Regional model simulations of marine boundary layer clouds over the southeast Pacific off South America. Part I: Control experiment

Abstract

A regional climate model is used to simulate boundary layer stratocumulus (Sc) clouds over the southeast Pacific off South America during August-October 1999 and to study their dynamical, radiative, and microphysical properties and their interaction with large-scale dynamic fields. Part I evaluates the model performance against satellite observations and examines physical processes important for maintaining the temperature inversion and Sc clouds in the simulation. The model captures major features of the marine boundary layer in the region, including a well-mixed marine boundary layer, a capping temperature inversion, Sc clouds, and the diurnal cycle. The Sc clouds develop in the lower half of and below the temperature inversion layer that increases its height westward off the Pacific coast of South America. The strength of the capping inversion is determined not only by large-scale subsidence and local sea surface temperature (SST), but also by cloud-radiation feedback. A heat budget analysis indicates that upward longwave radiation strongly cools the upper part of the cloud layer and strengthens the temperature inversion. This cloud-top cooling further induces a local enhancement of subsidence in and below the inversion layer, resulting in a dynamical warming that strengthens the temperature stratification above the clouds. While of secondary importance on the mean, solar radiation drives a pronounced diurnal cycle in the model boundary layer. Consistent with observations, boundary layer clouds thicken after sunset and cloud liquid water content reaches a maximum at 0600 local time just before the sunrise.