Numerical Study On The Microphysical Characteristics Mechanism Of Rainstorm Clouds And The Influence Of Stratus Clouds In The Warm Region Of South China During The Pre-flood Season

The pre-summer rainy season(April-June)in South China is the first rainy season of China.The rainfall amount in this rainy season accounts for 40%-50% of the total annual rainfall amount in South China.During this period,South China and its vicinity often experience the most frequent occurrences of extreme rainfall,leading to severe flooding and inundations.The warm-sector heavy rainfall is the most prominent feature of the pre-summer rainy season.Since warm-sector heavy rainfall is not directly affected by synoptic system,it is quite difficult to accurately simulate or forecast.In this study,two heavy rainfall events in South China during pre-summer rainy season are investigated using the Weather Research and Forecasting(WRF)model v4.0 with the comprehensive observation data,and we focus on the evolution mechanisms and microphysical properties of the warm-sector heavey rainfall and the influences of stratiform regions on evolution of mesoscale convective systems(MCSs)in two kinds of pre-summer heavy rainfall.A warm-sector heavy rainfall in South China on 15 June 2017 is investigated using the WRF simulations and observation data.The results show that the onset of heavy rainfall is consistent with the appearance of strong wind moving downward and low-level wind shear,and the center of strong radar reflectivity corresponds well with that of moisture divergence.In the growth and maturity stages of MCS,the outflow of cold pool that is not related to terrain can trigger new convections.Cloud microphysics analysis shows that at 1.5 km below the 0? layer,the conversion rate of ice-phase hydrometeor melting can reach twice that of the warm-rain process,indicating that it controls the rain formation in the melting layer,and the sedimentations of snow and graupel particles play a critical role.Due to the abundant water vapor and supercooled rain water over south China,the largest source of snow is the deposition of water vapor,and that of graupel is the accretion of supercooled raindrop by ice crystal and snow then converting into graupel.In addition,the phase change process significantly affects the temperature change of atmosphere.When the convective cloud base is low,the condensation of vapor in the lower layer will offset the cooling effect caused by the evaporation of rainwater and weaken the strength of ground cold pool.A series of sensitivity experiences are carried out to investigate the influences of stratiform on the evolution of MCS during another heavy rainfall event including warm-sector and cold-front stages on 8-10 May 2016.Results show that precipitation and the intensity of MCS are greatly weaker,and both warm-rain/cold-rain processes are also weakened in water vapor sensitivity experiments in which water vapor in stratiform regions are reduced than that in control.Precipitation and the intensity of MCS is slightly weaker in hydrometeors sensitivity experiments in which hydrometeors in stratiform regions are reduced to 10%,but it can reach the value of that in unmodified hydrometeor sensitivity experiments gradually,and it needs a longer time in weaker water vapor condition.The stratiform regions can be recovered gradually within an hour through the enhanced water vapor advection and reduced sedimentation/evaporation of rain water in hydrometeor sensitivity experiments.Weaker stratiform regions in hydrometeor sensitivity experiments are usually not conducive to the development of convective cell within it;however,it is found that stronger convective cell can also exists within weaker stratiform regions when they are converted from convection clouds and at their dissipating stage.Over all,the influence of hydrometeors in stratiform regions on evolution of MCS is limited,and the moisture environment in stratiform regions is more important.In this study,the evolution mechanisms and microphysical properties of warm-sector heavey rainfall and the influence of stratiform on evolution of MCS are investigated.The results can to some extent deepen our understanding in warm-sector heavy rainfall,and can also provide a reference for improving the numerical simulation in South China.