The 50 most-cited articles in Journal of Geophysical Research Atmospheres, 2018

Abstract We describe the construction of a new version of the Europe‐wide E‐OBS temperature (daily minimum, mean, and maximum values) and precipitation data set. This version provides an improved estimation of interpolation uncertainty through the calculation of a 100‐member ensemble of realizations of each daily field. The data set covers the period back to 1950 and provides gridded fields at a spacing of 0.25 ∘ × 0.25 ∘ in regular latitude/longitude coordinates. As with the original E‐OBS data set, the ensemble version is based on the station series collated as part of the ECA&D initiative. Station density varies significantly over the domain, and over time, and a reliable estimation of interpolation uncertainty in the gridded fields is therefore important for users of the data set. The uncertainty quantified by the ensemble data set is more realistic than the uncertainty estimates in the original version, although uncertainty is still underestimated in data‐sparse regions. The new data set is compared against the earlier version of E‐OBS and against regional gridded data sets…

Abstract Endorheic basins around the world are suffering from water and ecosystem crisis. To pursue sustainable development, quantifying the hydrological cycle is fundamentally important. However, knowledge gaps exist in how climate change and human activities influence the hydrological cycle in endorheic basins. We used an integrated ecohydrological model, in combination with systematic observations, to analyze the hydrological cycle in the Heihe River Basin, a typical endorheic basin in arid region of China. The water budget was closed for different landscapes, river channel sections, and irrigation districts of the basin from 2001 to 2012. The results showed that climate warming, which has led to greater precipitation, snowmelt, glacier melt, and runoff, is a favorable factor in alleviating water scarcity. Human activities, including ecological water diversion, cropland expansion, and groundwater overexploitation, have both positive and negative effects. The natural oasis ecosystem has been restored considerably, but the overuse of water in midstream and the use of environmental…

Abstract Air quality has been continuously improved in recent years in Beijing, yet severe haze episodes still frequently occur in winter. Here we deployed an Aerodyne high‐resolution aerosol mass spectrometer in two winter seasons during the same period to investigate the changes in aerosol chemistry from 2014 to 2016 in Beijing. Compared to 2014, submicron aerosol (PM1) species showed ubiquitous increases in mass concentrations by 10–130% in winter 2016, of which nitrate showed the largest increase among all aerosol species leading to a much higher NO3/SO4 ratio in 2016 (1.36 ± 0.90) than 2014 (0.72 ± 0.59). This result highlights an increasing role of nitrate in particulate matter pollution in recent years in Beijing. Aerosol composition and size distributions also changed significantly. Secondary inorganic species showed elevated contributions by ~10% in winter 2016 associated with corresponding decreases in organic aerosol (OA). Positive matrix factorization of OA illustrated the significant changes in both primary emissions and secondary production. While cooking OA decreased…

Abstract Evapotranspiration (ET) is a vital variable for land‐atmosphere interactions that links surface energy balance, water, and carbon cycles. The in situ techniques can measure ET accurately but the observations have limited spatial and temporal coverage. Modeling approaches have been used to estimate ET at broad spatial and temporal scales, while accurately simulating ET at regional scales remains a major challenge. In this study, we upscale ET from eddy covariance flux tower sites to the regional scale with machine learning algorithms. Five machine learning algorithms are employed for ET upscaling including artificial neural network, Cubist, deep belief network, random forest, and support vector machine. The machine learning methods are trained and tested at 36 flux towers sites (65 site years) across the Heihe River Basin and are then applied to estimate ET for each grid cell (1 km × 1 km) within the watershed and for each day over the period 2012–2016. The artificial neural network, Cubist, random forest, and support vector machine algorithms have almost identical…

Abstract Sea spray aerosol (SSA) generated by bubble bursting at the ocean surface is an important component of aerosol‐cloud interactions over remote oceans, providing the atmosphere with ice‐nucleating particles (INPs) or particles required for heterogeneous ice nucleation. Studies have shown that organic INPs are emitted during phytoplankton blooms, but changes in INP number concentrations (nINPs) due to ocean biological activity have not been directly demonstrated in natural SSA. In this study, a clean sector sampler was used to differentiate ice nucleation and composition of pristine SSA from terrestrial aerosol at the Mace Head Research Station in August 2015. Average nINPs active at −15 °C (nINPs,−15 °C) were 0.0011 L−1, and large variability (up to a factor of 200) was observed for INPs active warmer than −22 °C. Highest nINPs in the clean sector occurred during a period of elevated marine organic aerosol from offshore biological activity (M1, nINPs,−15 °C = 0.0077 L−1). A peak in nINPs was also observed in terrestrial organic aerosol (T1, nINPs,−15 °C = 0.0076 L−1). The…

Abstract This study analyzes a new high‐resolution general circulation model with regard to secondary gravity waves in the mesosphere during austral winter. The model resolves gravity waves down to horizontal and vertical wavelengths of 165 and 1.5 km, respectively. The resolved mean wave drag agrees well with that from a conventional model with parameterized gravity waves up to the midmesosphere in winter and up to the upper mesosphere in summer. About half of the zonal‐mean vertical flux of westward momentum in the southern winter stratosphere is due to orographic gravity waves. The high intermittency of the primary orographic gravity waves gives rise to secondary waves that result in a substantial eastward drag in the winter mesopause region. This induces an additional eastward maximum of the mean zonal wind at z ∼ 100 km. Radar and lidar measurements at polar latitudes and results from other high‐resolution global models are consistent with this finding. Hence, secondary gravity waves may play a significant role in the general circulation of the winter mesopause region.

