Impacts of aerosols from Beijing and the surrounding areas on urban precipitation

The aerosol properties and possible impacts on a convective precipitation case on 14 August 2008 over the Beijing region are investigated by using Weather Research and Forecasting Model (WRF). The results show that the domain-averaged precipitation amount can be increased up to 39.42%, 15.63% and 12.28% under slightly, moderately and heavily polluted conditions, respectively. Under polluted conditions, the increase of aerosols can lead to the decrease of the domain-averaged precipitation amount over the Beijing region. The domain-averaged precipitation amount can be reduced about 17% between light polluted and polluted conditions, which is mainly caused by the increase of the number of cloud condensation nuclei and cloud droplets and the decrease of cloud droplet effective radius. Many small cloud droplets nucleating from high aerosol concentrations may not grow larger, and coalesce very inefficiently into raindrops, and the cooling processes of the raindrops weaken, which is not favorable to the development and maintain of downdrafts and lead to the decreases of the surface outflow wind speed and cold pool near the surface, and the low-level convergence produced by the interactions between low-level cold outflows and large-scale low-level airflow weakens, which is not favorable to the vertical transport of water vapor and conversion of precipitation, and leads to the decrease of precipitation. Therefore, the mainly mechanism is the effects of the cloud and microphysical processes on the thermal and dynamic fields under polluted conditions.