The 50 most-cited articles in Journal of Geophysical Research Atmospheres, 2016

Abstract The Tibetan Plateau (TP) is undergoing significant warming since the 1950s. During the past two decades, extensive research has been conducted to investigate the climate change on the plateau. This review presents an overview of recent progress on climate change on the TP with the aim of providing a comprehensive understanding of changes in climate variables. Long‐term observation data from meteorological stations presented by the published literature were used to show the trends in various climate variables. The TP is overall getting warmer and wetter during the past decades. Temperature is significantly increased, especially since the 1980s. The overall warming rate ranges from 0.16 to 0.67°C decade −1 since the 1950s during different periods. The TP shows a uniform warming trend with the most significant warming in the northern part. Precipitation is slightly increased, and the spatial pattern of changes in precipitation is variable. The annual precipitation is increasing in most areas of the TP. Some subregions are becoming wetter, while some subregions are becoming…

Recent Mediterranean droughts have highlighted concerns that climate change may be contributing to observed drying trends, but natural climate variability in the region is still poorly understood. We analyze 900 years (1100-2012) of Mediterranean drought variability in the Old World Drought Atlas (OWDA), a spatiotemporal tree-ring reconstruction of the June-July-August self calibrating Palmer Drought Severity Index. In the Mediterranean, the OWDA is highly correlated with spring precipitation (April-June), the North Atlantic Oscillation (January-April), the Scandinavian Pattern (January-March), and the East Atlantic Pattern (April-June). Drought variability displays significant east-west coherence across the basin on multi-decadal to centennial time scales and north-south anti-phasing in the eastern Mediterranean, with a tendency for wet anomalies in the Black Sea region (e.g., Greece, Anatolia, the Balkans, etc) when coastal Libya, the southern Levant, and the Middle East are dry, possibly related to the North Atlantic Oscillation. Recent droughts are centered in the Western…

Abstract To investigate the seasonal characteristics of submicron aerosol (PM1) in Beijing urban areas, a high‐resolution time‐of‐flight aerosol‐mass‐spectrometer (HR‐ToF‐AMS) was utilized at an urban site in summer (August to September 2011) and winter (November to December 2010), coupled with multiple state of the art online instruments. The average mass concentrations of PM1 (60–84 µg m−3) and its chemical compositions in different campaigns of Beijing were relatively consistent in recent years. In summer, the daily variations of PM1 mass concentrations were stable and repeatable. Eighty‐two percent of the PM1 mass concentration on average was composed of secondary species, where 62% is secondary inorganic aerosol and 20% secondary organic aerosol (SOA). In winter, PM1 mass concentrations changed dramatically because of the different meteorological conditions. The high average fraction (58%) of primary species in PM1 including primary organic aerosol (POA), black carbon, and chloride indicates primary emissions usually played a more important role in the winter. However, aqueous…

Abstract Based on a uniquely dense network of surface towers measuring continuously the atmospheric concentrations of greenhouse gases (GHGs), we developed the first comprehensive monitoring systems of CO 2 emissions at high resolution over the city of Indianapolis. The urban inversion evaluated over the 2012–2013 dormant season showed a statistically significant increase of about 20% (from 4.5 to 5.7 MtC ± 0.23 MtC) compared to the Hestia CO 2 emission estimate, a state‐of‐the‐art building‐level emission product. Spatial structures in prior emission errors, mostly undetermined, appeared to affect the spatial pattern in the inverse solution and the total carbon budget over the entire area by up to 15%, while the inverse solution remains fairly insensitive to the CO 2 boundary inflow and to the different prior emissions (i.e., ODIAC). Preceding the surface emission optimization, we improved the atmospheric simulations using a meteorological data assimilation system also informing our Bayesian inversion system through updated observations error variances. Finally, we estimated the…

Abstract Accurate representation of global stratospheric aerosols from volcanic and nonvolcanic sulfur emissions is key to understanding the cooling effects and ozone losses that may be linked to volcanic activity. Attribution of climate variability to volcanic activity is of particular interest in relation to the post‐2000 slowing in the rate of global average temperature increases. We have compiled a database of volcanic SO 2 emissions and plume altitudes for eruptions from 1990 to 2014 and developed a new prognostic capability for simulating stratospheric sulfate aerosols in the Community Earth System Model. We used these combined with other nonvolcanic emissions of sulfur sources to reconstruct global aerosol properties from 1990 to 2014. Our calculations show remarkable agreement with ground‐based lidar observations of stratospheric aerosol optical depth (SAOD) and with in situ measurements of stratospheric aerosol surface area density (SAD). These properties are key parameters in calculating the radiative and chemical effects of stratospheric aerosols. Our SAOD calculations…

Abstract The radiative and microphysical effects of aerosols can affect the development of convective clouds. The objective of this study is to reveal if the overall aerosol effects have any discernible impact on the diurnal variations in precipitation and lightning by means of both observational analysis and modeling. As the first part of two companion studies, this paper is concerned with analyzing hourly PM10, precipitation, and lightning data collected during the summers of 2008–2012 in the Pearl River Delta region. Daily PM10 data were categorized as clean, medium, or polluted so that any differences in the diurnal variations in precipitation and lightning could be examined. Heavy precipitation and lightning were found to occur more frequently later in the day under polluted conditions than under clean conditions. Analyses of the diurnal variations in several meteorological factors such as air temperature, vertical velocity, and wind speed were also performed. They suggest that the influence of aerosol radiative and microphysical effects serve to suppress and enhance convective…

