The 50 most-cited articles in Journal of Geophysical Research Atmospheres, 2019

Abstract We outline a new and improved uncertainty analysis for the Goddard Institute for Space Studies Surface Temperature product version 4 (GISTEMP v4). Historical spatial variations in surface temperature anomalies are derived from historical weather station data and ocean data from ships, buoys, and other sensors. Uncertainties arise from measurement uncertainty, changes in spatial coverage of the station record, and systematic biases due to technology shifts and land cover changes. Previously published uncertainty estimates for GISTEMP included only the effect of incomplete station coverage. Here, we update this term using currently available spatial distributions of source data, state‐of‐the‐art reanalyses, and incorporate independently derived estimates for ocean data processing, station homogenization, and other structural biases. The resulting 95% uncertainties are near 0.05 °C in the global annual mean for the last 50 years and increase going back further in time reaching 0.15 °C in 1880. In addition, we quantify the benefits and inherent uncertainty due to the GISTEMP…

Abstract The year 2019 marks the thirtieth anniversary of the development of the first regional climate model (RCM), and here an overview is provided of the progress in regional modeling research and of the main challenges lying ahead. RCMs were primarily developed to provide fine‐scale climate information for impact studies, but they have evolved into general and multipurpose modeling tools. Among the main achievements in RCM research the focus is on: the development of community RCMs applicable to a wide variety of studies and regional contexts; the increase of model simulation length up to centennial scales and spatial resolutions up to convection‐permitting scales (few kilometers), leading to a better understanding of regional to local climate change signals; the development of fully coupled Regional Earth System Models; the inception of intercomparison projects culminating in the international Coordinated Regional Climate Downscaling Experiment; the extensive use of RCM simulations for impact assessments; and the involvement of the scientific community from developing countries…

Abstract The Whole Atmosphere Community Climate Model version 6 (WACCM6) is a major update of the whole atmosphere modeling capability in the Community Earth System Model (CESM), featuring enhanced physical, chemical and aerosol parameterizations. This work describes WACCM6 and some of the important features of the model. WACCM6 can reproduce many modes of variability and trends in the middle atmosphere, including the quasi‐biennial oscillation, stratospheric sudden warmings, and the evolution of Southern Hemisphere springtime ozone depletion over the twentieth century. WACCM6 can also reproduce the climate and temperature trends of the 20th century throughout the atmospheric column. The representation of the climate has improved in WACCM6, relative to WACCM4. In addition, there are improvements in high‐latitude climate variability at the surface and sea ice extent in WACCM6 over the lower top version of the model (CAM6) that comes from the extended vertical domain and expanded aerosol chemistry in WACCM6, highlighting the importance of the stratosphere and tropospheric chemistry…

Abstract New particle formation (NPF) represents the first step in the complex processes leading to formation of cloud condensation nuclei. Newly formed nanoparticles affect human health, air quality, weather, and climate. This review provides a brief history, synthesizes recent significant progresses, and outlines the challenges and future directions for research relevant to NPF. New developments include the emergence of state‐of‐the‐art instruments that measure prenucleation clusters and newly nucleated nanoparticles down to about 1 nm; systematic laboratory studies of multicomponent nucleation systems, including collaborative experiments conducted in the Cosmics Leaving Outdoor Droplets chamber at CERN; observations of NPF in different types of forests, extremely polluted urban locations, coastal sites, polar regions, and high‐elevation sites; and improved nucleation theories and parameterizations to account for NPF in atmospheric models. The challenges include the lack of understanding of the fundamental chemical mechanisms responsible for aerosol nucleation and growth under…

Abstract The stratosphere can have a significant impact on winter surface weather on subseasonal to seasonal (S2S) timescales. This study evaluates the ability of current operational S2S prediction systems to capture two important links between the stratosphere and troposphere: (1) changes in probabilistic prediction skill in the extratropical stratosphere by precursors in the tropics and the extratropical troposphere and (2) changes in surface predictability in the extratropics after stratospheric weak and strong vortex events. Probabilistic skill exists for stratospheric events when including extratropical tropospheric precursors over the North Pacific and Eurasia, though only a limited set of models captures the Eurasian precursors. Tropical teleconnections such as the Madden‐Julian Oscillation, the Quasi‐Biennial Oscillation, and El Niño–Southern Oscillation increase the probabilistic skill of the polar vortex strength, though these are only captured by a limited set of models. At the surface, predictability is increased over the United States, Russia, and the Middle East for…

Abstract Aerosols have significant and complex impacts on regional climate in East Asia. Cloud‐aerosol‐precipitation interactions (CAPI) remain most challenging in climate studies. The quantitative understanding of CAPI requires good knowledge of aerosols, ranging from their formation, composition, transport, and their radiative, hygroscopic, and microphysical properties. A comprehensive review is presented here centered on the CAPI based chiefly, but not limited to, publications in the special section named EAST‐AIRcpc concerning (1) observations of aerosol loading and properties, (2) relationships between aerosols and meteorological variables affecting CAPI, (3) mechanisms behind CAPI, and (4) quantification of CAPI and their impact on climate. Heavy aerosol loading in East Asia has significant radiative effects by reducing surface radiation, increasing the air temperature, and lowering the boundary layer height. A key factor is aerosol absorption, which is particularly strong in central China. This absorption can have a wide range of impacts such as creating an imbalance of…

