Effects of entrainment and droplet sedimentation on the microphysical structure of stratus and stratocumulus clouds

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