Multiple equilibria in a cumulus ensemble model employing the weak temperature gradient approximation

The existence of multiple equilibria in models of tropical precipitation is an interesting and potentially significant research problem. For instance, it may explain why some regions in the tropics are convectively inactive even under conditions which could support vigorous convection. Furthermore, investigating this problem using small scale numerical simulations which resolve convection could provide insight toward improving parameterizations which do not successfully capture this phenomenon.

We use a cloud resolving model in the context of the weak temperature gradient approximation (WTG) to investigate the conditions which support two possible stable equilibrium states: a dry, non-convecting and a precipitating one. Our work suggests that WTG is an important aspect in this problem. It provides a mechanism for the modeled domain to interact with the large scale by requiring that the vertical profiles of potential temperature relax to that of the large-scale environment. In the tropics, gravity waves redistribute buoyancy anomalies so as to maintain a horizontally homogeneous vertical profile. Imposing this effect in the model substantially modifies the evolution of convection, which in turn affects the equilibrium state.

We initiate experiments with either a dry moisture profile, or one that is obtained from radiative convective equilibrium. Whether the initially dry environment develops convection is strongly dependent on domain size, wind speed (surface fluxes), and on the rate that potential temperature relaxes to the background profile. We are also finding that the gross moist stability is an important parameter in characterizing the equilibrium state. In some cases, the dry equilibrium exhibits a negative gross moist stability.