An Analysis of the Structure and Dynamics of Inner Core Precipitation Features in a Tropical Cyclone

Abstract

Airborne Doppler radar observations of the stationary rainband complex and secondary eyewall in Hurricane Rita (2005) were analyzed to better understand the inner-core dynamics of tropical cyclones. In the upwind end of the rainband complex, convective cells displayed kinematic structures that varied with radius. Cells at smaller radii contained a low-level tangential jet constrained in altitude largely by tangential acceleration due to angular momentum conservation, while cells at larger radii contained a low-level and/or midlevel jet determined jointly by angular momentum conservation and vertical advection. These variations are attributable to vortex-scale dynamics in which convective buoyancy (associated with vertical advection) and vertical shear of the radial wind (associated with angular momentum conservation) change with radius. With jets constrained to low altitudes, inner cells are more likely to increase low-level convergence and amplify convection, possibly influencing the formation of a secondary eyewall. In the downwind end of the rainband complex, collapsing convective cells formed a mesoscale stratiform rainband that contained rising radial outflow within the stratiform cloud layer. Below the cloud layer, descending radial inflow was driven by horizontal buoyancy gradients, and thus horizontal vorticity generation, introduced by regions of sublimational and melting cooling. This inflow advected higher angular momentum inward, which resulted in the development of a midlevel tangential jet and broadening of the tangential wind field. This circulation may have also contributed to ventilation of the eyewall as inflow of low-entropy air continued past the rainband in both the boundary layer and midlevels. The stationary rainband complex soon evolved into a secondary eyewall, consisting of a ring of heavy precipitation outside the pre-existing eyewall. Enhanced radial outflow was located just above the boundary layer which modified the deeper overturning circulation of the secondary eyewall. This outflow was associated with a low-level tangential wind maximum which was strongly supergradient, mimicking the low-level circulation of the primary eyewall. Axisymmetric and asymmetric processes contributed comparably to strengthening the secondary eyewall tangential wind maximum. The evolution of these inner-core features likely played an important role in modifying the structure and intensity of the total vortex.