Reduction of the Entrainment Velocity by Cloud Droplet Sedimentation in Stratocumulus

The effect of sedimentation on stratocumulus entrainment is investigated using direct numerical simulations of a cloud-top mixing layer driven by radiative and evaporative cooling. The simulations focus on the meter and submeter scales that are expected to be relevant for entrainment, and the finest grid spacing is Δx = 26 cm. The entrainment velocity is investigated from the analysis of the integrated-buoyancy evolution equation, which is exactly derived from the flow evolution equations. The analysis shows that sedimentation interacts with entrainment through two different mechanisms. As previously reported, sedimentation prevents droplets from evaporating in the entrainment zone, which in turn reduces the entrainment velocity. Here it is shown that sedimentation also promotes a positive buoyancy flux that directly opposes entrainment. The strengths of both mechanisms are characterized by two different settling numbers, which allow for predicting which meteorological conditions favor the reduction of entrainment by sedimentation. These new insights allow for including sedimentation in a parameterization of the entrainment velocity. The reduction of the entrainment velocity by sedimentation predicted by the parameterization and observed in the simulations is 3 times larger than previously reported in large-eddy simulations, which implies that meter- and submeter-scale turbulence plays an important role in the interaction of entrainment with sedimentation. On the whole, analysis and simulations indicate that stratocumulus entrainment is more sensitive to the cloud droplet number density due to sedimentation than previously thought.