Radar-derived buoyancy in tropical cyclones

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 dynamic 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 multi-Doppler analyses, using a thermodynamic retrieval method tailored for tropical cyclones. This retrieval method can incorporate airborne Doppler radar, in situ and dropsonde data and uses a spline-based variational approach. Details of the retrieval methodology and analytic tests will be presented in a companion paper (Bell and Foerster). Here, data from the RAINEX and PREDICT/GRIP/IFEX field campaigns are used to retrieve the buoyancy perturbations relative to an axisymmetric, non-buoyant reference state in gradient wind balance at different stages of the TC's life-cycle. The characteristics of the convection at the different stages will be presented and implications of the results for understanding TC intensification will be discussed.