Dynamics and Thermodynamics of Tropical Cyclogenesis in the North Atlantic and the Caribbean

The purpose of this paper is to investigate mechanisms of tropical storm formation. The evolution of developing and non-developing tropical disturbances that were observed during the 2010 hurricane season in the North Atlantic and the Caribbean are investigated, using dropsonde data that were gathered during PREDICT. The data from the sondes were first processed by doing three dimensional variational analysis. Then various dynamic and thermodynamic variables were calculated: vorticity, saturation fraction, Okubo-Weiss parameter, vertical mass flux, moist entropy. Vorticity, moisture and entropy budget equations were used to calculate vorticity tendency and entropy tendency. The vorticity analyses show that tropical storm formation was preceded by the development of mid-level vorticity. We investigate the mechanism by which the mid-level vortex promotes intensification of the low-level vorticity. For this purpose additional dropsonde data from TCS08 were used together with the PREDICT data to explore correlations between dynamic and thermodynamic variables. The results suggest that the mid-level vortex is associated with more stable atmosphere, i. e. with cooler lower and warmer upper troposphere. Previous numerical results from a cloud resolving model, run in weak temperature gradient approximation mode, show that the resulting thermodynamic stratification is conducive to shallow convection which produces bottom heavy mass flux profile with the largest gradient near the surface. The mass continuity then enforces low-level mass convergence and subsequent spin up of the low-level vorticity.