Analysis of Hail Event Accompanied by Elevated Convection in Winter in East China

Based on conventional observational data, civil aviation airport message data, ERA5 reanalysis data, X-band phased array radar data, etc., a winter hail event in the airspace of East China on February 21, 2024 was analyzed. It was concluded that: (1) This event was a typical elevated thunderstorm. The abnormally strong southwest warm and humid air flow slowly climbed above the inversion layer along the deep cold cushion at the low level, creating a favorable environmental background for the formation of winter elevated convection hail. The hail occurred about 170 km behind the surface cold front. (2) This hail was caused by multiple instability mechanisms. Symmetric instability and convective instability were located at different heights. Conditional symmetric instability was mainly active below 700 hPa. When the warm and moist air ascended along the cold dome of a strong frontal zone, generating a sloping updraft. Convective instability was established in the middle layer (700 hPa and above), connecting the inclined convection formed by conditional symmetric instability at the low level. Under the combined action of the dynamic forcing of frontogenesis in the middle layer, the unstable energy was released and converted into deep and strong vertical convection, and the severe convective storm broke through the -20°C altitude layer. (3) The X-band phased array radar detected a thunderstorm group composed of similar supercell storms and multi-cell storms. The mesocyclone of the “supercell-like” storm had the characteristics of being shallow and short lasting, which was conducive to the enhancement of convergence above the cold cushion, promoting the formation of hail. (4) X-band dual polarization radar exhibits significant utility in identifying winter hail. The phase evolution and growth efficiency of hail particles were closely related to the dynamic-thermal structure of storms and the microphysical coupling processes. Within the storm, there existed differential reflectivity (ZDR) columns and differential phase shift rate (KDP) columns both tightly associated with strong updrafts. Hail embryos gathered and grew at high altitudes. Due to the presence of a mid-level warm layer, dry hailstones partially melted during descent, forming wet hailstones. This resulted in the presence of an area with elevated KDP values and depressed correlation coefficient (CC) values in the storm's rear flank.