Spiral Bands in a Simulated Hurricane. Part I: Vortex Rossby Wave Verification

An initially axisymmetric hurricane was explicitly simulated using the high-resolution PSU–NCAR nonhydrostatic mesoscale model (MM5). Spiral potential vorticity (PV) bands that formed in the model were analyzed. It was shown that PV bands and cloud bands are strongly coupled. The PV anomalies in and at the top of the boundary layer interact with friction to produce upward motion that gives rise to the inner cloud bands. The propagation properties of the PV bands were studied and found to be consistent with predictions of vortex Rossby wave theory. In the control simulation with full physics, continuous generation of PV through latent heat release in the eyewall and spiral rainbands maintain a “bowl-shape” PV field. Inward transport of high PV by the vortex Rossby waves and the process of nonlinear mixing tend to increase the inner-core PV and in turn intensify the hurricane. On the other hand, frictional and PV mixing processes acted linearly to spin down the hurricane to a midlevel vortex in a dry run, which indicates that a monopolar PV structure is the asymptotic stable state in the absence of condensation.