What is the trigger for tropical cyclogenesis?

Numerous disturbances migrate across each ocean basin during their respective hurricane seasons, yet only a small fraction of these make the transition to warm-cored, quasi-symmetric tropical cyclones. Even in the cases where "genesis" is anticipated, the actual process and timing of the transition remains somewhat mysterious.

Using a nested version of the WRF model with 2 km resolution in the innermost nest, we simulate the development of two weak vortices into tropical cyclones. The first has a 10 m/s surface vortex which decays with height, and the second has a 5 m/s surface vortex increasing to 10 m/s above the boundary layer, then decaying aloft. The evolution of the two vortices is remarkably similar. First, the surface vortex weakens and there is no apparent "surface development" for 48 hours. During this time, the mid-level vortex intensifies. Then, a smaller, more intense vortex forms in the inner core and extends downward to the surface, after which the cyclone develops rapidly.

Analysis of the two simulations shows that the development of the strong mid-level vortex, and the near saturation of the inner core by deep convection, are both required before rapid development can proceed, regardless of the strength of the surface vortex. The relative importance of surface moisture fluxes, latent heat release, inner-core relative humidity, and the merger of vorticity anomalies will be carefully evaluated.