Particle pH is a critical but poorly constrained quantity that affects many aerosol processes and properties, including aerosol composition, concentrations, and toxicity. We assess PM1 pH as a function of geographical location and altitude, focusing on the northeastern U.S., based on aircraft measurements from the Wintertime Investigation of Transport, Emissions, and Reactivity campaign (1 February to 15 March 2015). Particle pH and water were predicted with the ISORROPIA-II thermodynamic model and validated by comparing predicted to observed partitioning of inorganic nitrate between the gas and particle phases. Good agreement was found for relative humidity (RH) above 40%; at lower RH observed particle nitrate was higher than predicted, possibly due to organic-inorganic phase separations or nitrate measurement uncertainties associated with low concentrations (nitrate < 1 µg m−3). Including refractory ions in the pH calculations did not improve model predictions, suggesting they were externally mixed with PM1 sulfate, nitrate, and ammonium. Sample line volatilization artifacts were…

Abstract Variation in the seasonal cycle of African rainfall is of key importance for agriculture. Here an objective method of determining the timing of onset and cessation is, for the first time, extended to the whole of Africa. The method is applied to five observational data sets and the ERA‐Interim reanalysis. Compatibility with known physical drivers of African rainfall, consistency with indigenous methods, and generally strong agreement between satellite‐based rainfall data sets confirm that the method is capturing the correct seasonal progression of African rainfall. The biannual rainfall regime is correctly identified over the coastal region of Ghana and the Ivory Coast. However, the ERA‐Interim reanalysis exhibits timing biases over areas with two rainy seasons, and both ERA‐Interim and the ARCv2 observational data set exhibit some inconsistent deviations over West Africa. The method can be used to analyze both seasonal‐interannual variability and long‐term change. Over East Africa, we find that failure of the rains and subsequent humanitarian disaster is associated with…

Abstract An “offline” approach to DA is used, where static ensemble samples are drawn from existing CMIP climate‐model simulations to serve as the prior estimate of climate variables. We use linear, univariate forward models (“proxy system models (PSMs)”) that map climate variables to proxy measurements by fitting proxy data to 2 m air temperature from gridded instrumental temperature data; the linear PSMs are then used to predict proxy values from the prior estimate. Results for the LMR are compared against six gridded instrumental temperature data sets and 25% of the proxy records are withheld from assimilation for independent verification. Results show broad agreement with previous reconstructions of Northern Hemisphere mean 2 m air temperature, with millennial‐scale cooling, a multicentennial warm period around 1000 C.E., and a cold period coincident with the Little Ice Age (circa 1450–1800 C.E.). Verification against gridded instrumental data sets during 1880–2000 C.E. reveals greatest skill in the tropics and lowest skill over Northern Hemisphere land areas. Verification…

Abstract Soil moisture is a key state variable in many hydrological processes. The Global Land Data Assimilation System (GLDAS) can produce global and continuous soil moisture data sets which have been used in many applications. In this study, simulated soil moisture from four land surface models (LSM) (Mosaic, Noah, Community Land Model, and Variable Infiltration Capacity) in GLDAS‐1 and the more recent GLDAS‐2 were evaluated against in situ soil moisture measurements collected from two soil moisture networks located on the Tibetan Plateau at different soil depths. The two networks provide a representation of different climates and land surface conditions on the Tibetan Plateau which can make the evaluation results more robust and reliable. The results show that all the LSMs can well capture the temporal variation of observed soil moisture with the correlation coefficients mostly being above 0.5. However, they all display biases with the surface soil moisture being systematically underestimated in both of two network regions, and the Mosaic model always shows the largest bias that…

Abstract The usefulness of previous Coupled Model Intercomparison Project (CMIP) exercises has been hampered by a lack of radiative forcing information. This has made it difficult to understand reasons for differences between model responses. Effective radiative forcing (ERF) is easier to diagnose than traditional radiative forcing in global climate models (GCMs) and is more representative of the eventual temperature response. Here we examine the different methods of computing ERF in two GCMs. We find that ERF computed from a fixed sea surface temperature (SST) method (ERF_fSST) has much more certainty than regression based methods. Thirty year integrations are sufficient to reduce the 5–95% confidence interval in global ERF_fSST to 0.1 W m −2 . For 2xCO2 ERF, 30 year integrations are needed to ensure that the signal is larger than the local confidence interval over more than 90% of the globe. Within the ERF_fSST method there are various options for prescribing SSTs and sea ice. We explore these and find that ERF is only weakly dependent on the methodological choices. Prescribing…

Abstract Aerosol‐cloud interactions have the potential to modify many different cloud properties. There is significant uncertainty in the strength of these aerosol‐cloud interactions in analyses of observational data, partly due to the difficulty in separating aerosol effects on clouds from correlations generated by local meteorology. The relationship between aerosol and cloud fraction (CF) is particularly important to determine, due to the strong correlation of CF to other cloud properties and its large impact on radiation. It has also been one of the hardest to quantify from satellites due to the strong meteorological covariations involved. This work presents a new method to analyze the relationship between aerosol optical depth (AOD) and CF. By including information about the cloud droplet number concentration (CDNC), the impact of the meteorological covariations is significantly reduced. This method shows that much of the AOD‐CF correlation is explained by relationships other than that mediated by CDNC. By accounting for these, the strength of the global mean AOD‐CF relationship…

Abstract The recent International Panel on Climate Change report predicts the highly urbanized Northeastern U.S. to be at high risk to heat waves. Since urban residents and infrastructure are known to be highly vulnerable to extreme heat, the goal of this paper is to understand the interaction between the synoptic‐scale heat wave and the city‐scale urban heat island (UHI) effects. The study also qualitatively analyzes the primary factors that contribute to UHIs by comparing their intensities in different cities with distinct geo‐physical characteristics. Our results, generated by using the Weather Research and Forecasting model augmented with advanced urban surface parameterizations, confirm that the amplitude of UHI is related to the physical size of the city. However, the results suggest that cities of comparabale sizes might interact differently with heat waves: in New York City; Washington, DC; and Baltimore (but not in Philadelphia) the regular UHI was amplified more strongly during heat waves compared to smaller cities. The results also establish that the pattern of UHI in…

