黄土高原一次疑似龙卷天气的中尺度特征

Mesoscale Characteristics of a Doubtful Tornado over the Loess Plateau

利用高空、地面常规观测资料、分钟级加密自动气象站资料和榆林多普勒雷达资料,对2013年8月4日傍晚发生在榆林市的一次超级单体风暴天气进行中尺度分析。结果表明:(1)此次过程疑似一次超级单体龙卷天气过程;(2)从环流背景来看,榆林市上空中层强干冷平流配合低层切变线、西南急流,高层干冷、低层暖湿特征明显;从环境条件来看,强风暴发生前和发生期间,能量、抬升凝结高度、风切变满足龙卷发生所需的热力不稳定、垂直风切变条件;(3)雷达钩状回波结构清晰,并伴有强中气旋,>60 dBZ的回波和正负速度对已接地,呈现龙卷发生时的回波特征;(4)强风暴发生前后,由北向南经过榆林地区有多个龙卷涡旋TVS产品被识别;(5)气象要素场变化剧烈,地面气压明显降低,风速出现极值增强,风向发生突变,与龙卷发生期间风场观测特征基本一致,表明该区域出现龙卷的可能性较大。

The mesoscale characteristics of a severe convective weather event that occurred in Yulin on August 4th,2013 were analyzed using conventional observation data,including radiosonde and ground observations,as well as minute-resolution data from automatic meteorological stations and Doppler radar.The results show that this event was likely a tornado event triggered by a supercell.Considering the larger-scale circulation patterns,a northwesterly airflow in the middle troposphere led to strong dry-cold advection.Additionally,a southwesterly jet and a shear line in the lower layers contributed to a structure with dry-cold conditions at the top and wet-warm conditions at the bottom.In terms of the environmental conditions,prior to and during the formation of the supercell,the convective available potential energy(CAPE),lifting condensation level(LCL),and vertical wind shear met the necessary thermal instability and dynamic criteria for the development of a tornado event.The radar echoes exhibited a distinct hook-shaped pattern along with a strong mesocyclone.Moreover,the presence of positive and negative velocity pairs with echoes exceeding 60 dBZ indicated the echo pattern typically associated with a tornado.Before and after the formation of the supercell,several tornado vortex signatures(TVS)were detected as they moved through the Yulin area in a north-to-south direction.At the surface,meteorological conditions underwent significant changes,including a decrease in atmospheric pressure at the ground level and the rapid strengthening and rotation of winds at the 10-meter level.These changes corresponded to the observed characteristics of an ongoing tornado event,suggesting a high probability of atornado occurrence.