ALBERT

Beschreibung: Continued advancement in the realm of tropical cyclone (TC) forecasting requires a more accurate depiction of these storms at model initialization. This study examines the impact of precipitation assimilation on the representation of TCs in the North American Regional Reanalysis (NARR) before and after the 2004 introduction of precipitation assimilation over ocean in the vicinity of TCs. The probability distribution function of rainfall rates indicates that light (heavy) precipitation was overforecast (underforecast) in the early time period. Since the precipitation assimilation is applied through an adjustment to the latent heating distribution, the data assimilation system in the later time period initializes a low-level moisture and heating profile that is more conducive to the initiation of deep convection and the generation of precipitation. Consequently, the deep convection and enhanced latent heat release lead to a more robust warm core temperature perturbation and a better developed secondary circulation, which supplies the TC with larger quantities of moisture from the large-scale environment. Furthermore, the evolution of TC size, which was objectively estimated though the radius of outermost closed isobar (ROCI), is significantly more skillful ( p 〈0.05) in post-2003 storms. Based on this study, precipitation assimilation leads to a better analysis of temperature, winds, and moisture in the vicinity of TCs, resulting in improved representations of the water budget and storm lifecycle. Therefore, we conclude that efforts towards the development of precipitation assimilation techniques from radar and satellite datasets will be valuable toward the construction of improved TC forecasting tools with more authentic TC representation.

Beschreibung: Six years of aerosol size distribution measurements between 20 and 600 nm diameters and total aerosol concentration above 10 nm from March 2008 to February 2014 at the high-alpine site Jungfraujoch are presented. The size distribution was found to be typically bimodal with mode diameters and widths relatively stable throughout the year and the observation period. New particle formation was observed on 14.5% of all days without a seasonal preference. Particles typically grew only into the Aitken mode and did not reach cloud condensation nuclei (CCN) sizes on the time scale of several days. Growth of pre-existing particles in the Aitken mode, on average, contributed very few CCN. We concluded that the dominant fraction of CCN at Jungfraujoch originated in the boundary layer. A number of approaches were used to distinguish free tropospheric (FT) conditions and episodes with planetary boundary layer (PBL) influence. In the absence of PBL injections, the concentration of particles larger than 90 nm ( N 90 , roughly corresponding to the CCN concentration) reached a value ~40 cm −3 while PBL influence caused N 90 concentrations of several hundred or even 1000 cm −3 . Comparing three criteria for free tropospheric conditions, we found FT prevalence for 39% of the time with over 60% during winter and below 20% during summer. It is noteworthy that a simple criterion based on standard trace gas measurements appeared to outperform alternative approaches.

Beschreibung: Tropical convection has been observed to contain three cloud modes, the middle of which is cumulus congestus clouds. Congestus clouds act to moisten the tropical atmosphere, may be mixed-phase, and on occasion surpass the freezing level inversion from where they may develop into deeper convection. This study investigates the impacts of enhanced aerosol concentrations on the growth of congestus clouds produced in idealized cloud-resolving model simulations run under a state of radiative convective equilibrium (RCE). High resolution, long duration simulations were completed using the Regional Atmospheric Modeling System (RAMS). Aerosol concentrations between 2 and 4 km AGL were varied from clean to polluted conditions in order to represent the advection of Saharan dust over the Atlantic Ocean. The congestus populations within each aerosol simulation are statistically analyzed using ten days of model output after the simulation reaches RCE. Results indicate that congestus in more polluted conditions produce greater amounts of cloud water and ice mass, enhanced updraft strengths and an increase in the number of congestus cloud tops that extend above the freezing level. Enhanced vapor depositional growth on the populations of more numerous, smaller cloud droplets in the polluted conditions, and the subsequent increase in latent heat release in the warm phase regions of the cloud, are found to be important factors in convective invigoration of these cloud systems. Aerosol feedbacks associated with cold pools and condensate loading also influence the updraft strength, and act in opposition to the warm phase invigoration processes.

Beschreibung: With water vapor and clouds expected to effect significant feedbacks on climate, moisture transport through convective processes has important implications for future temperature change. The precipitation efficiency—the ratio of the rates at which precipitation and condensation form ( e  =  P / C )—is useful for characterizing how much boundary layer moisture recycles through precipitation versus mixes into the free troposphere through cloud detrainment. Yet, it is a difficult metric to constrain with traditional observational techniques. This analysis characterizes the precipitation efficiency of convection near the Big Island of Hawaii, USA, using a novel tracer: isotope ratios in water vapor. The synoptic circulation patterns associated with high and low precipitation efficiency are identified, and the importance of large-scale dynamics and local convective processes in regulating vertical distributions of atmospheric constituents important for climate is evaluated. The results suggest high e days are correlated with plume-like transport originating from the relatively clean tropics, while low e days are associated with westerly transport, generated by a branching of the jet stream. Differences in transport pathway clearly modify background concentrations of water vapor and other trace gases measured at Mauna Loa Observatory; however, local convective processes appear to regulate aerosols there. Indeed, differences between observed and simulated diurnal cycles of particle number concentration indicate precipitation scavenges aerosols and possibly facilitates new particle formation when e is high. As measurements of isotope ratios in water vapor expand across the subtropics, the techniques presented here can further our understanding of how synoptic weather, precipitation processes, and climate feedbacks interrelate.

Beschreibung: This study evaluates the ability of the Community Atmospheric Model version 5 (CAM5) to reproduce low clouds observed by the Atmospheric Radiation Measurement (ARM) cloud radar at Manus Island of the tropical western Pacific during the Years of Tropical Convection (YOTC). Here, low clouds are defined as clouds with their tops below the freezing level and bases within the boundary layer. Low-cloud statistics in CAM5 simulations and ARM observations are compared in terms of their general occurrence, mean vertical profiles, fraction of precipitating vs. non-precipitating events, diurnal cycle, and monthly time series. Other types of clouds are included to put the comparison in a broader context. The comparison shows that the model overproduces total clouds and their precipitation fraction but underestimates low clouds in general. The model, however, produces excessive low clouds in a thin layer between 954–930 hPa, which coincides with excessive humidity near the top of the mixed layer. This suggests that the erroneously excessive low clouds stem from parameterization of both cloud and turbulence mixing. The model also fails to produce the observed diurnal cycle in low clouds, not exclusively due to the model coarse grid spacing that does not resolve Manus Island. This study demonstrates the utility of ARM long-term cloud observations in the tropical western Pacific in verifying low clouds simulated by global climate models, illustrates issues of using ARM observations in model validation, and provides an example of severe model biases in producing observed low clouds in the tropical western Pacific.

