EFFECTS OF ENTRAINMENT AND DROPLET SEDIMENTATION ON THE MICROPHYSICALSTRUCTURE OF STRATUS AND STRATOCUMULUS CLOUDS

A new cloud model has been developed based upon a reconsideration of c loud-top entrainment mechanisms. A finite-thickness entrainment layer at cloud top allows the cloudy air to lose Liquid water to dry entrain ed air by sedimentation. The model uses a steady-state ensemble of one -dimensional updraughts and downdraughts, combined with a simplified t reatment of droplet sedimentation, autoconversion, and collection of c loud drops by drizzle, to examine vertical profiles of cloud liquid-wa ter content and droplet concentration. The model is demonstrated both for idealized cases and in comparison with a nocturnal stratocumulus c ase from the ASTEX. The sedimentation flux divergence in the entrainme nt layer at cloud top is shown to have a large impact on the cloud mic rophysics.