Impact of cloud overlap on cloud formation and lifetime through radiative coupling

Clouds influence the atmosphere's radiation balance, but cloud formation and lifetime itself is also influenced by radiation. Overlapping cloud layers, a common occurrence globally, are thus indirectly coupled through the individual layer's influence on radiative fluxes. In this work, we study the impacts of overlap between mid level (altocumulus and congestus) and high clouds (cirrus).

First, we identify tropical to subtropical West Africa as a hotspot of mid and high cloud overlap, based on CloudSat-CALIPSO observations. During the wet season, altocumulus and cirrus clouds overlap during at least 20% of all-sky conditions. Second, we design an idealized numerical setup that resolves two radiatively coupled cloud layers, allowing one cloud layer to evolve based on the influence of the other. We run experiments by varying the initial optical thickness of each cloud layer according to observed climatology. These experiment allows us to quantify how cloud overlap affects overall cloud lifetime, atmospheric radiative heating, and local radiative balance. 

Since both altocumulus and cirrus in this region find their origin in deep convection, we expect that cloud overlap, via radiative heating, affects subsequent deep convection. Given the difficulty of representing mid level clouds in models, our results have implications for cloud climatology and regional radiation balance in climate simulations as well.