Diurnal Evolution of the Cloud-Topped Marine Boundary Layer. Part I: Nocturnal Stratocumulus Development

Abstract

The structure and evolution of the extratropical marine atmosphere boundary layer (MABL) depend largely on the variability of stratus and stratocumulus clouds. Stratus clouds are generally associated with a well-mixed MABL, whereas daytime observations of stratocumulus-topped boundary layers generally indicate that the cloud and subcloud layers are decoupled. In the Atlantic Stratocumulus Transition Experiment, aircraft measurements show a surface-based mixed layer separated from the base of the stratocumulus by a layer that is stable to dry turbulent mixing. 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 decoupled stratcumulus-capped boundary layer. They are, however, very sensitive 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 sizes but may contain many large drops. Clouds increase at night in response to the combined effect of convection, which can transport drops to the top of the MABL, and outgoing longwave radiation, which cools the 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 convective 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 stratocumulus-capped MABL.