MODELING THE IMPACT OF MARINE STRATOCUMULUS ON BOUNDARY-LAYER STRUCTURE

An ensemble-average closure model intended for mesoscale studies is ap plied to a marine stratocumulus-capped PBL. The intention is to test t his model, in particular, for cases where cloud and subcloud layers ar e decoupled. The test is based on one case from the First ISCCP Region al 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. T hey show the response of the entire boundary layer dynamic structure t o stratocumulus formation as well as longwave and shortwave radiative heat transfers. The net result is that the entire turbulent layer in t he cloud-capped case is more vigorously mixed, more neutrally stratifi ed, and deeper compared to a cloud-free PBL developing under similar c onditions. Surface fluxes of sensible and latent heat, from the measur ements as well as simulations thus vary relatively little between the areas in spite of the observed substantial sea surface temperature dif ference. All simulations presented here reveal cloud decoupling during daytime. The multilayer structure is, however, seen almost only in pr ofiles 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 th e surface thus generally never penetrates to the cloud layer during da ytime but may eventually cause formation of a shallow layer of cumuli below the main cloud layer.