DIURNAL EVOLUTION OF THE CLOUD-TOPPED MARINE BOUNDARY-LAYER .1. NOCTURNAL STRATOCUMULUS DEVELOPMENT

The structure and evolution of the extratropical marine atmosphere bou ndary layer (MABL) depend largely on the variability of stratus and st ratocumulus clouds. Stratus clouds are generally associated with a wel l-mixed MABL, whereas daytime observations of stratoculmulus-topped bo undary layers generally indicate that the cloud and subcloud layers ar e decoupled. In the Atlantic Stratocumulus Transition Experiment, airc raft measurements show a surface-based mixed layer separated from the base of the stratocumulus by a layer that is stable to dry turbulent m ixing. This layer forms due to shortwave heating of the stratocumulus clouds. Cumulus clouds often develop in this transition layer and they play a fundamental role in the redistribution of heat in the decouple d stratocumulus-capped boundary layer. They are, however, very sensiti ve to small changes in the heat and moisture in the boundary layer and are generally transient features that depend directly on the surface sensible and latent heat fluxes. The cumulus contribute a bimodal drop -size distribution to the stratocumulus layer skewed to the smallest s izes but may contain many large drops. Clouds increase at night in res ponse to the combined effect of convection, which can transport drops to the top of the MABL, and outgoing longwave radiation, which cools t he boundary layer. The relationship between the cumulus clouds and the latent heat flux is complex. Small cumulus may enhance the flux, but as more water vapor is redistributed vertically by an increase in conv ective activity the latent heat flux decreases. This study illustrates the need for boundary-layer models to properly handle the occurrence of intermittent cumulus to predict the diurnal evolution of the strato cumulus-capped MABL.