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Anthropogenic aerosols have an important impact on summer heavy precipitation, but their impact and associated mechanisms are still greatly uncertain. In this study, the WRF-Chem including the aerosol-meteorology interactions was used to investigate the effects and possible mechanisms of aerosol direct, indirect, and total effects on a heavy rainfall event that occurred in Beijing during August 12—13, 2020. The results are as follows. (1) In the first stage of the precipitation (15∶00—18∶00 BT on August 12), due to the aerosol effects on solar radiation and cloud nuclei, the aerosol direct, indirect, and total effects all decrease the hourly precipitation intensity in the Beijing urban area, with a maximum reduction of 6 mm·h-1, 1 mm·h-1, and 4 mm·h-1, respectively. (2) In the second stage (20∶00 BT on August 12 to 04∶00 BT on August 13), both aerosol direct and indirect effects decrease the hourly precipitation intensity, with a maximum reduction of about 4 mm·h-1. The aerosol total effect has little impact on the hourly precipitation intensity but delays the precipitation time. Generally, the aerosol direct effect enhances the warm cloud process while weakening the cold cloud process, thus decreasing the precipitation and advancing the strong precipitation process in the second precipitation stage. The aerosol indirect effect decreases the effective radius of rain, thereby reducing the merging process and precipitation, and delaying the onset of heavy precipitation during the second precipitation stage. The aerosol total effect on precipitation is a nonlinear result of both aerosol direct and indirect effects, with the direct effect dominating in the first stage of precipitation and the indirect effect dominating in the second stage of precipitation.

The emission of black carbon (BC) particles, which cause atmospheric warming by affecting radiation budget in the atmosphere, is the result of an incomplete combustion process of organic materials. The recent wildfire event during the summer 2019–2020 in south-eastern Australia was unprecedented in scale. The wildfires lasted for nearly 3 months over large areas of the two most populated states of New South Wales and Victoria. This study on the emission and dispersion of BC emitted from the biomass burnings of the wildfires using the Weather Research Forecast–Chemistry (WRF–Chem) model aims to determine the extent of BC spatial dispersion and ground concentration distribution and the effect of BC on air quality and radiative transfer at the top of the atmosphere, the atmosphere and on the ground. The predicted aerosol concentration and AOD are compared with the observed data using the New South Wales Department of Planning and Environment (DPE) aethalometer and air quality network and remote sensing data. The BC concentration as predicted from the WRF–Chem model, is in general, less than the observed data as measured using the aethalometer monitoring network, but the spatial pattern corresponds well, and the correlation is relatively high. The total BC emission into the atmosphere during the event and the effect on radiation budget were also estimated. This study shows that the summer 2019–2020 wildfires affect not only the air quality and health impact on the east coast of Australia but also short-term weather in the region via aerosol interactions with radiation and clouds. [ABSTRACT FROM AUTHOR]