Abstract Statistical downscaling and dynamical downscaling are two approaches to generate high‐resolution regional climate models based on the large‐scale information from either reanalysis data or global climate models. In this study, these two downscaling methods are used to simulate the surface climate of China and compared. The Statistical Downscaling Model (SDSM) is cross validated and used to downscale the regional climate of China. Then, the downscaled historical climate of 1981–2000 and future climate of 2041–2060 are compared with that from the Weather Research and Forecasting (WRF) model driven by the European Center‐Hamburg atmosphere model and the Max Planck Institute Ocean Model (ECHAM5/MPI‐OM) and the L'Institut Pierre‐Simon Laplace Coupled Model, version 5, coupled with the Nucleus for European Modelling of the ocean, low resolution (IPSL‐CM5A‐LR). The SDSM can reproduce the surface temperature characteristics of the present climate in China, whereas the WRF tends to underestimate the surface temperature over most of China. Both the SDSM and WRF require further work…

Abstract The increase in winter haze over eastern China in recent decades due to variations in meteorological parameters and anthropogenic emissions was quantified using observed atmospheric visibility from the National Climatic Data Center Global Summary of Day database for 1980–2014 and simulated PM2.5 concentrations for 1985–2005 from the Goddard Earth‐Observing System (GEOS) chemical transport model (GEOS‐Chem). Observed winter haze days averaged over eastern China (105–122.5°E, 20–45°N) increased from 21 days in 1980 to 42 days in 2014 and from 22 to 30 days between 1985 and 2005. The GEOS‐Chem model captured the increasing trend of winter PM2.5 concentrations for 1985–2005, with concentrations averaged over eastern China increasing from 16.1 µg m−3 in 1985 to 38.4 µg m−3 in 2005. Considering variations in both anthropogenic emissions and meteorological parameters, the model simulated an increase in winter surface‐layer PM2.5 concentrations of 10.5 (±6.2) µg m−3 decade−1 over eastern China. The increasing trend was only 1.8 (±1.5) µg m−3 decade−1 when variations in…

Abstract Statistical bias‐adjustment of climate models' outputs is being increasingly used for assessing the impact of climate change on several sectors. It is known that these techniques may alter the mean climate signal of the adjusted variable; however, the effect on the projected occurrence of climate extremes is less commonly investigated. Here the outputs of an ensemble of high‐resolution (0.11°) regional climate models (RCM) from the Coordinated Regional‐climate Downscaling Experiment for Europe (EURO‐CORDEX) have been bias adjusted, and a number of climate indices from the Expert Team on Climate Change Detection and Indices have been calculated for the present (1981–2010) and future (2071–2100) climate. Indices include absolute‐thresholds indices, percentile‐based indices, and indices based on the duration of an event. Results show that absolute‐threshold indices are largely affected by bias adjustment, as they depend strongly on both the present mean climate value (usually largely biased in the original RCMs) and its shift under climate change. The change of…

Abstract The variability and trend in global precipitable water vapor (PWV) from 1979 to 2014 are analyzed using the PWV data sets from the ERA‐Interim reanalysis of the European Centre for Medium‐Range Weather Forecasts (ECMWF), reanalysis of the National Centers for Environmental Prediction (NCEP), radiosonde, Global Positioning System (GPS), and microwave satellite observations. PWV data from the ECMWF and NCEP have been evaluated by radiosonde, GPS, and microwave satellite observations, showing that ECMWF has higher accuracy than NCEP. Over the oceans, ECMWF has a much better agreement with the microwave satellite than NCEP. An upward trend in the global PWV is evident in all the five PWV data sets over three study periods: 1979–2014, 1992–2014, and 2000–2014. Positive global PWV trends, defined as percentage normalized by annual average, of 0.61 ± 0.33% decade −1 , 0.57 ± 0.28% decade −1 , and 0.17 ± 0.35% decade −1 , have been derived from the NCEP, radiosonde, and ECMWF, respectively, for the period 1979–2014. It is found that ECMWF overestimates the PWV over the ocean prior…

In the Arctic, impurities in the atmosphere and cryosphere can strongly affect the atmospheric radiation and surface energy balance. While black carbon has hence received much attention, mineral dust has been in the background. Mineral dust is not only transported into the Arctic from remote regions but also, possibly increasingly, generated in the region itself. Here we study mineral dust in the Arctic based on global transport model simulations. For this, we have developed a dust mobilization scheme in combination with the Lagrangian particle dispersion model FLEXPART. A model evaluation, based on measurements of surface concentrations and annual deposition at a number of stations and aircraft vertical profiles, shows the suitability of this model to study global dust transport. Simulations indicate that about 3% of global dust emission originates from high-latitude dust sources in the Arctic. Due to limited convection and enhanced efficiency of removal, dust emitted in these source regions is mostly deposited closer to the source than dust from for instance Asia or Africa. This…

Abstract Long‐term time series of key climate variables with a relevant spatiotemporal resolution are essential for environmental science. Moreover, such spatially continuous data, based on weather observations, are commonly used in, e.g., downscaling and bias correcting of climate model simulations. Here we conducted a comprehensive spatial interpolation scheme where seven climate variables (daily mean, maximum, and minimum surface air temperatures, daily precipitation sum, relative humidity, sea level air pressure, and snow depth) were interpolated over Finland at the spatial resolution of 10 × 10 km 2 . More precisely, (1) we produced daily gridded time series ( FMI _ ClimGrid ) of the variables covering the period of 1961–2010, with a special focus on evaluation and permutation‐based uncertainty estimates, and (2) we investigated temporal trends in the climate variables based on the gridded data. National climate station observations were supplemented by records from the surrounding countries, and kriging interpolation was applied to account for topography and water bodies. For…