Beschreibung: Solar influences on spatial patterns of Eurasian winter climate and possible mechanisms are investigated based on a multiple linear regression method, multi-sources observational and reanalysis data. Robust and significant solar signals are detected in Eurasian surface air temperature (SAT) and strong solar activity evidently warms most area of the continent. The spatial pattern of sea level pressure (SLP) responses to solar activity is similar but not identical to that of the North Atlantic Oscillation (NAO). Compared to the NAO, geographic distribution of solar-induced SLP anomalies shifts eastward, with significantly enhanced influences over northern Eurasia. Relatively weaker solar signals were also found in mid-to-upper troposphere. The spatial pattern of 500 hPa geopotential anomalies resembles a negative Scandinavia Teleconnection pattern and the 200 hPa subtropical jet is weakened, while zonal wind at high latitudes is enhanced due to strong solar activity. The anomalous zonal circulations can be attributed to the “top-down” mechanism. During high solar activity winters, an enhanced stratospheric zonal wind anomaly propagates downward, causing zonal wind anomalies in the troposphere. However, the “bottom-up” mechanisms may provide more reasonable explanations of the distinct solar influences on Eurasian climate. Solar-induced strong warm advection in lower atmosphere tends to increase SAT but decrease SLP, resulting in enhanced solar influences over northern Eurasia. Meanwhile, change in the land-ocean thermal contrast (LOTC) could also amplify the circulation anomaly. Inhomogeneous surface heating caused by anomalous solar activity modifies LOTC, which probably enhances the solar-induced circulation patterns. Such a positive feedback may potentially strengthen the solar influences.

Beschreibung: This study reports the chemical composition of particles present along Greenland's North Greenland Eemian Ice Drilling (NEEM) ice core, back to 110,000 years before present. Insoluble and soluble particles larger than 0.45 µm were extracted from the ice core by ice sublimation and their chemical composition was analyzed using scanning electron microscope and energy dispersive X-ray spectroscopy and micro-Raman spectroscopy. We show that the dominant insoluble components are silicates, whereas NaCl, Na 2 SO 4 , CaSO 4 , and CaCO 3 represent major soluble salts. For the first time, particles of CaMg(CO 3 ) 2 and Ca(NO 3 ) 2 •4H 2 O are identified in a Greenland ice core. The chemical speciation of salts varies with past climatic conditions. Whereas the fraction of Na-salts (NaCl + Na 2 SO 4 ) exceeds that of Ca-salts (CaSO 4 + CaCO 3 ) during the Holocene (0.6–11.7 kyr BP), the two fractions are similar during the Bølling-Allerød period (12.9–14.6 kyr BP). During cold climate such as over the Younger Dryas (12.0–12.6 kyr BP) and the Last Glacial Maximum (15.0–26.9 kyr BP), the fraction of Ca-salts exceeds that of Na-salts, showing that the most abundant ion generally controls the salt budget in each period. High-resolution analyses reveal changing particle compositions: those in Holocene ice show seasonal changes, and those in LGM ice show a difference between cloudy bands and clear layers, which again can be largely explained by the availability of ionic components in the atmospheric aerosol body of air masses reaching Greenland.

Beschreibung: Characterizing black carbon (BC) concentrations in the seasonal snowpack is of interest because BC deposition on snow can reduce albedo and accelerate melt. In Washington State, USA snowmelt from the seasonal snowpack provides an important source of water resources, but minimal work has been done characterizing BC concentrations in snow in this region. BC concentrations in snow were monitored over two winters (2012 and 2013) at Tronsen Meadow, located near Blewett Pass in the eastern Cascade Mountains in Central Washington, to characterize spatial and temporal variations in BC concentrations, and the processes affecting BC concentrations in the snowpack. BC concentrations were measured using a Single Particle Soot Photometer (SP2). Snowpit BC concentrations at spatial scales ranging from cm to 100 m scales were fairly homogenous during the accumulation season, with greater spatial variability during the melt season due to variable melt patterns. BC concentrations in snow increased in late winter-spring due to an increase in atmospheric BC concentrations and trapping of BC on the snow surface during melt. However, during a period of intense melt in 2013 BC concentrations decreased, likely caused by melt water scavenging. In summer 2012 the Table Mountain forest fire burned adjacent to the study site, and BC concentrations in the snowpack in 2013 were far higher than in previous years, with charred trees post-fire the likely source of the elevated BC.

Beschreibung: The interactions between idealized binary tropical cyclones (TCs) on f - and β -planes with different separation distance and thermodynamic soundings obtained from the NCEP/NCAR reanalysis data averaged over the western North Pacific (WNP) are investigated through ensemble three-dimensional numerical simulations with a horizontal resolution of 10 km in a single domain. In the simulations on the f -plane, two TCs show mutual cyclonic rotations with symmetric structures. Two TCs with thermodynamic profiles of larger convective available potential energy (CAPE) and maximum potential intensity (MPI) show greater interaction than those with a smaller CAPE and MPI due to the stronger tangential velocity near the TC center. In the simulations on the β -plane, the two TCs do not merge, because the beta effect prevents the attraction of the two TCs by generating asymmetric motions of the TC with northwestward forcing. The relative strengths of the two TCs change with time and depend on the low-level inflow influenced by the Coriolis parameter. Similar to the results on the f -plane, the two TCs only merge with the thermodynamic soundings of large CAPE and MPI.

Beschreibung: The tropical rain belt is a narrow band of clouds near the equator, where the most intense rainfall on the planet occurs. On seasonal timescales, the rain moves across the equator following the sun, resulting in wet and dry seasons in the tropics. The position of the tropical rain belt also varies on longer time scales. Through the latter half of the 20th century, for example, shifts in tropical rainfall have been associated with severe droughts, including the African Sahel and Amazon droughts. Here, I show that climate models project a northward migration of the tropical rain belt through the 21st century, with future anthropogenic aerosol reductions driving the bulk of the shift. Models that include both aerosol indirect effects yield significantly larger northward shifts than models that lack aerosol indirect effects. Moreover, the rate of the shift corresponds to the rate of the decrease of anthropogenic aerosol emissions across different time periods and future emission scenarios. This response is consistent with relative warming of the Northern Hemisphere, a decrease in northward cross-equatorial moist static energy transport, and a northward shift of the Hadley circulation, including the tropical rain belt. The shift is relatively weak in the Atlantic sector, consistent with both a smaller decrease in aerosol emissions and a larger reduction in northward cross equatorial ocean heat flux. Although aerosol effects remain uncertain, I conclude that future reductions in anthropogenic aerosol emissions may be the dominant driver of a 21st century northward shift of the tropical rain belt.

Beschreibung: Aerosol deposition over the Southern Ocean and Antarctica has the potential to alter marine productivity and thus ocean carbon uptake while also impacting radiative balance due to scattering and absorption from atmospheric particulates. Quantification of modern emission, transport and deposition of terrestrial dust and other airborne material from Southern Hemisphere sources is challenging due to low emission levels and poor detection from remote sensing platforms. Here, forward trajectory modeling is used to explore atmospheric transport, independent of deposition processes, from 1979 to 2013. Trajectories are initiated from known arid dust source areas in South America (Patagonia), Australia, and southern Africa, with detailed consideration of New Zealand as a potential source. Results suggest that Patagonian and New Zealand dust and other aerosol emissions share strong atmospheric transport during all seasons, allowing even potentially small New Zealand emissions to contribute significantly to Southern Ocean and Antarctic aerosol loading. We find that atmospheric transport controlling distribution of dust and other aerosols shows distinct spatial variability. New Zealand and Patagonia rapidly contribute a high proportion of trajectories to West Antarctica, while in interior East Antarctica source contributions are limited and highly mixed. The sensitivity of existing deep ice core sites to modern atmospheric transport is discussed. Finally, interannual variability of poleward trajectory extension over the Pacific and Atlantic sectors of the Southern Ocean highlights the association of both tropical Pacific sea-surface temperature and high-latitude wind variability (e.g. the Southern Annular Mode) with transport of dust and other aerosols to the Southern Ocean and Antarctica.