Using a single column model with ground‐based, aircraft, and satellite data sets we assess the combined role of smoke and dust aerosols, land degradation/aridization (LDA), and their impact on the planetary boundary layer (PBL) in influencing near‐surface air temperature over the Sahel. Our study is unique because it assesses the combined role of smoke and dust aerosols on PBL evolution and near‐surface air temperatures during both day and nighttime. More importantly, using a theoretical framework, we provide a careful explanation of the geophysical processes responsible for the changes in PBL and near‐surface air temperature. Our results indicate that during northern hemisphere winter months, dust, and smoke over Sahel radiatively combine to impact the PBL. We show that aerosol mixtures dominated by dust modify PBL height in a manner that minimizes/maximizes surface layer cooling/warming at times when daytime maximum/nocturnal minimum temperatures occur. Furthermore, we find that increasing smoke contribution to total column aerosol optical extinction counteracts nighttime warming through daytime cooling. When smoke constitutes half or more of to the total column aerosol optical extinction, the ratio of longwave to shortwave radiative forcing is less than 10%, and nighttime cooling ensues. Minimum temperature is most sensitive to changes in mid‐visible aerosol optical depth (AOD) values <1 and doubling of dust AOD within this range during the 1950–1980 Sahelian LDA event is estimated to have a nocturnal warming potential of 0.6°C. Plain Language Summary: We use a weather model to investigate how dust and smoke from agricultural fires impact the near‐surface air temperature of Sahel. As vegetation in the region is reduced due to drought and expansion of the Sahara, it is expected that more dusty days will occur. We find that so long as dust dominates over smoke, increasing the concentration of dust and smoke causes daytime cooling but nighttime warming compared to days without smoke and dust. The nighttime warming is caused by large dust particles that trap heat escaping from Earth's surface, while daytime cooling is due to shade produced by smoke and dust. Stagnant conditions near the surface at night concentrate the heat in a shallow layer, enhancing warming. When smoke dominates, however, both day and nighttime are cooler since dust cannot trap enough heat to offset the daytime shading. Importantly, shading during the daytime also increases stagnation at the surface during nighttime. This enhances nighttime cooling due to a thinner near‐surface layer and reduces mixing of warm air from aloft due to the increased stagnation. Through these mechanisms continued reduction of vegetation in the Sahel will lead to more warm nights, a result similar to greenhouse gas warming. Key Points: Dust and smoke in combination cause nighttime warming if extinction due to smoke is less than half of totalNear‐surface air temperature response is nonmonotonic during the daytime due to aerosol radiative forcing‐PBL feedbacksAerosols may be a substantial source of nocturnal warming during periods of enhanced land degradation and aridation [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the long‐term (1982–2015) variability of aerosol optical properties, as well as aerosol and greenhouse gases (GHG) direct radiative forcing (RF) in central Europe on the basis of MERRA‐2 reanalysis and ground‐based observation. Calculations of longwave (LW) and shortwave (SW) RF were made with the use of the off‐line Fu‐Liou radiative transfer model and were preceded by the sensitivity study of the code's input parameters. Then, the long‐term mean as well as the annual variability for selected regions of aerosol optical properties and radiative forcing were analysed. The mean AOD trend was −0.056 ± 9% per decade and AOD was reduced by 48% between 1982 and 2015. While for 1982–1990 the trend per decade was −0.12 ± 20%, for 1991–2000 it was −0.17 ± 17% and only −0.02 ± 31% in the last 15 years (2001–2015). The trend of the aerosol radiative forcing (ARF) was 1.52 ± 0.12 W·m−2/10 year and 1.21 ± 0.19 W·m−2/10 year at top of the atmosphere (TOA) and at Earth's surface, respectively. The trend for GHG was significantly smaller and it equalled 0.27 ± 0.01 W·m−2/10 year at TOA and 0.17 ± 0.01 W·m−2/10 year at the surface. Changes in GHG and aerosol direct effect produced additional 3.2 ± 0.2 W·m−2 at TOA, which could be associated with the observed regional climate warming. The change of the direct aerosol effect was about 4 times the GHG RF. The influence of aerosol loading reduction on the radiation budget was significantly higher in the 1990s of the 20th century in comparison to the first decades of the 21st. The effect of aerosol reduction has significant impact on air temperature changes during warm season and negligible during winter. [ABSTRACT FROM AUTHOR]