Abstract Water vapor‐weighted mean temperature, T m , is a vital parameter for retrieving precipitable water vapor (PWV) from the zenith wet delay (ZWD) of Global Navigation Satellite Systems (GNSS) signal propagation. In this study, the T m at 368 GNSS stations for 2000–2012 were calculated using three methods: (1) temperature and humidity profiles from ERA‐Interim, (2) the Bevis T m – T s relationship, and (3) the Global Pressure and Temperature 2 wet model. T m derived from the first method was used as a reference to assess the errors of the other two methods. Comparisons show that the relative errors of the T m derived from these two methods are in the range of 1–3% across more than 95% of all the stations. The PWVs were calculated using the aforementioned three types of T m and the GNSS‐derived ZWD at 107 stations. Again, the PWVs calculated using T m from the first method were used as the reference of the other two PWVs. The root‐mean‐square errors of these two PWVs are both in the range of 0.1–0.7 mm. The second method is recommended in real‐time applications, since its…

Abstract Spitsbergen has experienced some of the most severe temperature changes in the Arctic during the last three decades. This study relates the recent warming to variations in large‐scale atmospheric circulation (AC), air mass characteristics, and sea ice concentration (SIC), both regionally around Spitsbergen and locally in three fjords. We find substantial warming for all AC patterns for all seasons, with greatest temperature increase in winter. A major part of the warming can be attributed to changes in air mass characteristics associated with situations of both cyclonic and anticyclonic air advection from north and east and situations with a nonadvectional anticyclonic ridge. In total, six specific AC types (out of 21), which occur on average 41% of days in a year, contribute approximately 80% of the recent warming. The relationship between the land‐based surface air temperature (SAT) and local and regional SIC was highly significant, particularly for the most contributing AC types. The high correlation between SAT and SIC for air masses from east and north of Spitsbergen…

Abstract Current chemical transport models commonly undersimulate the atmospheric concentration of nitrous acid (HONO), which plays an important role in atmospheric chemistry, due to the lack or inappropriate representations of some sources in the models. In the present study, we parameterized up‐to‐date HONO sources into a state‐of‐the‐art three‐dimensional chemical transport model (Weather Research and Forecasting model coupled with Chemistry: WRF‐Chem). These sources included (1) heterogeneous reactions on ground surfaces with the photoenhanced effect on HONO production, (2) photoenhanced reactions on aerosol surfaces, (3) direct vehicle and vessel emissions, (4) potential conversion of NO 2 at the ocean surface, and (5) emissions from soil bacteria. The revised WRF‐Chem was applied to explore the sources of the high HONO concentrations (0.45–2.71 ppb) observed at a suburban site located within complex land types (with artificial land covers, ocean, and forests) in Hong Kong. With the addition of these sources, the revised model substantially reproduced the observed HONO levels.…

Abstract The Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) field mission based at Ellington Field, Texas, during August and September 2013 employed the most comprehensive airborne payload to date to investigate atmospheric composition over North America. The NASA ER‐2, DC‐8, and SPEC Inc. Learjet flew 57 science flights from the surface to 20 km. The ER‐2 employed seven remote sensing instruments as a satellite surrogate and eight in situ instruments. The DC‐8 employed 23 in situ and five remote sensing instruments for radiation, chemistry, and microphysics. The Learjet used 11 instruments to explore cloud microphysics. SEAC4RS launched numerous balloons, augmented AErosol RObotic NETwork, and collaborated with many existing ground measurement sites. Flights investigating convection included close coordination of all three aircraft. Coordinated DC‐8 and ER‐2 flights investigated the optical properties of aerosols, the influence of aerosols on clouds, and the performance of new instruments for satellite measurements of…

Abstract Forests are undergoing significant changes throughout the globe. These changes can modify water, energy, and carbon balance of the land surface, which can ultimately affect climate. We utilize satellite data to quantify the potential and actual impacts of forest change on land surface temperature (LST) from 2003 to 2013. The potential effect of forest change on temperature is calculated by the LST difference between forest and nearby nonforest land, whereas the actual impact on temperature is quantified by the LST trend difference between deforested (afforested) and nearby unchanged forest (nonforest land) over several years. The good agreement found between potential and actual impacts both at annual and seasonal levels indicates that forest change can have detectable impacts on surface temperature trends. That impact, however, is different for maximum and minimum temperatures. Overall, deforestation caused a significant warming up to 0.28 K/decade on average temperature trends in tropical regions, a cooling up to −0.55 K/decade in boreal regions, a weak impact in the…

Abstract The characteristics of raindrop size distributions (DSDs) and vertical structures of rainfall during the Asian summer monsoon season in East China are studied using measurements from a ground‐based two‐dimensional video disdrometer (2DVD) and a vertically pointing Micro Rain Radar (MRR). Based on rainfall intensity and vertical structure of radar reflectivity, the observed rainfall is classified into convective, stratiform, and shallow precipitation types. Among them, shallow precipitation has previously been ignored or treated as outliers due to limitations in traditional surface measurements. Using advanced instruments of 2DVD and MRR, the characteristics of shallow precipitation are quantified. Furthermore, summer rainfall in the study region is found to consist mainly of stratiform rain in terms of frequency of occurrence but is dominated by convective rain in terms of accumulated rainfall amount. Further separation of the summer season into time periods before, during, and after the Meiyu season reveals that intrasummer variation of DSDs is mainly due to changes in…

