京津冀一次暖区大暴雨的成因分析

Analysis on causes of a warm-sector torrential rain event in the Beijing-Tianjin-Hebei region

利用常规气象观测资料、区域自动站加密观测资料、NCEP 1°×1°逐6 h再分析资料,以及气象卫星、多普勒天气雷达、风廓线雷达探测资料与北京变分多普勒雷达分析系统(VDRAS)反演资料,对2020年8月12日京津冀地区一次区域性暖区大暴雨过程的降水特征、环流背景、中尺度系统演变特征及其成因进行了分析。结果表明:这次过程发生在副热带高压边缘、500 hPa以下暖气团中,主要影响系统为850 hPa低空急流和暖式切变线。强降水表现出明显的阶段性,主要分两个阶段,分别对应河北南部和京津冀北部两个暴雨区,其形成原因不同。第一阶段强降水由一α中尺度对流系统(MαCS)发生发展造成,降雨前局地水汽和能量充足,地面辐合线触发不稳定能量释放形成线状强回波,对应地面气旋性环流为螺旋状回波,其上的对流单体不断发生发展造成强降水。第二阶段强降水由多个β中尺度云团产生,北上加强的偏南风低空急流为暴雨的发生提供了充足的水汽和动力条件,边界层低空急流及中尺度低涡系统是第二阶段大暴雨的重要影响系统,雄安新区单站125.9 mm极端小时强降水是由“逗点状”回波尾部暖云降水叠加“列车效应”共同造成。

Based on conventional meteorological observations,observational data from the regional automatic weather stations,NCEP reanalysis data with spatial resolution of 1°×1°and temporal resolution of 6 h,satellite images,Doppler weather radar and wind profiler data,as well as the retrieval results from BeijingVariational Doppler Radar Analysis System(VDRAS),we have conducted analysis of the precipitation characteristics,atmospheric circulation,mesoscale system evolution features and the causes of a regional warm-sector torrential rain event in the Beijing-Tianjin-Hebei region on 12 August 2020.The results show that the event occurs in the warm air mass below 500 hPa at the edge of the subtropical high,and the main influencing systems are the low-level jet and the warm shear line at the 850 hPa.The whole severe precipitation event presented clearly the phased characteristics;it can be divided into two stages corresponding to the two different rainstorm areas,i.e.,the southern Hebei and the north part of the Beijing-Tianjin-Hebei region with different formation mechanisms.The severe precipitation in the first stage is caused by a meso-α-scale convective system(MαCS),which is triggered by the surface convergence line.Before the precipitation starts,the local water vapor and unstable energy is rich.When the surface convergence line triggers the release of unstable energy,a strong linear echo is formed.The severe precipitation is mainly caused by the continuous development of convective cells in the spiral echo corresponding to the surface cyclonic circulation.The severe precipitation in the second stage is generated by several β-scale cloud clusters,and the southerly low-level jet strengthened northward provides sufficient water vapor and better dynamic conditions for the occurrence of torrential rain.The low-level jet in the boundary layer and the mesoscale low vortex system are the crucial influence systems at the second stage of this event.The hourly extremely severe precipitation of 125.9 mm at a station in Xiongan New Area is caused jointly by the warm cloud precipitation at the tail of comma-shaped radar echo and the"train effect".