A case study on the radar characteristics and physical process involved in the genesis of a mini supercell tornado under the background of cold vortex

An EF1 tornado occurred in the northern coast of the Bohai under the background of cold vortex on 16 August 2019. Using the Yingkou S-band dual-polarization Doppler weather radar data, surface automatic weather station (AWS) observations at 5 min interval, the Panjin wind profile radar data and ERA5 reanalysis data, the environmental background, the structure and formation of the tornadic storm and the tornadogenesis are studied. The results show that the tornado occurred under the background of a cold vortex at 500 hPa, and it was located in the water vapor conveyor belt on the west side of the residual vortex of typhoon "Lekima". The environmental condition is characterized by weak vertical wind shear and strong low-level thermal instability. The Yingkou dual-polarization radar is located 15 km away from where the tornado originated. The hook echo, the descending reflectivity core (DRC), the weak echo hole (WEH), and the tornadic debris signature (TDS) in mini supercell are detected by the radar. The outflow of the decaying thunderstorm gust front moved westward, while the sea breeze front near Yingkou slowly moved eastward. The two boundary layer convergence lines merged, leading to the forming of γ-mesoscale vortex under the influence of horizontal shear instability. The intersection of outflow boundaries, the large positive ambient buoyancy and the vertical perturbation pressure gradient associated with the low pressure induced by the middle level mesocyclone jointly produced strong updrafts. The collocation of the updrafts and the γ-mesoscale vortex played a critical role for the genesis of misocyclone by strong stretching. The combination of the maximum rotation velocity and the minimum diameter of the misocyclone corresponded to the tornadogenesis, and the separation of the misocyclone and the mesocyclone in the middle level led to the dissipation of the tornado.