Dynamics and thermodynamics of tropical cyclogenesis

During a recent field campaign, PRE Depression Investigation of the Cloud systems in the Tropics (PREDICT), eight disturbances over the North Atlantic and the Caribbean were observed over the course of twenty-six mission flights. For this work we use dropsonde data from PREDICT to analyze the evolution of both the dynamics and thermodynamics of the disturbances that developed into tropical storms as well as of those that did not develop. For each mission we calculate vorticity, mass flux and saturation fraction and we use the equations for vorticity, entropy and moisture to derive the rainfall rate and the vorticity and entropy tendencies. Contrasting the developing versus non-developing cases we attempt to explain why each observed disturbance developed or failed to develop. The results show that, for a disturbance to spin up, a well established mid-level vortex is necessary. In thermal wind balance this corresponds to cooling of the lower and warming of the upper troposphere. Previous numerical results from a cloud resolving model, run in weak temperature gradient (WTG) approximation mode, show that such an environment favors convection producing strong precipitation with a maximum vertical mass flux in the lower troposphere. The strong gradient of the vertical mass flux in the low troposphere results in intense vorticity convergence and the spinup of a warm-core vortex as envisioned by Bister and Emanuel (1997).