Moist waves and mesovortices in a model simulation of Katrina (2005)

A high-resolution, multiply-nested, limited-area model weakly constrained by ensemble data assimilation is used to simulate the evolution of Hurricane Katrina (2005) near the time of its peak intensity on August 28th, 2005. Spectral decomposition of vorticity and precipitation fields indicates three general types of wave disturbances or departures from axial symmetry. The gravest azimuthal wavenumbers (1-2) describe the toroidal motion of the inner core with typical period of one hour. This motion represents a small wobble of ~10 km relative to a smoothed track that moves in a northwestward, then northward direction on this day, prior to landfall on the 29th. An intermediate band of wavenumbers (3-6) describes the azimuthal propagation and vacillation of eye / inner-eyewall mesovortices that tend to move together within a narrow band of phase speeds in the direction of tangential flow but slightly retrograde with respect to the peak velocity. The critical radius of these disturbances is located just inside the radius of maximum angular velocity in a region of reversed potential vorticity gradient, suggesting that a moist barotropic instability is responsible for the waves. Their vacillation, however, is insufficient to disrupt the persistent annular structure of the main tangential circulation of the vortex, which is maintained by the meridional overturning circulation. At larger radii the low and intermediate wavenumbers represent middle and outer rainbands. These bands are not uniform locally but contain well-resolved structures. A broad band of higher wavenumbers (7-64) describes oblique convective cells within the annular eyewall and adjacent bands, and their coupling to inertia-gravity waves that radiate outward and upward from the center of the storm. A weak environmental vertical shear over the entire storm is responsible for asymmetries in the azimuthal distribution of intermediate and high wavenumber power. A non-zero gravity-wave momentum flux in the Cartesian reference frame above the storm results from the imposed asymmetry of vertical shear, as viewed in the circular frame, and its effects on wave excitation and propagation at upper levels.