Convective Cold Pool Invigoration by Interactions with Land Surface-Intercepted Rainwater

Convective cold pools are generated by latent cooling from phase changes (e.g., evaporation and melting) that occur in association with convective clouds. This latent cooling combines with hydrometeor loading to increase the local air density. The resulting negatively buoyant pocket of air descends, forming a downdraft. Upon reaching the surface, the dense air collects, forming a laterally expanding cold pool. Cold pools are often associated with gusty near-surface winds, and they can help to trigger convective initiation along their peripheries while suppressing convection within their stably stratified interiors.

The rate of lateral expansion is driven in large part by the density difference between the cold pool air and the ambient air. All else equal, a denser cold pool is likely to expand more rapidly. By extension, a denser cold pool will likely exhibit stronger near-surface winds and exert a greater influence on the formation of future convection.

Latent cooling, which increases air density, invigorates cold pools. Crucially, this latent cooling need not be driven solely by phase changes of airborne hydrometeors. Upon falling onto a land surface, precipitation can face a variety of fates, including interception by vegetation and percolation into the soil. Despite having reached the surface, some precipitation (such as intercepted rainwater coating a plant’s leaf) may remain available to undergo further phase transitions. The resulting latent cooling can increase the local air density so as to further invigorate the cold pool, increasing the likelihood of convective triggering along the gust front.

In this study, we use idealized numerical simulations to estimate the magnitude of cold pool invigoration by intercepted precipitation. Daytime tropical rainforest convection is simulated at LES resolution using the Regional Atmospheric Modeling System (RAMS), which is coupled to the Land Ecosystem-Atmosphere Feedback, version 3 (LEAF-3) surface scheme. Mechanism denial experiments suggest that intercepted rainwater is responsible for profound cold pool invigoration in this setting. This presentation will discuss the mechanisms leading to this invigoration response, as well as sensitivity to factors such as soil moisture, land cover, and environmental CCN concentrations.