How spiral rainbands propagate in numerically simulated tropical cyclones

Early radar observations of tropical cyclones indicated that convection/precipitation outside the eyewall is often organized into spirally banded structures, known now as spiral rainbands. Some spiral rainbands are observed to move around the center of the storm and propagate radially outward, while some rainbands remain relatively stationary to the storm center. Three hypotheses have been proposed to explain the properties of spiral rainband propagation, and each views spiral rainbands as 1) internal gravity waves that are generated from the rotating asymmetries in the eyewall region; 2) vortex-Rossby waves that emanate radially outward from the eyewall along the sharp negative potential vorticity gradient; or 3) tropical squall lines that maintain their structures through mechanical lifting along the spreading, dense cold pools. Depending on how spiral rainbands are characterized, their propagation properties differ significantly. This study will examine how spiral rainbands propagate in high-resolution, full-physics numerical simulations of Hurricane Bill (2009) and make comparisons to previous hypotheses.