The Development of Warm Rain in a Cumulus Model

Abstract

A one-and-a-half-dimensional, time-dependent cloud model proposed by the authors is extended to investigate warm-rain formation. A total of 61 mass categories, corresponding to radii from 4 ?m to 4 mm, are used to determine the drop distribution. The distribution of hydrometeors evolves with time as a result of condensation, evaporation, stochastic coalescence, sedimentation, and drop breakup. The formation of liquid drops around condensation nuclei is parameterized to take a prescribed drop size distribution, though the number concentration of nuclei is predicted. Three different types of the prescribed initial size distribution of drops are considered to test their control of the subsequent hydrometeor distribution. Convection is initiated in a conditionally unstable atmosphere which represents tradewind conditions, and long time integrations of the model are performed to cover the entire life cycle of a simulated cumulus cloud. In a typical case, the maximal updraft resulting from the calculation is 4.7 m sec?1 and the rate of rainfall reaches its peak of 27 mm hr?1 10 min after rainfall has started at the ground. The rain continues for 25 min and the total amount of rainfall is 4.5 min. Some aspects of maritime warm cumulus and rainfall seem to he simulated, such as the sudden onset of large-drop rain in convective showers. The predicted size distributions for raindrops are compared with the Marshall-Palmer distribution with fairly good agreement. It is also found that the broad initial size distribution generates the onset and end of precipitation earlier than the narrow size distribution.