Beschreibung: We show the results and evaluation with independent measurements from assimilating both MOPITT (Measurements Of Pollution In The Troposphere) and IASI (Infrared Atmospheric Sounding Interferometer) retrieved profiles into the Community Earth System Model (CESM). We used the DART (Data Assimilation Research Testbed) ensemble Kalman Filter technique, with the full atmospheric chemistry CESM component CAM-chem (Community Atmospheric Model with Chemistry). We first discuss the methodology and evaluation of the current data assimilation system with coupled meteorology and chemistry data assimilation. The different capabilities of MOPITT and IASI retrievals are highlighted, with particular attention to instrument vertical sensitivity and coverage and how these impact the analyses. MOPITT and IASI CO retrievals mostly constrain the CO fields close to the main anthropogenic, biogenic and biomass burning CO sources. In the case of IASI CO assimilation, we also observe constraints on CO far from the sources. During the simulation time period (June and July 2008), CO assimilation of both instruments strongly improves the atmospheric CO state as compared to independent observations, with the higher spatial coverage of IASI providing better results on the global scale. However, the enhanced sensitivity of multispectral MOPITT observations to near surface CO over the main source regions provides synergistic effects at regional scales.

Beschreibung: Processes that influence the humidity and cirrus cloud abundance in the Tropical Tropopause Layer (TTL) during boreal winter 2006-07 are investigated in simulations of clouds along backward trajectories of parcels ending at the 372 K potential temperature (100 hPa) level in the tropics. Trajectories are calculated using offline calculations of seasonal mean tropical radiative heating rates along with reanalysis temperature and wind data with enhanced wave-driven variability in the TTL. The one-dimensional (vertical) time-dependent cloud microphysical model is initialized with water vapor measurements from the Microwave Limb Sounder and the evolution of clouds along each trajectory is simulated using temperature profiles extracted from reanalysis data and convective cloud top heights estimated from 3-hourly geostationary satellite imagery. Averaged over the tropics, waves dehydrate the 100 hPa level by 0.5 ppmv, while convection and cloud microphysical processes moisten by 0.3 and 0.7 ppmv, respectively. The tropical mean cloud occurrence frequencies in the mid to upper TTL agree well with those based on satellite observations (spatial correlation of 0.8). Waves and convection enhance cloud occurrence at the cold point tropopause by 4% and 2%, respectively. Temporal variability of the heating rates as indicated by the ERA-Interim 6-hourly heating rate fields dehydrates the TTL by 0.4 ppmv and decreases the cloud occurrence by 4% because parcels are more likely to encounter the coldest temperatures and dehydrate near the cold point, limiting cloud formation above. The final dehydration locations of parcels, concentrated near the dateline in the tropical Pacific, are insensitive to various model parameters.

Beschreibung: Increasing optical depth poleward of 45° is a robust response to warming in global climate models. Much of this cloud optical depth increase has been hypothesized to be due to transitions from ice-dominated to liquid-dominated mixed-phase cloud. In this study, the importance of liquid-ice partitioning for the optical depth feedback is quantified for 19 CMIP5 models. All models show a monotonic partitioning of ice and liquid as a function of temperature, but the temperature at which ice and liquid are equally mixed (the glaciation temperature) varies by as much as 40K across models. Models that have a higher glaciation temperature are found to have a smaller climatological liquid water path (LWP) and condensed water path, and experience a larger increase in LWP as the climate warms. The ice-liquid partitioning curve of each model may be used to calculate the response of LWP to warming. It is found that the re-partitioning between ice and liquid in a warming climate contributes at least 20% to 80% of the increase in LWP as the climate warms, depending on model. Inter-model differences in the climatological partitioning between ice and liquid are estimated to contribute at least 20% to the inter-model spread in the high-latitude LWP response in the mixed-phase region poleward of 45°S. It is hypothesized that a more thorough evaluation and constraint of GCM mixed-phase cloud parameterizations, and validation of the total condensate and ice-liquid apportionment against observations will yield a substantial reduction in model uncertainty in the high-latitude cloud response to warming.

Beschreibung: In this study, we adopt a parametric sensitivity analysis framework that integrates the quasi-Monte Carlo parameter sampling approach and a surrogate model to examine aerosol effects on the East Asian Monsoon climate simulated in the Community Atmosphere Model (CAM5). A total number of 256 CAM5 simulations are conducted to quantify the model responses to the uncertain parameters associated with cloud microphysics parameterizations and aerosol (e.g., sulfate, black carbon (BC), and dust) emission factors and their interactions. Results show that the interaction terms among parameters are important for quantifying the sensitivity of fields of interest, especially precipitation, to the parameters. The relative importance of cloud-microphysics parameters and emission factors (strength) depends on evaluation metrics or the model fields we focused on, and the presence of uncertainty in cloud microphysics imposes an additional challenge in quantifying the impact of aerosols on cloud and climate. Due to their different optical and microphysical properties and spatial distributions, sulfate, BC, and dust aerosols have very different impacts on East Asian Monsoon through aerosol-cloud-radiation interactions. The climatic effects of aerosol do not always have a monotonic response to the change of emission factors. The spatial patterns of both sign and magnitude of aerosol-induced changes in radiative fluxes, cloud, and precipitation could be different, depending on the aerosol types, when parameters are sampled in different ranges of values. We also identify the different cloud microphysical parameters that show the most significant impact on climatic effect induced by sulfate, BC and dust, respectively, in East Asia.

Beschreibung: Earth radiation management has been suggested as a way to rapidly counteract global warming in the face of a lack of mitigation efforts, buying time and avoiding potentially catastrophic warming. We compare six different radiation management schemes that use surface, troposphere and stratosphere interventions in a single climate model in which we projected future climate from 2020 to 2099 based on RCP4.5. We analyze the surface air temperature responses to determine how effective the schemes are at returning temperature to its 1986-2005 climatology and analyze precipitation responses to compare side effects. We find crop albedo enhancement is largely ineffective at returning temperature to its 1986-2005 climatology. Desert albedo enhancement causes excessive cooling in the deserts and severe shifts in tropical precipitation. Ocean albedo enhancement, sea-spray geoengineering, cirrus cloud thinning and stratospheric SO 2 injection have the potential to cool more uniformly, but cirrus cloud thinning may not be able to cool by much more than 1 K globally. We find that of the schemes potentially able to return surface air temperature to 1986-2005 climatology under future greenhouse gas warming, none has significantly less severe precipitation side effects than other schemes. Despite different forcing patterns, ocean albedo enhancement, sea-spray geoengineering, cirrus cloud thinning and stratospheric SO 2 injection all result in large scale tropical precipitation responses caused by Hadley cell changes and land precipitation changes largely driven by thermodynamic changes. Widespread regional scale changes in precipitation over land are significantly different from the 1986-2005 climatology and would likely necessitate significant adaptation despite geoengineering.

