The Spatiotemporal Variations Of Precipitation And Its Response To Topography And Atmospheric Aerosol In The Sichuan Basin

The spatiotemporal variations of precipitation over recent years in the Sichuan Basin are analyzed based on the Tropical Rainfall Measuring Mission(TRMM)3b42RT precipitation data between 2001 and 2017.Furthermore,15-year(2001-2015)aerosol optical depth(AOD)data from Moderate Resolution Imaging Spectroradiometer(MODIS)is used to study the characteristics of aerosol and to explore the relationships between precipitation variations and high aerosol concentrations in the Sichuan Basin.To understand the mechanisms of aerosol effects and topographic forcing on precipitation in the Sichuan Basin,the model of Weather Research and Forecasting(WRF)with Chemistry(Chem)is employed to simulate a typical southwest vortex rainstorm event occurring during 20-21 July 2012 over the northwest Sichuan Basin,two sensitivity simulation experiments excluding anthropogenic aerosol emissions and uplifting the Sichuan Basin topography respectively are designed to reveal the influences and their mechanisms of basin topography and aerosols on the process of rainstorm.The main conclusions are as follows:(1)Connections of precipitation variations with high aerosol levels in the Sichuan Basin:During recent years,the frequencies of light rain(daily precipitation amount of 0.1-10mm)and moderate rain(10-25mm)in the Sichuan Basin have decreased,while the frequencies of heavy rain(25-50 mm),rainstorm(50-100 mm),large rainstorm(100-250 mm)and extraordinary(>250 mm)have all shown an increasing trend.Precipitation in the northwest of the Sichuan Basin shows the significant increasing trends,while precipitation in the southwest Sichuan Basin decreased significantly.Precipitation fluctuation is most obvious in the southern part of the basin.It is suggested that the variations of precipitation may be related to the high aerosol concentrations,because the high AOD areas are consistent with the regions where precipitation changes significantly.The regression analysis between precipitation and AOD shows that there are significant positive correlations of heavy rain,rainstorm and large rainstorm with AOD respectively,confirming that the increasing aerosol concentrations increasing the frequency of heavy rain,rainstorm and large rainstorm.(2)The mechanism on influence of the special topography on rainstorm in the Sichuan Basin:The sensitivity simulation of basin topographic effect shows that the heavy rainstorm in control simulation starts later than that in sensitivity simulation,and the rainfall intensity in control simulation is strongly enhanced.The influence mechanism is mainly manifested in:1)As south airflow crosses the basin,a stronger positive vorticity center forms in the southern basin in the lower layer of troposphere in control simulation,with south wind weakening.Therefore,the water vapor and energy reach the northern part of the basin later,leading the precipitation to be delayed.2)The warmer air temperature and higher humidity condition make more moist static energy accumulates in the lower troposphere in control simulation.When airflow with more moist static energy reaches the northern part of basin,strongly upward airflow is triggered under the influence of the strong positive relative vorticity in the lower troposphere and the windward side of the topography.3)Stronger vertical velocity provides advantage for the vertical development of cloud system,more water vapor provides greater supersaturation for precipitation particles in control simulation.Under these favourable conditions,precipitation particles are generated and transformed in large quantities in the control simulation,its rain water,snow crystals and graupels are much more than the sensitivity simulation,precipitation intensity is enhanced to heavy rainstorm as it.(3)Effects of aerosol on precipitation in the Sichuan Basin:The experiment results of anthropogenic aerosol sensitivity simulation show that aerosols weaken the precipitation in the basin at the early stage and increases the rainstorm over the northwest basin at the later stage of rainstorm.During the initial period,precipitation occurred in the area of Sichuan Basin,where higher aerosol concentrations inhibit the condensation process of cloud droplets,thus releasing less latent heat,weakening vertical motion in the cloud.The related generation and transformation processes of rain water,snow crystals and graupels are suppressed,leading to the lower intensity of the precipitation in control simulation.Governed by south wind,the area of precipitation gradually moved towards the northwestern hillside of basin.The vertical velocity of sensitivity simulation which excludes anthropogenic aerosol emissions declined by the subsidence and drag of more precipitation particles which generated in the early stage of precipitation,while a strong vertical motion is forced by topographic uplift of upwind slope in control experiment with high concentration of aerosols.There is stronger condensation process of cloud droplets in control experiment,and these droplets are transformed to rain water,snow crystals and graupels more efficiently,increasing the intensity of rainstorm.