Abstract Greenland Ice Sheet (GrIS) mass loss has accelerated since the turn of the twenty‐first century. Several recent episodes of rapid GrIS ablation coincided with intense moisture transport over Greenland by atmospheric rivers (ARs), suggesting that these events influence the evolution of GrIS surface mass balance (SMB). ARs likely provide melt energy through several physical mechanisms, and conversely, may increase SMB through enhanced snow accumulation. In this study, we compile a long‐term (1980–2016) record of moisture transport events using a conventional AR identification algorithm as well as a self‐organizing map classification applied to MERRA‐2 data. We then analyze AR effects on the GrIS using melt data from passive microwave satellite observations and regional climate model output. Results show that anomalously strong moisture transport by ARs clearly contributed to increased GrIS mass loss in recent years. AR activity over Greenland was above normal throughout the 2000s and early 2010s, and recent melting seasons with above‐average GrIS melt feature positive…

Abstract Using volcanic sulfur dioxide emissions in an aerosol‐climate model, we derive a time series of global‐mean volcanic effective radiative forcing (ERF) from 1979 to 2015. For 2005–2015, we calculate a global multiannual mean volcanic ERF of −0.08 W/m2 relative to the volcanically quiescent 1999–2002 period, due to a high frequency of small‐to‐moderate‐magnitude explosive eruptions after 2004. For eruptions of large magnitude such as 1991 Mt. Pinatubo, our model‐simulated volcanic ERF, which accounts for rapid adjustments including aerosol perturbations of clouds, is less negative than that reported in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) that only accounted for stratospheric temperature adjustments. We find that, when rapid adjustments are considered, the relation between volcanic forcing and volcanic stratospheric optical depth (SAOD) is 13–21% weaker than reported in IPCC AR5 for large‐magnitude eruptions. Further, our analysis of the recurrence frequency of eruptions reveals that sulfur‐rich small‐to‐moderate‐magnitude…

Abstract Short‐term Quantitative Precipitation Forecasting is important for flood forecasting, early flood warning, and natural hazard management. This study proposes a precipitation forecast model by extrapolating Cloud‐Top Brightness Temperature (CTBT) using advanced Deep Neural Networks, and applying the forecasted CTBT into an effective rainfall retrieval algorithm to obtain the Short‐term Quantitative Precipitation Forecasting (0–6 hr). To achieve such tasks, we propose a Long Short‐Term Memory (LSTM) and the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN), respectively. The precipitation forecasts obtained from our proposed framework, (i.e., LSTM combined with PERSIANN) are compared with a Recurrent Neural Network (RNN), Persistency method, and Farneback optical flow each combined with PERSIANN algorithm and the numerical model results from the first version of Rapid Refresh (RAPv1.0) over three regions in the United States, including the states of Oregon, Oklahoma, and Florida. Our experiments indicate better statistics,…

Abstract Atmospheric rivers (ARs) are narrow, long, transient, water vapor‐rich corridors of the atmosphere that are responsible for over 90% of the poleward water vapor transport in and across midlatitudes. However, the role of ARs in modulating extratropical and polar hydroclimate features (e.g., water vapor content and precipitation) has not been fully studied, even though moistening of the polar atmosphere is both a key result and amplifier of Arctic warming and sea ice melt, and precipitation is key to the surface mass balance of polar sea ice and ice sheets. This study uses the Modern‐Era Retrospective analysis for Research and Applications, Version 2 reanalysis to characterize the roles of AR water vapor transport on the column‐integrated atmospheric water vapor budget in the extratropical and polar regions of both hemispheres. Meridional water vapor transport by ARs across a given latitude (examined for 40°, 50°, 60°, and 70°) is strongly related to variations in area‐averaged (i.e., over the cap poleward of the given latitude) total water vapor storage and precipitation…

Abstract The incoming solar radiation is the essential climate variable that determines the Earth's energy cycle and climate. As long‐term high‐quality surface measurements of solar radiation are rare, satellite data are used to derive more information on its spatial pattern and its temporal variability. Recently, the EUMETSAT Satellite Application on Climate Monitoring (CM SAF) has published two satellite‐based climate data records: Surface Solar Radiation Data Set‐Heliosat, Edition 2 (SARAH‐2), and Clouds and Radiation Data Set based on AVHRR (advanced very high resolution radiometer) Satellite Measurements, Edition 2 (CLARA‐A2). Both data records provide estimates of surface solar radiation. In this study, these new climate data records are compared to surface measurements in Europe during the period 1983–2015. SARAH‐2 and CLARA‐A2 show a high accuracy compared to ground‐based observations (mean absolute deviations of 6.9 and 7.3 W/m2, respectively) highlighting a good agreement considering the temporal behavior and the spatial distribution. The results show an overall…

Abstract Nocturnal dinitrogen pentoxide (N 2 O 5 ) heterogeneous chemistry impacts regional air quality and the distribution and lifetime of tropospheric oxidants. Formed from the oxidation of nitrogen oxides, N 2 O 5 is heterogeneously lost to aerosol with a highly variable reaction probability, γ (N 2 O 5 ), dependent on aerosol composition and ambient conditions. Reaction products include soluble nitrate (HNO 3 or NO 3 − ) and nitryl chloride (ClNO 2 ). We report the first‐ever derivations of γ (N 2 O 5 ) from ambient wintertime aircraft measurements in the critically important nocturnal residual boundary layer. Box modeling of the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the eastern United States derived 2,876 individual γ (N 2 O 5 ) values with a median value of 0.0143 and range of 2 × 10 −5 to 0.1751. WINTER γ (N 2 O 5 ) values exhibited the strongest correlation with aerosol water content, but weak correlations with other variables, such as aerosol nitrate and organics, suggesting a complex, nonlinear dependence on multiple…

