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Air pollution is a severe environmental problem in the Indian subcontinent. Largely caused by the rapid growth of the population, industrialization, and urbanization, air pollution can adversely affect human health and environment. To mitigate such adverse impacts, the Indian government launched the National Clean Air Programme (NCAP) in January 2019. Meanwhile, the unexpected city-lockdown due to the COVID-19 pandemic in March 2020 in India greatly reduced human activities and thus anthropogenic emissions of gaseous and aerosol pollutants. The NCAP and the lockdown could provide an ideal field experiment for quantifying the extent to which various levels of human activity reduction impact air quality in the Indian subcontinent. Here, we study the improvement in air quality due to COVID-19 and the NCAP in the India subcontinent by employing multiple satellite products and surface observations. Satellite data shows significant reductions in nitrogen dioxide (NO2) by 17% and aerosol optical depth (AOD) by 20% during the 2020 lockdown with reference to the mean levels between 2005–2019. No persistent reduction in NO2 nor AOD is detectable during the NCAP period (2019). Surface observations show consistent reductions in PM2.5 and NO2 during the 2020 lockdown in seven cities across the Indian subcontinent, except Mumbai in Central India. The increase in relative humidity and the decrease in the planetary boundary layer also play an important role in influencing air quality during the 2020 lockdown. With the decrease in aerosols during the lockdown, net radiation fluxes show positive anomalies at the surface and negative anomalies at the top of the atmosphere over most parts of the Indian subcontinent. The results of this study could provide valuable information for policymakers in South Asia to adjust the scientific measures proposed in the NCAP for efficient air pollution mitigation. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) plays a complicated impact on summer extreme precipitation in downstream areas like Yangtze Plain, and the convection-permitting modeling (CPM) over the TP has obvious added value for regional climate simulation. This study investigates whether the CPM over the TP can improve the simulation of a typical extreme Meiyu rainfall in Yangtze Plain and the possible mechanism. To address these issues, we conduct three experiments with the Weather Research and Forecasting (WRF) model using a combination of nesting feedback and different resolution topography. Results reveal that the CPM over the TP better reproduces the regional climate over the TP in terms of the precipitation and temperature. The more realistic representation of the regional climate over the TP can further improve the simulation of the spatial distribution of rainfall, the maximum center of rainfall and heavy rainfall processes of Meiyu rainfall in June 2011 but with a wet bias, which could be attribute to too much convective precipitation caused by the cumulus parameterizations. These improvements mostly benefit from the detailed representation of the complex topography over the TP, which contributes to capturing more reasonable eddy kinetic energy at 500 hPa. Consequently, middle atmospheric cold and downward wind bias over the TP as well as the bias of thermal and mechanical circulation fields (e.g., monsoon flow and South Asia High) are reduced. This work demonstrates the potential for the more realistic representation of atmosphere and land surface information over TP to improve the simulations and prediction of extreme Meiyu rainfall. • The CPM over TP has obvious added value for regional climate simulation in terms of precipitation and temperature over the TP. • The CPM over the TP significantly improves the simulation of Meiyu rainfall in June 2011, especially for the heavy rainfall processes. • The detailed representation of the complex topography in CPM plays a dominant role in the improvement of rainfall simulation. [ABSTRACT FROM AUTHOR]

Aerosol pollution is an acute environmental issue in developing countries. Asia has been experiencing rapid changes in anthropogenic aerosols during the past two decades due to fast growth in population and economy. It is still an open question how aerosol loadings, represented by aerosol optical depth (AOD), have evolved in this century, particularly during the past decade when China and India implemented a clean air act aiming to improve air quality. Based on Terra aerosol retrievals and aerosol reanalysis, a change point of AOD trend is detected at 2010 in East China versus a persistent increasing AOD trend in the Indian subcontinent with no detectable change point from 2000 to 2019. In East China, positive AOD trend (+0.11 ± 0.022 decade−1) is confirmed from 2000 to 2010 (hereinafter the former period) yet negative trend (−0.26 ± 0.027 decade−1) is identified from 2011 to 2019 (hereinafter the later period). In the Indian subcontinent, persistent positive trend (+0.04 ± 0.001) is detected from 2000 to 2019 (hereinafter the whole period). All of these trends are attributed mainly to changes in sulfate aerosols. Further analysis of the aerosol pollution extreme events (APEE; defined as daily AOD over the long-term local 90th AOD percentile) manifest a positive trend (+0.16 ± 0.029 decade−1) of the APEEs' magnitude in East China during the former period yet a negative trend (−0.11 ± 0.020 decade−1) during the latter period; the Indian subcontinent demonstrates a positive trend (+0.02 ± 0.004 decade−1) during the whole period due to increasing sulfate aerosols. The APEEs have become more frequent (+3.5 ± 0.53 day month−1 decade−1) in East China during the former period yet less frequent (−3.6 ± 0.39 day month−1 decade−1) during the latter period; in the Indian subcontinent, more frequent APEEs (+1.1 ± 0.25 day month−1 decade−1) have been detected during the whole period. Consistent with the AOD trends, clear-sky radiation in East China shows a negative trend at the surface (−3.2 ± 0.67 W m−2 decade−1), a positive trend in the atmosphere (+1.4 ± 0.68 decade−1), and a negative trend at the top of the atmosphere (−1.8 ± 0.43 decade−1) during the former period, respectively; opposite trends with much larger magnitude are seen during the latter period. In the Indian subcontinent, the clear-sky radiation trends during the whole period are −1.4 ± 0.38, +1.7 ± 0.31, and + 0.5 ± 0.16 W m−2 decade−1 at the surface, in the atmosphere, and at the top of the atmosphere, respectively. Comparison of radiation trends at clear-sky and all-sky conditions suggests that absorbing aerosols dominate the radiation budget in the atmosphere and the aerosol reanalysis of the Modern-Era Retrospective Analysis for Research ans Applications version 2 (MERRA-2) might overestimate the radiation response to clouds. This study provides an up-to-date analysis of the long-term trends in aerosols and their extreme events and radiation in two of the world's heavily polluted regions and the results have important implications for assessment of the environmental and climatic impacts of the ongoing clean air acts in Asia. • Fewer aerosol pollution extreme events were observed in China in the past decade. • Surface solar radiation increased in China but maintained in India for 2011–2019. • Clean Air Act has improved air quality in China but not in India in the past decade. [ABSTRACT FROM AUTHOR]