Mixing Mechanisms in Cumulus Congestus Clouds. Part II: Numerical Simulations

Abstract

The entrainment mechanisms of deep convective clouds in South Africa are simulated by using both axially and slab-symmetric cumulus models having very high spatial and temporal resolutions. The simulated clouds on three different days show a structured organization with small-scale features such as in-cloud downdrafts. The mixing processes are examined by analyzing the time variation of dynamic and thermodynamic quantities along computed parcel trajectories. The calculations indicate that most of the entrainment occurs at the cloud top. Evaporative cooling and perturbation pressure cause downdrafts that transport highly diluted air from the cloud top down to lower levels. The penetrative downdrafts are mainly located near the edges of the cloud. In the presence of wind shear the downshear portion of the clouds becomes cooler and more diluted, and it is associated with stronger downdrafts compared to the upshear side. The asymmetric organization is attributed to turbulent exchange of horizontal momentum near the cloud top.