Analysis of Evolution Structure of a Squall Line in Zhejiang Province Based on Multi-band Radar Observation

In this paper, the characteristics of a squall line that occurred in central and northern Zhejiang on 21 July 2022 are analyzed by using S-band dual-polarization radar, X-band phased array radar data, wind retrieval, lightning location and ERA5 reanalysis data. The results show that: The squall line affected the area for up to 6 hours, causing large-scale winds of 8-10 levels along the way. The pressure surge ahead of the strong winds and precipitation at the extremely strong wind station served as a precursor to extreme winds. The squall line occurred under the control of the subtropical high. The dry and cold air at the bottom of the high-level cold vortex invaded, with the high-energy and high humidity in the low-level forming an unstable layer structure, which provided favourable environmental conditions for the generation and maintenance of the squall line. The observation of the S-band dual polarisation radar showed that the evolution of squall lines went through processes such as initiation, merging, growth, maturity, and disappearance. The initial convection was formed in the mesoscale convergence lines in the northwest and east of Hangzhou. Strong storms in the northwest of Hangzhou moved faster than those in the east, leading to the rapid merging and development of the two into squall lines. After the squall line entered the south bank of the Hangzhou Bay, the storm outflow overlapped with the mesoscale convergence centre on the south bank of the Hangzhou Bay, leading to the rapid development of the squall line to its mature stage. In addition, the growth of vertical wind shear was also conducive to the rapid development of the squall line. Strong winds often occurred in areas with dense cloud flashes. The X-band phased array radar could finely detect the vertical structure of strong cells before and after extreme winds occurred. The development of ZDR and KDP columns in mature strong cells represented storm intensification, and the rapid decline of the echo centroid could indicate surface winds. The falling and dragging of precipitation particles intensified the sinking motion, which further enhanced the strong winds. The configuration of inclined upward and downward airflow in strong squall lines ensured that the falling of precipitation particles did not affect the upward airflow, which was beneficial for the development and maintenance of storms. The oblique sinking airflow met the near-surface environmental wind, triggering the development of frontal convection, which was conducive to the propagation of squall lines downstream.