Abstract Chemical transport associated with the dynamics of the Asian summer monsoon (ASM) system is investigated using model output from the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model run in specified dynamics mode. The 3‐D day‐to‐day behavior of modeled carbon monoxide is analyzed together with dynamical fields and transport boundaries to identify preferred locations of uplifting from the boundary layer, the role of subseasonal‐scale dynamics in the upper troposphere and lower stratosphere (UTLS), and the relationship of ASM transport and the stratospheric residual circulation. The model simulation of CO shows the intraseasonal east‐west oscillation of the anticyclone may play an essential role in transporting convectively pumped boundary layer pollutants in the UTLS. A statistical analysis of 11 year CO also shows that the southern flank of the Tibetan plateau is a preferred location for boundary layer tracers to be lofted to the tropopause region. The vertical structure of a model tracer (E90) further shows that the rapid ASM…

Abstract Land surface models incorporate a large number of process descriptions, containing a multitude of parameters. These parameters are typically read from tabulated input files. Some of these parameters might be fixed numbers in the computer code though, which hinder model agility during calibration. Here we identified 139 hard‐coded parameters in the model code of the Noah land surface model with multiple process options (Noah‐MP). We performed a Sobol' global sensitivity analysis of Noah‐MP for a specific set of process options, which includes 42 out of the 71 standard parameters and 75 out of the 139 hard‐coded parameters. The sensitivities of the hydrologic output fluxes latent heat and total runoff as well as their component fluxes were evaluated at 12 catchments within the United States with very different hydrometeorological regimes. Noah‐MP's hydrologic output fluxes are sensitive to two thirds of its applicable standard parameters (i.e., Sobol' indexes above 1%). The most sensitive parameter is, however, a hard‐coded value in the formulation of soil surface resistance…

The Goddard microphysics was recently improved by adding a fourth ice class (frozen drops/hail). This new 4ICE scheme was developed and tested in the Goddard Cumulus Ensemble (GCE) model for an intense continental squall line and a moderate, less organized continental case. Simulated peak radar reflectivity profiles were improved in intensity and shape for both cases, as were the overall reflectivity probability distributions versus observations. In this study, the new Goddard 4ICE scheme is implemented into the regional-scale NASA Unified-Weather Research and Forecasting (NU-WRF) model, modified and evaluated for the same intense squall line, which occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E). NU-WRF simulated radar reflectivities, total rainfall, propagation, and convective system structures using the 4ICE scheme modified herein agree as well as or significantly better with observations than the original 4ICE and two previous 3ICE (graupel or hail) versions of the Goddard microphysics. With the modified 4ICE, the bin microphysics-based rain…

Abstract Based on station observations and reanalysis data sets, the atmospheric moisture budget and its critical role in regulating the variability of summer precipitation over the Tibetan Plateau (TP) are investigated. Results indicate that the yearly variability of summer precipitation over the southern TP (STP) is mainly controlled by remote moisture transport. Local surface evaporation presents an infinitesimal interannual fluctuation, but it cannot be ignored since it is a large component of total precipitation amount in every summer. Although the incoming moisture transport at the western boundary of STP is much weaker than that at the southern boundary, it primarily influences the temporal variation of STP summer precipitation. Further analysis shows that the summer North Atlantic Oscillation (NAO) also possesses a significant impact on the variation of STP summer precipitation. A strong NAO apparently weakens the moisture transport at the western boundary, inducing less precipitation over the STP. When the NAO is strong, wave‐activity flux obviously diverges eastward from…

Abstract Nitryl chloride (ClNO2) plays potentially important roles in atmospheric chemistry, but its abundance and effect are not fully understood due to the small number of ambient observations of ClNO2 to date. In late autumn 2013, ClNO2 was measured with a chemical ionization mass spectrometer (CIMS) at a mountain top (957 m above sea level) in Hong Kong. During 12 nights with continuous CIMS data, elevated mixing ratios of ClNO2 (>400 parts per trillion by volume) or its precursor N2O5 (>1000 pptv) were observed on six nights, with the highest ever reported ClNO2 (4.7 ppbv, 1 min average) and N2O5 (7.7 ppbv, 1 min average) in one case. Backward particle dispersion calculations driven by winds simulated with a mesoscale meteorological model show that the ClNO2/N2O5‐laden air at the high‐elevation site was due to transport of urban/industrial pollution north of the site. The highest ClNO2/N2O5 case was observed in a later period of the night and was characterized with extensively processed air and with the presence of nonoceanic chloride. A chemical box model with detailed…

Abstract Knowledge about long‐term changes in climate extremes is vital to better understand multidecadal climate variability and long‐term changes and to place today's extreme events in a historical context. While global changes in temperature and precipitation extremes since the midtwentieth century are well studied, knowledge about century‐scale changes is limited. This paper analyses a range of largely independent observations‐based data sets covering 1901–2010 for long‐term changes and interannual variability in daily scale temperature and precipitation extremes. We compare across data sets for consistency to ascertain our confidence in century‐scale changes in extremes. We find consistent warming trends in temperature extremes globally and in most land areas over the past century. For precipitation extremes we find global tendencies toward more intense rainfall throughout much of the twentieth century; however, local changes are spatially more variable. While global time series of the different data sets agree well after about 1950, they often show different changes during the…

