云南一次非超级单体龙卷多尺度分析

Multiscale Analysis of a Non-Supercell Tornado in Yunnan

基于再分析资料、多源观测资料和文山多普勒天气雷达资料,对2021年5月31日午后出现在云南省文山州广南县的一次EF0级龙卷的环流背景、环境潜势、中尺度对流系统的演变过程以及形成机理进行研究。结果表明:(1)此次龙卷发生在副热带高压外围偏西气流,700 hPa风速辐合区,850 hPa切变线上,且中层大气有干冷空气的侵入,具备龙卷发生所需的热力和水汽条件,但动力条件不利于龙卷出现,即低层和深层垂直风切变均显著弱于我国中东部的非超级单体龙卷。(2)较高的地面温度、云图上的水汽边界和中尺度辐合线共同导致了滇东南地区对流风暴的触发。受复杂地形影响,对流单体出现分裂并加强,不断向西北方向海拔较高的广南县城推进。(3)龙卷形成的可能机制是:广南盆地内较高的地面温度形成对流性不稳定,盆地东侧对流单体下方的冷池出流进入盆地并向西扩张,冷暖空气的交汇使新的单体开始沿辐合切变线触发,其下方的冷池与环境温度差有利于龙卷生成。随后该对流单体沿盆地西侧的地形发展成狭长形态,并在风暴低层形成初始涡旋,地形强迫作用加强了初始涡旋上空的抬升运动,使其向上倾斜拉伸的同时涡度增加,最终形成此次龙卷。

Based on reanalysis data,multi-source observation data,and Wenshan Doppler radar data,a study was conducted on the circulation background,environmental potential,evolution process,and formation mechanism of a EF0 tornado that occurred in Guangnan County,Wenshan Prefecture,Yunnan Province,in the afternoon of May 31,2021.The results showed that:(1)The tornado occurred in the western flow at edge of subtropical anticyclone,the 700 hPa wind speed convergence region,and the 850 hPa shear line.There was a dry and cold air intrusion in the middle atmosphere,and having the necessary thermal and water vapor conditions for a tornado to occur.However,the dynamic conditions were not conducive to the occurrence of a tornado,that is,the low-level and deep vertical wind shear were significantly weaker than that of non-supercell tornadoes in the central and eastern regions of China.(2)Higher ground temperatures,water vapor boundaries on cloud images,and mesoscale convergence lines jointly triggered convective storms in southeastern Yunnan Province.Affected by complex terrain,the convective cells appeared to be split and strengthened,and continued to advance towards the Guangnan County,which has a higher elevation in the northwest.(3)The possible mechanism for the formation of the tornado is that the high ground temperature in the Guangnan Basin created convective instability,and the cold pool outflow below the convective cells on the eastern side of the basin entered the basin and expanded westward.The intersection of warm and cold air caused new cells to begin triggering along the convergence shear line,and the temperature difference between the cold pool and the surrounding environment below it was conducive to the formation of the tornado.Subsequently,the convective cells developed into a narrow and long shape along the terrain on the western side of the basin,then an initial vortex formed in the lower level of the storm.The topographic forcing enhanced the uplift motion above the initial vortex,causing it to tilt and stretch while increasing vorticity,ultimately forming this tornado.