利用多普勒天气雷达资料,对2003年7月8日夜间发生在安徽无为县的强烈龙卷过程进行了详细的分析。该龙卷发生前的主要天气背景是江淮梅雨期暴雨的天气形势:一个东移的高空槽、强烈的对流不稳定和低空的西南风急流。低层垂直风切变很大并且抬升凝结高度较低,有利于强龙卷的产生。产生该强龙卷的对流系统最初是一条位于大片层状云降水区中的长对流雨带。在随后的演变中,对流雨带的南段逐渐消散,北段逐渐变宽,最终成为一个团状的对流系统,而龙卷产生自该系统南端的一个超级单体。最初的中层中气旋形成于7月8日22:49(北京时,下同),相应对流单体的反射率因子尚没有呈现出超级单体的特征。随后中气旋迅速加强,在22:55,反射率因子形态呈现出经典超级单体的特征:明显的低层入流缺口和其左侧的阵风锋,入流缺口位于超级单体移动方向(东北方向)的右后侧,低层的弱回波区和中高层的回波悬垂结构,最大反射率因子超过55 dBz。在龙卷产生前8min,即23:12中气旋达到强中气旋标准,相应的垂直涡度值达到2.3×10^-2/s。在龙卷产生前几分钟和龙卷进行过程中,中气旋保持很强,但相应的反射率因子强度减弱,低层入流缺口渐渐消失。在龙卷进行过程中的23:29,雷达速度图像呈现出一个强烈中气旋包裹着一个更小尺度的龙卷式涡旋特征TVS,与TVS对应的垂直涡度值达5.0×10^-2/s。上述导致龙卷的中层中气旋局限于4 km以下的低层大气,前后共持续了1 h 49 min,相应超级单体的高反射率因子区局限在6 km以下,属于低质心的对流系统,产生的天气是强烈龙卷,伴随有暴雨,但没有冰雹。文中还对此次龙卷的生成机制进行了探讨。

Based on the Hefei Doppler weather radar data,the tornadoes occurred in Wuwei county,Anhui province in the night of 8 July 2003 are analyzed in detail.From the night of 8 to the early morning of 9 July,associated with the organized convective rain belts embodied in the large area of stratiform precipitation,several meso-γ-scale vortexes formed along the leading edge of the convective rain-belt within the lower atmosphere.A mesocyclone formed at the south tip of the convective rain-belt led to the severe tornado occurred at 23:20 in Wuwei county,Anhui province.This tornado,ranked F3,killed 16 and wounded 166 persons.The convective system which caused the Wuwei tornado is a classic supercell located at the south end of a convective rainbelt.During the lasting period of the tornado,a TVS formed within the strong mesocyclone,the vertical vorticity associated with the TVS is about 5.0×10^-2/s.This supercell is a low centroid convective system,the above 45 dBz reflectivity region is limited to below 6 km,the severe tornado is the major severe weather,accompanied by heavy rain,without hail.The analysis indicate that,for the Wuwei tornado,a tornado warning could be issued with 8 min lead time,mainly based on the strong mesocyclone on velocity map,illustrating the greatly enhanced capability to detect tornadoes with the deployment of the China new generation Doppler weather radar.The mechanisms for this tornado occurrence are also discussed.