两次强龙卷过程的环境背景场和多普勒雷达资料的对比分析

THE CONTRASTIVE ANALYSIS OF SYNOPTIC SITUATION AND DOPPLER RADAR DATA FOR TWO INTENSE TORNADO CASES

关键词:  龙卷  大气对流参数  多普勒天气雷达  中气旋

Key words:  tornado  convective parameter  Doppler weather radar

摘要:

通过分析发生在安徽省灵璧县(2005年7月30日)和无为县(2003年7月8日)的两次强龙卷过程的环境背景场和雷达资料发现:(1)在环境背景场方面,两次过程都发生在高空有低槽和切变线、地面较为暖湿的环境中,且龙卷发生地附近都有低层边界(冷锋或雷暴外流边界等),低层垂直风切变较大。不同之处是灵璧县龙卷0～1km的平均垂直风切变较大,0～6km的平均垂直风切变较小,而无为县龙卷的高、低层的平均垂直风切变都很大。进一步分析得出,在对流有效位能达到一定值后,密度加权垂直风切变对对流的组织和维持起到非常重要的作用。(2)在雷达特征方面,两次龙卷过程的母体雷达反射率因子回波不同于经典超级单体。虽然回波形态、强度和高度各异,但都存在强的中气旋,且其强核都出现在雷达可探测的最低高度。中气旋内部的垂直涡度都很大,达到2～6个中气旋单位(一个中气旋单位=10^-2s^-1)。另外,雷达导出产品中的中气旋识别产品对强对流天气的监测有重要的应用价值,雷达超前于龙卷发生约半小时识别出中气旋,这对龙卷的预警非常有意义。

Abstract:

Two F2～F3 tornadoes,which occurred on 30 July 2005 in Lingbi and on 8 July in Wuwei,respectively,brought serious disaster to Anhui Province and greatly influenced the society.By analyzing the synoptic situation and Doppler radar data,it is found that the two cases have the same characteristics in synoptic situation.They both happened where there were high-level troughs and shears with warm,moist atmosphere on the surface,boundary-layer convergence line(such as cold front or thunderstorm outflow boundary etc) where tornadoes happened,and large low-level vertical wind shear.However,the difference between the two cases is that for Lingbi the vertical wind shear(VWS) is large at the 0～1km low level but is small on the 0～6km deep layer;for Wuwei VWS is large at both the 0～6km and 0～1km levels.With comprehensive analysis,the result displays that vertical wind shear has an important effect on the organization and maintenance of convective systems when CAPE exceeds a threshold.The radar base reflectivity factors of the two tornadoes are different from classical supercells,and the shape,intensity,and height of the echos are also different.But strong mesocyclone appeared in both tornado events and all the strong mesocyclone cores located at the lowest detectable height of radar.Vertical vortex of mesocyclone in the two tornadoes is about 2～6 mesocyclone units(one mesocyclone unit is 10^-2s^-1),and vertical vortex on 8 July 2003 is greater than that on 30 July 2005.Radar-derived products,such as products of mesocyclone,are very useful to detect strong convective weather,Especially,the appearance of mesocyclone is half an hour earlier than that of tornado,which is quite important for tornado warning.