Using the WRF model with chemistry (WRF-Chem), this study investigates the potential contributions of aerosol direct (ADE) and aerosol indirect (AIE) effects on the nocturnal convective precipitation occurred on 9–10 September 2019 in Beijing. It shows that ADE plays a more important role in spatiotemporal changes of heavy precipitation than AIE in urban areas. Further simulation analyses show that ADE suppresses precipitation by inducing greater surface shortwave radiative cooling and stability before 13:00 (UTC, hereafter) on 9 September, which reduces convective available potential energy (CAPE) losses and then increases the later CAPE. The ADE-induced CAPE increase makes updrafts stronger, increasing cloudiness and transporting more cloud water into the upper troposphere. This, in turn, causes greater latent heat release from freezing and then stronger convection, producing higher rain rate during 13:00–21:00. During 12:00–18:00, AIE causes a decrease in CAPE, which weakens updrafts. AIE decreases the snow mixing ratio above 700 hPa except at 15:00–16:00 on 9 September, which can decrease the release of latent heat from freezing, contributing to the decrease in precipitation after 18:00. Due to the greater stability before 10:00 and the longer time required for updrafts to overcome this stability, ADE necessitates 2–3 h to adjust the simulated rainfall time series. • ADE plays more important roles than AIE in change of heavy precipitation studied. • ADE suppresses daytime precipitation with peak rate delayed by 2 h. • ADE enhances while AIE slightly suppresses nighttime precipitation. [ABSTRACT FROM AUTHOR]

Liguria, located in the Northwest of Italy, is one of the Italian regions hit in the recent past by some of the most severe precipitation events among the ones observed in Italy. From a synoptic point of view, similar configurations characterize the extreme events that affected Liguria, i.e. the simultaneous presence of a deep pressure minimum west of the region and a strong high pressure over eastern Europe. Such conditions are favorable to the triggering of a quasi-stationary mesoscale V-shaped convective system over the Ligurian Sea. Furthermore, this kind of configuration is favorable to the formation and intrusion of wide plumes of aerosol, mainly mineral dust from the Sahara Desert and sea salt aerosols generated under high wind conditions in the Mediterranean basin. The present study aims at evaluating the impact that these aerosol plumes can have on meteorological fields during the triggering and evolution of the deep convective systems responsible for the Liguria flooding events. This study is carried out through numerical simulations performed with the WRF-Chem model, version 4.0. In particular, our main aim is to investigate the influence that the so-called direct (aerosol-radiation) and indirect (aerosol-cloud) interactions may have on mesoscale V-shape convective systems and on the associated rainfall events. In the simulations in which the direct aerosol effects are switched on, a consistent weakening signal is identified in the wind speed in all simulations; a major role seems to be played by the increase of downward longwave radiation in the presence of aerosol, that, by reducing the land-sea temperature gradient, causes the weakening of surface winds. More complex are the consequences of aerosol on the amount/intensity of the resulting precipitations. Unexpectedly, a reduced convergence is not necessarily associated with a weakening of precipitation and in half of the studied cases the interaction between aerosol, moisture and radiation increases the instability despite the reduced dynamical forcing due to the weakening of the convergence. [Display omitted] • Aerosol direct/indirect effects on deep convective systems are investigated via coupled chemistry-meteorology simulations. • The direct effects associated with the aerosol-radiation feedback appear to overcome the aerosol indirect effects. • Longwave radiation emitted by the aerosol layer attenuates land–sea thermal contrast. • Aerosol-radiation feedback is responsible for a significant attenuation of the winds. • Aerosol direct and indirect effects on precipitation associated on deep convective systems are not univocal. [ABSTRACT FROM AUTHOR]

