Understanding the Entrainment and Detrainment Processes in Maritime Shallow Cumulus Clouds using Lagrangian Trajectories: Buoyancy Sorting or Acceleration Sorting?

Entrainment and detrainment processes remain one of the biggest challenges in convective cloud dynamics and need to be parameterized in mass-flux-based convection schemes. A common assumption in convection parameterizations is that the fate of a mixture between cloud and environmental air is determined by the "buoyancy sorting" hypothesis, and further that the net effect of entrainment and detrainment is to reduce the vertical momentum in the cloud. In this study, Lagrangian trajectories are investigated to understand the entrainment and detrainment processes in maritime shallow cumulus clouds and to examine hypotheses in convection parameterization schemes. Analysis of vertical momentum in cumulus clouds using Lagrangian trajectories and using a bulk budget approach both indicate that the overall impact of entrainment and detrainment on momentum is to accelerate the cloud updraft, rather than acting as a drag force. Following the trajectories, it is found that the entrained air has larger mean vertical velocity than the detrained air, in contradiction with the typical assumption in the mass-flux based plume models. This finding indicates the necessity for a careful treatment of the dynamical properties in the near cloud environment. Investigating the buoyancy of entraining and detraining trajectories, we find that the widely accepted "buoyancy sorting" hypothesis is not able to correctly describe both entrainment and detrainment processes, regardless of how the cloud objects are defined. Instead, whether a mixed parcel is likely to be entrained into or detrained out of the cloud depends on its vertical acceleration. More specifically, vertically accelerated parcels near cloud edge are more likely to be entrained and vertically decelerated parcels are more likely to be detrained. Thus, the "acceleration sorting" hypothesis is proposed. Decomposition of the vertical momentum budget for the entrained and detrained trajectories shows that it is the pressure gradient acceleration, especially the dynamical pressure gradient acceleration associated with the flow structure and the effective buoyancy, rather than the buoyancy alone, that dominate the "acceleration sorting". Our results suggest that the flow structure of cloud thermals might be a potential candidate responsible for "acceleration sorting" processes during the entrainment and detrainment. These findings provide new insights to the understanding of the physical processes during both entrainment and detrainment.