冷涡背景下华北平原一次弓形回波致灾大风过程分析

Case analysis of damaging high winds generated by bow echoes in the presence of a cold vortex over the North China Plain

为了提高对弓形回波致灾大风环境演变和致灾机理的认识,综合利用多源观测和ERA5再分析资料,研究了2020年6月25日华北平原夜间弓形回波的风暴环境演变特征及地面致灾大风的成因机制。结果表明:此次过程发生在高空冷涡背景下,华北平原处于中层干冷气流与低层西南暖湿气流叠加区域,因此有利于强对流天气的发生;对流风暴演变可归结为“超级单体-弓形回波-逗点回波”三个阶段,风暴环境逐渐从中等强度的对流有效位能和深层风垂直切变向弱的对流有效位能和强的风垂直切变演变;超级单体阶段,探空曲线呈“X”型分布,负浮力效应为地面大风的产生做主要贡献,动量下传和冷池密度流的作用为辅;弓形回波阶段,由于低层暖平流和地面辐射降温的共同作用,近地面出现较强逆温,850—500 hPa垂直温度直减率增大,负浮力、动量下传和冷池密度流作用均较前一阶段明显加强,导致地面13级致灾大风的形成;逗点回波阶段,850—700 hPa的干层减弱,负浮力作用与超级单体阶段相当,动量下传和冷池密度流作用与弓形回波阶段相当,造成地面大风的形成。最后给出本次弓形回波环境演变和致灾机理的物理模型。

In order to improve the study of environmental evolution and mechanism of strong winds induced by bow echo,multisource observations(Doppler radar,wind profiler,meteorological tower in Tianjin and automatic weather station data at 5 min interval)and reanalysis data of the ECMWF ERA5(spatial resolution of 0.25°and temporal resolution of 1 h)are comprehensively used to analyze the nocturnal bow echo event in North China Plain on 25 June 2020.Results indicate that this bow echo event developed under favorable weather conditions.The cold dry air behind the 500 hPa cold vortex superimposed on the warm moist southwesterly airflow at 850 hPa,which is conducive to the occurrence of convective storm.Characteristics of the convective storm evolution can be summed up as three stage:Supercell,bow echo and comma echo.The environmental condition at the supercell stage is characterized by moderate Convective Effective Potential Energy(CAPE)and 0—6 km wind shears,while at the bow echo and comma echo stages CAPE is low and 0—6 km wind shears are high.In the supercell stage,the sounding curve shows an"X"-shaped distribution.The negative buoyancy effect is the primary contributor to the surface gale.Meanwhile,the downward transport of momentum and cold pool high-density air are helpful for the formation of surface gale.In the bow echo stage,due to the combined effects of low-level warm advection and surface radiative cooling,a strong temperature inversion layer occurred near the surface,and the vertical temperature lapse rate between 850—500 hPa increased.The effects of negative buoyancy,downward transportation of momentum and cold pool high-density air all strengthened compared with that in the previous stage,resulting in the formation of the13th grade disastrous gale.In the comma echo stage,the 850—700 hPa dry layer weakened,and the negative buoyancy effect is similar to that in the supercell stage.The effects of downward transport of momentum and cold pool density are similar to that in the bow echo stage,resulting in the formation of surface gale.Finally,the environmental evolution and mechanism of bow echo conceptual model are summarized.