Generation of Moving Spiral Bands in Tropical Cyclones

A theory based on the shallow water equation is developed to explain the generation of moving spiral-banded structures in tropical cyclones. The theory proposes that fluctuation of the vorticity distribution in the compact core region can act as a source, generating gravity waves that produce the banded structures. This mechanism is usually called the Lighthill radiation mechanism. A numerical experiment using the Fifth Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5) code is used to test this theory. The moving bands in the simulated tropical cyclone show good agreement with the theoretically calculated gravity waves that emerge from a core region with an elliptical distribution of vorticity. Results of the simulated cyclone also illustrate that spiral cloud bands are triggered by the propagating gravity waves.