A numerical study of the role of vertical structure of vorticity during tropical cyclone genesis

An eight-level axisymmetric model with simple parameterizations for clouds and the atmospheric boundary layer was developed to examine the evolution of vortices that are precursors to tropical cyclones. The effect of vertical distributions of the vorticity, especially that arising from a merger of mid-level vortices, has been studied to provide support for a new vortex-merger theory of tropical cyclone genesis. The basic model was validated with the analytical results available for the spin-down of axisymmetric vortices. With the inclusion of the cloud and boundary layer parameterizations, the evolution of deep vortices into hurricanes and the subsequent decay is simulated quite well. Effects of several parameters such as initial vortex strength, radius of maximum winds, sea surface temperature and latitude (Coriolis parameter) on the evolution were examined. A new finding is the manner in which mid-level vortices of the same strength decay and how, on simulated merger of these mid-level vortices, the resulting vortex amplifies to hurricane strength in a realistic time-frame. The importance of the sea-surface temperature on evolution of full vortices has been studied and explained. Also it is found that the strength of the surface vortex determines the time taken by the deep vortex to amplify to hurricane strength