An Equilibrium Raindrop Size Distribution Associated with a Heavy-Rain-Producing Convective System in Japan

Abstract

We investigated the microphysical mechanisms for the enhancement of rainfall in a precipitating system that spawned heavy rainfall over the central part of the Kanto Plain in eastern Japan on 12 July 2022. Optical disdrometer observations confirmed the existence of an equilibrium state in the raindrop size distribution, especially when the precipitation intensity was high (e.g., 20 mm hr⁻¹). As the raindrop size distribution approached equilibrium, raindrop sizes centered around 1.5 mm were observed initially, and then the number concentration of smaller and larger size raindrops increased simultaneously. The raindrop size distribution was closely related to the vertical profile of the parameters of raindrop size distribution that were estimated from polarimetric radar observations. These results suggest that the formation of equilibrium raindrop size distributions is important to produce substantial rainfall in the midlatitude as well as in the tropics.