Surface rainfall processes as simulated in a cloud-resolving model

Abstract

Surface rain rate can be simply formulated with the sum of moisture and cloud sources/sinks. In this study the moisture sink comprises the local moisture change, moisture convergence (with an imposed vertical velocity), and surface evaporation, whereas the cloud source/sink comprises the local hydrometeor change since the cyclic boundary condition leads to zero hydrometeor convergence. The sources/sinks and their contributions to the surface rain rate are examined based on hourly zonal mean simulation data from a two-dimensional cloud-resolving model. The model is forced by the large-scale vertical velocity, zonal wind, and horizontal advections obtained from Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). Although variation in the moisture sink largely accounts for much of the variation in the surface rain rate, the cloud source/sink may modify the surface rain rate significantly. The magnitude of the cloud source/sink increases when the zonal mean surface rain rate increases from 0 to 1 mm h-1, and it decreases when the rain rate increases from 1 to 2 mm h-1. The cloud source/sink is further analyzed by breaking it into ice and water hydrometeors. The ice hydrometeors may account for more contributions to the cloud variations than the water hydrometeors, and their growth may lead the surface rain rate by 1-2 hours. Copyright 2005 by the American Geophysical Union.