Comparison of Characteristics and Extreme Gale Mechanisms of Two Strong Squall Lines in Zhejiang During Spring

Two significant squall line events occurred in Zhejiang from afternoon until nightfall on 25 March and 2 April in 2024. In order to deepen the understanding of characteristics of the spring squall line and the mechanism of extreme wind, two processes are analyzed based on ERA5 reanalysis data, observations of automatic weather stations, S-band radar and X-band phased array radar. Main conclusions are as follows. 0325 squall line occurs in front of the upper trough and near the surface cold front. The mesoscale circulation generates near the front triggers the convection. 0402 squall line occurs in advance of the upper south trough and within the warm region near the surface. The strong warm advection and deep convective instability stratification in the middle to lower layers are important mechanisms for development. 0325 squall line initially appears as a small bow echo. The southward movement of the cold front leads to the formation of strong storm cells near the front line and the merging with the severe storm, making the size of the storm rapidly grow into a squall line, with an obvious bow structure. 0402 squall line is formed by the convergence of two linear convective lines. It develops and matures under the action of the topographic convergence line, without obvious bow structure. The strong cell causing extreme wind and hail during 0325 squall line is a supercell, which is mainly characterized by high development height of the storm, strong oblique updraft, backward inflow jet and mesocyclone, ZDR arc and KDP foot. On the eve of extreme wind, the horizontal scale contraction with vertical scale extension of the meso-cyclone and sudden drop of the storm's center of mass indicate a strong downburst flow in the storm, and the horizontal pressure gradient increases due to the superimposed effect of the cold pool and the background field cold front, which result in extreme wind. In 0402 squall line, the development height of the strong storm cell causing extreme wind is relatively low, but the oblique updraft and the rear inflow jet still form a mid-level convergence velocity pair (mid-level radial convergence), and there are also ZDR arcs and KDP feet at the lower level. Prior to the occurrence of extreme winds, characteristics of downbursts, such as the stretching of velocity pairs and the decrease in the storm center, are also observed. Effects of precipitation drag and the narrow pipe phenomenon also contribute to the further intensification of surface winds.