2021年7月21日保定清苑龙卷雷达观测和环境条件分析

ENVIRONMENTAL CONDITIONS AND RADAR MONITORING OF THETORNADO ON JULY 21, 2021 IN QINGYUAN DISTRICT,BAODING CITY

利用石家庄新乐SA型多普勒天气雷达资料、雄安新区安新东白庄X波段相控阵多普勒天气雷达资料、ERA5再分析资料和常规观测资料对2021年7月21日河北省保定市清苑区东闾村龙卷过程进行了详细分析。结果如下:(1)龙卷于15时43分开始,历时22分钟,根据灾情判定此次龙卷为EF2级。造成此次龙卷的影响系统不典型,500 hPa及以下为切变或风速辐合区,700 hPa急流不仅输送了暖湿空气,加强了不稳定,还加大了垂直风切变,有利于强对流的生成和发展。(2)造成龙卷的微型超级单体风暴环境场特征:CAPE达到1680.3 J/kg,0~3 km垂直风切变较大为17.1 m/s,0~1 km垂直风切变为7.1 m/s,0~6 km垂直风切变较小为7 m/s,湿层相对深厚,抬升凝结高度为316.1 m。(3)SA型多普勒天气雷达和X波段相控阵雷达观测到的1 km以上中气旋起止时间、强度、伸展高度和演变趋势基本一致。(4)X波段相控阵雷达径向分辨率很高,同时龙卷距离雷达较近,因而方位角方向分辨率也很高,观测更精细。X波段相控阵雷达在龙卷发生前5分钟到龙卷结束持续观测到1 km以下的低层中气旋,对发布龙卷预警有很好的指示意义。X波段相控阵雷达清晰地观测到钩状回波顶点附近低层的辐合流场,辐合高度可达2 km以上,证实了沿龙卷路径地面是辐合流场的结论。(5)在龙卷形成前14分钟X波段相控阵雷达观测到超单体风暴后侧下沉气流,后侧下沉气流温度低于环境温度,同时在龙卷形成位置中低层观测到强的辐合上升运动,这些观测特征验证了美国学者关于低层中气旋的形成机理。(6)低层中气旋生成后,X波段相控阵雷达观测到低层辐合迅速加强,达到最强时龙卷形成,验证了美国学者关于低层中气旋形成后还需要低层强辐合以及中层强上升气流的向上垂直拉升,龙卷才能形成的机理。

The current study presented a detailed analysis of the tornado events on July 21,2021,in DonglüVillage,Qingyuan District,Baoding City,Hebei Province,by using data from the SA-band Doppler Weather Radar in Shijiazhuang City and the X-Band Phased Array Doppler Weather Radar in Xiong’an New Area,ERA5 Reanalysis data and conventional observations data.The results were as follows:(1)The tornado began at 15:53 and lasted 22 minutes.According to the extent of damage,the tornado was classified as EF2.The tornado was not caused by a typical weather system;instead,there were shear or wind speed convergence areas below 500hPa.The 700 hPa jet stream brought warm and moist air,enhancing instability and increasing vertical wind shear,which facilitated the development of strong convection.(2)The environmental characteristics of the mini-supercell storm that generated the tornado included a convective available potential energy of 1680.3 J/kg.The vertical wind shear for 0~3 km was 17.1 m/s,indicating a strong shear.The vertical wind shear for 0~1 km was 7.1 m/s,while for 0~6 km,it was 7 m/s,indicating a smaller shear.The wet layer was relatively deep,and the lifting condensation level was observed at 316.1 km.(3)The starting and ending time,intensity,extension height,and evolution trend of the mesocyclone above 1 km observed by the SA-band Doppler Weather Radar and the X-band Phased Array Radar were basically consistent.(4)The X-band Phased Array Radar provided precise observations with its high radial resolution and proximity to the tornado.It continuously monitored a low-level mesocyclone below 1 km from 5 minutes before the tornado formation until its end,indicating the imminent occurrence of the tornado.The X-band Phased Array Radar also clearly observed the convergent flow field near the apex of the hook echo with convergence heights reaching more than 2 km,confirming the presence of a convergent flow field along the tornado’s path.(5)The rear-side downdraft of the supercell storm was observed by the X-band Phased Array Radar 14 minutes before tornado formation.The temperature of the rear-side downdraft was lower than the environmental temperature.Strong convergence and upward movement were observed in the middle and lower layers of the area where the tornado formed,consistent with the formation mechanism of mesocyclones in the lower layers proposed by American scholars.(6)After the mesocyclone was formed in the lower layer,the X-band Phased Array Radar observed a rapid strengthening of convergence in the lower layers.The tornado formed when the convergence reached its peak,validating the mechanism proposed by American scholars that strong convergence in the lower layers and upward vertical uplift of the strong updraft in the middle layers are necessary for tornado formation.