Moving spiral structure in tropical cyclones

The thesis is focused on the asymmetric dynamics of tropical cyclones. The first step of this research was to set up an idealized numerical model based on the PSU/NCAR MM5 model. Based on the internal structures of the simulated tropical cyclone, it has been shown that evolution of the tropical cyclone may be classified into three stages. Each stage has distinct structural characteristics.;A theory developed based on the shallow water equations is proposed to explain the generation of moving spiral banded structure in tropical cyclones. The theory shows that fluctuation of vorticity distribution in a compact region can act as a source to generate gravity waves. This mechanism of gravity waves generation is usually called Lighthill radiation. The moving bands in the simulated tropical cyclone show good agreement with the theoretical gravity waves, calculated by using an elliptical distribution of vorticity in the core region. Analyse of the numerical results suggest that cloud bands can be triggered to form due to the propagating gravity waves generated in the core region by an elliptical vortex.;Theoretical results show that spiral waves can effectively spin down a vortex, and the spin down rate is dependent on the strength of the vortex. Angular momentum budgets of the simulated tropical cyclone show that the wave losses due to spiral waves can be a substantial fraction of the net gain or loss of total angular momentum. The results suggest the possibility that radiation of spiral waves may affect the rate of change of total angular momentum of the tropical cyclone. Nevertheless, the rate of intensity change of a tropical cyclone should mainly be determined by the approximate balance of the horizontal convergence of angular momentum by the mean flow and the dissipation by surface friction.