Convective-Scale Variations in the Inner-Core Rainbands of a Tropical Cyclone

Abstract

irborne Doppler radar documented a variety of convective-scale structures within the inner-core rainbands of Hurricane Rita (2005). As predicted by past studies, wind shear determined azimuthal variations in the convection. All convective-scale circulations had radial inflow at low levels, upward motion, and outflow in the midtroposphere. Convective cells at smaller radii contained a low-level tangential jet determined largely by tangential acceleration due to angular momentum conservation (u?/r term), while cells at larger radii contained a low-level and/or midlevel jet determined jointly by the u?/r and vertical advection terms. The outflow was at a higher (lower) altitude for the outer (inner) cells.Radial variations in the convective cells are attributable to differences in buoyancy and vertical shear of the radial wind (?u/?z). More buoyant updrafts at larger radii enhance vertical advection of ?, creating local tangential jets at midlevels. At smaller radii the stronger low-level radial inflow contributes to a greater ?u/?z, confining convectively generated jets to low levels. The low-level tangential jet and convectively generated pressure gradients produce outward-pointing supergradient acceleration that decelerates the boundary layer inflow. Consequently, this supergradient flow will enhance convergence and convection at the radius of inner rainband cells, increasing the likelihood of secondary eyewall formation. It is hypothesized that a critical zone for secondary eyewall formation exists where sufficiently high ?u/?z consistently constrains the altitudes of convectively generated supergradient flow so that convection in this radial zone leads to a newly developed eyewall. Once an incipient secondary eyewall forms at a certain radius, subsidence occurring along its inner edge separates it from the primary eyewall.