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 spatio-temporal resolution, we present in detail the synoptic situation, environmental conditions, triggering and evolution characteristics of the convective storm, and 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 and 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-profiling radar and radial velocity of Shenyang weather radar both show that the enhanced 0-1 km vertical wind shear had a mesoscale feature indicating that the coupling between the boundary layer jet and the mid-level jet formed strong vertical wind shear favorable for the EF4 tornado. The Kaiyuan tornado was generated by an isolated supercell with typical supercell radar echo features, an intense mesocyclone and tornadic vortex signature, etc. The strom was triggered by both a dryline and a convergence line of gust front to the east of the dryline. The rainfall produced in the front of the storm first made the atmosphere rapidly saturated over Kaiyuan and its vicinity. When the hook echo part at the back of the storm moved to Kaiyuan and its vicinity, the significantly improved low-level moisture condition was good for the less strong downdraft of the storm to generate the cold pool with suitable intensity. So the storm, with the suitable cold pool, strong warm and moist boundary-layer air inflow, strong low-level and mid-level vertical wind shear, and intense updraft, produced the EF4 tornado in Kaiyuan. The temperature distribution from automatic weather stations shows that the temperature differences between cold pool of the storm and the ambient atmosphere were 2-4℃, favorable for 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.