Microphysical characteristics of a supercell hailstorm

Based on precipitation observations of CINRAD/SA-D dual-polarization weather radar and ground automatic stations as well as disaster surveys and ERA5 data, the environmental conditions for a supercell hailstorm event are analyzed. Reliable observational data on the size distribution of hails are obtained by the particle identification technology, and the hail spectrum distribution is fitted. The study investigates the relationship of ground particle spectrum distribution in different regions and stages of the hailstorm with parameters of dual-polarization radar and obtains an in-depth understanding of the microphysical processes, thermodynamics, and dynamics of supercell hailstorms. The results indicate that: (1) The hailstorm is jointly influenced by an upper-level trough and a surface convergence line. It is a right-moving supercell storm generated in a low Convective Available Potential Energy (CAPE) and strong vertical wind shear environment, leading to disastrous large hails, heavy local rainfall and strong winds. (2) The identified hail particle size distribution in Gaotang and Changqing exhibits a monotonically decreasing pattern, and the Gamma distribution shows a significant fitting advantage for hailstones with diameter≥5 mm. Due to the inclusion of graupels with diameter＜5 mm, the intercept parameter (N₀) is generally larger than that in previous studies; the shape factor (µ) is negative. (3) The differential reflectivity arc of the supercell hailstorm is characterized by a large reflectivity factor gradient (Z_H), a significant differential reflectivity (Z_DR), and a low correlation coefficient (CC) and specific differential phase (K_DP). A considerable number of very large to large raindrops are observed at the ground level, which originated from a large amount of ice particles above the melting layer, accompanied by a significant number of smaller raindrops. The microphysical processes are primarily dominated by melting and evaporation. (4) Hails are concentrated in the narrow area at the front of the forward downdraft in the supercell hailstorm, with low-level Z_H≥60 dBz, Z_DR≤0 dB, CC≤0.92, K_DP＞1.7°/km, and a core with K_DP＞3.0°/km below the melting layer. Large hails, small hails, and graupels appear sequentially with a high proportion of large and very large raindrop density. The microphysical processes are mainly dominated by melting and shedding. (5) The period of maximum rainfall intensity occurs in the strong echo area of the forward downdraft after concentrated hailfall, where the number density of raindrops of various sizes increases significantly, leading to a marked increase in rainfall intensity. The peak of rainfall intensity occurred during the period when medium-sized raindrops were predominant. The microphysical processes are primarily dominated by melting and shedding, accompanied by coalescence and fragmentation processes caused by strong winds. (6) At the tail of the forward downdraft, as precipitation is about to end, Z_H, Z_DR, and K_DP decrease synchronously, indicating the presence of medium and light raindrops. The role of the particle size sorting mechanism is pronounced and persistent in the particle distribution of hailstorms. Melting and shedding are important microphysical processes in the hail falling phase. The processes such as evaporation, coalescence, and fragmentation are phased and localized, contributing to the complexity and diversity of the raindrop spectrum.