Entrainment and detrainment in numerically simulated cumulus congestus clouds. Part I: General results

This paper is the first in a three-part series in which a three-dimensional numerical model is run at high resolution to simulate cumulus congestus cl ouds in three dimensions with the principal goal of understanding the mecha nisms associated with entrainment and detrainment. The clouds are contained within a nested grid having a 50-m uniform gild spacing; the model does no t allow precipitation or ice formation and achieves saturation through bulk condensation. The prescribed environment is that associated with nonprecip itating New Mexican cumulus clouds observed on 9 and 10 August 1987. The convection is initiated using continuous surface heating, including a c entral Gaussian component to represent the effects of an isolated mountain range. Several regions of concentrated surface heating are used during the first hour to condition the environment. The turbulent motion thereby intro duced into the boundary layer is crucial for the accurate simulation of the clouds. The simulated clouds extend vertically up to 4 km, and model results genera lly agree with aircraft observations in quantities such as cloud base and t op height and the presence or absence of pronounced detrainment layers at m idlevels. Further, the pulsating nature of the convection, in which the clo uds strengthen and decay over periods of several minutes, is also similar t o observations. The cloud-top height is generally nor correlated with the l evel of neutral buoyancy for hypothetical parcels ascending undilute. Spatial resolution at least as fine as that used here appears necessary in order to capture the details of cumulus entrainment, although clouds simula ted on a single coarse grid exhibited a. substantial degree of similarity t o their nested grid counterparts and were at times somewhat more vigorous.