Beschreibung: The wind velocity structure in the upper stratosphere, mesosphere and lower thermosphere (MLT) is studied with the recently developed method of infrasound probing of the atmosphere. The method is based on the effect of infrasound scattering from highly anisotropic wind velocity and temperature inhomogeneities in the middle and upper atmosphere. The scattered infrasound field propagates in the acoustic shadow zones, where it is detected by microbarometers. The vertical profiles of the wind velocity fluctuations in the upper stratosphere (30–52 km) and MLT (90–140 km) are retrieved from the wave forms and travel times of the infrasound signals generated by explosive sources such as volcanoes and surface explosions. The fine-scale wind layered structure in these layers was poorly observed until present time by other remote sensing methods, including radars and satellites. It is found that the MLT atmospheric layer (90–102 km) can contain extremely high vertical gradients of the wind velocity, up to 10 m/s per 100 m. The effect of a fine-scale wind velocity structure on the wave forms of infrasound signals is studied. The vertical wavenumber spectra of the retrieved wind velocity fluctuations are obtained for the upper stratosphere. Despite the difference in the locations of the explosive sources all the obtained spectra show the existence of high vertical wavenumber spectral tail with a −3 power law decay. The obtained spectral characteristics of the wind fluctuations are necessary for improvement of gravity wave drag parametrizations for numerical weather forecast.

Beschreibung: We have computed lightning electromagnetic pulses (LEMPs), including the azimuthal magnetic field H ϕ , vertical electric field E z , and horizontal (radial) electric field E h that propagated over 5 to 200 km of flat lossy ground, using the finite-difference time-domain (FDTD) method in the 2D cylindrical coordinate system. This is the first systematic full-wave study of LEMP propagation effects based on a realistic return-stroke model and including the complete return-stroke frequency range. Influences of the return-stroke wavefront speed (ranging from c /2 to c , where c is the speed of light), current risetime (ranging from 0.5 to 5 µs), and ground conductivity (ranging from 0.1 mS/m to ∞) on H ϕ , E z , and E h have been investigated. Also, the FDTD-computed waveforms of E h have been compared with the corresponding ones computed using the Cooray-Rubinstein formula. Peaks of H ϕ , E z , and E h are nearly proportional to the return-stroke wavefront speed. The peak of E h decreases with increasing current risetime, while those of H ϕ and E z are only slightly influenced by it. The peaks of H ϕ and E z are essentially independent of the ground conductivity at a distance of 5 km. Beyond this distance, they appreciably decrease relative to the perfectly conducting ground case, and the decrease is stronger for lower ground conductivity values. The peak of E h increases with decreasing ground conductivity. The computed E h /E z is consistent with measurements of Thomson et al. [1988]. The observed decrease of E z peak and increase of E z risetime due to propagation over 200 km of Florida soil [ Uman et al. 1976; Lin et al. 1979] are reasonably well reproduced by the FDTD simulation with ground conductivity of 1 mS/m.

Beschreibung: This study investigates the impact of temperature and moisture profiles from Atmospheric Infrared Sounder (AIRS) on the prediction of the Indian Summer Monsoon, using the variational data assimilation system annexed to the Weather Research and Forecasting (WRF) model. In this study, three numerical experiments are carried out. The first is the control (CTRL) and includes no assimilation; in the second, named Conv, assimilation of conventional Global Telecommunication System data is performed. The third one, named ConvAIRS, is identical to the Conv except that it also includes assimilation of AIRS profiles. The initial fields of tropospheric temperature and water vapour mixing ratio showed significant improvement over the model domain. Assimilation of AIRS profiles has significant impact on predicting the seasonal mean monsoon characteristics such as tropospheric temperature, low level moisture distribution, easterly wind shear and precipitation. The vertical structure of the root mean square error (RMSE) is substantially affected by the assimilation of AIRS profiles, with smaller errors in temperature, humidity and wind magnitude. The consequent improved representation of moisture convergence in the boundary layer (and deep convection as well) causes an increase in precipitation forecast skill. The fact that the monsoonal circulation is better captured, thanks to an improved representation of thermal gradients, which in turn leads to more realistic moisture transport, is particularly noteworthy. Several previous data impact studies with AIRS and other sensors have focused on the short- or medium-range of the forecast. The demonstrated improvement in all the predicted fields associated with the Indian Summer Monsoon, consequent to the assimilation of AIRS profiles, is an innovative finding with large implications to the operational seasonal forecasting capabilities over the Indian subcontinent.

Beschreibung: The oceans cover most of the Earth's surface, contain nearly half the total global primary biomass productivity, and are a major source of atmospheric aerosol particles. Here we experimentally investigate links between biological activity in seawater and sea spray aerosol (SSA) flux, a relationship of potential significance for organic aerosol loading and cloud formation over the oceans, and thus for climate globally. Bubbles were generated in laboratory mesocosm experiments either by recirculating impinging water jets or glass frits. Experiments were conducted with Atlantic Ocean seawater collected off the eastern end of Long Island, NY, and with artificial seawater containing cultures of bacteria and phytoplankton Thalassiosira pseudonana , Emiliania huxleyi , and Nannochloris atomus . Changes in SSA size distributions occurred during all phases of bacterial and phytoplankton growth, as characterized by cell concentrations, dissolved organic carbon (DOC), total particulate carbon (TPC), and transparent exopolymer particles (TEP, gel-forming polysaccharides representing a major component of biogenic exudate material). Over a 2 week growth period, SSA particle concentrations increased by a factor of less than 2 when only bacteria were present and by a factor of about 3 when bacteria and phytoplankton were present. Production of jet generated SSA particles of diameter less than 200 nm increased with time, while production of all particle diameters increased with time when frits were used. The implications of a marine biological activity dependent SSA flux are discussed.

Beschreibung: The OSIRIS instrument on the Odin satellite, launched in 2001 and currently operational, measures limb scattered sunlight from which profiles of stratospheric aerosol extinction are retrieved. SAGE II was launched in 1984 and provided measurements of stratospheric aerosol extinction until mid 2005. This provides approximately four years of mission overlap which has allowed us to consistently extend the SAGE II version 7.00 record to the present using OSIRIS aerosol extinction retrievals. In this work we first compare coincident aerosol extinction observations during the overlap period by interpolating the SAGE II 525nm and 1020nm channels to the OSIRIS extinction wavelength of 750nm. In the tropics to mid latitudes mean differences are typically less than 10%, although larger biases are seen at higher latitudes and at altitudes outside the main aerosol layer. OSIRIS aerosol extinction retrievals at 750nm are used to create a monthly time series zonally averaged in 5 degree bins and qualitatively compared to SAGE II 525nm observations averaged in the same way. The OSIRIS time series is then translated to 525nm with an Angstrom exponent relation and bias corrected. For most locations, this provides agreement during the overlap time period to better than 15%. Uncertainty in the resulting OSIRIS time series is estimated through a series of simulation studies over the range of aerosol particle size distributions observed by in-situ balloon instruments, and is found to be approximately 20% for background and moderately volcanic aerosol loading conditions for the majority of OSIRIS measurement conditions.

