Stationary and Moving Convective Bands in Hurricanes

Abstract

Aircraft observations in hurricanes indicate that the hurricane vortex may be subdivided into an inner gyre where the air trajectories form closed paths and an outer envelope where they do not. In the closed gyre, a core of air moves with the vortex; in the envelope, environmental air passes through the vortex and around the core. A system of spiral bands, termed the stationary band complex (SBC), forms near the boundary between the core and the envelope where the Rossby number is of order unity. The SBC differs dynamically both from convective rings because it is asymmetric and from propagating gravity-wave bands because its Doppler-shifted frequency is below the local inertia frequency. In more intense systems with stronger convective instability, the SBC may evolve into a convective ring and move into the vortex core. Outward propagating gravity-wave bands have also been observed. Such bands are often associated with track oscillations as the storm makes landfall or recurves. Spiral-shaped entities within the SBC tend to lie across the streamlines when the convective instability is small and along them when it is large. Storms moving through an environmental flow with westerly vertical shear exhibit an east-to-west drift across the vortex. This phenomenon is expressed in the asymmetric streamfunction as an anticyclonic eddy northeast of the center and a cyclonic eddy south of the center. The velocity potential has a divergent cell west of the center and convergent cell that extends along the inside of the SBC east of the center. This pattern is apparently forced by potential vorticity conservation along the trajectories of the rotational flow and by heating in the SBC. The irrotational flow between the two cells substantially cancels the rotational drift within the vortex core.