To quantify the impact of the direct aerosol effect accurately, this study incorporated the Geostationary Ocean Color Imager (GOCI) aerosol optical depth (AOD) into a coupled meteorology‐chemistry model. We designed three model simulations to observe the impact of AOD assimilation and aerosol feedback during the KORUS‐AQ campaign (May–June 2016). By assimilating the GOCI AOD with high temporal and spatial resolutions, we improve the statistics from the comparison AOD and AERONET data (root‐mean‐square error: 0.12, R: 0.77, index of agreement: 0.69, mean‐absolute error: 0.08). The inclusion of the direct effect of aerosols produces the best model performance (root‐mean‐square error: 0.10, R: 0.86, index of agreement: 0.72, mean‐absolute error: 0.07). AOD values increased as much as 0.15, which is associated with an average reduction in solar radiation of ‐31.39 W/m2, a planetary boundary layer height (‐104.70 m), an air temperature (‐0.58 °C), and a surface wind speed (‐0.07 m/s) over land. In addition, concentrations of major gaseous and particulate pollutants at the surface (SO2, NO2, NH3,SO42−, NO3−, NH4+, and PM2.5) increase by 7.87–34%, while OH concentration decreases by ‐4.58%. Changes in meteorology and air quality appear to be more significant in high‐aerosol loading areas. The integrated process rate analysis shows decelerated vertical transport, resulting in an accumulation of air pollutants near the surface and the amount of nitrate, which is higher than that of sulfate because of its response to reduced temperature. We conclude that constraining aerosol concentrations using geostationary satellite data is a prerequisite for quantifying the impact of aerosols on meteorology and air quality. Key Points: Both GOCI AOD assimilation and the direct effect of aerosols improve the performance of the coupled meteorology‐chemistry modelDecelerated ventilation results in an accumulation of air pollutants near the surfaceThe increase in nitrate concentrations near the surface is greater than sulfate because of the response of nitrate formation to reduced temperatures [ABSTRACT FROM AUTHOR]

Historically, cargo ships have been powered by low-grade fossil fuels, which emit particles and particle-precursor vapors that impact human health and climate. We used a global chemical-transport model with online aerosol microphysics (GEOS-Chem-TOMAS) to estimate the aerosol health and climate impacts of four emission-control policies: (1) 85% reduction in sulfur oxide (SO _x ) emissions (Sulf); (2) 85% reduction in SO _x and black carbon (BC) emissions (Sulf-BC); (3) 85% reduction in SO _x , BC, and organic aerosol (OA) emissions (Sulf-BC-OA); and (4) 85% reduction in SO _x , BC, OA, and nitrogen oxide (NO _x ) emissions (Sulf-BC-OA-NO _x ). The SO _x reductions reflect the 0.5% fuel-sulfur cap implemented by the International Maritime Organization (IMO) on 1 January 2020. The other reductions represent realistic estimates of future emission-control policies. We estimate that these policies could reduce fine particulate matter (PM _2.5 )-attributable mortalities by 13 300 (Sulf) to 38 600 (Sulf-BC-OA-NO _x ) mortalities per year. These changes represent 0.3% and 0.8%, respectively, of annual PM _2.5 -attributable mortalities from anthropogenic sources. Comparing simulations, we estimate that adding the NO _x cap has the greatest health benefit. In contrast to the health benefits, all scenarios lead to a simulated climate warming tendency. The combined aerosol direct radiative effect and cloud-albedo indirect effects (AIE) are between 27 mW m ^−2 (Sulf) and 41 mW m ^−2 (Sulf-BC-OA-NO _x ). These changes are about 2.1% (Sulf) to 3.2% (Sulf-BC-OA-NO _x ) of the total anthropogenic aerosol radiative forcing. The emission control policies examined here yield larger relative changes in the aerosol radiative forcing (2.1%–3.2%) than in health effects (0.3%–0.8%), because most shipping emissions are distant from populated regions. Valuation of the impacts suggests that these emissions reductions could produce much larger marginal health benefits ($129–$374 billion annually) than the marginal climate costs ($12–$17 billion annually).

Studying precipitation diurnal variation characteristics is crucial to better understand their formation mechanism. Moreover, it is fundamental in assessing regional climate variability and to validate the effectiveness of cloud and precipitation parameterization schemes in weather and climate models. Based on observational data from 2008 to 2017, this manuscript objective is to assess the aerosol effects on summer precipitation on an hourly-scale diurnal variation basis in the Beijing metropolitan area. The results put in evidence that the precipitation frequency and duration on polluted days were 25.0% and 14.8% lower with respect to clean days. No significant differences were observed in total daily accumulated rainfall between clear and polluted days. While heavy precipitation mean intensity on polluted days increased by 13.5%, which is potentially linked with aerosol microphysical effects. Note that precipitation occurs earlier on polluted days, with peak time of 1 ∼ 2-hour in advance compared with clean days over the urban areas of Beijing, which may be primarily ascribed to the influence of advanced turning local circulation of mountain-valley breezes and urban heat island circulation due to aerosol-radiation effects.

