical two-moment scheme. This allows a more realistic representation of microphysical processes, such as deposition and sedimentation, and therefore of the microphysical and optical properties of contrail cirrus. The simulated contrail microphysical and optical properties agree well with in situ and satellite observations. As compared to estimates using an older version of the contrail cirrus scheme, the optical depth of contrail cirrus is significantly higher, particularly in regions with high air traffic density, due to high ice crystal number concentrations on the main flight routes. Nevertheless, the estimated radiative forcing for the year 2002 supports our earlier results. The global radiative forcing of contrail cirrus for the year 2006 is estimated to be 56mW/m 2 . A large uncertainty of the radiative forcing estimate appears to be connected with the, on average, very small ice crystal radii simulated in the main air traffic areas, which make the application of a radiative transfer parameterization based on geometric optics questionable.

Description: The Lake Siling Co is currently the largest endorheic lake in Tibet and the lake surface area has expanded by about 40% since the 1970s, with a remarkable acceleration after 1999. In this study, a hydrologic modeling framework was established by linking the Variable Infiltration Capacity (VIC) land surface hydrologic model with the degree-day glacier-melt model over the Lake Siling Co basin, with the aim to quantify the contribution of each runoff component to changes in the lake storage. We found that glacier melt contributed to less than 10% of the total water input to the lake during 1979–2013, while precipitation-induced runoff in non-glacierized area was responsible for about 67–75%. The mean annual water input to the lake increased by 2.15 × 10 9 m 3 yr −1 in 2000–2013 relative to that in 1979–1999. The amount of precipitation over the lake surface, precipitation-induced runoff, and glacier-melt runoff accounted for 13%, 82% and 5% of this total increase, respectively, suggesting that the substantial expansion of Siling Co in the 2000s was mostly due to the increase in precipitation-induced runoff. When modeling lake level changes during 1979–2013, we found that the water level rose by 14.1 m when glacier melt was included and only 10.5 m, a reduction of about 1/4, when glacier melt was removed. It is concluded that glacier melt played an important role in controlling the water level of Siling Co, although it only contributed less than 10% of water input to the lake during 1979–2013.

Description: High elevations are thought to be warming more rapidly than lower elevations, but there is a lack of air temperature observations in high mountains. This study compares instantaneous values of land surface temperature (1030/2230 and 0130/1330 local solar time) as measured by MODIS MOD11A2/MYD11A2 at 1 km resolution from the TERRA and AQUA platforms respectively with equivalent screen level air temperatures (in the same pixel). We use a transect of 22 in situ weather stations across Kilimanjaro ranging in elevation from 990 to 5803 m, one of the biggest elevational ranges in the world. There are substantial differences between LST and T air , sometimes up to 20˚C. During the day/night LST tends to be higher/lower than T air . LST-T air differences (ΔT) show large variance, particularly during the daytime, and tend to increase with elevation, particularly on the NE slope which faces the morning sun. Differences are larger in the dry seasons (JF and JJAS), and reduce in cloudy seasons. Healthier vegetation (as measured by NDVI) and increased humidity lead to reduced daytime surface heating above air temperature and lower ΔT, but these relationships weaken with elevation. At high elevations transient snow cover cools LST more than T air . The predictability of ΔT therefore reduces. It will therefore be challenging to use satellite data at high elevations as a proxy for in situ air temperatures in climate change assessments, especially for daytime T max. ΔT is smaller and more consistent at night, so it will be easier to use LST to monitor changes in T min.