Abstract Having recognized the limitations in spatial representativeness and/or temporal coverage of (i) current ground ETa observations and (ii) land surface model‐ and remote sensing‐based ETa estimates due to uncertainties in soil and vegetation parameters, a calibration‐free nonlinear complementary relationship (CR) model is employed with inputs of air and dew‐point temperature, wind speed, and net radiation to estimate 0.1°, monthly ETa over China during 1982–2012. The modeled ETa rates were first validated against 13 eddy‐covariance measurements, producing Nash‐Sutcliffe efficiency values in the range of 0.72–0.94. On the basin scale, the modeled ETa values yielded a relative bias of 6%, and a Nash‐Sutcliffe efficiency value of 0.80 in comparison with water‐balance‐derived evapotranspiration rates across 10 major river basins in China, indicating the CR‐simulated ETa rates reliable over China. Further evaluations suggest that the CR‐based ETa product is more accurate than seven other mainstream ETa products. The 31‐year mean annual ETa value decreases from the southeast to the…

Abstract One of the largest sources of uncertainty in estimates of global temperature change is that associated with the correction of systematic errors in sea surface temperature (SST) measurements. Despite recent work to quantify and reduce these errors throughout the historical record, differences between analyses remain larger than can be explained by the estimated uncertainties. We revisited the method used to estimate systematic errors and their uncertainties in version 3 of the Met Office Hadley Centre SST data set, HadSST. Using comparisons with oceanographic temperature profiles, we make estimates of biases associated with engine room measurements and insulated buckets and constrain the ranges of two of the more uncertain parameters in the bias estimation: the timing of the transition from uninsulated to insulated buckets in the middle twentieth century and the estimated fractions of different measurement methods used. Here, we present HadSST.4.0.0.0, based on release 3.0.0 and 3.0.1 of the International Comprehensive Ocean‐Atmosphere Data Set supplemented by drifting buoy…

Abstract Atmospheric rivers (ARs) are now widely known for their association with high‐impact weather events and long‐term water supply in many regions. Researchers within the scientific community have developed numerous methods to identify and track of ARs—a necessary step for analyses on gridded data sets, and objective attribution of impacts to ARs. These different methods have been developed to answer specific research questions and hence use different criteria (e.g., geometry, threshold values of key variables, and time dependence). Furthermore, these methods are often employed using different reanalysis data sets, time periods, and regions of interest. The goal of the Atmospheric River Tracking Method Intercomparison Project (ARTMIP) is to understand and quantify uncertainties in AR science that arise due to differences in these methods. This paper presents results for key AR‐related metrics based on 20+ different AR identification and tracking methods applied to Modern‐Era Retrospective Analysis for Research and Applications Version 2 reanalysis data from January 1980 through…

Abstract Global reanalysis products are extensively used for hydrologic applications in sparse data regions. European Centre for Medium‐Range Weather Forecasts Re‐Analysis version 5 (ERA‐5), among the new‐era reanalysis products, has been significantly improved for horizontal and vertical resolutions and data assimilation. However, the new‐era reanalysis products (ERA‐5, Climate Forecast System Reanalysis, ERA‐Interim, Modern Era Retrospective Analysis for Research and Applications version 2, and Japanese 55‐year Reanalysis Project) have not been evaluated for the hydrologic applications in India specially to understand if ERA‐5 outperforms the other reanalysis products or not. Here we use the five new‐era reanalysis products for the monsoon (June–September) season precipitation, maximum ( T max ) and minimum ( T min ) temperatures, total runoff, evapotranspiration, and soil moisture against the observations from India Meteorological Department in India for 1980–2018. We use a well‐calibrated and evaluated hydrological model (the Variable Infiltration Capacity model) to simulate…

Abstract The African continent continuously experiences extreme aerosol load conditions, during which the World Health Organization clean air standard of 10 μg/m3 of PM2.5 mass is systematically exceeded. Africa holds the world largest source of desert dust emissions, undergoes strong industrial growth, and produces approximately a third of the Earth's biomass burning aerosol particles. Sub‐Saharan biomass burning is driven by agricultural practices, such as burning fields and bushes in the postharvest season for fertilization, land management, and pest control. Thus, these emissions are predominantly anthropogenic. Here we use global atmospheric composition, climate, and health models to simulate the chemical composition of the atmosphere and calculate the mortality rates for Africa by distinguishing between purely natural, industrial/domestic, and biomass burning emissions. Air quality‐related deaths in Africa rank within the top leading causes of death in Africa. Our results of ~780,000 premature deaths annually point to the extensive health impacts of natural emissions, high…

Abstract A primary goal of the Deep Blue (DB) project is to create consistent long‐term aerosol data records, suitable for climate studies, using multiple satellite instruments. In order to continue Earth Observing System (EOS)‐era aerosol products into the Joint Polar Satellite System era, we have successfully ported the DB algorithm to process data from the Visible Infrared Imaging Radiometer Suite (VIIRS). Although the basic structure of the VIIRS algorithm is similar to that for the Moderate Resolution Imaging Spectroradiometer (MODIS), many enhancements have been made compared to the MODIS collection 6 (C6) version. Most have also been implemented in the latest MODIS Collection 6.1 (C6.1). For example, a new smoke mask was developed based on the spectral curvature of measured reflectance to distinguish biomass burning smoke from weakly absorbing urban/industrial aerosols. Consequently, a new aerosol‐type flag was added into the VIIRS DB data set. In addition, new dust models have been developed to account for the nonsphericity of mineral dust. As a result, a discontinuity in…

Abstract The Deep Blue (DB) algorithm has been used to retrieve aerosol optical depth (AOD) and Ångström exponent (AE) over land from multiple satellite instruments, including the Moderate Resolution Imaging Spectroradiometers (MODIS) aboard the Terra and Aqua platforms and the Visible Infrared Imaging Radiometer Suite (VIIRS). This study first validates the latest MODIS (Collection 6.1) and VIIRS (Version 1) DB data products against Aerosol Robotic Network observations. On global average, the typical level of uncertainty in AOD is slightly better than ±(0.05 + 20%) relative to Aerosol Robotic Network. AE is quantitatively more uncertain but qualitatively shows skill at distinguishing between fine‐mode and coarse‐mode dominated aerosol columns. Results are also compared with the previous MODIS Collection 6. The stability of the three DB data sets ranges from 0.005–0.01 AOD per decade. Second, spatial and temporal patterns in AOD and AE are compared between the three data sets. It is found that they all show similar patterns of spatial coverage, which is predominantly linked to cloud…