Abstract: In this study, all‐sky ShortWave Direct Aerosol Radiative Forcing (SWDARF) at the top of atmosphere is estimated using the method of Oikawa et al. (2013) applied to two generations of Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Level 2 products, i.e., version 2 (V2) and version 3 (V3), and the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud product. The estimated SWDARF in Oikawa et al. (2013) was based on CALIPSO V2 product, which contained significant errors in cloud clearing and low‐altitude aerosols. This error was corrected in V3, resulting in greatly improved and significantly different aerosol and cloud distributions. In clear‐sky conditions, the magnitude of aerosol optical thickness underestimation becomes smaller and SWDARF becomes more negative using the V3 product. In addition, above‐cloud aerosols, which cause positive SWDARF, are less frequently detected and below‐cloud aerosols are more frequently detected in the V3 product than in the V2 product, so that cloudy‐sky SWDARF becomes more negative using the V3 product. From these results, clear‐sky, cloudy‐sky, and all‐sky SWDARFs become more negative using the V3 product than the V2 product. The magnitude of negative SWDARF using the V3 product is more than twice as large as the V2 product under all‐sky conditions due to V3 improvements in the lidar retrieval algorithms. Considering the uncertainties of aerosol and cloud measurements, annual zonal averages of clear‐sky, cloudy‐sky, and all‐sky SWDARFs from 60°S to 60°N are estimated as −4.0 ± 0.2, −1.1 ± 0.3, and −2.1 ± 0.2 Wm−2 from the V3 product. [ABSTRACT FROM AUTHOR]

The prospect of global climate change will have wide scale impacts, such as ecological stress and human health hazards. One aspect of concern is future changes in air quality that will result from changes in both meteorological forcing and air pollutant emissions. In this study, the GU-WRF/Chem model is employed to simulate the impact of changing climate and emissions following the IPCC AR4 SRES A1B scenario. An average of 4 future years (2020, 2030, 2040, and 2050) is compared against an average of 2 current years (2001 and 2010). Under this scenario, by the Mid-21st century global air quality is projected to degrade with a global average increase of 2.5 ppb in the maximum 8-hr O 3 level and of 0.3 μg m −3 in 24-hr average PM 2.5 . However, PM 2.5 changes are more regional due to regional variations in primary aerosol emissions and emissions of gaseous precursor for secondary PM 2.5 . Increasing NO x emissions in this scenario combines with a wetter climate elevating levels of OH, HO 2 , H 2 O 2 , and the nitrate radical and increasing the atmosphere’s near surface oxidation state. This differs from findings under the RCP scenarios that experience declines in OH from reduced NO x emissions, stratospheric recovery of O 3 , and increases in CH 4 and VOCs. Increasing NO x and O 3 levels enhances the nitrogen and O 3 deposition, indicating potentially enhanced crop damage and ecosystem stress under this scenario. The enhanced global aerosol level results in enhancements in aerosol optical depth, cloud droplet number concentration, and cloud optical thickness. This leads to dimming at the Earth’s surface with a global average reduction in shortwave radiation of 1.2 W m −2 . This enhanced dimming leads to a more moderate warming trend and different trends in radiation than those found in NCAR’s CCSM simulation, which does not include the advanced chemistry and aerosol treatment of GU-WRF/Chem and cannot simulate the impacts of changing climate and emissions with the same level of detailed treatments. This study indicates that effective climate mitigation and emission control strategies are needed to prevent future health impact and ecosystem stress. Further, studies that are used to develop these strategies should use fully coupled models with sophisticated chemical and aerosol-interaction treatments that can provide a more realistic representation of the atmosphere. [ABSTRACT FROM AUTHOR]