Abstract Chlorine atoms (Cl) are highly reactive toward hydrocarbons in the Earth's troposphere, including the greenhouse gas methane (CH 4 ). However, the regional and global CH 4 sink from Cl is poorly quantified as tropospheric Cl concentrations ([Cl]) are uncertain by ~2 orders of magnitude. Here we describe the addition of a detailed tropospheric chlorine scheme to the TOMCAT chemical transport model. The model includes several sources of tropospheric inorganic chlorine (Cl y ), including (i) the oxidation of chlorocarbons of natural (CH 3 Cl, CHBr 2 Cl, CH 2 BrCl, and CHBrCl 2 ) and anthropogenic (CH 2 Cl 2 , CHCl 3 , C 2 Cl 4 , C 2 HCl 3 , and CH 2 ClCH 2 Cl) origin and (ii) sea‐salt aerosol dechlorination. Simulations were performed to quantify tropospheric [Cl], with a focus on the marine boundary layer, and quantify the global significance of Cl atom CH 4 oxidation. In agreement with observations, simulated surface levels of hydrogen chloride (HCl), the most abundant Cl y reservoir, reach several parts per billion (ppb) over polluted coastal/continental regions, with…

Abstract The accuracy of any gridded climatic data sets is as important as their availability for regional climate and ecological studies. In this study, the accuracy of estimated precipitation in central Asia from three recently developed reanalysis data sets, Modern‐Era Retrospective Analysis for Research and Applications (MERRA), ECMWF Interim Re‐Analysis (ERA‐Interim), and Climate Forecast System Reanalysis (CFSR), is evaluated through comparisons with observations from 399 stations during 1979–2010. An interpolated precipitation data set from station observations and a satellite remotely sensed data set, Tropical Rainfall Measuring Mission (TRMM) 3B42, are included in the evaluation. Major results show that MERRA data have higher accuracy than ERA‐Interim and CFSR, although they all overestimate the observed precipitation especially in late spring and early summer months, suggesting errors in their ways of representing convective precipitation in that region. In comparison, the interpolated and satellite‐sensed data, which provide no upper air information/data, have higher…

Abstract Observations of surface direct solar radiation (DSR) and visibility, particulate matter with aerodynamic diameters less than 2.5 µm (PM2.5), together with the aerosol optical thickness (AOT) taken from Moderate‐Resolution Imaging Spectroradiometer and Multiangle Imaging Spectroradiometer, were investigated to gain insight into the impact of aerosol pollution on surface solar radiation in China. The surface DSR decreased during 2004–2014 compared with 1993~2003 over eastern China, but no clear reduction was observed in remote regions with cleaner air. Significant correlations of visibility, PM2.5, and regionally averaged AOT with the surface DSR over eastern China indicate that aerosol pollution greatly affects the energy available at the surface. The net loss of surface solar radiation also reduces the surface ground temperature over eastern China. However, the slope of the linear variation of the radiation with respect to atmospheric visibility is distinctly different at different stations, implying that the main aerosol type varies regionally. The largest slope value…

Abstract In this paper, we describe a new flexible and robust NH 3 retrieval algorithm from measurements of the Infrared Atmospheric Sounding Interferometer (IASI). The method is based on the calculation of a spectral hyperspectral range index (HRI) and subsequent conversion to NH 3 columns via a neural network. It is an extension of the method presented in Van Damme et al. (2014a) who used lookup tables (LUT) for the radiance‐concentration conversion. The new method inherits the advantages of the LUT‐based method while providing several significant improvements. These include the following: (1) Complete temperature and humidity vertical profiles can be accounted for. (2) Third‐party NH 3 vertical profile information can be used. (3) Reported positive biases of LUT retrieval are reduced, and finally (4) a full measurement uncertainty characterization is provided. A running theme in this study, related to item (2), is the importance of the assumed vertical NH 3 profile. We demonstrate the advantages of allowing variable profile shapes in the retrieval. As an example, we analyze how…

Abstract Aircraft and satellite observations indicate the presence of ppt (ppt ≡ pmol/mol) levels of BrO in the free troposphere with important implications for the tropospheric budgets of ozone, OH, and mercury. We can reproduce these observations with the GEOS‐Chem global tropospheric chemistry model by including a broader consideration of multiphase halogen (Br‐Cl) chemistry than has been done in the past. Important reactions for regenerating BrO from its nonradical reservoirs include HOBr + Br − /Cl − in both aerosols and clouds, and oxidation of Br − by ClNO 3 and ozone. Most tropospheric BrO in the model is in the free troposphere, consistent with observations and originates mainly from the photolysis and oxidation of ocean‐emitted CHBr 3 . Stratospheric input is also important in the upper troposphere. Including production of gas phase inorganic bromine from debromination of acidified sea salt aerosol increases free tropospheric Br y by about 30%. We find HOBr to be the dominant gas‐phase reservoir of inorganic bromine. Halogen (Br‐Cl) radical chemistry as implemented here in…

Abstract This study investigated the particle size‐ and age‐dependent mixing structures of individual particles in clean and polluted air. Aerosols were classified into eight components: sea salt, mineral dust, fly ash, metal, soot, sulfates, nitrates, and organic matter (OM). Based on our aerosol classification, a particle that consists of two or more aerosol components can be defined as an internally mixed particle. Otherwise, it is considered to be an externally mixed particle. Within the internally mixed particle class, we identified four heterogeneous mixing structures: core‐shell, dumbbell, OM coating, and dispersed OM, as well as one homogeneous‐like mixing structure. Homogeneous‐like mixing mainly occurred in fine particles (<1 µm), while the frequency of heterogeneously mixed particles increased with particle size. Our study demonstrated that particle mixing structures depend on particle size and location and evolve with time. OM‐coating and core‐shell structures are important indicators for particle aging in air as long as they are distant from specific emission sources.…

