Spatial and statistical distribution of convective and stratiform clouds in the gyre-pouch of incipient tropical cyclones

Tropical cyclones developing from within tropical waves in the Atlantic and eastern Pacific generally form within the critical layer of the parent wave. The critical layer represents a closed recirculation region in a frame of reference moving westward at the phase speed of the wave. This region favors storm development for dynamical and thermodynamical reasons: (i) the nearby environmental flow is cyclonic and characterized by large Okubo-Weiss parameter, indicating predominantly rotational flow rather than shearing or straining deformation, and (ii) this flow is relatively moist, with average saturation fraction of 70-95%, and contains abundant and persistent deep convective precipitation as retrieved from TRMM satellite measurements. These properties are illustrated for 55 named storms in August-September of 1998-2001. Further examination of the circulation patterns and cloud populations in developing vs non-developing environments indicates extension of the closed gyre to the surface, accompanied by surface convergence, and amplification of the convective cloud component. Evidence is presented that the closed recirculation is important to the cloud population and its associated vertical profiles of diabatic heating and mass convergence. These anomalies further enhance surface convergence and create a positive feedback for development of an intense diabatic vortex within the gyre-pouch of the parent wave's critical layer. In most cases the vortex and gyre initially move together, owing to weak relative flow in the critical layer, but after a gestation period of a few days the vortex emerges from the wave, having acquired characteristics of a tropical depression extending into the upper troposphere, with divergent anticyclonic outflow in that region.