It is known that aerosol and precursor gas emissions over East Asia may be underestimated by 50% due to the absence of data on regional rural and township industries. As the most important element of anthropogenic emissions, sulphur dioxide (SO2) can form sulfate aerosols through several chemical processes, thus affecting the regional and global climate. In this study, we use the Community Atmospheric Model 5.1 (CAM5.1) to investigate the effects of anthropogenic aerosols on radiative forcing and the climate over East Asia, taking into consideration various SO2 emission levels, including double the amount of SO2 emissions over East Asia. Numerical experiments are performed using high-resolution CAM5.1 with pre-industrial (PI) and present day (PD) aerosol emission levels, and with PD aerosol emission levels with double SO2 emissions over East Asia (PD2SO2). The simulated aerosol optical depth and surface sulfate concentrations over East Asia are significantly increased in PD2SO2, which is in better agreement with the observational results. The simulation results show extensive aerosol direct and indirect radiative forcing for PD--PI (the difference between PI and PD), which significantly weakens the large-scale intensity of the East Asian summer monsoon (EASM) and reduces the summer precipitation. Compared to PD, the aerosol direct radiative forcing is significantly increased in PD2SO2, whereas the aerosol indirect radiative forcing is markedly decreased due to the inhibition of cloud formation, especially over North China. The increase in aerosol direct radiative forcing and decrease in aerosol indirect radiative forcing result in insignificant changes in the total amount of aerosol radiative forcing. These results also show that the large-scale intensity of the EASM and the associated summer precipitation are insensitive to the doubling of current SO2 emissions. [ABSTRACT FROM AUTHOR]

The promotion of renewable energy as a substitute for fossil fuels is the key solution to achieve the goals established during the United Nations Climate Change Conference in Glasgow (COP26) based on which member countries agreed to phase down coal power and achieve net-zero carbon emissions. Among various renewable energy sources, solar energy is an attractive option that will have a significant effect on the future energy supply and energy use. Therefore, we selected the period of 2016–2020 during which the aerosol concentration gradually decreased due to strict pollutant control measures to evaluate solar energy simulations based on the Weather Research Forecast-Chemistry (WRF-Chem) model. We also analyzed the contributions of the aerosol direct effect (ADE), aerosol indirect effect (AIE), and cloud radiation effect (CRE) to solar energy trends by conducting sensitivity experiments. The results show that the WRF-Chem model performs well for the 2 m temperature (T 2), cloud fraction, PM 2.5 , solar energy trends during 2016–2020. There are regional and seasonal differences in the contributions of ADE, AIE, and CRE to solar energy trends, with a decrease in ADE contributions and an increase in CRE contributions from north to south in China, and the AIE contribution being relatively slight. On an annual scale, ADE is the main contributor to the increase in solar energy trends in the Beijing-Tianjin-Hebei (89%) and Fenwei Plains (83.9%) from 2016 to 2020, which is related to the horizontal distribution of PM 2.5. In the Yangtze River Delta and other regions, ADE and CRE contributed equally to the increase in solar energy trends, about 40%. In the Pearl River Delta and Sichuan Basin, the contribution of CRE is larger than that of AIE and ADE, the Pearl River Delta region is the largest contributor of CRE to the annual solar energy trends among the five major urban agglomerations, with a contribution of 78.4%, and Sichuan basin is the only region where CRE has a negative contribution to the annual solar energy trends (−59.1%). On the seasonal scale, the contribution of CRE is dominant except for the greater positive contribution of ADE to the solar energy trends in spring, summer, and autumn in Beijing-Tianjin-Hebei and in autumn in Fenwei Plain. • The WRF-Chem model captures the solar energy trends well • The aerosol effects contributes positively to solar energy trends under strict control of air pollution • The contribution of clouds to solar energy trends is regional and seasonal [ABSTRACT FROM AUTHOR]