Beschreibung: An intensive field campaign was conducted on Deokjeok Island off the west coast of the Korean Peninsula during spring 2009 to characterize the optical and hygroscopic properties of Asian continental outflows. A slightly high wavelength dependence of light absorption coefficient, α of 1.6 ± 0.05 (average ± 1·standard deviation), and a low humidity-dependent light scattering enhancement factor at 80% relative humidity, f (80%) (2.0 ± 0.2), were obtained when air masses originated from the northern part of China (N China), compared to those obtained when air masses originated from the eastern part of China (E China) (α = 1.4 ± 0.1; f (80%) = 2.4 ± 0.2). The relatively high α and low f (80%) during the N China compared to those during the E China were consistent with a relatively high mass ratio of organic aerosol to sum of SO 4 2− , NO 3 − and NH 4 + during the N China (1.01 ± 0.17) compared to the E China episode (0.25 ± 0.13). This result indicates the importance of organic aerosol on aerosol optical and hygroscopic properties of haze plumes. Single scattering albedo (SSA) of dry PM 2.5 (0.92 ± 0.01) and mass scattering efficiency (MSE) of dry PM 2.5 at 550 nm wavelength during the E China episode (3.6 ± 0.3 m 2 g −1 ) were higher than those previously obtained at the air mass source regions in China (SSA = ~0.8; MSE = ~3.0 m 2 g −1 ), implying that optical properties of PM 2.5 were significantly altered during long-range atmospheric transport.

Beschreibung: This study utilizes a version of the Advanced Regional Prediction System (ARPS) with a canopy sub-model (ARPS-CANOPY) to evaluate the sensitivity of cold-air pool evolution to forest canopy density and valley geometry, and elucidate the underlying processes. Numerical experiments are conducted with forest canopy structures spanning from bare ground to dense canopies and terrain configurations ranging from small, shallow valleys to broad, deep valleys. In a set of experiments in which forest canopy density is varied, the minimum potential temperature in the cold-air pool is found to be as much as 15K warmer with a dense canopy than with no canopy. Analysis of the thermodynamic budget reveals weaker cooling rates in the dense canopy case than the sparse canopy case, with differences in cooling rates persisting through the duration of the simulation. An additional experiment in which cooling of canopy elements and the influence of the canopy on the ground radiation budget are neglected highlights the important role the canopy plays in limiting radiative loss from the ground surface along the sidewall and valley floor. In experiments in which valley geometry is varied, the cold-air pool is found to be strongest in a medium valley (10 km wide, 187.5 m deep), and weakest in a small valley (2 km wide, 37.5 m deep). Analysis of along-slope buoyancy suggests that downslope-flow driven cooling in a medium valley is more efficient than in a large valley (30 km wide, 500 m deep).

Beschreibung: Under a changing environment, seasonal droughts have been exacerbated with devastating impacts. However, the understanding of drought mechanism and predictability is limited. Based on the hindcasts from multiple climate models, the predictability and forecast skill for drought over China are investigated. The 3-month standardized precipitation index (SPI3) is used as the drought index, and the predictability is quantified by using a perfect model assumption. Ensemble hindcasts from multiple climate models are assessed individually and the grand multi-model ensemble is also evaluated. Drought forecast skill for model ensemble mean is higher than individual ensemble members, and NMME grand ensemble performs the best. Predictability is higher than forecast skill, indicating the room for improving drought forecast. Drought predictability and forecast skill are positively correlated in general, but they vary depending on seasons, regions and forecast leads. Higher drought predictability and forecast skill are found over regimes where ENSO has significant impact. For the ENSO-affected regimes, both drought predictability and forecast skill in ENSO years are higher than that in neutral years. This study suggests that predictability not only provides a measure for selecting climate models for ensemble drought forecast in ENSO-affected regimes, but also serves as an indicator for forecast skill especially when in-situ and/or remote sensing measurements for the hindcast verifications are considered unreliable.

Beschreibung: Arid and semi-arid regions located in subtropical zones are projected to experience the most adverse impacts of climate change. During the warm season, observations and IPCC global climate models (GCMs) generally support a “wet gets wetter, dry gets drier” hypothesis in these regions, which acts to amplify the climatological transitions in the context of the annual cycle. In this study, we consider changes in U.S. early warm season precipitation in the observational record and regional climate model (RCM) simulations driven by two “well performing” dynamically downscaled CMIP3 models (HadCM3 and MPI ECHAM5) that have a robust climatological representation of the North American Monsoon System (NAMS). Both observations and model results show amplification in historical seasonal transitions of temperature and precipitation associated with NAMS development, with WRF-MPI better representing the observed signal. Assuming the influence of remote Pacific sea surface temperature forcing (SST) associated with the El Niño Southern Oscillation and Pacific Decadal Variability (ENSO-PDV) on U.S. regional climate remains the same in the 21 st century, similar extreme trends are also projected by WRF-MPI for the next 30 years. A methodology is also developed to objectively analyze how climate change may be synergistically interacting with ENSO-PDV variability during the early warm season.. Our analysis suggests that interannual variability of warm season temperature and precipitation associated with Pacific SST forcing is becoming more extreme and the signal is stronger in the observed record.

Beschreibung: Present study documents the intraseasonal variability of sea surface temperature (SST) in the South China Sea (SCS) during boreal winter and its association with the East Asian winter monsoon (EAWM) variability. In northern Tropics, the largest intraseasonal variability of SST during boreal winter is found in the SCS, with two localized regions of large standard deviation, one extending westward from the Luzon Strait and the other extending southward from the coast of central Vietnam. Correspondingly, large intraseasonal variability in surface heat fluxes is observed in the above regions. Analysis shows that the formation of large intraseasonal SST anomalies in these regions is attributed largely to wind-related surface latent heat flux changes, with supplementary contribution from cloud-related surface shortwave radiation changes. Wind-induced Ekman advection has a negative effect and the Ekman upwelling pattern differs from the intraseasonal SST anomaly pattern. The intraseasonal variations of SST in the SCS display a close association with the East Asian winter monsoon (EAWM) change with a time lag of 3–5 days. In a weak (strong) phase of the EAWM, decrease (increase) in surface wind speed and suppression (enhancement) in surface latent heat flux lead to intraseasonal SST warming (cooling). This intraseasonal SST signal displays a southward propagation with the SST change in northern SCS leading that in southern SCS by about 2 days. A similar southward propagation is seen in surface wind speed and latent heat flux anomalies. The southward propagation of cloud and shortwave radiation anomalies is limited to northern part of the SCS.

