Convective Aggregation and Tropical Climate in RCEMIP

The Radiative-Convective Equilibrium (RCE) Model Intercomparison Project (RCEMIP) leveraged the simplicity of RCE to focus attention on moist convective processes and their interactions with radiation and circulation across a wide range of model types including cloud-resolving models, general circulation models, single column models, global cloud-resolving models, and large eddy simulations. We briefly review some of the robust results that emerged across the spectrum of models that participated in the first phase of RCEMIP (RCEMIP-I). Convective aggregation consistently warms and dries the atmosphere, reduces the coverage of high clouds, and increases mean precipitation and precipitation extremes. With warming, anvil clouds rise while decreasing in extent, large-scale circulations weaken, and CAPE increases. Aggregation also modulates cloud feedbacks and climate sensitivity.

However, RCEMIP-I also revealed (1) a strikingly large diversity in simulated climate states and (2) the strong imprint of convective self-aggregation, yet the lack of consensus in its structure and response to warming. Gaining a deeper understanding of convective aggregation and tropical climate will require reducing the degrees of freedom with which convection can vary. Therefore, Phase II of RCEMIP (RCEMIP-II) utilizes a prescribed sinusoidal sea surface temperature (SST) pattern to provide a constraint on the structure of convection and move one critical step up the model hierarchy of configuration complexity. This so-called ``mock-Walker'' configuration generates features that resemble observed tropical circulations. The specification of the mock-Walker protocol for RCEMIP-II is described, along with preliminary results. Under weak SST gradients, unforced self-aggregation emerges across the entire domain but as the SST gradient increases, the convective region narrows and is confined to the warmest SSTs. The prescribed SST boundary condition is the only difference in the set-up between RCEMIP-II and RCEMIP-I. As a result, RCEMIP-II will enable analysis of the influence of a forced circulation on the simulated tropical climate state, cloud-circulation coupling, and influence of aggregation on climate.