A series of 60-year numerical experiments starting from 1851 was conducted using a global climate model coupled with an aerosol-cloud-radiation model to investigate the response of the Asian summer monsoon to variations in the secondary organic aerosol (SOA) flux induced by two different estimations of biogenic volatile organic compound (BVOC) emissions. One estimation was obtained from a pre-existing archive and the other was generated by a next-generation model (the Model of Emissions of Gases and Aerosols from Nature, MEGAN). The use of MEGAN resulted in an overall increase of the SOA production through a higher rate of gasto-particle conversion of BVOCs. Consequently, the atmospheric loading of organic carbon (OC) increased due to the contribution of SOA to OC aerosol. The increase of atmospheric OC aerosols was prominent in particular in the Indian subcontinent and Indochina Peninsula (IP) during the pre- and early-monsoon periods because the terrestrial biosphere is the major source of BVOC emissions and the atmospheric aerosol concentration diminishes rapidly with the arrival of monsoon rainfall. As the number of atmospheric OC particles increased, the number concentrations of cloud droplets increased, but their size decreased. These changes represent a combination of aerosol-cloud interactions that were favorable to rainfall suppression. However, the modeled precipitation was slightly enhanced in May over the oceans that surround the Indian subcontinent and IP. Further analysis revealed that a compensating updraft in the surrounding oceans was induced by the thermally-driven downdraft in the IP, which was a result of surface cooling associated with direct OC aerosol radiative forcing, and was able to surpass the aerosolcloud interactions. The co-existence of oceanic ascending motion with the maximum convective available potential energy was also found to be crucial for rainfall formation. Although the model produced statistically significant rainfall changes with locally organized patterns, the suggested pathways should be considered guardedly because in the simulation results, 1) the BVOC-induced aerosol direct effect was marginal; 2) cloud-aerosol interactions were modeldependent; and 3) Asian summer monsoons were biased to a nonnegligible extent. [ABSTRACT FROM AUTHOR]

Impacts of aerosol direct (ADE) and aerosol indirect (AIE) effects on an extreme rainfall event over Beijing on 19–21 July 2016 were analyzed by conducting four numerical experiments with the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). The analysis shows that the ADE and AIE change the distribution of the rainfall centers and concentrate rainfall in urban areas. The ADE-induced hourly rainfall intensity increases by about 1 mm h−1, 10 mm h−1, and 5 mm h−1, whereas AIE-induced hourly rainfall intensity decreases by about 1.4 mm h−1, 10 mm h−1, and 3 mm h−1 in Beijing, central and eastern part of Beijing during 1200–1800 BJT on 19 July, respectively. The ADE has a slight impact (variation <0.3 mm h−1) on the hourly rainfall intensity, whereas the AIE increases the hourly rainfall intensity by about 1 mm h−1 in Beijing; the ADE and AIE increase the hourly rainfall intensity by about 12 mm h−1 and 20 mm h−1 in central part of Beijing while decrease by about 10 mm h−1 and 21 mm h−1 in eastern part of Beijing during 1200–1800 BJT on 20 July, respectively. The ADE destabilizes the atmosphere due to the increase in atmospheric diabatic heating, leading to an increase in rainfall in Beijing during 1200–1800 BJT on 19 July. The nucleation of more aerosols to cloud droplets, which decreases the size of the droplets and inhibits collision, decreasing the amount of rainfall. The impact of the ADE on rainfall is weakened due to the wet deposition caused by rainfall decreasing the aerosol concentration during 1200–1800 BJT on 20 July. The AIE increases the number of collisions and therefore increases the amount of rainfall. The difference in vertical speed is responsible for the difference in the local hourly rainfall intensity for central and eastern part of Beijing during 1200–1800 BJT on 20 July. • The impacts of direct and indirect aerosol effects on an extreme heavy rainfall in Beijing are revealed in detail. • Pollution, meteorology, and thermo-dynamical of this rainfall event on 19–21 July 2016 were revealed by WRF-Chem. • Relative effects of direct and indirect aerosol effects on rainfall were investigated. • Differences in aerosol effects on vertical speed and collision are the main reasons for the differences in rainfall. [ABSTRACT FROM AUTHOR]