Beschreibung: Emissions of methane (CH 4 ) and volatile organic compounds (VOCs) from oil and gas production may have large impacts on air quality and climate change. Methane and VOCs were measured over the Haynesville and Marcellus shale gas plays onboard the NCAR C-130 and NOAA WP-3D research aircraft in June-July of 2013. We used an eddy covariance technique to measure in situ fluxes of CH 4 and benzene from both C-130 flights with high-resolution data (10 Hz) and WP-3D flights with low-resolution data (1 Hz). Correlation (R=0.65) between CH 4 and benzene fluxes was observed when flying over shale gas operations and the enhancement ratio of fluxes was consistent with the corresponding concentration observations. Fluxes calculated by the eddy covariance method show agreement with a mass balance approach within their combined uncertainties. In general, CH 4 fluxes in the shale gas regions follow a lognormal distribution, with some deviations for relatively large fluxes (〉10 µg m −2 s −1 ). Statistical analysis of the fluxes shows that a small number of facilities (i.e. ~10%) are responsible for up to ~40% of the total CH 4 emissions in the two regions. We show that the airborne eddy covariance method can also be applied in some circumstances when meteorological conditions do not favor application of the mass balance method. We suggest that the airborne eddy covariance method is a reliable alternative and complementary analysis method to estimate emissions from oil and gas extraction.

Beschreibung: Convective features (CFs) observed by the Tropical Rainfall Measuring Mission (TRMM) satellite between 2004 and 2011 are analyzed to determine the relative roles of thermodynamics and aerosols as they modulate radar reflectivity and lightning. We studied the simultaneous impacts of normalized convective available potential energy (NCAPE) and warm cloud depth (WCD) as well as cloud condensation nuclei concentrations (D ≥ 40 nm; N40) on total lightning density (TLD), average height of 30 dBZ echoes (AVGHT30), and vertical profiles of radar reflectivity (VPRR) within individual CFs. The results show that TLD increases by up to 600% and AVGHT30 increases by up to 2–3 km with increasing NCAPE and N40 for fixed WCD. The partial sensitivity of TLD/AVGHT30 to NCAPE and N40 separately were comparable in magnitude, but account for a fraction of the total range of variability (i.e., when the influences of NCAPE and N40 are considered simultaneously). Both TLD and AVGHT30 vary inversely with WCD such that maxima of TLD and AVGHT30 are found for the combination of high NCAPE , high N40 , and shallower WCD . The relationship between lightning and radar reflectivity was shown to vary as a function of N40 for a fixed thermodynamic environment. Analysis of VPRRs shows that reflectivity in the mixed phase region is up to 5.0-5.6 dB greater for CFs in polluted environments compared to CFs in pristine environments (holding thermodynamics fixed). This analysis favors a merged hypothesis for the simultaneous roles of thermodynamics and aerosols as they influence deep convective clouds in the Tropics.

Beschreibung: The aim of this study is to investigate the influence of ozone depletion and recovery on the Southern Annular Mode (SAM) and stratosphere-troposphere coupling. Using the NIWA-UKCA chemistry-climate model, we compare reference runs that include forcing due to greenhouse gases and ozone depleting substances to sensitivity simulations in which ozone depleting substances are fixed at their 1960 levels. We find that ozone depletion leads to an increased frequency of extreme anomalies and increased persistence of the SAM in the stratosphere as well as stronger, more persistent stratosphere-troposphere coupling. Currently the stratosphere provides an appreciable amount of predictability to the troposphere on time scales of one or two months, however we find that this effect reduces over time as stratospheric ozone recovers to pre-ozone hole levels towards the latter part of this century.

Beschreibung: Despite the importance of the coupling between vegetation dynamics and root-zone soil moisture in land–atmosphere interactions, there is no land data assimilation system (LDAS) that currently addresses this issue, limiting the capacity to positively impact weather and seasonal forecasting. We develop a new LDAS that can improve the skill of an ecohydrological model to simulate simultaneously surface soil moisture, root-zone soil moisture, and vegetation dynamics by assimilating passive microwave observations that are sensitive to both surface soil moisture and terrestrial biomass. This LDAS first calibrates both hydrological and ecological parameters of a land surface model, which explicitly simulates vegetation growth and senescence. Then, it adjusts the model states of soil moisture and leaf area index (LAI) sequentially using a genetic particle filter. We can adjust the subsurface soil moisture, which is not observed directly by satellites, because we simulate the interactions between vegetation dynamics and subsurface water dynamics. From a point-scale evaluation, we succeed in improving the performance of our land surface model and generate ensembles of the model state whose distribution reflects the combined information in the land surface model and satellite observations. We show that the adjustment of the subsurface soil moisture significantly improves the capacity to simulate vegetation dynamics in seasonal forecast timescales. This LDAS can contribute to the generation of ensemble initial conditions of surface and subsurface soil moisture and LAI for a probabilistic framework of weather and seasonal forecasting.

Beschreibung: Effectively modeling the influence of terrestrial snow on climate in general circulation models (GCMs) is limited by imperfect knowledge and parameterization of arctic and sub-arctic climate processes, and a lack of reliable observations for model evaluation and improvement. This study uses a number of satellite-derived datasets to evaluate how well the current generation of climate models from the fifth Coupled Model Intercomparison Project (CMIP5) simulate the seasonal cycle of climatological snow cover fraction (SCF) and surface albedo over the Northern Hemisphere snow season (September – June). Using a variety of metrics, the CMIP5 models are found to simulate SCF evolution better than that of albedo. The seasonal cycle of SCF is well reproduced despite substantial biases in simulated surface albedo of snow-covered land (α sfc_snow ), which affect both the magnitude and timing of the seasonal peak in α sfc_snow during the fall snow accumulation period, and the springtime snow ablation period. Insolation-weighting demonstrates that the biases in α sfc_snow during spring are of greater importance for the surface energy budget. Albedo biases are largest across the boreal forest, where the simulated seasonal cycle of albedo is biased high in 15/16 CMIP5 models. This bias is explained primarily by unrealistic treatment of vegetation masking and subsequent overestimation (more than 50% in some cases) of peak α sfc_snow , rather than by biases in SCF. While seemingly straightforward corrections to peak α sfc_snow could yield significant improvements to simulated snow albedo feedbacks, changes in α sfc_snow could potentially introduce biases in other important model variables such as surface temperature.

