Analysis of the circulation background and vorticity source for the Wuhan Tornado Storm on 14 May, 2021

At 20:39 BT on 14 May 2021, a rare EF3 tornado struck Wuhan, causing severe casualties and significant property damage. Both the X-band dual-polarization weather radar at China University of Geosciences (Wuhan) and the S-band weather radar at Wuhan Observatory indicated that the parent storm of this tornado was a supercell with a hook echo. Using ERA5 reanalysis data combined with a mesoscale numerical model, the weather condition, circulation background, and vorticity sources of this tornado event were analyzed. And the tornado’s circulation background was compared with that of another EF3 tornado which occurred in Hubei Province. The results are as follows. (1) The tornado occurred within the southern branch flow on the periphery of the western Pacific subtropical high. The development of a low-pressure system east of the Tibetan Plateau enhanced the pressure gradient between the low-pressure system and the subtropical high, which favored the formation of a low-level jet and strengthened low-level vertical wind shear, providing favorable moisture and dynamic conditions for the tornado storm. The coupling of upper- and low-level jets created an excellent environment for deep convection with low-level convergence and upper-level divergence, while the Jianghuai inverted trough served as a triggering mechanism for the convective storm. (2) The circulation background of this Wuhan tornado shares similarities with that of another EF3 tornado that occurred in Hubei on 16 April 1977, particularly in terms of the coupling of upper- and low-level jets, the influence of the Jianghuai inverted trough, and strong southwest-northeast oriented low-level vertical wind shear. (3) During the tornado storm genesis, both the low-level storm-relative velocity and the environmental horizontal vorticity were oriented southeast–northwest. The environmental horizontal vorticity had a large projection component along the storm-relative velocity direction, and a high-value area of storm-relative helicity extended from the southeast into the storm interior, providing a vorticity source for mesocyclone development. This underscores the importance of high spatiotemporal resolution vertical wind profile observations in the nowcasting and warning of tornadoes.