Multiple equilibria in a cloud resolving model

Multiple equilibria in limited domain simulations (Sobel et al. 2007; Sessions et al. 2009) are believed to be related to some aspects of large-scale organization of convection in the tropics, including bimodality in the distribution of water vapor in the tropics (Zhang et al. 2003), multiple equilibria in the existence and location of the intertropical convergence zone (ITCZ; Bellon and Sobel 2009), and the tendency for randomly distributed convection to self-aggregate in radiative convective equilibrium simulations (Bretherton et al. 2005). In the limited domain simulations, multiple equilibria correspond to a state characterized by persistent precipitating deep convection or one which remains completely dry even in the presence of positive convective available potential energy.

This work investigates the existence of multiple equilibria in a cloud resolving model (CRM) employing the weak temperature gradient (WTG) approximation. We have found that there is a limited range of boundary conditions which support multiple equilibria in our CRM, and that WTG is essential for the existence of both equilibrium states. For boundary conditions which can sustain either a dry or precipitating state, the state that is realized in the model depends on the initial moisture in the free troposphere. For a given surface wind speed, there is a minimum moisture threshold below which subsidence and implied lateral export removes the existing moisture, and above which the troposphere continues to moisten as convection develops and the system evolves to the precipitating state.