What Controls Local Entrainment and Detrainment Rates in Simulated Shallow Convection?

Abstract

The lack of consensus as to how entrainment and detrainment must be represented in convection parameterizations highlights the need for in-depth investigations of the processes driving lateral mixing in clouds. Direct estimates of entrainment and detrainment rates are here obtained from high-resolution simulations of shallow cumulus convection using a method that isolates the contributions from various competing processes. Moreover, cloud-averaged entrainment and detrainment rates are computed and correlated with bulk cloud and environmental properties. Detrainment is found to dominate in the middle of the cloud layer, and is locally driven by evaporation along cloud edges. Entrainment and detrainment events also occur due to changes in updraft velocity, but vertical acceleration and deceleration balance each other on average to yield no net entrainment/detrainment. In contrast, entrainment at cloud base is mostly related to condensation in dry updrafts surrounding the clouds, whereas detrainment at the top of the cloud layer is driven by buoyancy reversal. Cloud-averaged entrainment and detrainment rates both correlate preferentially with in-cloud vertical pressure gradients at all altitudes, but not with cloud-core buoyancy. No significant influence of environmental moisture on entrainment/detrainment was found, probably because of the very humid shell surrounding each cloud. Overall, these results suggest that entrainment and detrainment result from two concurring processes: a cloud-scale circulation driven by vertical pressure gradients, and small-scale, turbulent-like motions along the core edges generating mixing and, again, vertical pressure gradients.