Analysis of supercell and meso-scale convective system causing an extreme thunderstorm gale in Tianjin on 25 June 2020

Abstract

An extreme thunderstorm gale event (hereinafter referred to as the "6.25" event) occurred in Tianjin on 25 June 2020. Based on the intensive stational data and the data from Doppler weather radar, wind profile radar, microwave radiometer, and VDRAS (Variational Doppler Radar Analysis System), we conducted an analysis of supercell and meso-scale convective system causing the"6.25"event. The main results are as follow: Before the occurrence of gale, the echo pattern of the supercell changed from hook to bow. The increase of the disturbance temperature gradient in the front of mesoscale convective system (MCS) and the mesoscale convergence center formed by the cold pool outflow and the ambient wind field merge with the convergence area in the southeast of Tianjin, which is beneficial to strengthen and maintain the strength of MCS. The strong vertical wind shear at 0—3 km enhanced the vertical vorticity at the Xiqing station, which also played an important role in the development of MCS. The merging of the incoming air flow at the height of 3 km behind the storm and the downdraft inside the storm enhanced effectively the downdraft velocity, which was conducive to the formation of thunderstorm winds. Mid-altitude Meridional Convergence strengthened, the diameter of mesocyclone decreased, and the variation of V40up6 (total volume of strong echoes greater than 40 dBz over 6 km) and V40down6(total volume of strong echoes greater than 40 dBz below 6 km) was nearly 30 minutes earlier than the occurrence of extreme gale at the Xiqing station, which has some value for prediction.

Keywords

• supercell
• bow echo
• extreme wind
• vertical wind shear
• vertical vorticity