Diagnostic Determination of Cumulus Cloud Populations from Observed Large-Scale Variables

Abstract

A method is proposed whereby some properties of cumulus cloud populations are determined from observed large-scale meteorological variables. This method combines large-scale heat and moisture balance considerations with a simple steady-state one-dimensional model for an individual cumulus cloud. When applied to the mean summertime conditions over the Marshall Islands, the method gives a bimodal distribution of the vertical mass flux at the cloud base in terms of the cloud top height: one group of clouds penetrates up to the 200–300 mb level and another group stays below the 600 mb level, while relatively few cloud tops lie. between 400 and 600 mb. The total percentage area covered by all clouds is found to be a few percent and the vertical velocity inside the cloud at the cloud base is estimated to be somewhere near 1 m sec⁻¹ for most clouds. Because the observed large-scale averaged upward velocity at that level is of the order of 0.1 cm sec⁻¹, this result indicates that the total mass entering into ascending clouds through the cloud base is one order of magnitude larger than the mass brought into the subcloud layer by large-scale convergence. This excess mass is compensated by the downward motion with a velocity of the order of 1 cm sec⁻¹ in the region between clouds at the cloud base.