Abstract The South American low‐level jet (SALLJ) east of the Andes plays an important role in regional weather and hydrology by transporting large amounts of moisture from the Amazon to the subtropics. A new methodology is introduced for detecting SALLJ events based on seasonal‐percentile thresholds of wind speed and wind shear. Direct comparisons are made between new and conventional fixed‐threshold methods. Identification of SALLJ events is compared between five different reanalysis products and validated against available radiosonde observations. A new climatology of the SALLJ is presented for one particular reanalysis during a 38‐year period (1979–2016). Based on this new definition, the present study analyzes the spatiotemporal variability of the SALLJ on seasonal and interannual timescales, as well as trends in the jet over recent decades. Results show that the interannual variability of the jet's strength and frequency is significantly modulated by the El Niño Southern Oscillation, especially during spring. Trends in the vertically integrated moisture transport during SALLJ…

Abstract Observations from a wintertime and summertime field campaign are used to assess the relationship between black and brown carbon (BC and BrC, respectively) optical properties and particle composition and coating state. The wintertime campaign, in Fresno, CA, was impacted by primary emissions from residential wood burning, secondary organic and inorganic particle formation, and BC from motor vehicles. Two major types of BrC were observed in wintertime. One occurred primarily at night—the result of primary biomass burning emissions. The second was enhanced in daytime and strongly associated with particulate nitrate and the occurrence of fog. The biomass‐burning‐derived BrC absorbed more strongly than the nitrate‐associated BrC but had a weaker wavelength dependence. The wintertime BC‐specific mass absorption coefficient (MAC BC ) exhibited limited dependence on the ensemble‐average coating‐to‐BC mass ratio ( R coat‐rBC ) at all wavelengths, even up to R coat‐rBC of ~5. For the summertime campaign, in Fontana, CA, BC dominated the light absorption, with negligible BrC…

Abstract The Tracking Atmospheric Rivers Globally as Elongated Targets (tARget) algorithm is further developed to Version 3, adding the capability to track atmospheric river (AR) life cycles along with other refinement. The results indicate AR genesis is more frequent toward the western boundaries of midlatitude ocean basins and nearby upstream land areas (~1 month −1 ) compared to the eastern boundaries (~0.5 month −1 ) and least frequent in tropical and polar areas (reaching toward 0). AR termination is more frequent toward the northeastern sectors of North Pacific/Atlantic and adjacent downstream land areas and in the Southern Ocean near Antarctica (~1 month −1 ) compared to the adjacent ocean sectors (~0.5 month −1 ) and least frequent in tropical areas and interior Antarctica where AR genesis is similarly infrequent. ARs tend to be longer‐lived when the genesis (termination) occurs toward the western (eastern) boundaries of midlatitude ocean basins and adjacent land areas (maximum lifetime >72 hr) compared to the opposite side of the ocean basins (24–72 hr) and when terminated…

Abstract The stratosphere has been identified as an important source of predictability for a range of processes on subseasonal to seasonal (S2S) time scales. Knowledge about S2S predictability within the stratosphere is however still limited. This study evaluates to what extent predictability in the extratropical stratosphere exists in hindcasts of operational prediction systems in the S2S database. The stratosphere is found to exhibit extended predictability as compared to the troposphere. Prediction systems with higher stratospheric skill tend to also exhibit higher skill in the troposphere. The analysis also includes an assessment of the predictability for stratospheric events, including early and midwinter sudden stratospheric warming events, strong vortex events, and extreme heat flux events for the Northern Hemisphere and final warming events for both hemispheres. Strong vortex events and final warming events exhibit higher levels of predictability as compared to sudden stratospheric warming events. In general, skill is limited to the deterministic range of 1 to 2 weeks.…

Abstract Heat waves lead to major impacts on human health, food production, and ecosystems. To assess their predictability and how they are projected to change under global warming, it is crucial to improve our understanding of the underlying processes affecting their occurrence and intensity under present‐day climate conditions. Beside greenhouse gas forcing, processes in the different components of the climate system—in particular the land surface, atmospheric circulation, and the oceans—may play a key role in changing the odds for a particular event. This study aims to identify the role of the individual drivers for five heat waves (and, in some cases, of concurrent droughts) in the recent decade. Simulations are performed with the Community Earth System Model using nudging of horizontal atmospheric circulation and prescription of soil moisture. The fully constrained model accurately reproduces how anomalous an event was. Factorial experiments, which force the model toward observations for one or several key components at a time, allow us to identify how much of the observed…

Abstract One of the world's largest revegetation programs, the Grain for Green Project (GfGP), has been taking place on the Loess Plateau of China since 1999. Such massive revegetation causes changes in the region's hydrological cycle, water availability, and ecological sustainability through enhanced evapotranspiration ( ET ). Here we quantify effects of the GfGP's revegetation on ET over this water‐stressed region. Our approach involves use of a modified Priestley‐Taylor Jet Propulsion Laboratory (PT‐JPL) model, incorporating vegetation dynamics as a new component. The original PT‐JPL model has been expanded from site scale to regional scale, thereby allowing its application to the Loess Plateau. The modified PT‐JPL model was calibrated and validated against flux tower‐measured and water balance‐based ET observations. The model performed well at a regional scale with the incorporation of vegetation dynamics. To quantify the net effect of revegetation on evaporative water consumption after the GfGP, we compared scenarios with and without revegetation. We find the revegetation has…

