Simulating the transition from shallow to deep convection across scales: the role of congestus clouds

Abstract An idealized case of gradual oceanic transition from shallow to deep convection based on Kuang and Bretherton (2006) is simulated at three different horizontal resolutions: one that resolves most of the turbulent eddies, one typical of cloud-resolving models and one typical of general circulation models. The former serves as a reference, and allows the identification of clouds as individual objects, distinguishing shallow cumulus, congestus and cumulonimbus. At coarser resolutions, parameterizations of convection are included and assessed, with a particular focus on congestus clouds and precipitation associated with shallow convective clouds. Congestus clouds are found to contribute the most to turbulent transport during the transition, while occupying a volume comparable to shallow cumulus and cumulonimbus. Kilometer-scale horizontal resolutions prove to be insufficient to resolve congestus, and parameterization schemes of shallow and deep convection are not necessarily appropriate to represent those intermediate clouds. The representation of rainfall in the shallow convection scheme plays a key role in the transition. Sensitivity experiments show that enhanced rainfall inhibits convection in single-column simulations, while it favors resolved convection and spatial heterogeneities in three-dimensional simulations with kilometer-scale resolution. Results highlight the need for an appropriate parameterization of congestus in both kilometer-scale and large-scale models. The case study and the methods presented here are proposed as a useful framework to evaluate models and their parameterizations in a shallow-to-deep convection transition context.