Dynamics of Downdrafts Around a Growing Convective Cloud and their Role in Vertical Transport: A Numerical Study

This study examines the dynamics of downdrafts around isolated growing cumuli, focusing on cloud-scale quasi-toroidal circulations and their impact on tracer transport. Two scenarios are explored: a quasi-laminar thermal forced by a warm bubble perturbation in a quiescent flow and a turbulent cloud forced by surface fluxes in a well developed boundary layer. In the early stages of cumuli growth, updrafts are consistently surrounded by downdrafts associated with cloud-scale quasi-toroidal circulations. As the cumuli deepen, these toroidal downdrafts concentrate around the upper half of the cloud. At the level of minimum vertical velocity, approximately 50-70% of the upward mass flux is compensated within a radius of approximately twice the radius of the updraft in the laminar-bubble simulations, while this compensation fraction is approximately 20-60% within the same region of the turbulent simulations. Analysis of the vertical momentum budget reveals that downdrafts in the turbulent setup are primarily mechanically forced (dynamic pressure acceleration). In contrast, an enhanced role of buoyant acceleration in the laminar-bubble setup is due to the forcing method. In the laminar-bubble and turbulent simulations, these downdrafts cause little net downward transport of environmental tracers; instead, the downdraft air is subsequently entrained and transported upward in the cloud's toroidal circulation. Regions with sustained descent from cloud top to base were typically located more than ∼4 km away from the cloud, with a subsidence speed of ∼0.1 m/s. This does not cause significant downward transport within the cloud's lifetime.