2018年8月徐州一次龙卷多普勒雷达特征分析

Analysis on the Doppler radar characteristics of a tornado in Xuzhou on August 2018

2018年8月18日18—20时在江苏省徐州市出现两次龙卷天气过程。两次龙卷持续时间均在1 min左右,强度为EF1—EF2级别,由于第二次龙卷发生位置距雷达站较远,因此仅分析第一次龙卷过程。通过常规观测资料和徐州多普勒天气雷达资料等进行分析表明:龙卷发生前,风暴经过两次加强后最终产生龙卷,说明对流单体加强有利于诱发龙卷。上干下湿的不稳定结构、中低层较强的垂直风切变和低的抬升凝结高度提供了有利条件。反射率因子呈超级单体特征且出现钩状回波。在龙卷发生前18 min在雷达回波图检测到了中气旋并维持4个体扫且有明显的从高层到低层发展的趋势,在最低仰角检测到了相邻像素间的强速度切变,对龙卷的预警有良好的参考意义。

Two tornado processesoccurred in Xuzhou City,Jiangsu Province from 18:00 BST to 20∶00 BST on August 18,2018.The duration of both two cases is about 1min,and the intensity is EF1-EF2.The location of the second tornado is far from the radar station,so only the first onewas analyzed.The analysis of conventional observation data and Xuzhou Doppler weather radar data shows that:before the tornado,the thunderstorm finally produce a tornado was after it strengthened twice,which indicates that the reinforcement of convective monomer is beneficial to induce tornado.The unstable environmental condition of upper dry and lower wet,strong vertical wind shear in the mid-low layers and low uplift condensation height provide favorable conditions for the generation of tornadoes.The radar reflectivity factor presents super-cell characteristics and the hook echo.In the 18 minutes before the tornado occur,a mesocyclone is detected in the radar echo map and maintains four scans with a obvious trend from high to low level.At the lowest elevation,a strong velocity shear between adjacent pixels is detected,which provides a good reference for the early forecasting of tornado.