2012年7月27日陕北佳县特大暴雨天气的成因

利用常规高低空气象观测资料、地面逐小时降水、物理量、NCEP再分析资料和卫星云图,对2012年7月27日发生在陕北佳县的一次特大暴雨天气进行诊断分析,结果表明：暴雨前期副热带高压较强,当西风带的冷空气与副热带高压西北侧的西南暖湿气流交汇时,触发对流产生;低层700 hPa西南气流、850 hPa东南气流为特大暴雨提供了比较充沛的水汽;大气低层存在大量不稳定能量;高空200 hPa陕西北部位于浅槽的前部、急流入口区的右侧,存在正涡度平流,这一区域有强烈的辐散,提供了较强的上升运动,是特大暴雨产生的主要机制;在卫星云图上表现为一个水平尺度为200~400km、云顶亮温最低为-60℃,生命史为14 h的中-α尺度对流云团。

Cause Analysis of an Extremely Heavy Rainstorm in Jiaxian County in the North Shaanxi Province on July 27 2012

Based on the routine observation data,hourly rainfall data,physical quantity fields and satellite images as well as NCEP reanalysis data,an extremely heavy rainstorm in Jiaxian County in the north Shaanxi Province on July 27,2012 was analyzed. The conclusions are as follows： In the early stage of rainstorm the subtropical high was stronger,when the cold air from westerly zone encountered the southwest warm and moist air from the northwest of subtropical high,convection was triggered. The southwest airflow on 700hPa and the southeast airflow on 850 hPa provided a relatively abundant water vapor for the rainstorm. There were lots of unstable energy in the lower level,and on the level of 200 hPa the north of Shaanxi Province was located in the front of the shallow trough and the right side of the jet stream’s entrance,where positive vorticity advection and strong divergence existed,which resulted in a strong vertical movement. On the satellite cloud images,there was a meso-α-scale convective cloud cluster,whose horizontal scale was from200 to 400 km,and the lowest brightness temperature was-60 ℃ and sustained time was 14 hours.