Comparative Analysis of Two Rare Large Hail Supercells in Winter

Two large-hail supercells that occurred before spring in Zhejiang, on February 21, 2024 (“24·2·21”), and March 3, 2025 (“25·3·3”), were analyzed using the reanalysis data, automatic weather station observations, lightning location data, and S-band dual-polarization radar data. The results indicate that both convection events occurred after the passage of a cold wave front and were elevated convection systems. Above the inversion layer, conditionally unstable stratification and positive convective available potential energy were present. Deep convection was rapidly triggered under the influence of the 850 hPa shear line. Strong, deep-layer vertical wind shear favored the development of supercells. The dry layer in the “25·3·3” case was deeper than that in the “24·2·21” case. In both supercell hail processes, hailfall was accompanied by a decrease in storm centroid height and in the maximum shear height of the mesocyclone. The “24·2·21” supercell exhibited a long maintenance time, a mid-level double vortex structure, and differential reflectivity (Z_DR) and specific differential phase (K_DP) columns extending above 6 km. The Z_DR and K_DP values within the overhanging echo were small, and the cross-correlation coefficient (C_C) decreased with decreasing height, indicating the presence of melting small ice phase particles, which favored moist hail growth. In contrast, the “25·3·3” supercell had a shorter maintenance time, a deep single vortex (mesocyclone) structure, and a pronounced rear inflow. The Z_DR and K_DP columns were confined below 5 km. Small Z_DR and K_DP values accompanied by large C_C values at upper levels indicated a greater abundance of dry ice phase particles. This was manifested by an increased frequency of intracloud lightning flashes, which may be associated with accelerated freezing caused by rear-inflow dry cold air and was conducive to dry hail growth in the upper cold cloud region.