Abstract We examine the characteristics of secondary gravity waves (GWs) excited by a localized (in space) and intermittent (in time) body force in the atmosphere. This force is a horizontal acceleration of the background flow created when primary GWs dissipate and deposit their momentum on spatial and temporal scales of the wave packet. A broad spectrum of secondary GWs is excited with horizontal scales much larger than that of the primary GW. The polarization relations cause the temperature spectrum of the secondary GWs generally to peak at larger intrinsic periods τ I r and horizontal wavelengths λ H than the vertical velocity spectrum. We find that the one‐dimensional spectra (with regard to frequency or wave number) follow lognormal distributions. We show that secondary GWs can be identified by a horizontally displaced observer as “fishbone” or “>” structures in z − t plots whereby the positive and negative GW phase lines meet at the “knee,” z knee , which is the altitude of the force center. We present two wintertime cases of lidar temperature measurements at McMurdo,…

Abstract Renewable energy production is strongly influenced by weather and climate. Regional climate projections can be useful to quantify climate change impacts on renewable energies. With this aim, we analyze future changes of wind speed and wind energy potentials using a multimodel ensemble of EURO‐CORDEX simulations at 12 km and three‐hourly resolution, considering nine different global and regional climate model chains. A comparison between modeled historical 10 m wind speeds and ERA‐Interim‐driven evaluation runs for the same regional climate models uncovers some substantial model biases. The bias‐corrected 10 m wind speeds are extrapolated to the hub height of a wind turbine to derive gridded wind energy output (Eout). The ensemble mean responses project only small changes of mean annual and winter Eout for large parts of Europe in future decades, but a considerable decrease for summer Eout. In terms of variability, increasing intraannual and interdaily variabilities are projected for large parts of northern, central, and eastern Europe. While the ensemble spread is quite…

Abstract Drought is the most recurrent and destructive hazard in arid and semiarid regions and will only become more complex under climate change. It is vital to characterize the various types of drought, to investigate the potential factors affecting different types of drought, and to assess the relationship between drought types. In this study, the Standardized Precipitation Index and the Standardized Runoff Index were used to characterize meteorological and hydrological drought, respectively, to investigate drought characteristics and mechanisms in 17 catchments on the Loess Plateau from 1961 to 2013. Furthermore, the propagation time from meteorological to hydrological drought was explored and the potential factors influencing drought propagation time were investigated. The results indicate that the Loess Plateau has experienced an increased tendency toward both meteorological and hydrological droughts over the period 1961–2013, with hydrological drought more serious than meteorological drought at various drought assessment time scales. Moreover, average drought duration and…

Abstract Crop residue burning (CRB) is a recurring problem, during October–November, in the northwestern regions (Punjab, Haryana, and western Uttar Pradesh) of India. The emissions from the CRB source regions spread in all directions through long‐range transport mechanisms, depending upon the meteorological conditions. In recent years, numerous studies have been carried out dealing with the impact of CRB on the air quality of Delhi and surrounding areas, especially in the Indo‐Gangetic Basin (also referred to as Indo‐Gangetic Plain). In this paper, we present detailed analysis using both satellite‐ and ground‐based sources, which show an increasing impact of CRB over the eastern parts of the Indo‐Gangetic Basin and also over parts of central and southern India. The increasing trends of finer black carbon particles and greenhouse gases have accelerated since the year 2010 onward, which is confirmed by the observation of different wavelength dependent aerosol properties. Our study shows an increased risk to ambient air quality and an increased spatiotemporal extent of pollutants in…

Abstract Due to coarse spatial resolution, the application of Gravity Recovery and Climate Experiment (GRACE) data in local groundwater resource management has been limited. To overcome this issue, a downscaling approach is presented to improve the spatial resolution of GRACE‐derived groundwater storage anomalies using evapotranspiration (ET) data. The statistical downscaling method is only applied in areas where there is a strong relationship between GRACE‐derived groundwater storage (GWS) and ET, and the relationship can be established under different spatial resolutions. In this study, GWS anomalies are downscaled from 110 km to 2 km in the North China Plain, capturing subgrid heterogeneity in groundwater storage changes. The downscaled results are further validated using data from 111 observation wells from 2003 to 2014 in the Beijing Plain. The results show that the in situ groundwater levels agree with the Gravity Recovery and Climate Experiment (GRACE) downscaled results for the Beijing Plain in both interannual and monthly scales, with the correlation coefficient of 0.94 and…

Abstract Nonmethane hydrocarbons (NMHCs) have an important influence on the formation of photochemical smog and secondary organic aerosols in the atmosphere. We investigated 55 NMHCs in Chengdu from 27 October 2016 to 30 September 2017 to identify their sources, pollution levels, and secondary transformation potential. The average concentration of NMHCs in Chengdu was 41.8 ± 20.8 ppbv. Meteorological conditions play a vital role in the formation and dispersion of NMHCs, PM2.5, and O3. The photochemical conversion of NMHCs was inhibited by lower solar radiation, leading to low O3 concentrations and high NMHC concentrations. Aromatics and alkenes played a leading role in the secondary transformation, accounting for 60 ± 16% and 29 ± 11% of the total secondary formation potential, respectively. The southwest air mass trajectory was the most significant transportation path affecting the NMHC concentration in Chengdu. The results of the positive matrix factorization model indicated that there were seasonal differences in the sources of NMHCs in Chengdu. Four major emission sources were…

Abstract We present a method using Doppler lidar data for identifying the main sources of turbulent mixing within the atmospheric boundary layer. The method identifies the presence of turbulence and then assigns a turbulent source by combining several lidar quantities: attenuated backscatter coefficient, vertical velocity skewness, dissipation rate of turbulent kinetic energy, and vector wind shear. Both buoyancy‐driven and shear‐driven situations are identified, and the method operates in both clear‐sky and cloud‐topped conditions, with some reservations in precipitation. To capture the full seasonal cycle, the classification method was applied to more than 1 year of data from two sites, Hyytiälä, Finland, and Jülich, Germany. Analysis showed seasonal variation in the diurnal cycle at both sites; a clear diurnal cycle was observed in spring, summer, and autumn seasons, but due to their respective latitudes, a weaker cycle in winter at Jülich, and almost non‐existent at Hyytiälä. Additionally, there are significant contributions from sources other than convective mixing, with…