Abstract Heat waves and urban heat islands (UHIs) may interact together, but the dependence of their interaction on background climate is unclear. Hourly meteorological observations in June to August from 2013 to 2015 collected in the megacities of Beijing (temperate semihumid monsoon climate), Shanghai (subtropical humid monsoon climate), and Guangzhou (marine subtropical monsoon climate) in China were used to study the interaction. At each megacity, eight rural stations and eight urban stations, respectively, were selected to study the UHI. Although under different background climates, UHIs in Beijing and Guangzhou shared a similar diurnal variability, that is, higher in the nighttime. However, the diurnal cycle is opposite for Shanghai if rural coastal stations were selected as rural reference stations. During heat wave periods, daytime (10:00–16:00) UHIs were intensified by 0.9 ± 0.13 (mean ± 1 standard deviation) °C in Shanghai, nighttime (22:00–4:00) UHIs were intensified by 0.9 ± 0.36 and 0.8 ± 0.20 °C in Beijing and Guangzhou, respectively. The surface solar radiation during…

Abstract Satellite precipitation estimates (SPEs) have been widely used in various applications. However, when applied to small basins and regions, the spatial resolution of SPEs is too coarse. In this study, we present three downscaling algorithms based upon the relationships between SPEs and cloud optical and microphysical properties in northeast Austria. Different downscaling techniques, namely, multiple linear regression, artificial neural networks, and spline interpolation, were adopted for the downscaling of Integrated Multi‐satellitE Retrievals for GPM (IMERG) precipitation data. In this respect, linear and nonlinear relationship among IMERG data and different cloud variables, such as cloud effective radius, cloud optical thickness, and cloud water path, was evaluated. Downscaled SPEs, as well as the original IMERG product, were subsequently validated using 54 rain gauges at a daily timescale. According to the results, all downscaled products were more accurate than the original IMERG data. Furthermore, all downscaling techniques captured the spatial patterns of precipitation…

Abstract Geoengineering methods could potentially offset aspects of greenhouse gas‐driven climate change. However, before embarking on any such strategy, a comprehensive understanding of its impacts must be obtained. Here, a 20‐member ensemble of simulations with the Community Earth System Model with the Whole Atmosphere Community Climate Model as its atmospheric component is used to investigate the projected hydroclimate changes that occur when greenhouse gas‐driven warming, under a high emissions scenario, is offset with stratospheric aerosol geoengineering. Notable features of the late 21st century hydroclimate response, relative to present day, include a reduction in precipitation in the Indian summer monsoon, over much of Africa, Amazonia and southern Chile and a wintertime precipitation reduction over the Mediterranean. Over most of these regions, the soil desiccation that occurs with global warming is, however, largely offset by the geoengineering. A notable exception is India, where soil desiccation and an approximate doubling of the likelihood of monsoon failures occurs.…

Abstract The Tibetan Plateau (TP) is an ecologically fragile region that is sensitive to climate change. In the context of global climate change, the climate change trends of the TP and the vegetation dynamic response need to be investigated. Based on in situ meteorological data, Satellite Pour l'Observation de la Terre vegetation data, and Moderate Resolution Imaging Spectroradiometer land cover data, a comprehensive analysis was conducted to determine the trends of climate parameters in the TP region at different time scales (long term: 1960–2014; midterm: 1980–2014; short term 1999–2014). A consistent warming trend was observed for different temporal scales, while a warming slowdown was identified during 1999 and 2014. The warming rate was also shown to be much higher in the high‐altitude regions (>4,000 m), especially at midterm and short‐term time scales. A new cloud‐free time series of vegetation index data sets was reconstructed, and the vegetation density showed a general increasing trend along with a warming trend in the TP. The regions showing significant increases…

Abstract Investigating the spatiotemporal link among different drought types is of great significance for understanding the causative mechanism of droughts. In this study, we propose a new approach for connecting meteorological and hydrological drought events by considering their overlaps in drought persistence time and spatial coverage in three dimensions (i.e., latitude, longitude, and time). The variable infiltration capacity model with daily hydrometeorological observations in and around the Yellow River basin for the period of 1961–2012 is used to derive the standardized precipitation evapotranspiration index and standardized runoff index. Results show that minor meteorological drought events with short durations and small areas generally are not prone to cause a hydrological drought, especially in winter for the source region, leading to failure matches between the two drought types. By contrast, meteorological droughts of large magnitudes are more likely to influence a hydrological drought in different ways. In three dimensions, the transformation from meteorological to…

Abstract Deposition of mineral dust into ocean fertilizes ecosystems and influences biogeochemical cycles and climate. In situ observations of dust deposition are scarce, and model simulations depend on the highly parameterized representations of dust processes with few constraints. By taking advantage of satellites' routine sampling on global and decadal scales, we estimate African dust deposition flux and loss frequency (a ratio of deposition flux to mass loading) along the trans‐Atlantic transit using the three‐dimensional distributions of aerosol retrieved by spaceborne lidar (Cloud‐Aerosol Lidar with Orthogonal Polarization [CALIOP]) and radiometers (Moderate Resolution Imaging Spectroradiometer [MODIS], Multiangle Imaging Spectroradiometer [MISR], and Infrared Atmospheric Sounding Interferometer [IASI]). On the basis of a 10‐year (2007‐2016) and basin‐scale average, the amount of dust deposition into the tropical Atlantic Ocean is estimated at 136‐222 Tg/year. The 65‐83% of satellite‐based estimates agree with the in situ climatology within a factor of 2. The magnitudes of…

