A Simple Theoretical Model for Tropical Cyclogenesis

Tropical cyclogenesis is associated with the transition from a mid-level convective vortex “seed” to a surface-based vortex, during which the seed core becomes saturated and the vertical mass flux profile becomes bottom-heavy. This genesis process involves a two-way interaction between convection and its environment that currently lacks a rigorous theory. Recent work developed an analytic theory for the vertical profile of buoyancy in individual convective updrafts that accounts for the effects of entrainment of environmental air. Here we extend this theory to consist of a modeled interaction between a tropical cyclone "seed envelope" and an ensemble of clouds represented by entraining plumes. The model accounts for both how environmental humidity modifies the buoyancy profile via entrainment and how the detrainment of water from convection moistens the environment. There are two proposed mechanisms by which the gradual moistening of the environment leads to genesis: the buoyancy maximum shifts downwards promoting inflow of higher MSE air and a shift in the distribution of convective plumes towards deeper convection. We test these mechanisms in our simple model as well as in a recent set of ensembles simulations of real world tropical cyclogenesis events that find low-mid level relative humidity to be a strong predictor of the onset genesis.