Abstract Almost all remote sensing atmospheric PM2.5 estimation methods need satellite aerosol optical depth (AOD) products, which are often retrieved from top‐of‐atmosphere (TOA) reflectance via an atmospheric radiative transfer model. Then, is it possible to estimate ground‐level PM2.5 directly from satellite TOA reflectance without a physical model? In this study, this challenging work was achieved based on a machine learning model. Specifically, we established the relationship between PM2.5, satellite TOA reflectance, observation angles, and meteorological factors in a deep learning architecture (denoted as Ref‐PM modeling). This relationship was trained with station PM2.5 measurements, and then the PM2.5 values of those locations without stations could be retrieved. Taking the Wuhan Urban Agglomeration as a case study, the results demonstrate that, compared with AOD‐PM modeling, the Ref‐PM modeling obtains a competitive performance, with sample‐based cross‐validated R2 and root‐mean‐square error values of 0.87 and 9.89 μg/m3, respectively. Also, the TOA‐reflectance‐derived…

Abstract Raindrop size distribution (RSD) characteristics of summer and winter seasons over north Taiwan are analyzed by using long‐term (~12 years) raindrop spectra from Joss‐Waldvogel disdrometer located at National Central University (24°58′N, 121°10′E), Taiwan. Along with the disdrometer data, radar reflectivity mosaic from six ground‐based radars, Tropical Rainfall Measuring Mission, Moderate Resolution Imaging Spectroradiometer, and ERA‐Interim data sets are used to establish the dynamical and microphysical characteristics of summer and winter rainfall. Significant differences in raindrop spectra of summer and winter rainfall are noticed. Winter rainfall has a higher concentration of small drops and a lower concentration of midsize and large drops when compared to summer rainfall. RSD stratified on the basis of rain rate showed a higher mass‐weighted mean diameter ( D m ) and a lower normalized intercept parameter (log 10 N w ) in summer than winter. Similarly, diurnal variation of RSD showed higher D m and lower log 10 N w values in summer as compared to winter rainfall. In…

Abstract A record‐setting temperature of 17.5°C occurred on 24 March 2015 at the Esperanza station located near the northern tip of the Antarctic Peninsula (AP). We studied the event using surface station data, satellite imagery, reanalysis data, and numerical simulations. The Moderate Resolution Imaging Spectroradiometer Antarctic Ice Shelf Image Archive provides clear evidence for disintegration and advection of sea ice, as well as the formation of melt ponds on the ice sheet surface at the base of the AP mountain range. A deep low‐pressure center over the Amundsen‐Bellingshausen Sea and a blocking ridge over the southeast Pacific provided favorable conditions for the development of an atmospheric river with a northwest‐southeast orientation, directing warm and moist air toward the AP, and triggering a widespread foehn episode. A control simulation using a regional climate model shows the existence of local topographically induced warming along the northern tip of the AP (∼60% of the full temperature signal) and the central part of the eastern AP (>90% of the full temperature…

Abstract Satellite‐based active fire data are a viable tool to understand the role of global fires in the biosphere and atmosphere. The Moderate Resolution Imaging Spectroradiometer (MODIS) sensors on Aqua and Terra satellites are nearing the end of their lives. The Visible Infrared Imaging Radiometer Suite (VIIRS) sensor on the Suomi National Polar‐orbiting Partnership satellite and the subsequent Joint Polar Satellite System series is expected to extend the MODIS active fire record. Thus, understanding the similarities of and discrepancies between the two data sets during their overlap period is important for existing applications. This study investigated the dependence of the MODIS and VIIRS fire characterization capabilities on satellite view zenith angle and the relationship between the two sensors' fire radiative power (FRP) from individual fire clusters to fire data on continental and global scales. The results indicate that the VIIRS fire characterization capability is similar across swath, whereas MODIS is strongly dependent on view zenith angle. Statistical analyses reveal…

Abstract Desert dust is one of the most important atmospheric ice‐nucleating aerosol species around the globe. However, there have been very few measurements of ice‐nucleating particle (INP) concentrations in dusty air close to desert sources. In this study we report the concentration of INPs in dust laden air over the tropical Atlantic within a few days' transport of one of the world's most important atmospheric sources of desert dust, the Sahara. These measurements were performed as part of the Ice in Clouds Experiment‐Dust campaign based in Cape Verde, during August 2015. INP concentrations active in the immersion mode, determined using a droplet‐on‐filter technique, ranged from around 10 2 m −3 at −12°C to around 10 5 m −3 at −23°C. There is about 2 orders of magnitude variability in INP concentration for a particular temperature, which is determined largely by the variability in atmospheric dust loading. These measurements were made at altitudes from 30 to 3,500 m in air containing a range of dust loadings. The ice active site density ( n s ) for desert dust dominated aerosol…

Abstract Clouds containing ice are vital for precipitation formation and are important in determining the Earth's radiative budget. However, primary formation of ice in clouds is not fully understood. In the presence of ice nucleating particles (INPs), the phase change to ice is promoted, but identification and quantification of INPs in a natural environment remains challenging because of their low numbers. In this paper, we quantify INP number concentrations in the free troposphere (FT) as measured at the High Altitude Research Station Jungfraujoch (JFJ), during the winter, spring, and summer of the years 2014–2017. INPs were measured at conditions relevant for mixed‐phase cloud formation at T = 241/242 K. To date, this is the longest timeline of semiregular measurements akin to online INP monitoring at this site and sampling conditions. We find that INP concentrations in the background FT are on average capped at 10/stdL (liter of air at standard conditions [T = 273 K and p = 1013 hPa]) with an interquartile range of 0.4–9.6/stdL, as compared to measurements during times when…

