Pathways to Tropical Cyclogenesis in Rotating Radiative-Convective Equilibrium Simulations

In a modeled environment of rotating radiative-convective equilibrium (RCE), convective self-aggregation may take the form of spontaneous tropical cyclogenesis. We investigate the dynamic and thermodynamic processes leading to tropical cyclogenesis in idealized simulations with a three-dimensional cloud-permitting model configured in rotating RCE. The background planetary vorticity is varied across f-plane cases to represent a wide range of tropical and near-equatorial environments. Convection is initialized randomly in an otherwise homogeneous environment, with no background wind, precursor disturbance, or other large-scale forcing.

All simulations with Coriolis parameter corresponding to latitudes from 10°-20° generate intense tropical cyclones. A mid-level vortex emerges spontaneously in the days leading to genesis, which has dynamic and thermodynamic implications on its environment that facilitate the spinup of a low-level vortex. Cyclogenesis is also possible in our model at near-equatorial values of the Coriolis parameter. In these experiments, convection first self-aggregates into a non-rotating quasi-circular cluster. Vorticity then preferentially develops at low levels until tropical storm intensity is achieved, aided by strong near-surface inflow and a nearly-saturated column. These results emphasize that the role of the mid-level vortex is to saturate the column and facilitate low-level inflow. If those conditions are created by a different means such as self-aggregation, cyclogenesis is possible without the mid-level vortex pathway. Nevertheless, at values of the Coriolis parameter corresponding to latitudes from 10°-20°, the formation of a mid-level vortex is the preferred way of creating an environment conducive to cyclogenesis – even when it is allowed to form spontaneously rather than being imposed.