Abstract The spatial‐temporal changes in terrestrial water storage (TWS) over the Tibetan Plateau (TP) and six selected basins during 2003–2014 were analyzed by applying the Gravity Recovery and Climate Experiment data and the extended Variable Infiltration Capacity‐glacier model, including the upstream of Yangtze (UYA), Yellow (UYE), Brahmaputra (UB), and Indus river basins and the Inner TP and the Qaidam Basin. The possible causes of TWS changes were investigated from the perspective of surface water balance and TWS components through multisource data and the Variable Infiltration Capacity‐glacier model. There was a strong spatial heterogeneity in changes of Gravity Recovery and Climate Experiment TWS in the TP—with apparent mass accumulation in central and northern TP and a sharp decreasing trend in southern and northwestern TP. The TWS changes in the TP were mostly attributed to variations in precipitation and evapotranspiration from the perspective of land‐surface water balance. Precipitation played a dominant role on the TWS accumulation in the UYA and UYE, while…

Abstract Current nuclear arsenals used in a war between the United States and Russia could inject 150 Tg of soot from fires ignited by nuclear explosions into the upper troposphere and lower stratosphere. We simulate the climate response using the Community Earth System Model‐Whole Atmosphere Community Climate Model version 4 (WACCM4), run at 2° horizontal resolution with 66 layers from the surface to 140 km, with full stratospheric chemistry and with aerosols from the Community Aerosol and Radiation Model for Atmospheres allowing for particle growth. We compare the results to an older simulation conducted in 2007 with the Goddard Institute for Space Studies ModelE run at 4° × 5° horizontal resolution with 23 levels up to 80 km and constant specified aerosol properties and ozone. These are the only two comprehensive climate model simulations of this scenario. Despite having different features and capabilities, both models produce similar results. Nuclear winter, with below freezing temperatures over much of the Northern Hemisphere during summer, occurs because of a reduction of…

Abstract Using multiple data sources, favorable conditions for the 2019 SSW event and its predictive skill from 11 subseasonal to seasonal (S2S) forecast models are explored in this study. This mixed‐type (displacement to split) SSW event occurred under moderate El Niño, easterly quasi‐biennial oscillation (QBO) together with solar minimum, and phases 4–6 of the Madden‐Julian oscillation (MJO). A strong positive PNA formed and developed in the troposphere before this SSW event, which is associated with enhanced and wave‐1‐dominated eddy heat flux. The predictive limit to this SSW onset is beyond 18 days in most S2S models, longer than the average predictive limit in existing literature. This high predictive skill may originate from the favorable initial conditions (QBO, MJO) and boundary conditions (moderate El Niño, solar minimum). Although some difference in the predictability of the 2019 SSW onset is found between models, most models well forecast its onset at a lead time of 18 days. More than 50% of the ensemble members initialized on 13 December forecast the zonal wind…

Abstract A method has been developed to extend the Global Inundation Estimate from Multiple Satellites (GIEMS). The method presented here is based on retrieval principals similar to GIEMS but with an updated estimation of microwave emissivity in order to be less dependent on ancillary data and with some changes to the final surface water estimation to correct a known overestimation over low vegetation areas. The new methodology, GIEMS‐2, provides monthly estimates of surface water extent, including open water, wetlands, or rice paddies, and it has been applied to the Special Sensor Microwave/Imager and the Special Sensor Microwave Imager Sounder intercalibrated observations to produce a global data record of surface water extent from 1992 to 2015, on an equal area grid of 0.25° × 0.25° at the equator (∼25 km). The time series have been thoroughly evaluated: they are seamless and do not show any obvious artifact related to changes in satellite instrumentation over the ∼25 years. Comparisons with precipitation estimates show good agreement, displaying expected patterns related to…

Abstract Amazonian deforestation is causing notable changes in the hydrological cycle by altering important precipitation characteristics. This study uses daily rainfall time series data from 112 rain gauges and a recent yearly 1‐km land use data set covering the period from 1974 to 2012 to evaluate the effects of the extent of deforestation at different spatial scales on the onset of the rainy season and on the duration of dry spells in southern Amazonia. Correlation analyses indicate a delay in the onset of 0.12–0.17 days per percent increase in deforestation. Analysis of cumulative probability density functions emphasizes that the likelihood of rainy season onset occurring earlier than normal decreases as the local deforestation fraction increases. In addition, the probability of occurrence of dry spells in the early and late rainy season is higher in areas with greater deforestation. The delayed onset and longer dry spell events in highly deforested areas increase the climate risk to agriculture in the region.

Abstract Atmospheric visibility is often used as a proxy for ambient air quality. However, in addition to the concentrations of particulate matter, visibility is also affected by meteorological conditions. The relative contributions of PM 2.5 concentrations and meteorological conditions to visibility are not yet clear. In this study, the individual contributions of PM 2.5 concentrations and the relative humidity (RH) to the visibility are measured based on observations at 12 stations in Beijing. We find that visibility decreases quickly as PM 2.5 concentrations increase at clean condition and decreases slowly when PM 2.5 concentrations exceed 100 μg/m 3 . The visibility decreases linearly as PM 2.5 concentrations increase when PM 2.5 concentrations lower than 50 μg/m 3 ; however, there tends to be an exponential relationship as PM 2.5 concentrations increase. The PM 2.5 concentrations can explain 50% of the variance in visibility in clean and high‐humidity environments, whereas this fraction decreases to 10–15% when the PM 2.5 concentrations exceed 200 μg/m 3 . In contrast, RH has…

Abstract Sudden stratospheric warming (SSW) events can exhibit long‐lasting surface impacts that promise improvements in medium‐range to seasonal predictability. Their surface impact is dominated by the negative phase of the North Atlantic Oscillation (NAO). Hence, the question arises if stratospheric variability, and in particular the frequency of SSW events, can in turn be estimated from surface NAO conditions. This is especially relevant for the period before frequent upper air observations became available, while daily surface observations of the NAO date back to 1850. The surface impact is here quantified by NAO characteristics that are commonly observed after SSW events: a switch from a positive to a negative NAO and an extended persistence of the negative NAO, termed NAO events. Two thirds of SSW events are found to be followed by either a persistence or switch NAO event, and a quarter of SSW events are followed by both. On the other hand, less than 25% of winter surface NAO events are preceded by a SSW event. Based on these findings, an index purely based on surface NAO…

