Resolving Droplet Sedimentation Effects in Stratocumulus Clouds

Abstract

We use direct numerical simulations to quantify the effects that droplet sedimentation has on the stratocumulus-topped boundary layer. Our analysis includes both ends of the length-scale spectrum that are deemed important for representing turbulence in stratocumulus clouds, spanning from meter scales at the cloud top to large energy-containing eddies the size of the boundary layer. We conduct sensitivity experiments that involve varying the droplet sedimentation strength and the Reynolds number. Consistent with previous studies, we find that increasing sedimentation causes a decrease in mean entrainment velocity, with an observed effect of at least $20\%$. Interestingly, the turbulence kinetic energy and the turbulent entrainment flux are enhanced by sedimentation. To reconcile the apparent contradiction of turbulent flux increasing and mean entrainment velocity decreasing, we quantify the various mean fluxes of the liquid water static energy in the cloud-top region, as needed for the evaluation of the entrainment-rate equation. As sedimentation strength intensifies, the magnitude of the sedimentation flux undergoes a more rapid increase than the turbulent flux, effectively compensating for the increase in turbulent flux. To explain the increase in turbulence intensity, we show that sedimentation increases the contrast between descending dry, warm air in cloud holes and the moist, cold air within cloudy puffs. This increased contrast intensifies evaporative cooling near the cloud hole edges, which accelerates the downdrafts, drives turbulence, and distributes moisture more evenly between the cloud and subcloud layers. Overall, we show that microphysical effects are as important as turbulent effects at meter-scale resolution.