Abstract Aromatic hydrocarbons are important anthropogenic precursors of tropospheric ozone and secondary organic aerosols. Here we measured ambient aromatic hydrocarbons from March 2012 to February 2014 at six rural sites in China's developed coastal regions. On average, benzene (B) comprised > 50% of total benzene (B), toluene (T), ethylbenzene (E), and xylenes (X) (BTEX) at sites in the Northeast China Plain (NECP) or in the North China Plain (NCP), whereas T, E, and X accounted for > 77% of total BTEX at sites in the Yangtze River Delta (YRD) and the Pearl River Delta in the south. BTEX at the northern sites was significantly correlated ( p < 0.01) with combustion tracer‐carbon monoxide (CO) but weakly correlated with traffic marker‐methyl tert‐butyl ether (MTBE), suggesting that their main sources were coal and biofuel/biomass burning with substantially elevated B levels during the winter heating period. In contrast, BTEX at the southern sites originated mainly from traffic‐related and/or industrial emission sources, as indicated by the poor correlations with CO but highly…

Abstract Beijing has been experiencing serious air pollution in recent years, resulting in serious impacts on the local environment and climate and on human health. In addition to individual pollution sources and weather systems, feedback between aerosols and downwelling solar radiation (DSR) and between aerosols and winds also contribute to heavy aerosol pollution. By using atmospheric visibility (VIS) to represent the relative amount of aerosol pollution during a 5 week observation around the Asia‐Pacific Economic Cooperation (APEC) period (22 October to 25 November 2014) over a site in south Beijing, China, we show clear positive relationships between DSR and VIS and between winds and VIS. The sensitivities of daily DSR and surface winds to VIS are approximately 15.42 W/m2/km and 0.068 m/s/km, respectively. The strengthening contributions to atmospheric visibility by surface DSR‐VIS interactions and between surface wind‐aerosol interactions are estimated at approximately 15% and 12%, respectively, in south Beijing around the APEC period.

Abstract New consistent and precise computations of the production of five cosmogenic radioisotopes, 7 Be, 10 Be, 14 C, 22 Na, and 36 Cl, in the Earth's atmosphere by cosmic rays are presented in the form of tabulated yield functions. For the first time, a detailed set of the altitude profiles of the production functions is provided which makes it possible to apply the results directly as input for atmospheric transport models. Good agreement with most of the earlier published works for columnar and global isotopic production rates is shown. Altitude profiles of the production are important, in particular for such tasks as studies of strong solar particle events in the past, precise reconstructions of solar activity on long‐term scale, tracing air mass dynamics using cosmogenic radioisotopes, etc. As an example, computations of the 10 Be deposition flux in the polar region are shown for the last decades and also for a period around 780 A.D. and confronted with the actual measurements in Greenland and Antarctic ice cores.

Abstract The considerable impact of permafrost degradation on hydrology and water resources, ecosystems, human engineering facilities, and climate change requires us to carry out more in‐depth studies, at finer spatial scales, to investigate the issue. In this study, regional differences of the future permafrost changes are explored with respect to the regions (high altitude and high latitude, and in four countries) based on the surface frost index (SFI) model and multimodel and multiscenario data from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Results show the following: (1) Compared with seven other sets of driving data, Climatic Research Unit air temperature combined with Climate Forecast System Reanalysis snow data (CRU_CFSR) yield a permafrost extent with the least absolute area bias and was thus used in the simulation. The SFI model, driven by CRU_CFSR data climatology plus multimodel mean anomalies, produces a present‐day (1986–2005) permafrost area of 15.45 × 10 6 km 2 decade −1 , which compares reasonably with observations of 15.24 × 10 6 km 2…

Abstract The ability of 11 models in simulating the aerosol vertical distribution from regional to global scales, as part of the second phase of the AeroCom model intercomparison initiative (AeroCom II), is assessed and compared to results of the first phase. The evaluation is performed using a global monthly gridded data set of aerosol extinction profiles built for this purpose from the CALIOP (Cloud‐Aerosol Lidar with Orthogonal Polarization) Layer Product 3.01. Results over 12 subcontinental regions show that five models improved, whereas three degraded in reproducing the interregional variability in Z α 0–6 km , the mean extinction height diagnostic, as computed from the CALIOP aerosol profiles over the 0–6 km altitude range for each studied region and season. While the models' performance remains highly variable, the simulation of the timing of the Z α 0–6 km peak season has also improved for all but two models from AeroCom Phase I to Phase II. The biases in Z α 0–6 km are smaller in all regions except Central Atlantic, East Asia, and North and South Africa. Most of the models…

Abstract Modeling of extreme events and its dynamic behavior have always been an intriguing topic. Increase in the magnitude and frequency of extreme events has widely been reported in recent decades, which is attributed to abrupt changes in climate. Numerous studies on extreme Indian monsoon characteristics, using a coarse‐resolution data set, have pointed out significant changes in heavy precipitation pattern over India. However, these studies differ in their conclusions, emphasizing the need for a fine‐resolution analysis. The present study aims to analyze the spatiotemporal variations and trends in the extreme (wet and dry) Indian monsoon precipitation, using 0.25° × 0.25° high‐resolution gridded data for a period of 113 years (1901–2013). Significant increase in the maximum intensity of rainfall and spatial heterogeneity is observed over the past half century. In addition, significant negative trends in wet spell durations and positive trends in dry spell durations are observed over wet regions; whereas contrasting trends are observed over dry regions. A shift in the frequency…

Abstract Measurements of hydroxyl (OH) and hydroperoxy (HO 2 *) radical concentrations were made at the Pasadena ground site during the CalNex‐LA 2010 campaign using the laser‐induced fluorescence‐fluorescence assay by gas expansion technique. The measured concentrations of OH and HO 2 * exhibited a distinct weekend effect, with higher radical concentrations observed on the weekends corresponding to lower levels of nitrogen oxides (NO x ). The radical measurements were compared to results from a zero‐dimensional model using the Regional Atmospheric Chemical Mechanism‐2 constrained by NO x and other measured trace gases. The chemical model overpredicted measured OH concentrations during the weekends by a factor of approximately 1.4 ± 0.3 (1 σ ), but the agreement was better during the weekdays (ratio of 1.0 ± 0.2). Model predicted HO 2 * concentrations underpredicted by a factor of 1.3 ± 0.2 on the weekends, while measured weekday concentrations were underpredicted by a factor of 3.0 ± 0.5. However, increasing the modeled OH reactivity to match the measured total OH reactivity…

