Diagnosis of the Net Cloud Mass Flux in Gate

Abstract

A diagnostic analysis of the net cloud mass flux for the mean state of Phase III is presented. In the upper troposphere, the environmental mass flux is shown to be slightly larger than the adiabatic sinking motion required by radiative cooling. At the outflow layer of tall clouds, if the moistening effect due to detrainments is balanced primarily by the drying caused by adiabatic sinking in the environment, the computed net detrainment rate would be underestimated. It is hypothesized that a considerable portion of the moisture detrained from tall clouds is transported horizontally toward higher latitudes by horizontal eddy fluxes. The diagnosed environmental subsidence has a maximum in the midtroposphere and is less than 1.5 times the sinking motion required by the radiative cooling in low levels. The vertical profile of the diagnosed net detrainment rate suggests three primary detrainment layers: below 800 mb, between 700 and 500 mb, and above 300 mb. It is shown that overprediction of the cloud liquid water content from a one-dimensional entrainment cloud model will lead to spurious shallow cloud population and an overestimate of environmental sinking motion in low levels. However, the computed detrainment rate in middle and high levels and the computed net cloud mass flux in high levels are not sensitive to the predicted cloud liquid water content.