Abstract Carbonyls are an important group of volatile organic compounds (VOCs) that play critical roles in tropospheric chemistry. To better understand the formation mechanisms of carbonyl compounds, extensive measurements of carbonyls and related parameters were conducted in Beijing in summer 2008. Formaldehyde (11.17 ± 5.32 ppbv), acetone (6.98 ± 3.01 ppbv), and acetaldehyde (5.27 ± 2.24 ppbv) were the most abundant carbonyl species. Two dicarbonyls, glyoxal (0.68 ± 0.26 ppbv) and methylglyoxal (MGLY; 1.10 ± 0.44 ppbv), were also present in relatively high concentrations. An observation‐based chemical box model was used to simulate the in situ production of formaldehyde, acetaldehyde, glyoxal, and MGLY and quantify their contributions to ozone formation and RO x budget. All four carbonyls showed similar formation mechanisms but exhibited different precursor distributions. Alkenes (mainly isoprene and ethene) were the dominant precursors of formaldehyde, while both alkenes (e.g., propene, i‐butene, and cis‐2‐pentene) and alkanes (mainly i‐pentane) were major precursors of…

Abstract Reprocessed ozonesonde data from eight SHADOZ (Southern Hemisphere ADditional OZonesondes) sites have been used to derive the first analysis of uncertainty estimates for both profile and total column ozone (TCO). The ozone uncertainty is a composite of the uncertainties of the individual terms in the ozone partial pressure ( P O3 ) equation, those being the ozone sensor current, background current, internal pump temperature, pump efficiency factors, conversion efficiency, and flow rate. Overall, P O3 uncertainties (Δ P O3 ) are within 15% and peak around the tropopause (15 ± 3 km) where ozone is a minimum and Δ P O3 approaches the measured signal. The uncertainty in the background and sensor currents dominates the overall Δ P O3 in the troposphere including the tropopause region, while the uncertainties in the conversion efficiency and flow rate dominate in the stratosphere. Seasonally, Δ P O3 is generally a maximum in the March–May, with the exception of SHADOZ sites in Asia, for which the highest Δ P O3 occurs in September–February. As a first approach, we calculate sonde…

Abstract Retrievals of ice and snow are made from Ka‐ and W‐band zenith‐pointing Doppler radars at Hyytiälä, Finland, during the snow experiment component of the Biogenic Aerosols: Effects on Clouds and Climate (2014) field campaign. In a novel optimal estimation retrieval, mean Doppler velocity is exploited to retrieve a density factor parameter, which modulates the mass, shape, terminal velocity, and backscatter cross sections of ice particles. In a case study including aggregate snow and graupel we find that snow rate and ensemble mean ice density can be retrieved to within 50% of in situ measurements at the surface using dual‐frequency Doppler radar retrievals. While Doppler measurements are essential to the retrieval of particle density, the dual‐frequency ratio provides a strong constraint on particle size. The retrieved density factor is strongly correlated with liquid water path, indicating that riming is the primary process by which the density factor is modulated. Using liquid water path as a proxy for riming, profiles classified as unrimed snow, rimed snow, and graupel…

Abstract Recent studies highlight the importance of humidity in accounting for extreme values of wet‐bulb temperature, a proposed indicator of heat stress. But this result may be sensitive to the measure of heat stress used—an ongoing source of confusion.

Abstract We present atmospheric measurements of methane (CH 4 ) and ethane (C 2 H 6 ) taken aboard a National Oceanic and Atmospheric Administration WP‐3D research aircraft in 2015 over oil‐ and natural gas‐producing regions of the central and western United States. We calculate emission rates from the horizontal flux of CH 4 and C 2 H 6 in the planetary boundary layer downwind of five of these oil‐ and gas‐producing regions: the Bakken in North Dakota, the Barnett in Texas, the Denver Basin in Colorado, the Eagle Ford in Texas, and the Haynesville in Texas and Louisiana. In general, we find that the enhancement of C 2 H 6 relative to CH 4 in the atmosphere is similar to their relative abundances in locally produced natural gas. For the Bakken and Barnett regions, both absolute CH 4 emissions and the percentage of produced natural gas emitted to the atmosphere are consistent with previous studies. The percentage of produced natural gas emitted to the atmosphere was lower than in previous studies in the Denver Basin and the Haynesville regions, which may be due to a decrease in…

Abstract Convection‐permitting simulations are used to investigate the key mechanism of Meiyu precipitation diurnal cycle over China. Six days from the 2014 Meiyu season are used to produce a “north” composite rainband over the Yangtze‐Huaihe River Basin and another 6 days used to produce a “south” composite band. Both rainbands have peak rainfall in the early morning, while the south band has a secondary peak in the afternoon. Low‐level ageostrophic winds (AGWs) are found to exhibit diurnal cycles with clockwise rotations and their directions, and magnitudes depend on the background geostrophic monsoon flows. Net moisture flux into a control volume enclosing each rainband is almost purely due to AGWs. For both rainbands, net flux reaches maximum at ~04 LST, about 3–4 hr before morning precipitation peak. For the north band, a prominent minimum occurs at ~19 LST, 4 hr before the precipitation minimum. The moisture fluxes through the southern control volume boundary make the largest contributions to the net flux and its diurnal variations. The diurnal variations of the AGWs and their…