Abstract Lake ice is a robust indicator of climate change. The availability of information contained in Moderate Resolution Imaging Spectroradiometer daily snow products from 2000 to 2017 could be greatly improved after cloud removal by gap filling. Thresholds based on open water pixel numbers are used to extract the freezeup start and breakup end dates for 58 lakes on the Tibetan Plateau (TP); 18 lakes are also selected to extract the freezeup end and breakup start dates. The lake ice durations are further calculated based on freezeup and breakup dates. Lakes on the TP begin to freezeup in late October and all the lakes start the ice cover period in mid‐January of the following year. In late March, some lakes begin to break up, and all the lakes end the ice cover period in early July. Generally, the lakes in the northern Inner‐TP have earlier freezeup dates and later breakup dates (i.e., longer ice cover durations) than those in the southern Inner‐TP. Over 17 years, the mean ice cover duration of 58 lakes is 157.78 days, 18 (31%) lakes have a mean extending rate of 1.11 day/year,…

Abstract Research has revealed systematic changes in warming rates with elevation (EDW) in mountain regions. However, weather stations on the Tibetan plateau are mostly located at lower elevations (3,000‐4,000 m) and are nonexistent above 5,000 m, leaving critical temperature changes unknown. Satellite LST (Land Surface Temperature) can fill this gap but needs calibrating against in situ air temperatures (Tair). We develop a novel statistical model to convert LST to Tair, developed at 87 high‐elevation Chinese Meteorological Administration stations. Tair (daily maximum/minimum temperatures) is compared with Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua LST (1330 and 0130 local time) for 8‐day composites during 2002‐2017. Typically, 80‐95% of the difference between LST and Tair (ΔT) is explained using predictors including LST diurnal range, morning heating/nighttime cooling rates, the number of cloud free days/nights, and season (solar angle). LST is corrected to more closely represent Tair by subtracting modeled ΔT. We validate the model using an AWS on Zhadang Glacier…

Abstract After smoke from burning biomass is emitted into the atmosphere, chemical and physical processes change the composition and amount of organic aerosol present in the aged, diluted plume. During the fourth Fire Lab at Missoula Experiment, we performed smog‐chamber experiments to investigate formation of secondary organic aerosol (SOA) and multiphase oxidation of primary organic aerosol (POA). We simulated atmospheric aging of diluted smoke from a variety of biomass fuels while measuring particle composition using high‐resolution aerosol mass spectrometry. We quantified SOA formation using a tracer ion for low‐volatility POA as a reference standard (akin to a naturally occurring internal standard). These smoke aging experiments revealed variable organic aerosol (OA) enhancements, even for smoke from similar fuels and aging mechanisms. This variable OA enhancement correlated well with measured differences in the amounts of emitted volatile organic compounds (VOCs) that could subsequently be oxidized to form SOA. For some aging experiments, we were able to predict the SOA…

Abstract Sudden stratospheric warmings (SSWs) have been linked with anomalously cold temperatures at the surface in the middle to high latitudes of the Northern Hemisphere as climatological westerly winds in the stratosphere tend to weaken and turn easterly. However, previous studies have largely relied on reanalyses and model simulations to infer the role of SSWs on surface climate and SSW relationships with extremes have not been fully analyzed. Here, we use observed daily gridded temperature and precipitation data over Europe to comprehensively examine the response of climate extremes to the occurrence of SSWs. We show that for much of Scandinavia, winters with SSWs are on average at least 1 °C cooler, but the coldest day and night of winter is on average at least 2 °C colder than in non‐SSW winters. Anomalously high pressure over Scandinavia reduces precipitation on the northern Atlantic coast but increases overall rainfall and the number of wet days in southern Europe. In the 60 days after SSWs, cold extremes are more intense over Scandinavia with anomalously high pressure and…

Abstract Nitrate (NO3−), which is mainly oxidized from NO2 by the OH radical (OH·) and O3 in the atmosphere, is a major component of inorganic aerosols. However, the contributions of the OH· and O3 pathways to NO3− in urban aerosols and the influence of air pollution to both pathways remain unclear. Oxygen isotopes of NO3− were measured for PM2.5 in Beijing in 2014. The Δ17O‐NO3− values (17.0–32.8‰) were significantly higher in winter (27.2 ± 3.6‰) than in summer (24.2 ± 1.3‰). By estimating contributions of O3 to the NOx cycle, the Δ17O values of NO3− endmembers produced via the NO2 + OH· (P1), NO3· + DMS/HC (P2), and N2O5 hydrolysis (P3) pathways were calculated for each observation. The contributions of the three pathways (P1: 32 ± 10%, P2: 34 ± 10%, and P3: 34 ± 20% annually) were calculated using the Stable Isotope Analysis in R model. We found that NO3− formation was dominated by the O3 oxidation pathways (P2 + P3; 68 ± 23% annually, 73 ± 21% in spring, 59 ± 23% in summer, 75 ± 20% in autumn, and 69 ± 22% in winter). Moreover, PM2.5, NO2, and NO3− pollution decreased the…

