Numerical Simulation Of Aerosol Effects On East Asian Summer Monsoon

In this study,numerical experiments are conducted with different regional domain sizes during the summer over East Asia using WRF-Chem to study the sensitivity of aerosol effect to domain size,which has not been paid enough attention on before.First of all,the domain size has a great influence on the simulated background circulation field.In the small domain simulation,the boundary forcing is strong,the background circulation field and precipitation are in good agreement with the reanalysis data and observations,while in the large domain simulation,due to the influence of boundary condition and increased internal variability,the simulated EASM(East Asian Summer Monsoon)is weak,the circulation system moves to the south,and the precipitation decreases in southern China,and increases in the adjacent ocean.The change of circulation system results in the increase of aerosol in the southern China and the decrease in the northern China in the large domain simulation,as well as the decrease of water vapor meridional transport.The decrease of aerosol and water vapor in northern China leads to the sharp decrease of aerosol water content and the decrease of AOD(Aerosol Optical Depth).In terms of the climate effect of aerosols,aerosol caused the land surface temperature decrease and reduced the thermal difference between land and sea,weaken the southwest monsoon in the small domain simulation,causing an updraft and a downdraft in South China as well as northern China,and a weak upward compensating airflow in the middle of China.Updraft corresponds to negative precipitation anomaly,while downdraft corresponds to positive precipitation anomaly.Therefore,in East China,the precipitation changes caused by aerosols show a "+-+-+" pattern in the small domain simulation.The weakening of monsoon brings about the decrease of precipitation latent heat release,which makes the temperature over East Asia show a downward trend,and there is a cooling center anomaly over East Asia,which further weakens the East Asian summer monsoon.In the small domain simulation,Aerosol-Radiation interactions weakens the monsoon on the land of southern China,while strengthens the monsoon at sea.Aerosol-Cloud interactions enhances the monsoon over the sea,and the circulation on the land not changed much.Aerosol-Cloud interactions has a great influence on land surface temperature through the change of cloud cover.The surface temperature will affect the lower atmosphere temperature through the mixing of boundary layer and adjust the vertical circulation.In the large domain simulation,the aerosol effect is similar to that in the small domain simulation,but due to the different background circulation field,the precipitation and circulation changes induced by aerosols are more southerly in the south and more northerly in the north than in the small domain simulation,the precipitation anomaly distribution still presents a "+-+-+" pattern.