Polarimetric insights into pre-monsoon convective clouds and hail storm microphysics over central India using polarimetric radar observation

The present study characterizes the pre-monsoon convective clouds (PMCC) observed in the monsoon core zone and a hail-producing storm during the pre-monsoon month observed using C-band dual-polarization (CDPR) radar at the IITM's Atmospheric Research Testbed (ART) facility in Silkheda, 60 km north of Bhopal. The composite analysis of PMCC has shown that the maximum height of 35- and 50-dBZ echo tops (ETHs) is around 13 km and 11 km, respectively, indicating intense and deep convection. The mean value of Convective Available Potential Energy (CAPE) is also high for the pre-monsoon season. The composite contoured-frequency-by-altitude diagrams (CFADs) of PMCC show a high frequency of smaller values of differential reflectivity (Z_DR ∼ 0 dB) and specific differential phase (K_DP) above the melting layer, indicating that intense convective updrafts dominate the PMCC, causing hail/graupel particles to be rimmed heavily above the melting layer and reach greater heights. The wider distribution or higher values of Z_H, Z_DR, K_DP, and small values of correlation coefficient ρ_HV (<0.95) near the surface indicate the presence of a rain-hail mixture during the pre-monsoon month.

The microphysical characteristic of a hailstorm event that produced hailstones over Bhopal on 30th April 2023 is explored. It is found that the CAPE value was building up before the hailstorm event, and it was high just before the hailstone was observed over Bhopal. Time evolution of storms indicates the dominant presence of graupel/hail at 5-11 km. Graupel/hail will form due to the riming or accretion processes of cloud droplets, supercooled water, or ice particles. It has been observed that the riming process is dominant during the storm's evolution, which produces very low values of Z_DR and K_DP. The high value of ρ_HV (>0.99) at these altitudes suggests that the graupel/hail is dry. The updraft associated with a convectively unstable atmosphere exaggerates the growth of ice crystals by a rapid collection of supercooled water droplets. That ultimately led to forming graupel/hailstones through the riming process. The wet hailstones and large raindrops signatures close to the surface are also observed and are characterized by higher values of Z_H (> 50 dBZ), enhanced Z_DR (∼ 3–4 dB), and a depleted ρ_HV (< 0.95). Additionally, a K_DP exceeding 1.8°/km near the surface indicates the process of hail melting/hail coated with water.