Abstract The precipitation in Sichuan Basin (SB), China, exhibits pronounced diurnal variation, including minimum rainfall in daytime and a prominent peak near midnight. This study investigates the primary mechanism of precipitation diurnal variation in SB using forecasts from three summer months of 2013 produced at a 4‐km grid spacing. The model forecasts reproduce the observed spatial distributions and diurnal cycles well, including the peak precipitation in SB at around 02 local solar time (LST). Contrary to the common belief that emphasizes the solenoidal effects associated with the Tibetan and Yunnan‐Guizhou Plateaus, prominent diurnal inertial oscillations of boundary layer south‐southwesterly low‐level jet into SB are shown to play more important roles in modulating the diurnal cycles of precipitation in SB. A basinwide moisture budget analysis is performed to reveal that the moisture flux from the southeast side of the basin dominates within the diurnal oscillations of the net moisture flux into the basin, and the much enhanced nocturnal low‐level jet plays a crucial role in…

Abstract Dehydration at the tropical cold point tropopause primarily controls the entry value of water vapor to the stratosphere, with additional (uncertain) contributions from subtropical monsoonal circulations and extreme deep convection. Here we quantify the links of observed stratospheric water vapor with near‐equatorial cold point temperature ( T CP ), based on interannual variations of monthly zonal averages over the period 1993–2017. Water vapor observations are from combined Halogen Occultation Experiment and Aura Microwave Limb Sounder satellite measurements, and cold point temperatures are from high quality radiosondes and GPS satellite data. Interannual water vapor anomalies are highly correlated with T CP , and coherent patterns can be traced in space and time away from the tropical tropopause to quantify transport in the Brewer‐Dobson circulation, including diagnosing seasonal changes in circulation. Lagged regressions with T CP are used to reconstruct water vapor variations directly tied to the cold point, and these reconstructions account for a majority of the…

Abstract Spatiotemporally resolved urban fossil fuel CO2 (FFCO2) emissions are critical to urban carbon cycle research and urban climate policy. Two general scientific approaches have been taken to estimate spatiotemporally explicit urban FFCO2 fluxes, referred to here as “downscaling” and “bottom‐up.” Bottom‐up approaches can specifically characterize the CO2‐emitting infrastructure in cities but are labor‐intensive to build and currently available in few U.S. cities. Downscaling approaches, often available globally, require proxy information to allocate or distribute emissions resulting in additional uncertainty. We present a comparison of a downscaled FFCO2 emission data product (Open‐source Data Inventory for Anthropogenic CO2 (ODIAC)) to a bottom‐up estimate (Hestia) in four U.S. urban areas in an effort to better isolate and understand differences between the approaches. We find whole‐city differences ranging from −1.5% (Los Angeles Basin) to +20.8% (Salt Lake City). At the 1 km × 1 km spatial scale, comparisons reveal a low‐emission limit in ODIAC driven by saturation of the…

Abstract Results from 22 airborne field campaigns, including more than 10 million high‐resolution particle images collected in cirrus formed in situ and in convective anvils, are interpreted in terms of particle shapes and their potential impact on radiative transfer. Emphasis is placed on characterizing ice particle shapes in tropical maritime and midlatitude continental anvil cirrus, as well as in cirrus formed in situ in the upper troposphere, and subvisible cirrus in the upper tropical troposphere layer. There is a distinctive difference in cirrus ice particle shapes formed in situ compared to those in anvils that are generated in close proximity to convection. More than half the mass in cirrus formed in situ are rosette shapes (polycrystals and bullet rosettes). Cirrus formed from fresh convective anvils is mostly devoid of rosette‐shaped particles. However, small frozen drops may experience regrowth downwind of an aged anvil in a regime with RHice > ~120% and then grow into rosette shapes. Identifiable particle shapes in tropical maritime anvils that have not been impacted by…

Abstract Geoengineering with stratospheric sulfate aerosols can, to some extent, be designed to achieve different climate objectives. Here we use the state‐of‐the‐art Community Earth System Model, version 1, with the Whole Atmosphere Community Climate Model as its atmospheric component (CESM1(WACCM)), to compare surface climate and stratospheric effects of two geoengineering strategies. In one, SO2 is injected into the tropical lower stratosphere at the equator to keep global mean temperature nearly constant under an RCP8.5 scenario, as has been commonly simulated in previous studies. In another, the Geoengineering Large Ensemble (GLENS), SO2 is injected into the lower stratosphere at four different locations (30°N/S and 15°N/S) to keep global mean temperature, the interhemispheric temperature gradient, and the equator‐to‐pole temperature gradient nearly unchanged. Both simulations are effective at offsetting changes in global mean temperature and the interhemispheric temperature gradient that result from increased greenhouse gases, but only GLENS fully offsets changes in the…

Abstract We use World Wide Lightning Location Network (WWLLN) data on the radiated radio frequency electromagnetic energy per stroke to identify the upper tip of the global lightning stroke energy distribution. The mean stroke energy is about 1,000 J per stroke in the very low frequency band between 5 and 18 kHz, while the distribution used in this paper is limited to strokes in that band above 1 MJ, about 3 orders of magnitude above the mean. It is shown that these energies are representative of the tip of the optical distribution, first identified by Turman (1977) above 10 GW per stroke, which he termed “superbolts.” The distribution peaks globally in the Northern Hemisphere winter (November–February) with most superbolts being found in the North Atlantic west of Europe, the winter Mediterranean Sea, and a strong local maximum over the Andes in South America. We identify regions with somewhat fewer superbolts in the North Pacific east of Japan in winter, along the equator of the Atlantic and Indian Oceans and south of South Africa. We find very few superbolts during April–October…

Abstract We use the Weather Research and Forecasting Model with the chemistry/Data Assimilation Research Testbed (WRF‐Chem/DART) chemical weather forecasting/data assimilation system with multiconstituent data assimilation to investigate the improvement of air quality forecasts over eastern China. We assimilate surface in situ observations of sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), carbon monoxide (CO), particulate matter with diameters less than 2.5 μm (PM2.5) and 10 μm (PM10), and satellite aerosol optical depth to adjust the related anthropogenic emissions as well as the chemical initial conditions. We validate our forecast results out to 72 hr by comparison with the in situ observations. Results show that updated emissions improve the model performance between 10% and 65% root mean square error reduction for the assimilated species except particulate matter with a diameter between 2.5 and 10 μm (PM2.5‐10), which is slightly improved due to the limited anthropogenic contribution to it. In a sensitivity experiment with a different update interval, the CO…