Beschreibung: Understanding critical processes that contribute to the organization of mesoscale convective systems (MCSs) is important for accurate weather forecasts and climate predictions. In this study, we investigate the effects of wind shear at different vertical levels on the organization and properties of convective systems using the Weather Research and Forecasting (WRF) model with spectral-bin microphysics. Based on a control run for a MCS with weak wind shear (Ctrl), we find that increasing wind shear at the lower troposphere (L-shear) leads to a more organized quasi-line convective system. Strong wind shear in the middle troposphere (M-shear) tends to produce large vorticity and form a mesocyclone circulation and an isolated strong storm that leans toward supercellular structure. By increasing wind shear at the upper vertical levels only (U-shear), the organization of the convection is not changed much, but the convective intensity is weakened. Increasing wind shear in the middle troposphere for the selected case results in a significant drying, and the drying is more significant when conserving moisture advection at the lateral boundaries, contributing to the suppressed convective strength and precipitation relative to Ctrl. Precipitation in the L- and U-shear does not change much from Ctrl. Evident changes of cloud macro- and micro-physical properties in the strong wind shear cases are mainly due to large changes in convective organization and water vapor. The insights obtained from this study help us better understand the major factors contributing to convective organization and precipitation.

Beschreibung: The Sahelian Sudan is an arid to semi-arid region that depends on the seasonal rainfall as the main source of water, but its rainfall has large interannual variability. Such dry regions usually have their main moisture sources elsewhere; thus the rainfall variability is directly related to the moisture transport. This study seeks to identify source regions of water vapor for Sahelian Sudan during the monsoon period, from July to September. We have used the Lagrangian trajectory model FLEXPART driven by ERA-interim results for the time period 1998 to 2008. The results show that most of the air masses that reach this region during the monsoon period have their major origins over the Arabian Peninsula, Central Africa or are associated with the tropical easterly jet. Flow associated with ITCZ contributes almost half of the total precipitated water; most of it comes from Central Africa. This suggests that moisture recycling is the major contributor, compared to Oceanic sources. The flows from the northeast (Arabian Peninsula and north Asia) and east (Horn of Africa and north Indian Ocean) contribute about one third of the precipitated water. The rest of precipitated water comes from the Mediterranean, subtropical Atlantic and western Sahel, all with smaller contribution. Our results also indicate that different sub-regions of Sahelian Sudan have different moisture sources. Such result needs to be taken into account in seasonal forecasting practices.

Beschreibung: A combination of ray theory and 2D time-dependent simulations is used to investigate the linear effects of a time-dependent, vertically and horizontally inhomogeneous background horizontal wind field on the propagation, refraction and reflection of small-scale gravity wave packets. Interactions between propagating waves of different scales are likely to be numerous and important. We find that a static medium scale wave wind field of sufficient amplitude can channel and/or critical level filter a small scale wave or cause significant reflection, depending upon both waves’ parameters. However, the inclusion of a time-dependent phase progression of the medium scale wave can reduce energy loss through critical level filtering by up to ~70% and can also reduce reflection by up to ~60% for the cases simulated. We also find that the relative direction of propagation between the small scale and medium scale wave can significantly affect small-scale wave filtering. When the phases are progressing in the same horizontal direction, the small-scale wave is far more likely to become trapped and ultimately critical level filtered than if the phases are propagating in opposite horizontal directions unless reflection occurs first. Considerations of time-dependent winds associated with medium scale propagating waves and their directionality are important for assessing the propagation and dispersion of small-scale waves over large horizontal distances.

Beschreibung: A theoretical study is presented to investigate lightning strikes to towers, with focus on wave interactions occurring at the return-stroke front due to the arrival of current pulses that propagate upward on the channel after being transmitted from the tower. The lightning channel is represented as a transmission line including corona and nonlinear losses. Analyses for the hypothetical case of a lossless channel considering matched tower and channel impedances show that the arrival of current pulses at the upward moving return-stroke front leads to an increase in corona currents leaving the channel. This transient process generates current pulses whose arrival at the tower top can be interpreted as the effect of a current reflection at the upward moving front, even though no impedance discontinuity exists at that point if return-stroke and leader channel properties are assumed the same. In the more realistic case of a lossy channel considering unmatched channel and tower impedances, the nonlinear channel resistance modifies the current pulses that propagate along the channel so that they merge smoothly with the return-stroke front. In this case, the interaction of the current pulses transmitted from the tower to the channel with the upward-moving return-stroke front does not lead to features that can be clearly interpreted as the result of current reflections at that point in the evaluated conditions. Finally, it is argued that the current reflection coefficient at the tower top should be viewed as a current dependent parameter as opposed to a constant, linear value.

Beschreibung: Measurements made by microwave imaging radiometers can be used to retrieve several environmental parameters over the world's oceans. In this work, we calculate the uncertainty in retrievals obtained from the Special Sensor Microwave Imager Sounder (SSMIS) instrument caused by uncertainty in the input parameters to the retrieval algorithm. This work applies to the Version-7 retrievals of surface wind speed, total column water vapor, total column cloud liquid water, and rain rate produced by Remote Sensing Systems. Our numerical approach allows us to calculate an estimated input-induced uncertainty for every valid retrieval during the SSMIS mission. Our uncertainty estimates are consistent with the differences observed between SSMIS wind speed and vapor measurements made by SSMIS on the F16 and F17 satellites, supporting their accuracy. The estimates do not explain the larger differences between the SSMIS measurements of wind speed and vapor and other sources of these data, consistent with the influence of more sources of uncertainty.

Beschreibung: Frozen soil was simulated at six seasonally frozen and seven permafrost stations over the northern Tibetan Plateau using the Variable Infiltration Capacity (VIC) model for the period of 1962 – 2009. The VIC model resolved the seasonal cycle and temporal evolution of the observed soil temperatures and liquid soil moisture well. The simulated long-term changes during 1962–2009 indicated mostly positive trends for both soil temperature and soil moisture, and negative trends for soil ice content at annual and monthly time scales, although differences existed among the stations, soil layers and seasons. Increases in soil temperature were due mainly to increases in daily air temperature maxima and internal soil heat conduction, while decreases in soil ice content were related to warming of frozen soil. For liquid soil moisture, increases in the cold months can be attributed to increases in soil temperature and enhanced soil ice melt while changes in the warm months were the results of competition between positive precipitation and negative soil temperature effects. Precipitation and liquid soil moisture were strongly correlated with evapotranspiration and runoff, but had various degrees of correlations with baseflow during May-September. Seasonally frozen stations displayed longer and more active hydrological processes than permafrost stations. Slight enhancement of the surface hydrological processes at the study stations was indicated, due to the combined effects of precipitation changes, which were dominant, and frozen soil degradation.

Beschreibung: In the long term, precipitation in the central U.S. decreases by about 25% during the seasonal transition from June to July. This precipitation decrease is observed to have intensified since 1979, and such intensification could have enhanced spring drought occurrences in the central U.S., in which conditions quickly evolve from being abnormally dry to exceptionally dry. Various atmospheric and land reanalysis datasets were analyzed to examine the trend in the June-to-July seasonal transition. It was found that the intensified deficit in precipitation was accompanied by increased downward shortwave radiation flux, tropospheric subsidence, enhanced evaporative fraction, and elevated planetary boundary layer height, all of which could and did lead to surface drying. The change in tropospheric circulation was characterized by an anomalous ridge over the western U.S. and a trough on either side – a pattern known to suppress rainfall in the central U.S. The pattern of these trends calculated from 1979 to 2011 shows similarity with the evolution of the 2012 record drought from spring to summer.

