How Do Outer Spiral Rainbands Affect Tropical Cyclone Structure and Intensity?

A tropical cyclone typically consists of both inner and outer spiral rainbands. Spiral rainbands within a radius of about 2-3 times of the radius of maximum wind are generally referred to as inner spiral rainbands, while those farther outward are referred to as outer spiral rainbands. A predominant view of inner spiral rainbands is the activity of convectively coupled vortex Rossby waves. Outer spiral rainbands are predominantly viewed dynamically as inertia-gravity waves. They most frequently form and develop between 80 km and 150 km from the tropical cyclone center and propagate mainly outward. Outer spiral rainbands may affect tropical cyclone intensity both dynamically and thermodynamically. In this study, we try to understand how outer spiral rainbands affect the overall structure and intensity of a tropical cyclone. The quadruply nested, movable mesh, full compressible, nonhydrostatic tropical cyclone model TCM4 was used to perform a series of well designed numerical experiments to understand the impact of outer spiral rainbands on both the structure and intensity of tropical cyclones. We will show how the downdrafts and how the stratiform anvil clouds affect the overall size and intensity of the simulated tropical cyclones. The implications of our findings will be also discussed.