Multiple equilibria in weak temperature gradient simulations of deep tropical convection: The role of radiation

Multiple equilibria in precipitation refers to the ability of a limited domain model to sustain either a steady state with persistent precipitating deep convection, or one which remains completely dry under identical boundary and forcing conditions. It has been shown to exist in several models which parameterize the large scale via the weak temperature gradient (WTG) approximation, and it is believed to be analogous to dry and moist regions in larger radiative convective equilibrium (RCE) simulations in which convection has organized into a small number of intensely convecting regions surrounded by regions of strong descent (often refered to as self-aggregation). Organized tropical convection describes a wide range of phenomena, including tropical cyclones, oceanic squall lines, and the Madden-Julian Oscillation. Understanding the mechanisms which permit multiple equilibria in limited domain WTG simulations may provide insight to the mechanisms governing self-aggregation, and consequently, the many phenomena that are considered manifestations of organized convection.

Most modeling studies which have generated self-aggregation have reported that convective-radiative feedbacks are a key mechanism for convection to organize (though at least one unpublished study has shown that self-aggregation can occur with fixed radiative cooling). Given the analogy between self-aggregation on larger domain RCE simulations and multiple equilibria on smaller domain WTG experiments, this would suggest that multiple equilibria should also be sensitive to whether radiation is fixed or permitted to interact with the tropospheric moisture. Different models have demonstrated the existence of multiple equilibria with either interactive or fixed radiation.

Here, we report results from a series of WTG experiments using a cloud system resolving model to systematically explore the role of radiation on convection and multiple equilibria. We find that whether radiative cooling is fixed or permitted to cool according to tropospheric moisture content has a huge impact on the existence and robustness of multiple equilibria. The results are mostly--but not entirely--consistent with the conclusions taken from self-aggregation simulations.