Tropical Rain Characteristics and Microphysics in a Three-Dimensional Cloud Model

Abstract

The rain characteristics of convective clouds have been investigated numerically. Five different microphysical model settings have been used to study the roles of various mechanisms influencing rain formation: maritime ice (full microphysics, low cloud nuclei), maritime frozen (neglecting ice nuclei, enhancing drop freezing), maritime warm (only warm rain), continental ice (high cloud nuclei), and continental warm. Rain patterns and accumulation, drop growth modes, cell organization, heating rate profiles, and the alignment of rain cells in rainbands all differ greatly with different microphysics. Rainfall amounts were highest with maritime ice. With ice, a single, large rain cell was formed by absorbing small cells from the front and sides of the main cell. Forward in the cell, frozen drop formation dominated. Graupel-based hail fell from the storm center. A unique rain accumulation process produced heavy rain in a sloped updraft. Graupel fell from above, producing a high hail water content near the freezing level by collecting many supercooled drops from merging cells. The apparent heating source peaked at two different levels: one by drop condensation growth at about 3 km, and the other by depositional growth at about 5 km. With maritime ice, a 60-km rain ring appeared and persisted. In the cases of the rainband model, lines of rain cells moved intermittently through the formation of cells at the cloud's leading edge in maritime ice. The results of these investigations, with and without ice, indicates the importance of diffusive and riming growth of ice particles and the associated release of latent heat in the development of convection and rain.