A Determination of the Trade-Wind Cumuli Population Using BOMEX Data and an Axisymmetric Cloud Model

Abstract

A cumulus cloud population is determined over the trade-wind region under an undisturbed weather condition from the large-scale heat and moisture budgets. The BOMEX data during the period 22?23 June 1969 are used. Six classes of clouds (A?F) are considered. Each class of clouds changes the large-scale mean temperature and humidity through condensation, evaporation and convective transport processes. The change due to each class of clouds is determined using the time-dependent, two-dimensional, axisymmetric cloud model developed by Soong and Ogura (1973). It is found that the cloud population, consisting of 66.4 A clouds, 8.9 D clouds and 1.6 E clouds km?2 DAY?1, satisfies approximately the large-scale heat and moisture budget requirements. The cloud top heights of the three classes of clouds are 0.8, 1.6 and 2.0 km, respectively, and the cloud radii at the cloud base level during the mature stage are 62.5, 150 and 225 m, respectively. Consequently, the fractional area coverage of the predicted cloud population is 4.5%. Among the three classes of clouds, only E clouds penetrate into the trade-wind inversion layer. Approximately 18% of the total convective mass flux at the cloud base level is associated with the E clouds. The estimated Bowen ratio from the cloud population is less than 0.1. The estimated evaporation rate is 7.4 mm day?1 and the precipitation rate 0.29 mm day?1. These values are in good agreement with those observed during the period considered.