Numerical simulations are performed to investigate the radiative and cloud microphysical effects (direct and indirect effects respectively) of the Saharan dust aerosols and the subsequent perturbations of the radiation budget over the tropical East Atlantic Ocean. Simulations are conducted for July 1–31, 2010 over three two-way nested domains covering 0–25°N, 30–60°W using the Weather Research and Forecasting model coupled with Chemistry module (WRF-Chem-V3.6.1). Simulations of the dust direct effect over the outermost domain suggest that the dust-induced lower level heating increases the stability of the lower troposphere and reduces the cloud cover (up to 9%) underneath the maximum dust layer. Once the direct and indirect effects are considered together, dust particles act as cloud condensation nuclei and produce smaller sized cloud droplets. Overall, for the innermost domain, the mean cloud droplet radius becomes ~14% smaller in the presence of dust (12.4 μm and 10.7 μm in dust-free and dust-laden conditions respectively). Contrary to the direct effect, the coupled direct and indirect effects of dust not only reduces (up to 17%) the cloud cover below the maximum dust layer but also enhances (up to 9%) the cloud cover above the dust layers. The direct radiative forcing of dust excluding (including) cloud feedback are −13.66 (−4.83) and −11.25 (−1.22) W/m2 at the surface and at the top of the atmosphere, respectively. Coupled direct and indirect effects of dust result in the net radiative forcing of dust to −8.14 and −5.03 W/m2 at the surface and at the top of the atmosphere, respectively. • Radiative and cloud microphysical impacts of Saharan dust are investigated. • Climate forcing of dust are negative at the surface and top of the atmosphere. • Dust reduce the cloud cover below the maximum dust layer. • Coupled direct-indirect effects of dust reduce cloud droplet size. [ABSTRACT FROM AUTHOR]

In November 2016, a large area of wildfire occurred in the southeastern United States, concomitant with the occurrence of severe drought during the same period. Whereas the previous studies on biomass burning over this region mainly focused on the prescribed fire, this study investigated the impact of wildfire using the two-way-coupled Weather Research and Forecasting model and Community Multiscale Air Quality model. Two episodic wildfire burning events (November 6 to 9 and November 13 to 16, 2016) were selected, and the mean contribution to fine particulate matter (PM 2.5) in the southeastern United States from wildfires reached 9.6 to 42.5 μg m−3 and 10.9 to 26.1 μg m−3, with mean relative contributions of 41% and 49%, respectively, during these two events. The effect of wildfire propagates along the path of the smoke plume, which is determined by the wind speed and direction. For instance, during the first event, the dominant low-altitude wind vector displayed an anticyclonic-type flow with low wind speed, resulting in relatively localized influence and high intensity. In contrast, during the second event, relatively fast eastward wind, particularly over the latter part of the event, strengthened the diffusion and affected larger areas in comparison with the first event. Moreover, differently from the previous studies, this study took a further step to reveal the mechanism of the aerosol direct effect on the deterioration of air quality during wildfire, mainly through the modulation of reduction in surface downward shortwave radiation, planetary boundary layer height and wind speed, subsequently, facilitating pollution accumulation. Quantification analysis showed an average of 10% to 14% extra enhancement of PM 2.5 during the November 6 to 8 episode. Considering that more frequent drought is projected to occur in the southeastern United States, wildfire may play an even more important role in modulating the air quality in this region. Unlabelled Image • The impact of wildfire on PM 2.5 concentrations over southeastern U.S. was quantified. • During the episodes in November 2016, the contribution of wildfire to PM 2.5 is 45%. • The radiative effect of aerosol may further enhance the impact resulting from wildfire. [ABSTRACT FROM AUTHOR]