Observing And Simulating The Microphysical Structure Of A Subtropical Squall Line In Eastern China

A squall line is a highly organized mesoscale convective system(MCS),which usually consists of a leading convective line with trailing stratiform precipitation.Past squall line studies mostly focused on their dynamic and thermodynamic structures.On the other hand,the microphysical structures of squall lines in China remain largely unexplored,mainly due to the lack of microphysical structure observations.Eastern China is affected by the Asian summer monsoon,when warm and moist air is transported from the ocean into the continent,increasing convective instability in the region.Knowledge of the microphysical processes in squall lines is critical in improving quantitative precipitation estimation(QPE),microphysical parameterization in numerical weather prediction models,and ultimately more accurate short-term forecasts.In this paper,we document the evolution of microphysical characteristics of a subtropical squall line observed during the 2014 Observation,Prediction and Analysis of Severe Convection of China(OPACC)field campaign in Eastern China.The focus of this study are microphysical processes in the convective region and their evolution from formation to mature stages of a squall line.First,the data collected from a C-band,polarimetric Doppler radar(reflectivity Z,differential reflectivity ZDR,and specific differential phase KDP)and a disdrometer are used to investigate the variations of microphysical characteristics within the convective region during the formative,intensifying,and mature stages of the squall line.The microphysical characteristics of the squall line are noticeably different among these three stages.When the squall line develops from the formative stage to the mature stage,its radar-derived drop-size distribution(DSD)in the convective region evolves from continental-like convection to more maritime-like convection.Contrary to previous studies,DSD characteristics of a convective line may not be simply locked to a geographical location but varied extensively throughout its life cycle.Secondly,based on the polarimetric radar,an interesting feature in this squall line in Eastern China is the displaced Z and ZDR columns and the collocated Z and KDP columns,which is different from previous observational studies in West Africa and in Taiwan.The displaced patterns with moderate Z but large ZDR suggest that low concentrations of large drops are located along the leading edge of the squall line,which may be due to the size sorting effect and evaporation effect.The collocated Z and KDP columns with moderate ZDR suggest that high concentrations of medium drops are located in the convective center of the squall line.Meantime,the polarimetric radar derived liquid water content below the freezing level in the convective region is three times higher than the ice water content above the freezing level.This,in conjunction with a low cloud base(-0.68 km)and a high freezing level(-5 km),indicates a deep warm cloud layer and the dominance of the warm rain process within this squall line.Finally,using ARPS model 3DVAR and forecasts,three bulk schemes of one-moment,two-moment and three-moment schemes are compared in this squall line case.It is found that the choice of different moment of schemes has great impact.on the forecast.The models rely on the choose of microphysical schemes.Meantime,the three-moment scheme reproduced the observed characteristics of the squall line including the formative stage,developing stage,mature stage,and the whole evolution of the squall line.This can indicate that the three-moment scheme comes closer to reality.However,there is still a lot of room to improve for the microphysical research of the squall line in Eastern China.