一次山区龙卷的双部雷达回波监测分析

Monitoring of Two Radar Echoes for a Tornado in Mountain

利用秦皇岛S波段和承德C波段天气雷达资料、NCEP 1°×1°6h再分析资料、中尺度物理量及灾情报告,对2009年7月20日河北承德平泉县一次龙卷天气过程进行了综合分析。结果表明,龙卷发生在西高东低的山谷中,冷空气南压,高空西南急流动量下传,使得低层中尺度水平风场-V分量沿山谷西部增大,产生不对称小尺度山谷风次级环流,增强的龙卷发生区域气旋性旋转涡度导致了多个中气旋的发生;08:00北京探空站0～6km垂直风切变为19.44m.s-1,K指数为24℃,SI指数为4℃;14:00K指数为28℃,SI指数为0℃;前期强降水在中午强辐射作用下迅速蒸发,为龙卷风暴的发生提供了短时水汽条件;C波段雷达监测数据显示,13:06～14:36山谷东侧先后有5个中气旋生成并伴有2个普通风暴单体,TVS分别出现在13:30,14:06和14:36,低层最大旋转速度为29m.s-1,高度为0.8km,风暴向北北西-南南东方向移动,移速为60～70km.h-1,龙卷强度在F0～F1级之间;对比径向速度与中气旋显示:1km高度以下,龙卷接触地面持续时间为10min,4km高度以上,龙卷空中盘旋持续时间为45min。

Using the radar data of CINRAD/SA in Qinhuangdao and CINRAD/CC in Chengde, NCEP reanalysis data, mesoscale physical diagnosis and disaster report, a tornado in Pingquan county of Chengde on 20 July 2009 is analyzed, the preliminary conclusions indicate that the tornado take place in valley which is higher terrain in the west and lower terrain in the east, cold air move southtoward, the momentum of upper-level NW jet stream go down, which make the component V of low-level wind field in mesoscale system to increase along the western valley,＇ product an asymmetric secondary circulation of small-scale valley breeze, enhance cyclonic vortices and occur much more mesocyclone in outbreak zone of tornado. At Bei jing station, the vertical wind shear is 19.44 m · s -1 at the heights of 0-6 km, the index K is 24, SI is 4 ℃ at 08：00, K is 28 ℃, SI is 0 ℃ at 14：00. Because the rainfall quickly is evaporated under the action of strong radiation in the afternoon, so it supply the short-time vapor conditions for keeping tornado. C wave rang radar displays that there are five mesocyclones and two general storm cells in the east of valley from 13：06 to 14：36, and three times onTVSdata are at 13：30, 14：06, 14：36, respectively. The maximum vorticity in the lower level is 29 m · s- 1, height is 0.8 km, the moving direction of storm is from NNW to SSE, horizontal moving speed is 60-70 km · h -1 , maximum echo intensity is 69 dBZ, tornado strength limit within F0-F1. Comparing with the special profile of the Doppler radial velocity and mesocyclone, it is found the duration time until tornado touching the ground below 1 km is 10 min and the duration time of rotation above 4 km height is 45 min.