Abstract We developed an updated nonstationary bias‐correction method for a monthly global climate model of temperature and precipitation. The proposed method combines two widely used quantile mapping bias‐correction methods to eliminate potential illogical values of the variable. Instead of empirical parameter estimation in the more‐common quantile mapping method, our study compared bias‐correction performance when parametric or nonparametric procedures were used to estimate the probability distribution. The results showed our proposed bias‐correction method to be very effective in reducing the model bias: it removed over 80% and 83% of model bias for surface air temperature and precipitation, respectively, during the validation period. Compared with a widely used method of bias correction (delta change), our proposed technique demonstrates improved correction of the distribution of variables. In addition, nonparametric estimation procedures further reduced the mean absolute errors in temperature and precipitation during the validation period by approximately 2% and 0.4%,…

An intercomparison of high-latitude precipitation characteristics from observation-based and reanalysis products is performed. In particular the precipitation products from CloudSat provide an independent assessment to other widely used products, these being the observationally-based GPCP, GPCC and CMAP products and the ERA-Interim, MERRA and NCEP-DOE R2 reanalyses. Seasonal and annual total precipitation in both hemispheres poleward of 55° latitude is considered in all products, and CloudSat is used to assess intensity and frequency of precipitation occurrence by phase, defined as rain, snow or mixed phase. Furthermore, an independent estimate of snow accumulation during the cold season was calculated from the Gravity Recovery and Climate Experiment (GRACE). The intercomparison is performed for the 2007-2010 period when CloudSat was fully operational. It is found that ERA- Interim and MERRA are broadly similar, agreeing more closely with CloudSat over oceans. ERA-Interim also agrees well with CloudSat estimates of snowfall over Antarctica where total snowfall from GPCP and CloudSat…

Abstract Global soil moisture records are essential for studying the role of hydrologic processes within the larger earth system. Various studies have shown the benefit of assimilating satellite‐based soil moisture data into water balance models or merging multisource soil moisture retrievals into a unified data set. However, this requires an appropriate parameterization of the error structures of the underlying data sets. While triple collocation (TC) analysis has been widely recognized as a powerful tool for estimating random error variances of coarse‐resolution soil moisture data sets, the estimation of error cross covariances remains an unresolved challenge. Here we propose a method—referred to as extended collocation (EC) analysis—for estimating error cross‐correlations by generalizing the TC method to an arbitrary number of data sets and relaxing the therein made assumption of zero error cross‐correlation for certain data set combinations. A synthetic experiment shows that EC analysis is able to reliably recover true error cross‐correlation levels. Applied to real soil…

Abstract Emissions from 15 agricultural fires in the southeastern U.S. were measured from the NASA DC‐8 research aircraft during the summer 2013 Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC 4 RS) campaign. This study reports a detailed set of emission factors (EFs) for 25 trace gases and 6 fine particle species. The chemical evolution of the primary emissions in seven plumes was examined in detail for ~1.2 h. A Lagrangian plume cross‐section model was used to simulate the evolution of ozone (O 3 ), reactive nitrogen species, and organic aerosol (OA). Observed EFs are generally consistent with previous measurements of crop residue burning, but the fires studied here emitted high amounts of SO 2 and fine particles, especially primary OA and chloride. Filter‐based measurements of aerosol light absorption implied that brown carbon (BrC) was ubiquitous in the plumes. In aged plumes, rapid production of O 3 , peroxyacetyl nitrate (PAN), and nitrate was observed with ΔO 3 /ΔCO, ΔPAN/ΔNO y , and Δnitrate/ΔNO y reaching ~0.1, ~0.3,…

Abstract Modeling the climate of urban areas is of interest for studying urban heat islands (UHIs). Reliable assessment of the primary causes of UHIs and the efficacy of various heat mitigation strategies requires accurate prediction of urban temperatures and realistic representation of land surface physical characteristics in models. In this study, we expand the capabilities of the Weather Research and Forecasting (WRF) model by implementing high‐resolution, real‐time satellite observations of green vegetation fraction (GVF) and albedo. Satellite‐based GVF and albedo replace constant values that are assumed for urban pixels in the default version of WRF. Simulations of urban meteorology in Los Angeles using the improved model show marked improvements relative to the default model. The largest improvements are for nocturnal air temperatures, with a reduction in root‐mean‐square deviation between simulations and observations from 3.8 to 1.9°C. Utilizing the improved model, we quantify relationships between surface and 2 m air temperatures versus urban fraction, GVF, albedo, distance…

We investigate particulate matter (PM2.5) data reliability in five major Chinese cities: Beijing, Shanghai, Guangzhou, Chengdu, and Shenyang by cross-validating data from the U.S. diplomatic posts and the nearby Ministry of Environmental Protection sites based on 3 years' data from January 2013. The investigation focuses on the consistency in air quality assessment derived from the two data sources. It consists of studying (i) the occurrence length and percentage of different PM2.5 concentration ranges; (ii) the air quality assessment for each city; and (iii) the winter-heating effects in Beijing and Shenyang. Our analysis indicates that the two data sources produced highly consistent air quality assessment in the five cities. This is encouraging as it would inject a much needed confidence on the air pollution measurements from China. We also provide air quality assessments on the severity and trends of the fine particulate matter pollution in the five cities. The assessments are produced by statistically constructing the standard monthly meteorological conditions for each city,…