Modeling the Impact of Marine Stratocumulus on Boundary Layer Structure

Abstract

An ensemble-average closure model intended for mesoscale studies is applied to a marine stratocumulus-capped PBL. The intention is to test this model, in particular, for cases where cloud and subcloud layers are decoupled. The test is based on one case from the First ISCCP Regional Experiment, where solid cloud-capped and clear sky areas were found in close proximity. The model results compare favorably both with the measurements and with results from more complex model formulations. They show the response of the entire boundary layer dynamic structure to stratocumulus formation as well as longwave and shortwave radiative heat transfers. The net result is that the entire turbulent layer in the cloud-capped case is more vigorously mixed, more neutrally stratified, and deeper compared to a cloud-free PBL developing under similar conditions. Surface fluxes of sensible and latent heat, from the measurements as well as simulations thus vary relatively little between the areas in spite of the observed substantial sea surface temperature difference. All simulations presented here reveal cloud decoupling during daytime. The multilayer structure is, however, seen almost only in profiles of second-order moments. The mean profiles indicate one single, deep well-mixed layer, while the turbulence profiles clearly show two separate well-mixed layers. The turbulent flux of water vapor from the surface thus generally never penetrates to the cloud layer during daytime but may eventually cause formation of a shallow layer of cumuli below the main cloud layer.