Thermodynamics of tropical cyclogenesis in the northwest Pacific

1 This paper presents analyses of five tropical disturbances of various types, derived from observations made over the northwest Pacific in August and September of 2008. Various dynamic and thermodynamic products were derived from dropsonde and airborne Doppler radar data, with the goal of increasing our understanding of tropical cyclogenesis. From these analyses we draw the following tentative conclusions: The formation of a strong midlevel circulation, with its associated cold core at low levels and warm core aloft, greatly aids the spin-up of a tropical cyclone by changing the vertical mass flux profile of deep convection from top heavy to bottom heavy. This has two effects: (1) the enhancement at low levels of the convergence of mass and hence vorticity, thus aiding the spin-up of a warm-core vortex and (2) the suppression of the lateral export of moist entropy by deep convective inflows and outflows from the core of the developing system. This allows the relative humidity to build up, resulting in more intense convection and further development. Our results also suggest that strong horizontal strain rate at middle levels, as measured by a form of the Okubo-Weiss parameter, is detrimental to tropical cyclogenesis. Not only can such flow tear apart the midlevel vortex, it can also import air with low moist entropy. In our small sample, the Okubo-Weiss parameter was the best indicator of the potential for development. Vertical shear appeared to play a less important role in the systems we investigated.