A Dynamical Theory for Hurricane Spiral Bands

A dynamical theory is presented for two classes of hurricane spiral bands - inner and outer. Inner bands, which typically form within 500 km of the vortex center, can result from both potential vorticity PV wave breaking and vortex merger. PV wave breaking is found to occur when the PV field is asymmetric. This process ejects relatively high PV air down gradient into regions of relatively low PV air in the form of spiral bands. Vortex merger occurs when a vortex moves sufficiently dose to a PV source. This process also produces inner spiral bands. The inner bands modeled in this study were found to project almost entirely onto the rotational modes, thus suggesting they are a slow-manifold phenomenon rather than a gravity wave phenomenon as previously proposed. Outer bands, which typically form at a radius greater than 500 km from the vortex center, are found to occur as a result of the intertropical convergence zone ITCZ breakdown. Both types of bands are found to be stabilized by the adverse shear created by the main vortex. All theories are tested through the integration of a limited area, normal mode, spectral model.