Spontaneous tropical cyclogenesis in rotating radiative-convective equilibrium simulations

In numerical simulations of tropical cyclones, a broad vortex or saturated column is often used to initialize the circulation. Here, we instead allow a circulation to develop spontaneously from a homogeneous environment in 3-d cloud-resolving simulations of radiative-convective equilibrium in a rotating framework, with interactive radiation and surface fluxes and fixed sea surface temperature. The goals of this study are two-fold: to study tropical cyclogenesis in an unperturbed environment free from the influence of a prescribed initial vortex or external disturbances, and to compare cyclogenesis to non-rotating self-aggregation. “Self-aggregation” is a mode of convective organization in which there is a spontaneous transition from randomly distributed to organized convection despite homogeneous boundary conditions.

We quantify the feedbacks leading to tropical cyclogenesis using a variance budget equation for the vertically integrated frozen moist static energy. In the initial development of a broad circulation, the feedbacks involving longwave radiation and surface enthalpy fluxes dominate, which is similar to the initial phase of non-rotating self-aggregation. Sensitivity tests in which the degree of interactive radiation is modified are also performed to determine the extent to which the radiative feedbacks that are essential to non-rotating self-aggregation are important for tropical cyclogenesis. Finally, we examine the evolution of the rotational and divergent flow, to determine the point at which rotation becomes important and the cyclogenesis process begins to differ from non-rotating aggregation.