Radar-derived Buoyancy in Hurricane Rita (2005)
Annette M. Foerster and Michael M. Bell
University of Hawaii at Manoa, Honolulu, HI
Deep convection and stratiform precipitation are integral parts of a tropical cyclone (TC), but our understanding of how these different modes of convection contribute to intensification is still limited. The forcing of convection depends on an environment that changes throughout the TC's life-cycle. Ordinary tropical convection is generally forced by positive buoyancy, but rotational forces and a strengthening warm core play a progressively important role in organizing convection as a TC intensifies. In contrast to the balanced vortex model with moist-neutral ascent in the eyewall, there are modeling (Braun 2002) and observational (Eastin et al. 2005) studies that suggest that buoyant convection still makes a significant contribution to vertical motion in the mature TC eyewall. Further observational clarification is needed to determine the role of buoyancy in TC intensification throughout its life-cycle.
Direct calculation of buoyancy in a tropical cyclone is very difficult, because simultaneous measurements of kinematic and thermodynamic fields with sufficiently high resolution and accuracy are required. An indirect approach is to retrieve buoyancy and pressure perturbations from airborne Doppler radar data, using a thermodynamic retrieval method especially tailored for tropical cyclones, where buoyancy is defined with respect to a mean vortex in hydrostatic and gradient wind balance. Details of this novel retrieval methodology will be presented along with results illustrating the changing characteristics of buoyant convection at different stages of the life-cycle of Hurricane Rita (2005) using observational data from the Hurricane Rainband and Intensity Change Experiment (RAINEX) field campaign.