Multiscale Analysis of a Non-Supercell Tornado in Yunnan

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.