A probabilistic analysis of the dynamics and predictability of tropical cyclogenesis

Using methods unique for tropical cyclone studies in peer-reviewed literature, this study examines the dynamics and predictability of a nondeveloping tropical disturbance in the Gulf of Mexico during the 2004 hurricane season. Short-range ensemble forecasts are performed with a mesoscale model at low resolution with parameterized moist convection and at high resolution with explicit convection. Taking advantage of discrepancies between ensemble members, statistical correlation is used to elucidate why some ensemble members strengthen the disturbance into a tropical cyclone or hurricane and others do not. It is found that the two most important factors in the initial conditions for genesis in this case are the presence of deep moisture and high CAPE. These factors combine to yield more active initial convection and a quick spinup during the first 6-12 h. Because these factors result in quicker genesis in some ensemble members than others, they are also the primary source for spread early in the ensemble. Discrepancies after 12 h are amplified by differences in convection that are related to fluxes of sensible and latent heat. Eventually the wind-induced surface heat exchange mechanism results in even larger ensemble spread.