WRF-Chem simulations of aerosol impacts on summer monsoon precipitation over China

East Asia is one of the most heavily polluted regions in the world. Many previous studies have shown that East Asian monsoon precipitation is affected by aerosols, but contradictory results exist. In this study, the WRF-Chem model is used to examine the aerosol impacts on precipitation over China during the East Asian summer monsoon (EASM). The 4-month model simulations reasonably reproduce the precipitation amount and the latitudinal shift of major rain bands associated with EASM. The model approximately simulates the aerosol distributions but significantly underestimates aerosols over East China during the second half of the simulations, probably too much wet scavenging in the model.

In the first phase of EASM when the rain band is over South China, aerosols cool surface by scattering solar radiation. The cooling effect decreases the land-sea temperature difference, and thus weakens the strength of the monsoon circulation, causing a pattern of precipitation change resembling “southern flood and northern drought”. The precipitation changes are mainly resulted from the aerosol radiative effect while the aerosol microphysical effect acts in the opposite way with a smaller magnitude. In the second phase of EASM, the rain band shifts to the Yangtze River Basin and Northeast China. Aerosols absorb solar radiation and warm the atmosphere. In the meantime, aerosols serve as cloud condensation nuclei and delay rain formation by producing smaller cloud particles. Both the aerosol radiative and microphysical effects contribute to the northward shift of clouds and precipitation by inducing a pattern of precipitation change as “southern drought and northern flood”. Moisture budget analysis shows the change of precipitation in both phases of EASM is mainly contributed by the changes in vertical velocity associated with convection, while the horizontal advection of moisture plays a non-negligible role.