Study On The Microphysical Processes Of Extreme Rainfall Of The Pre-monsoon Season In Southern China

Extreme rainfall events occur commonly during the pre-monsoon season in southern China,which brings great trouble to people's life and local economic development.Owning to the interactions of different systems in the middle and low latitudes,the causes and physical mechanisms of the extreme rainfalls are complex.It is often difficult to explain the formation mechanisms of extreme rainfall with theories of frontal uplift and geopotential instability,although the baroclinicity of the atmosphere is not strong,the water vapor content is abundant,and the thermal instability is significant.At present,there are great difficulties and challenges in the accurate prediction of the extreme rainfall,which may be related to the cloud microphysical processes during the pre-monsoon season in South China.Therefore,the study of cloud microphysical processes is one of the difficulties and hotspots in the study of rainfall.According to the influence of the East Asian summer monsoon on rainfall in South China,the pre-monsoon was divided into three stages: the onset of the South China Sea summer monsoon(April),the transition period of the South China Sea summer monsoon onset(May),and after the onset of the South China Sea summer monsoon(June).Then,based on the average cloud structure from the Cloud Sat satellite data during the period from April to June 2006–2016,the results show that the cloud height in South China was significantly elevated after the onset of the South China Sea summer monsoon,and the particle size in the cloud increased.In addition,the clouds in South China have the characteristics of both continental and oceanic clouds by comparing to those over the South China Sea,middle reach of Yangtze River,and Northern China.Based on the understanding of cloud-rainfall microphysical processes obtained from the previous statistics of cloud vertical structure in Southern China,numerical simulations were carried out for an extreme rainfall in Guangzhou-Dongguan from May 21 to 22,2020(3 days after the onset of the summer East Asian monsoon).In view of multi-observations,this sudden and extremely strong rainfall occurred in the warm sector,which has the characteristics of rapid development and strong intensity,and ice phase processes are active during the rainfall process.Comparative experiments with different initial and boundary conditions and different cloud microphysical processes were launched by using the WRF(v4.2.1)model.The results show that using ERA5 data as the initial boundary conditions performs much better than the NCEP-FNL data.Besides,four widely used cloud microphysics schemes(WSM7,Thompson,NSSL 2-mom,and Goddard 4-ice)were tested,and the WSM7 scheme produced the best results.The WSM7 captured the occurrence and development of the convective system and their temporal and spatial evolution,although there were location errors toward south.The location error can be explained by the overestimated cold pool in the planetary boundary layer,and the evaporative cooling of rainwater was mainly responsible for cold pool.In view of this,several sensitivity experiments were carried out by reducing the influence of rainwater evaporation on the simulations.With the gradual decreases of the rainwater evaporation,the cold pool become weaker and weaker,and the location errors become less and less.The results gained in this paper are helpful for the understanding of the cloud microphysical processes in the extreme rainfall during the pre-monsoon season in South China and provides guidance for the improvement of the cloud microphysical scheme in the numerical models.