Abstract Streamflow is controlled by multiple factors concurrently. However, the multivariate relationship between global streamflow and meteorological factors/ocean signals is rarely explored at different temporal scales. Determining a suite of factors that explain most of the variations in global streamflow at multiple scales will be of great significance for water‐resource management and prediction. Temporally dependent multivariate relationships between streamflow and meteorological factors/ocean signals in 16 of the world's large rivers were identified using wavelet transform coherence and multiple‐wavelet coherence. Prior to that, the continuous wavelet transform was used to detect temporal patterns in streamflow. The continuous wavelet transform results showed that significant annual oscillations occurred in all streamflow series over the study period, either with continuous annual periodicity or with intermittent breaks. Oscillations with periodicities of around 4 to 6 months were also found in many rivers. A comparison of the results from the wavelet transform coherence and…

Abstract Afforestation‐induced changes in water resources have attracted worldwide attention. However, a clear picture of quantitative attribution of terrestrial water storage (TWS) variation from hydroclimatic and anthropogenic factors is still lacking. In this study, a quantitative analysis of TWS variation was conducted in the Yellow River basin of China under the Grain for Green project with consideration of irrigation. The results showed that the TWS has decreased (increased) more and more quickly (slowly) in the Loess Plateau (headwater region). The TWS increase corresponded to increased runoff and soil moisture in the headwaters, and the TWS depletion corresponded to decreased runoff and groundwater in the Loess Plateau and downstream regions. Regarding the TWS change (TWSC), it exhibited a negative trend across the basin. The increase in evapotranspiration (ET) dominated the basin‐averaged TWSC reduction, while the increase in ET was highly related to the increases in vegetation cover and irrigation water use. For spatial TWSC variations, the value of precipitation minus ET…

Abstract It has recently been recognized that, in addition to greenhouse gases, anthropogenic emissions of ozone‐depleting substances (ODS) can induce long‐term trends in the Brewer‐Dobson circulation (BDC). Several studies have shown that a substantial fraction of the residual circulation acceleration over the last decades of the twentieth century can be attributed to increasing ODS. Here the mechanisms of this influence are examined, comparing model runs to reanalysis data and evaluating separately the residual circulation and mixing contributions to the mean age of air trends. The effects of ozone depletion in the Antarctic lower stratosphere are found to dominate the ODS impact on the BDC, while the direct radiative impact of these substances is negligible over the period of study. We find qualitative agreement in austral summer BDC trends between model and reanalysis data and show that ODS are the main driver of both residual circulation and isentropic mixing trends over the last decades of the twentieth century. Moreover, aging by isentropic mixing is shown to play a key role…

Abstract Brown carbon (BrC) affects the Earth's radiative balance due to its strong light absorption at short wavelengths. A custom‐made combustion chamber was used to simulate biomass and coal burning and to investigate the emission characteristics of BrC absorption. Absorption Ångström exponents (AAEs) at the wavelength pair of 370 and 880 nm ranged from 1.19 to 3.25, suggesting the possible existence of BrC in biomass‐ and coal‐burning emissions. Based on the assumption that AAEBC = 1.0, BrC from biomass burning contributed to 41–85% of the total particles light absorption at 370 nm, which is much higher than that from coal burning (15–18%). The estimated absorption emission factors of BrC at 370 nm for biomass and coal burning were 15–47 and 2–13 m2/kg, respectively. A 10 × 10 km gridded BrC optical emission inventory for biomass and coal burning in China for 2015 was developed based on the measured absorption emission factors of BrC values and high‐resolution activity data. The total annual BrC absorption cross section emissions from biomass and residential coal burning were…

Abstract Organic aerosol (OA) constituted a large fraction of aerosol particles during severe haze episodes in winter in northern China, yet our understanding of its physical and chemical processing was limited. Here we investigate the sources and processes of OA during four haze episodes in winter in 2016 using high‐resolution aerosol mass spectrometer. The PM2.5 reached 400 μg/m3 during the severest episode (Ep1) when Beijing issued a red alert and implemented strict emission controls. Our results showed that secondary OA (SOA) dominated OA during haze episodes on average accounting for 46–66% of OA and was comparable to secondary inorganic aerosol (SIA) with the SOA/SIA ratios being 0.51–0.72. Primary OA from fossil‐fuel combustion, biomass burning, and cooking presented very strong diurnal variations during haze episodes and contributed up to 60% in OA at night. Comparatively, the changes in semivolatile and low‐volatility SOA were relatively small except a substantial increase in aqueous phase‐related oxidized OA (aq‐OOA) during Ep1 with high relative humidity and aerosol water…

Abstract Quantifying the efficacy of different climate forcings is important for understanding the real‐world climate sensitivity. This study presents a systematic multimodel analysis of different climate driver efficacies using simulations from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP). Efficacies calculated from instantaneous radiative forcing deviate considerably from unity across forcing agents and models. Effective radiative forcing (ERF) is a better predictor of global mean near‐surface air temperature (GSAT) change. Efficacies are closest to one when ERF is computed using fixed sea surface temperature experiments and adjusted for land surface temperature changes using radiative kernels. Multimodel mean efficacies based on ERF are close to one for global perturbations of methane, sulfate, black carbon, and insolation, but there is notable intermodel spread. We do not find robust evidence that the geographic location of sulfate aerosol affects its efficacy. GSAT is found to respond more slowly to aerosol forcing than CO2 in the early stages of…