NASA - National Aeronautics and Space Administration

The Weather Research and Forecasting model with Chemistry is used to examine aerosol impacts on precipitation over China during the East Asian summer monsoon (EASM). In the first EASM phase (6 May to 13 June), monsoonal rainband is located to the south of high aerosol areas. The surface cooling by the aerosol radiative effect induces an anticyclonic circulation in northern China, causing a precipitation decrease over southern China. The aerosol microphysical effect tends to broaden the precipitation region. In the second EASM phase (14 June to 5 August), the precipitation band jumps to northern China, where heavy pollution locates. By serving as cloud condensation nuclei, the aerosol microphysical effect causes a reduction of local cloudiness and induces ascent to the north and descent to the south, leading to a precipitation shift from south to north. The aerosol radiative effect strengthens such a meridional asymmetry. Moisture budget analysis shows that the aerosol-induced anomalies of precipitation in both phases are mainly contributed by the changes in vertical velocity associated with convection, while the horizontal advection of moisture plays a nonnegligible role. Our study highlights that aerosol effects on EASM precipitation depend on the location of monsoon precipitation band and its relative location to aerosols, which may be the main factor that contributed to the discrepancy in literature regarding the aerosol impacts on the so-called “southern flood and northern flood” over China.