Abstract Lake surface water temperature (LSWT) measurements from various sources illustrate that lakes are warming in response to climate change. Most previous studies of geographical distributions of lake warming have tended to utilize data with limited spatial resolution of LSWTs, including single‐point time series. Spatially resolved LSWT time series are now available from satellite observations, and some studies have investigated previously the intralake warming patterns in specific lakes (e.g., North American Great Lakes). However, across‐lake comparisons of intralake warming differences have not yet been investigated at a large, across‐continental scale, thus limiting our understanding of how intralake warming patterns differ more broadly. In this study, we analyze up to 20 years of satellite data from 19 lakes situated across the Northern Hemisphere, to investigate how LSWT changes vary across different lake surfaces. We find considerable intralake variability in warming trends across many lakes. The deepest areas of large lakes are characterized by a later onset of thermal…

Abstract Accurate representing freeze‐thaw (FT) process is of great importance in cold region hydrology and climate studies. With the STEMMUS‐FT model (Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil), we investigated the coupled water and heat transfer in the variably saturated frozen soil and the mechanisms of water phase change along with both evaporation and FT process, at a typical meadow ecosystem on the Tibetan Plateau. The STEMMUS‐FT showed its capability of depicting the simultaneous movement of soil moisture and heat flow in frozen soil. The comparison of different parameterizations of soil thermal conductivity indicated that the de Vries parameterization performed better than others in reproducing the hydrothermal dynamics of frozen soils. The analysis of water/vapor fluxes indicated that both the liquid water and vapor fluxes move upward to the freezing front and highlighted the crucial role of vapor flow during soil FT cycles as it connects the water/vapor transfer beneath the freezing front and above the evaporation front. The liquid/vapor…

Abstract Stratosphere‐troposphere coupling in the Southern Hemisphere (SH) polar vortex is an important dynamical process that provides predictability of the tropospheric Southern Annular Mode (SAM) and its associated surface impacts. SH stratosphere‐troposphere coupling is explored by height‐time domain empirical orthogonal function (EOF) analysis applied to the zonal mean‐zonal wind anomalies averaged over the Antarctic circumpolar region (55–65°S; U 55–65°S ). The leading EOF explains 42% of the height‐time variance of U 55–65°S and depicts the variations of the vortex that is tightly tied to the seasonal breakdown of the vortex during late spring. The leading EOF pattern, defined here as the stratosphere‐troposphere coupled mode, is characterized by variations in U 55–65°S that develop in early winter near the stratopause, change sign from late winter to early spring, gain maximum amplitude during October in the upper stratosphere, and then extend downward to the surface from October to January. This stratosphere‐troposphere coupling during the spring months appears to be…

Abstract Most intensive field studies investigating aerosols have been conducted in summer, and thus, wintertime aerosol sources and chemistry are comparatively poorly understood. An aerosol mass spectrometer was flown on the National Science Foundation/National Center for Atmospheric Research C‐130 during the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) 2015 campaign in the northeast United States. The fraction of boundary layer submicron aerosol that was organic aerosol (OA) was about a factor of 2 smaller than during a 2011 summertime study in a similar region. However, the OA measured in WINTER was almost as oxidized as OA measured in several other studies in warmer months of the year. Fifty‐eight percent of the OA was oxygenated (secondary), and 42% was primary (POA). Biomass burning OA (likely from residential heating) was ubiquitous and accounted for 33% of the OA mass. Using nonvolatile POA, one of two default secondary OA (SOA) formulations in GEOS‐Chem (v10‐01) shows very large underpredictions of SOA and O/C (5×) and overprediction of POA…

Abstract Ammonium sulfate (AS) and ammonium nitrate (AN) are key components of urban fine particles. Both field and model studies showed that heterogeneous reactions of SO 2 , NO 2 , and NH 3 on wet aerosols accelerated the haze formation in northern China. However, little is known on phase transitions of AS‐AN containing haze particles. Here hygroscopic properties of laboratory‐generated AS‐AN particles and individual particles collected during haze events in an urban site were investigated using an individual particle hygroscopicity system. AS‐AN particles showed a two‐stage deliquescence at mutual deliquescence relative humidity (MDRH) and full deliquescence relative humidity (DRH) and three physical states: solid before MDRH, solid‐aqueous between MDRH and DRH, and aqueous after DRH. During hydration, urban haze particles displayed a solid core and aqueous shell at RH = 60–80% and aqueous phase at RH > 80%. Most particles were in aqueous phase at RH > 50% during dehydration. Our results show that AS content in individual particles determines their DRH and AN content determines…

Abstract Central Asia, which is one of the most complex regions in the world, is crisscrossed with transboundary rivers shared by several countries. This paper analyzes the effects of climate change and human activities on hydrological processes and water resources in these transboundary rivers over the past half century. The results show that the average temperature in the Tienshan Mountains experienced “sharp” increases in 1998, with the average temperature after 1998 being 1.0°C higher than that during 1960–1998. This rapid warming has accelerated glacier shrinkage and decreases in snow cover. These changes influenced the hydrological processes, causing an earlier runoff peak and aggravated extreme hydrological events. Changes in mountainous hydrological processes affected regional water availability and intensified future water crisis in Central Asia. The mismatched spatial distributions of water and land resources, along with the intense human activities (e.g., overexploitation of water resources), have ultimately led to the present water crisis in Central Asia's river basins.…

Abstract Integrated Multi‐Satellite Retrievals for Global Precipitation Measurement (IMERG) is the last generation precipitation data source for a wide array of research, operational, and societal applications. The Global Precipitation Measurement mission provides these global and high‐resolution precipitation estimates through advanced satellite‐based radar and radiometers. The degree of improvement of the new IMERG products needs to be investigated to further advance the algorithm's development and application. This study focuses on systematically and extensively evaluating the uncalibrated Version 3 Late Run IMERG product, which has both backward and forward morphing, and highlights the level of improvement in comparison to its predecessor Version 7 Tropical Rainfall Measurement Mission (TRMM)‐based Multi‐satellite Precipitation Analysis real‐time product. Retrievals from different passive microwave (PMW) and infrared (IR) sensors contributing to IMERG are evaluated over the conterminous United States using ground‐based sensor precipitation estimates derived from the Multi‐Radar…

