Environmental Conditions, Evolution and Mechanisms of the EF4 Tornado in Kaiyuan of Liaoning Province on 3 July 2019

Based on multi-source observations with high spatial and temporal resolution, we present in detail the synoptic situation, the environmental conditions, the triggering and evolution characteristics of the convective storm, and the formation and demise mechanisms of the EF4 tornado in Kaiyuan, Liaoning province on the afternoon of 3 July, 2019 in this paper. The Kaiyuan tornado occurred under the 500-hPa northwesterly airflow and the 850-hPa shear line in the southwest side of a cold vortex over Northeast China, and in the strong warm moist southwesterly airflow at the surface. Except for the low relative humidity in the middle and lower troposphere and the higher lifting condensation level which were unfavorable for the tornado, other favorable environmental conditions for mesocyclonic tornado were all satisfied. However, winds of the Shenyang wind profiler radar and radial velocity of the Shenyang weather radar both show that the enhanced 0-1 km vertical wind shear owned a mesoscale feature, indicating that the coupling between the boundary layer jet and the mid-level jet formed strong vertical wind shear was favorable for the EF4 tornado. The Kaiyuan tornado was produced by an isolated supercell with typical supercell radar echo features, an intense mesocyclone and tornadic vortex signature, etc. The storm was triggered by both a dryline and a convergence line of gust front to the east of the dryline. Rainfall produced in the front of the storm first made the atmosphere rapidly saturated over Kaiyuan and its vicinity, and when the hook echo part at the back of the storm moved to Kaiyuan and its vicinity, the significantly improved low-level moisture condition favored the cold pool with suitable intensity generated by the relatively weak downdraft of the storm . So the storm, with the suitable cold pool, the strong warm moist boundary-layer air inflow, the strong low-level and mid-level vertical wind shear, and the intense updraft, produced the EF4 tornado in Kaiyuan. The temperature distribution from weather automatic stations shows that the temperature differences between the cold pool of the storm and the ambient atmosphere were 2-4 ℃ and were favorable for the Kaiyuan tornadogenesis. While the strong downdraft of the storm produced the intense cold pool with the temperature difference up to 7℃ from the environmental atmosphere, it destroyed the maintenance of vertical vorticity near the ground and then caused the demise of the Kaiyuan tornado.