Formation of Tropical Cyclone Spiral Rainbands in a 3-D Cloud-Resolving Model

Spiral rainbands are curved patterns of clouds and precipitation outside of the inner-core region of tropical cyclones (TCs). These features are often the source of various TC-related threats, such as flash flooding, landslides, and tornadoes. Their appearance in satellite imagery is often used to estimate TC intensity. Moreover, previous studies suggest that examining spiral rainbands can help understand changes in TC intensity.

Numerous ideas have been proposed to explain the formation of TC rainbands. Two of these hypotheses have been extensively investigated. The first characterizes rainbands as manifestations of inertia-gravity waves. The second describes rainbands as propagating Rossby waves on a circular vortex, called vortex-Rossby waves. Despite these and many other efforts, there is still no widespread agreement in the literature about how TC spiral rainbands form. This study first approached the problem by analyzing the stability of an idealized vortex in a shallow-water system. The purpose was to understand the characteristics of perturbations to a simple flow that grow as spiral bands. Insights from this analysis were applied to three-dimensional simulations of TCs produced by the System for Atmospheric Modeling (SAM). SAM represents the state of the art in numerical simulation of clouds and thus offers a promising tool to clarify the formation of TC spiral rainbands. Preliminary results from these TC simulations will be discussed.