Abstract Strategically applied geoengineering is proposed to reduce some of the known side effects of stratospheric aerosol modifications. Specific climate goals could be reached depending on design choices of stratospheric sulfur injections by latitude, altitude, and magnitude. Here we explore in detail the stratospheric chemical and dynamical responses to injections at different altitudes using a fully coupled Earth System Model. Two different scenarios are explored that produce approximately the same global cooling of 2°C over the period 2042–2049, a high‐altitude injection case using 24 Tg SO2/year at 30 hPa (≈25‐km altitude) and a low‐altitude injection case using 32 Tg SO2/year injections at 70 hPa (between 19‐ and 20‐km altitude), with annual injections divided equally between 15°N and 15°S. Both cases result in a warming of the lower tropical stratosphere up to 10 and 15°C for the high‐ and low‐altitude injection case and in substantial increases of stratospheric water vapor of up to 2 and 4 ppm, respectively, compared to no geoengineering conditions. Polar column ozone in…

Tropospheric aerosol optical depth (AOD) over India was simulated by Goddard Earth Observing System (GEOS)-Chem, a global 3-D chemical-transport model, using SMOG (Speciated Multi-pOllutant Generator from Indian Institute of Technology Bombay) and GEOS-Chem (GC) (current inventories used in the GEOS-Chem model) inventories for 2012. The simulated AODs were ~80% (SMOG) and 60% (GC) of those measured by the satellites (Moderate Resolution Imaging Spectroradiometer and Multi-angle Imaging SpectroRadiometer). There is no strong seasonal variation in AOD over India. The peak AOD values are observed/simulated during summer. The simulated AOD using SMOG inventory has particulate black and organic carbon AOD higher by a factor ~5 and 3, respectively, compared to GC inventory. The model underpredicted coarse-mode AOD but agreed for fine-mode AOD with Aerosol Robotic Network data. It captured dust only over Western India, which is a desert, and not elsewhere, probably due to inaccurate dust transport and/or noninclusion of other dust sources. The calculated AOD, after dust correction, showed…

Abstract A major stratospheric sudden warming (SSW) event was observed in February 2018 after a 4‐year absence since the winter of 2013/2014. Based on the reanalysis data, the polar night jet changed from a very strong state to a moderate state during 12–19 January, and the moderate westerlies directly reversed to easterlies during 5–15 February. The intensified East Asian trough, Alaskan blocking, and East U.S. trough amplified the extratropical climatological wave 2, which propagated upward into the stratosphere, leading to a vortex‐splitting SSW event. Predictions of the February 2018 SSW event are explored in hindcasts initialized 0–4 weeks in advance by the Beijing Climate Center Climate System Model. Less than 20% of the 28 ensemble members predict the reversal of [U]60°N, 10hPa in hindcasts initialized 3 or 4 weeks in advance if a 5‐day error is allowed, while this ratio increases to 43% in hindcasts initialized 1 week in advance. Based on the climatological occurrence of SSW events in the forecast system, the maximum deterministic predictable limit of this event is 1–2 weeks…

Abstract Uncertainties in climate projections exist due to natural variability, scenario uncertainty, and model uncertainty. It has been argued that model uncertainty can be decreased by giving more weight to those models in multimodel ensembles that are more skillful and realistic for a specific process or application. In addition, some models in multimodel ensembles are not independent. We use a weighting approach proposed recently that takes into account both model performance and interdependence and apply it to investigate projections of summer maximum temperature climatology over North America in two regions of different sizes. We quantify the influence of predicting diagnostics included in the method, look at ways how to choose them, and assess the influence of the observational data set used. The trend in shortwave radiation, mean precipitation, sea surface temperature variability, and variability and trend in maximum temperature itself are the most promising constraints on projections of summer maximum temperature over North America. The influence of the observational data…

Abstract We have augmented the existing capabilities of the integrated Weather Research and Forecasting (WRF)‐urban modeling system by coupling three urban canopy models (UCMs) available in the WRF model with the new community Noah with multiparameterization options (Noah‐MP) land surface model (LSM). The WRF‐urban modeling system's performance has been evaluated by conducting six numerical experiments at high spatial resolution (1 km horizontal grid spacing) during a 15 day clear‐sky summertime period for a semiarid urban environment. To assess the relative importance of representing urban surfaces, three different urban parameterizations are used with the Noah and Noah‐MP LSMs, respectively, over the two major cities of Arizona: Phoenix and Tucson metropolitan areas. Our results demonstrate that Noah‐MP reproduces somewhat better than Noah the daily evolution of surface skin temperature and near‐surface air temperature (especially nighttime temperature) and wind speed. Concerning the urban areas, bulk urban parameterization overestimates nighttime 2 m air temperature compared to…

Abstract This study advances the previous understanding of the role of climate variability on Indonesian fire activity, by considering (i) the presence of different types of El Niño and (ii) the interaction between El Niño and the Indian Ocean Dipole (IOD). We classify 12 El Niño events during 1979–2016 into Eastern Pacific (EP) and Central Pacific (CP) types (four and eight El Niño events, respectively) and analyze observational data of sea surface temperature, precipitation, drought code, biomass burning carbon emission, visibility, and aerosol optical depth accordingly. We find that more intense and prolonged Indonesian drought and fires occur in the EP type, during which the emitted carbon amounts almost double those in the CP type. By further separating the CP type El Niño according to the phase of the IOD, that is, positive and negative, we show that fire seasons with less burning intensity and shorter duration are predominantly associated with weakly positive or even negative phase of the IOD phenomena. Moreover, fire intensity exhibits geographic diversity: fires are always…

