Numerical analysis of aerosol direct and indirect effects on an extreme rainfall event over Beijing in July 2016

Abstract

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.