Entrainment and Detrainment in Numerically Simulated Cumulus Congestus Clouds. Part I: General Results

Abstract

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 clouds in three dimensions with the principal goal of understanding the mechanisms associated with entrainment and detrainment. The clouds are contained within a nested grid having a 50-m uniform grid spacing; the model does not allow precipitation or ice formation and achieves saturation through bulk condensation. The prescribed environment is that associated with nonprecipitating New Mexican cumulus clouds observed on 9 and 10 August 1987. The convection is initiated using continuous surface heating, including a central 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 introduced 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 generally agree with aircraft observations in quantities such as cloud base and top height and the presence or absence of pronounced detrainment layers at midlevels. Further, the pulsating nature of the convection, in which the clouds strengthen and decay over periods of several minutes, is also similar to observations. The cloud-top height is generally not correlated with the level 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 simulated on a single coarse grid exhibited a substantial degree of similarity to their nested grid counterparts and were at times somewhat more vigorous.