Clouds and Precipitation: Extreme Rainfall and Rain from Shallow Clouds

This chapter reviews present knowledge on extreme precipitation and moderate rainfall from low-level clouds. Primary focus is on the statistics of precipitation characteristics rather than on a detailed description of individual case studies. First, observed variability of precipitation from low-level clouds and the existing techniques to separate different microphysical stages from remote-sensing measurements are reviewed. Over the tropical areas of Pacific and Atlantic oceans, the global distribution of shallow rainfall exhibits a “butterfly” pattern. This feature encompasses heavily precipitating regions such as the intertropical, south Pacific, and south Atlantic convergence zones (ITCZ, SPCZ, and SACZ, respectively); the northern hemispheric counterpart of SPCZ and SACZ emerges only when shallow rain is isolated.

The nature of extreme precipitation varies temporally. On a timescale of about a day, extreme precipitation is associated with synoptic-scale disturbances, including a notable example known as tropical plumes or moist conveyer belt, which could give rise to extreme daily precipitation in downstream arid regions. On an hourly timescale, extreme precipitation is caused by mesoscale moisture convergence, which is so intense that it maintains a continuous overturning of saturated air. Satellite observations imply that the global distribution of extreme precipitation shows a systematic difference from the total rainfall map in terms of, for example, the contrast between land and ocean. The distribution of low-level, precipitation-related latent heating associated with warm rain coincides with the butterfly pattern. Its cohabitation and separation with the deep heating suggests that warm rain plays a role in providing a thick layer of moist static energy source to the convection, and that it is also related to the tropical plumes which cause extreme precipitation in the semiarid west coasts of continents.