How do the ups control the downs within deep convective clouds?

Evaporation of condensate from a cloud can cause air to become negatively buoyant and sink. This downward movement of air is called a downdraft. By injecting air from higher levels into the atmospheric boundary layer, downdrafts can modify the atmospheric conditions in the boundary layer. Downdrafts can also end the life cycle of a cloud and result in strong, sometimes destructive, wind gusts at the surface. The basic driving forces for downdrafts are well understood. For example, we know that evaporation of rain and the associated cooling of air is usually critical in causing the air to sink. Even though the basic driving forces are known, many interrelated processes contribute simultaneously to the strength of the downdraft, making it difficult to predict the strength of a downdraft under specific conditions. This prediction is, however, required to strengthen our conceptual understanding of convective systems.

Given the central role of downdrafts, Windmiller et al. improve our understanding of how best to predict downdraft properties as a function of updraft and environmental properties. Previous studies have investigated this question using simplified theoretical models or high-resolution but small-domain atmospheric simulations. This study addresses this question using an atmospheric simulation whose model domain spans the globe but still explicitly resolves rain-forming clouds. Focusing on convection over tropical oceans, the authors study 30 000 rain-forming clouds, and their associated downdrafts, under very different, naturally varying environmental conditions. The resulting average time evolution of the precipitation rate at the surface as well as the profiles of rain water, updraft and downdraft mass flux are shown in Fig. 1.