Contributions of Cumulus Cloud Life-Cycle Effects to the Large-Scale Heat and Moisture Budget Equations

Abstract

Effects of the life cycles of cumulus clouds, on the large-scale heat and moisture equations are derived by introducing a cloud distribution function which describes the cloud fractional coverage distribution in terms of cloud-top height as well as cloud age. It is found, in addition to the heating and drying effects described in previous formulations using a steady-state cloud model, that cumulus clouds have a moistening effect due to the finite life spans of cumulus clouds. The magnitude of this effect is inversely proportional to the lifetimes of clouds. The formulation has been applied to a typical trade wind weather situation observed during Phase III of BOMEX to determine the total cloud mass flux, the fractional cloud coverage distribution, as well as the mean lifetime of the shallow cumulus population. It is found that cumulus clouds covered about 2% of the total observational area. The mean life span of the cloud population is of the order of 9 min. The atmospheric air near the cloud-base level was recycled about three times a day by cumulus clouds. This recycling rate decreases rapidly with height. The recycling rate is about 0.5 day⁻¹ at the bottom of the trade inversion, and decreases to zero near the top of the inversion layer.