Spatiotemporal Variations of the Effects of Aerosols on Clouds and Precipitation in an Extreme-Rain-Producing MCS in South China

Previous studies focus mostly on the storm-scale-averaged precipitation responses to aerosols. Yet, the spatiotemporal variations of the aerosol effects can lead to localized and short-duration precipitation changes that are more relevant for improving rainfall forecasts. Here, we investigate the cloud and precipitation responses to aerosols during different life stages and in subregions with various cloud top heights of an extreme-rain-producing mesoscale convective system (MCS) in South China using the coupled WRF-Chem model. Results show mostly similar MCS-averaged precipitation responses between the polluted and clean conditions due to compensations among the subregions. However, the spatiotemporally discretized changes are divergent. Specifically, during the developing stage, aerosols increase precipitation in all subregions through generating larger precipitating hydrometers produced from the accretion of more cloud droplets. The most prominent precipitation enhancement occurs in the subregion with the strongest clean-condition rainfall. In the mature stage, the CCN activation abates, and so does the aerosol-induced precipitation increase. In the mixed-phase (-40 degrees C < T-top < 0 degrees C) and cold (T-top <= -40 degrees C) cloud top subregions, aerosols also reduce the melting of the ice-phase precipitating hydrometers, which process becomes more important to precipitation formation during the mature stage. Therefore, the drop of rainfall rate is more significant in these ice-phase-involved subregions, bringing the polluted precipitation to be less than that in the clean condition during the mature stage. The substantial spatiotemporal variations of the aerosol effects and the early intensification of heavy precipitation suggest the importance of incorporating aerosols in the modeling and prediction of regional heavy rainfall events.