Comparative Analysis of Two Rare Large Hail Supercells in Winter

Using reanalysis data, automatic stations, lightning positioning, and S-band dual-polarization radar data,? two large hail supercells that occurred in Zhejiang before the beginning of spring on February 21, 2024 and March 3, 2025 were analyzed. The results show that: Both processes happened under the circumstances where cold waves and cold fronts moved southward. "24·2·21" occurred near the surface cold front, and the convection was triggered by the cold front and developed rapidly in the convective unstable stratification. While "25·3·3" occurred 100 kilometers behind the cold front, which was an elevated convection above the cold front. The convection-symmetrical instability mechanism led to the development of deep convection above the inversion layer. The primary storm of "24·2·21" was grounded, but the primary storm of "25·3·3" was above 3 kilometers. Then it developed upwards and downwards, with the strong echo peak developing at a higher height. Both storms developed into supercells and dropped large hailstones during their mature stages. The process of hail dropping was accompanied by the decrease of the storm"s center of mass and the maximum shear height of the mesocyclone. A double-vortex structure was at the middle layer of the "24·2·21" supercell , while a single-vortex structure that of the "25·3·3" supercell , and the mesocyclone is deeper and stronger. Both storms had ZDR arc at the lower level. The "24·2·21" storm had obvious ZDR columns and KDP columns above the melting layer, with many liquid particles. Hail particles undergo significant wet growth in the updraft. The "25·3·3" storm had short ZDR columns and KDP columns, and the frequency of cloud flashes at the upper level was significantly higher than that of the "24·2·21" storm, with more solid particles in the cold clouds at the upper level. Hail enters the cold cloud and undergoes dry growth.