Abstract The radiative forcing caused by a volcanic eruption is dependent on several eruption source parameters such as the mass of sulfur dioxide (SO 2 ) emitted, the eruption column height, and the eruption latitude. General circulation models with prognostic aerosol and chemistry schemes can be used to investigate how each parameter influences the volcanic forcing. However, the range of multidimensional parameter space that can be explored is restricted because such simulations are computationally expensive. Here we use statistical emulation to explore the radiative impact of eruptions over a wide covarying range of SO 2 emission magnitudes, injection heights, and eruption latitudes based on only 30 simulations. We use the emulators to build response surfaces to visualize and predict the sulfate aerosol e ‐folding decay time, the stratospheric aerosol optical depth, and net radiative forcing of thousands of different eruptions. We find that the volcanic stratospheric aerosol optical depth and net radiative forcing are primarily determined by the mass of SO 2 emitted, but eruption…

Abstract We analyze the results of the gravity wave (GW)‐resolving, high‐resolution Kühlungsborn Mechanistic general Circulation Model in July at McMurdo Station (166.69°E and 77.84°S), where strong downslope eastward winds create strong mountain wave (MW) events. These MWs have horizontal wavelengths of λH≃230 km, propagate to z∼ 40–60 km, and can have upward phases in time if the eastward wind accelerates in time. Additionally, inertia‐GWs (IGWs) with λH∼ 500–800 km and ground‐based periods of τr∼ 5–6 hr are generated in the troposphere from unbalanced, large‐scale flow. The density‐scaled GW amplitudes are ∼10 times smaller at z∼ 80–100 km than at z < 50 km because of severe wave dissipation. “Fishbone” structures are seen at z∼ 30–60 km with upward (downward) phases in time below (above) the “knee” at zknee. We horizontally filter the perturbations to isolate the GWs in a fishbone structure for a particular MW event. We find that these GWs have strikingly similar parameters below and above zknee=46 km, with ground‐based horizontal phase speeds of cH∼ 40–60 m/s, τr∼ 9–10 hr, λH∼…

Abstract We characterized the chemical composition and optical properties of particulate matter (PM) emitted by a marine diesel engine operated on heavy fuel oil (HFO), marine gas oil (MGO), and diesel fuel (DF). For all three fuels, ∼80% of submicron PM was organic (and sulfate, for HFO at higher engine loads). Emission factors varied only slightly with engine load. Refractory black carbon (rBC) particles were not thickly coated for any fuel; rBC was therefore externally mixed from organic and sulfate PM. For MGO and DF PM, rBC particles were lognormally distributed in size (mode at d rBC ≈120 nm). For HFO, much larger rBC particles were present. Combining the rBC mass concentrations with in situ absorption measurements yielded an rBC mass absorption coefficient MAC BC,780nm of 7.8 ± 1.8 m 2 /g at 780 nm for all three fuels. Using positive deviations of the absorption Ångström exponent (AAE) from unity to define brown carbon (brC), we found that brC absorption was negligible for MGO or DF PM (AAE(370,880 nm)≈1.0 ± 0.1) but typically 50% of total 370‐nm absorption for HFO PM. Even…

Abstract Fire regimes across the globe have great spatial and temporal variability, and these are influence by many factors including anthropogenic management, climate, and vegetation types. Here we utilize the satellite‐based “active fire” product, from Moderate Resolution Imaging Spectroradiometer (MODIS) sensors, to statistically analyze variability and trends in fire activity from the global to regional scales. We split up the regions by economic development, region/geographical land use, clusters of fire‐abundant areas, or by religious/cultural influence. Weekly cycle tests are conducted to highlight and quantify part of the anthropogenic influence on fire regime across the world. We find that there is a strong statistically significant decline in 2001–2016 active fires globally linked to an increase in net primary productivity observed in northern Africa, along with global agricultural expansion and intensification, which generally reduces fire activity. There are high levels of variability, however. The large‐scale regions exhibit either little change or decreasing in fire…

Abstract To investigate the thermal characteristics and dynamics of permafrost as well as seasonally frozen ground over the upper reaches of the Heihe River Basin (URHR), an observation network with 14 boreholes was established during 2011–2014. The in situ measurements indicated mean annual air temperature that ranged from −5.2 to −2.3°C at the monitored elevation range of ∼3,600–4,150 m, and mean annual ground surface temperature that ranged from −1.3 to 1.7°C during 2013–2017. The mean annual ground temperature at 16‐ to 18‐m depth ranged from −1.71°C on the high (>4,000 m above sea level) north facing slope to about 0‐C around areas near the lower limit of permafrost. Active layer thickness at the monitored sites varied significantly with the range of 0.77–4.90 m during 2011–2017, and maximum frozen depth in seasonally frozen ground was about 5 m. Permafrost thickness was between ∼136 m and less than 10 m. Both permafrost and seasonally frozen ground were found to be subject to serious warming during the measured period in the URHR. This study provides new quantitative insights…

Abstract This study is the first attempt to investigate the characteristics of the drop size distribution (DSD) and drop shape relation (DSR) of seven typhoons after making landfall in China. Four typhoons were sampled by a C‐band polarimetric radar (CPOL) and a two‐dimensional video disdrometer (2DVD) in Jiangsu Province (East China) while three typhoons were sampled by two 2DVDs in Guangdong Province (south China). Although the DSD and DSR are different in individual typhoons, the computed DSD parameters in these two groups of typhoons possess similar characteristics. The DSR is more spherical, and the shape‐slope (μ‐Λ) relation has a significantly lower value of μ for a given Λ than that in typhoons in the Taiwan area, indicating different microphysical processes of typhoons between continental China and other regions (western Pacific and Atlantic). The convective precipitation of typhoons contains higher raindrop concentration and lower raindrop diameter than that of the maritime convection. A Z (reflectivity factor)‐R (rain rate) relationship, Z = 147.28R1.38, is derived for…