江苏典型下击暴流风暴结构特征统计分析

Statistical analysis of the structural characteristics of typical downbursts in Jiangsu province,China

为了研究江苏地区下击暴流的结构特征,利用常规天气资料、雷达探测资料、自动气象站观测资料和ERA5再分析资料等,选取2007—2018年江苏地区19个典型下击暴流过程进行统计分析。结果表明:江苏下击暴流的分布呈北多南少,以湿下击暴流为主,7月是下击暴流的高发月份,孤立风暴型下击暴流具有弱的天气尺度强迫和上干下湿的结构,风暴移速较慢,飑线镶嵌型下击暴流具有很强的天气尺度强迫特征,风暴移速较快。下击暴流影响期间地面温度变化剧烈,温度降低伴随有明显风速增大过程。统计显示,产生下击暴流风暴的环境温度平均垂直递减率为6.8℃/km,能够保证负浮力的维持,干冷空气被中层辐合气流夹卷进入风暴内进一步加强了下沉气流,使得下击暴流得以维持和加强。下击暴流的初生阶段,强反射率因子核心和中层径向辐合出现在下击暴流发生前20-30 min,成熟阶段,强反射率因子核心高度有明显降低,低层呈辐散结构。

19 typical downburst events in Jiangsu province from 2007 to 2018 are statistically analyzed using conventional observations,radar data,automatic meteorological station(AMS)data and ERA5 reanalysis product to reveal their structural characteristics.The results show high frequency of downburst in the north of Jiangsu and low frequency in the south,while wet downbursts account for most of the cases.July is the month with high downburst occurrences.The downburst caused by an isolated storm is featured by weak synoptic-scale forcing,wet low levels and dry mid-to high levels and slow storm movement.However,the downburst embedded in a squall line is featured by strong synoptic-scale forcing and fast storm movement.The ground temperature changes drastically during the downburst,and the temperature drop is accompanied by significant increase in wind speed.By statistics,the average vertical lapse rate of temperature is 6.8℃/km,which can ensure the maintenance of negative buoyancy.The dry and cold air is entrained into the storm by the mid-level convergent airflow and further strengthens the downdraft,which helps maintain and strengthen the downburst.In the initial stage,strong reflectivity and mid-level radial convergence appear 20—30 min prior to the downburst.In the mature stage,the height of the strong reflectivity core significantly descends,and the lower layer presents a divergence structure.