Beschreibung: A detailed time-resolved chemical characterization of ambient non refractory submicron aerosols (NR-PM 1 ) was conducted for the first time in India. The measurements were performed during the winter (November 2011 to January 2012) in a heavily polluted city of Kanpur, which is situated in the Indo-Gangetic Plain (IGP). Real-time measurements provided new insights into the sources and evolution of organic aerosols (OA) that could not be obtained using previously deployed filter-based measurements at this site. The average NR-PM 1 loading was very high (〉100 µg/m 3 ) throughout the study, with OA contributing approximately 70% of the total aerosol mass. Source apportionment of the OA using positive matrix factorization (PMF) revealed large contributions from fresh and aged biomass burning OA throughout the entire study period. A back trajectory analysis showed that the polluted air masses were affected by local sources and distant source regions where the burning of paddy residues occurs annually during winter. Several fog episodes were encountered during the study, and the OA composition varied between foggy and non-foggy periods, with higher oxygen to carbon (O/C) ratios during the foggy periods. The evolution of OA and their elemental ratios (O:C and H:C) were investigated for the possible effects of fog processing.

Beschreibung: Quantifying the sensitivity of warm rain to aerosols is important for constraining climate model estimates of aerosol indirect effects. In this study, the precipitation sensitivity to cloud droplet number concentration ( N d ) in satellite retrievals is quantified by applying the precipitation susceptibility metric to a combined CloudSat/MODIS dataset of stratus and stratocumulus that cover the tropical and subtropical Pacific Ocean and Gulf of Mexico. Consistent with previous observational studies of marine stratocumulus, precipitation susceptibility decreases with increasing liquid water path (LWP), and the susceptibility of the mean precipitation rate R is nearly equal to the sum of the susceptibilities of precipitation intensity and of probability of precipitation. Consistent with previous modeling studies, the satellite retrievals reveal that precipitation susceptibility varies not only with LWP but also with N d . Puzzlingly, negative values of precipitation susceptibility are, however, found at low LWP and high N d . There is marked regional variation in precipitation susceptibility values that cannot simply be explained by regional variations in LWP and N d . This suggests other controls on precipitation apart from LWP and N d and that precipitation susceptibility will need to be quantified and understood at the regional scale when relating to its role in controlling possible aerosol-induced cloud lifetime effects.

Beschreibung: We investigate the observed trends and interannual variability in surface ozone over the United States using the Global Modeling Initiative chemical transport model. We discuss the roles of meteorology, emissions, and transport from the stratosphere in driving the interannual variability in different regions and seasons. We demonstrate that a hindcast simulation for 1991–2010 can reproduce much of the observed variability and the trends in summertime ozone, with correlation coefficients for seasonally and regionally averaged median ozone ranging from 0.46 to 0.89. Reproducing the interannual variability in winter and spring in the western United States may require higher resolution models to adequately represent stratosphere-troposphere exchange. Hindcast simulations with fixed versus variable emissions show that changes in anthropogenic emissions drive the observed negative trends in monthly median ozone concentrations in the eastern United States during summer, as well as the observed reduction in the amplitude of the seasonal cycle. The simulation underestimates positive trends in the western United States during spring, but excluding the first four years of data removes many of the statistically significant trends in this region. The reduction in the slope of the ozone versus temperature relationship before and after major emission reductions is also well represented by the model. Our results indicate that a global model can reproduce many of the important features of the meteorologically induced ozone variability as well as the emission-driven trends, lending confidence to model projections of future changes in regional surface ozone.

Beschreibung: This study presents a comprehensive evaluation of five widely used multisatellite precipitation estimates (MPEs) against 1°x1° gridded-rain gauge dataset as ground truth over India. One decade observations are used to assess the performance of various MPEs (climate prediction center (CPC)-South Asia dataset, CPC morphing technique (CMORPH), precipitation estimation from remotely sensed information using artificial neural networks (PERSIANN), tropical rainfall measuring mission's (TRMM) multi-satellite precipitation analysis (TMPA-3B42) and global precipitation climatology project (GPCP)). All MPEs have high detection skills of rain with larger probability of detection (POD) and smaller ‘misses’ values. However, the detection sensitivity differs from one product (and also one region) to the other. While the CMORPH has the lowest sensitivity of detecting rain, CPC shows highest sensitivity and often over detects rain, as evidenced by large POD and false alarm ratio (FAR) and small ‘misses’ values. All MPEs show higher rain sensitivity over eastern India than western India. These differential sensitivities are found to alter the biases in rain amount differently. All MPEs show similar spatial patterns of seasonal rain bias and root mean square error (RMSE), but their spatial variability across India is complex and pronounced. The MPEs overestimate the rainfall over the dry regions (northwest and southeast India) and severely underestimate over mountainous regions (west coast and northeast India), whereas the bias is relatively small over the core monsoon zone. Higher occurrence of virga rain due to sub-cloud evaporation and possible missing of small-scale convective events by gauges over the dry regions are the main reasons for the observed overestimation of rain by MPEs. The decomposed components of total bias show that the major part of overestimation is due to false precipitation. The severe underestimation of rain along the west coast is attributed to the predominant occurrence of shallow rain and underestimation of moderate to heavy rain by MPEs. The decomposed components suggest that the missed precipitation and hit bias are the leading error sources for the total bias along the west coast. All evaluation metrics are found to be nearly equal in two contrasting monsoon seasons (southwest and northeast), indicating that the performance of MPEs doesn't change with the season, at least over southeast India. Among various MPEs, the performance of TMPA is found to be better than others, as it reproduced most of the spatial variability exhibited by the reference.

Beschreibung: Cloud masks serve as a basis for estimates of cloud amount, which is an essential parameter for studying the Earth's radiation budget. The most commonly-used cloud mask is a simple thematic classification, which includes qualitative information on the presence of clouds in the satellite's instantaneous field of view (IFOV). Cloud mask classes have to be ‘translated’ into a quantitative measure, in order to be used for cloud amount calculations. The assignment of cloud fractions to cloud mask classes is a subjective process, and increases uncertainty in cloud amount estimates. We evaluated this degree of uncertainty using the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask product. Together with the operational MODIS cloud mask interpretation, we investigated two extreme alternatives: ‘rigorous’ (only ‘confident cloudy’ IFOVs were 100% cloudy), and ‘tolerant’ (only ‘confident clear’ IFOVs were 0% cloudy). Results showed that in Europe the range of uncertainty was 14.3% in Europe, and controlled by the frequency of small convective clouds. Comparison with surface-based observations suggests that the ‘rigorous’ interpretation of the cloud mask is more accurate than that used operationally for MODIS Level 3 product generation. The ‘rigorous’ approach resulted in the smallest bias (−0.7%), the smallest RMSE (4.6%), the small standard deviation (6%), and the strongest correlation (0.935). These results suggest that for climatological applications the ‘rigorous’ scenario should be considered as a more accurate ‘best guess’ over land.