Comparative analysis of supercells associated with two different types of wind disaster

Surface observations at minute-resolution from automatic weather stations, weather radar data from Yancheng, Huai'an, Yueyang and Jingzhou and the ECMWF ERA-Interim global high-resolution reanalysis data are used in this study. Characteristics of the circulation environment and structures of two supercells are comparatively analyzed. One supercell induced a tornado on 23 June 2016 and the other triggered a downburst on 1 June 2015. The results are as follows. (1) The two convective disasters occurred in similar circulation environments. The supercells with disastrous convective winds appeared to the east of the 850 hPa low vortex and in the warm zone to the left of the low-level jet. The distance between the center of the low vortex and the disaster area is about 600 to 700 km. In both cases, the convective available potential energy (CAPE) was higher than 2000 J/kg. However, features of the two wind disasters are different, i.e., the disaster in Funing was mainly caused by the tornado, and the accident of the "Eastern Star" on the Yangtze River was directly associated with the downburst. The relative position of maximum flash precipitation versus wind disaster is different in the two convective activities. The maximum flash heavy precipitation corresponding to the supercell center occurred on the left side of the moving direction of the tornado in Funing. The place where the ship capsized was coincident with the center of heavy precipitation during the severe convective weather. The position of the wind disaster relative to the instantaneous strong precipitation center is helpful for distinguishing downbursts from tornados caused by supercells. (2) The vertical wind shear of the environment has an important influence on the structure, development and maintenance of convective storm. The environmental vertical wind shear within 0-6 km reached 4×10^-3 s^-1 just before the occurrence of the tornado in Funing. The main body of the supercell tilted forward with height, corresponding to a strong inclined updraft and bounded weak echo region (BWER). However, the environmental vertical wind shear was only about 2.3×10^-3 s^-1 in Jianli, and the updraft of the storm cell was almost vertical. Before the tornado in Funing happened, the supercell with mesocyclone was first monitored in northwestern Jiangshu province, corresponding to the environment with strong vertical wind shear under 1.5 km and large storm relative helicity within 0-3 km. As the storm moved eastward with lower LCL (lifting condensation level), the bottom of the mesocyclone sank lower gradually, and the tornado formed finally. However, the 0-3 km relative storm helicity and 0-1.5 km vertical wind shear along the storm moving direction in Jianli were much weaker than that associated with the tornado in Funing. The bottom level of the mesocyclone in the storm was not lower than 1.6 km, and the storm intensity was weak with a short duration of mesocyclone maintenance. (3) The vertical structure characteristic of environmental humidity is an important factor to distinguish different types of disastrous convective wind. During the storm activity in Jianli, observations collected at automatic weather stations show that the pressure evolution exhibited a cone-shape with rapid jumping up and dropping down while temperature kept decreasing. The pressure peak appeared four minutes earlier than the precipitation peak appeared. It was associated with strong evaporation of hydrometers when the deep dry environmental air in the mid-troposphere was entrained in the convective storm. For the disaster in Funing, the ground cold pool effect was relatively weak, corresponding to a very shallow dry layer in the middle level and a deep saturated atmosphere in the lower level of the environmental atmosphere.