Three-dimensional structure of an equilibrium drop size distribution within a convective system in Japan

This study investigated three-dimensional structure of an equilibrium drop size distribution within a convective system that spawned heavy rainfall over northern Kyushu in western Japan on 10 July 2023. Ground-based optical disdrometer observations showed that the drop size distribution shape became bimodal (the peaks are at 0.7 and 1.0 mm in diameter) and then reached an equilibrium state during the rapid increase in precipitation intensity. Analyses of vertical profiles of polarimetric measurements showed that within the convective system collisional coalescence was dominant mainly at 1.5-4 km height, whereas collisional breakup was dominant below 1.5 km height. These processes were inferred to enhance the precipitation intensity. The equilibrium drop size distribution continued at least one minute during the event, and its spatial scale, diagnosed by a radar-derived parameter to be several kilometers, suggested that the equilibrium drop size distribution was a meso-γ-scale phenomenon.