Impacts of mesoscale forcing on cumulus entrainment and vigor

English Abstract:

Mesoscale circulations are thought to invigorate cumuli by increasing their horizontal area and thereby reducing the suppressive effects of entrainment. Previous research on this subject has focused on mesoscale circulations that are internal to the cloud field itself, namely those driven by precipitation and consequent evaporative cold pools. Herein we consider the impacts of external mesoscale forcing, specifically forced ascent driven by airflow over a narrow mountain ridge representing a tropical island. Large-eddy simulations of trade-wind cumuli traversing such a ridge reveal that, despite little to no increase in the cloud-layer depth, all metrics of cloud vigor (buoyancy, perturbation vertical velocity, cloud liquid water content, and precipitation) undergo major enhancements over the island. Analysis of the simulations suggests that much of the cumulus enhancement is owing to reduced cumulus entrainment over the island, which may be rationalized from two different perspectives: (i) the island's forced ascent saturates broad moisture anomalies in the subcloud layer, causing a widening of the cloud cores and subsequent reduction in their entrainment, and (ii) energy constraints demand that entrainment decreases as the ratio of cloud-base mass flux to cloud updraft velocity decreases. These findings may partially explain the strong preference for deep convection to develop in areas of organized mesoscale ascent.