Examining the Role of Eyewall Convection in Hurricane Intensity Change Using Airborne Radar Observations

Forecasting tropical cyclone (TC) intensity remains challenging, because TC intensity is determined by many internal and external factors. The maximum potential intensity is controlled by environmental variables, but internal mechanisms seem to plays a critical role in TC intensity change. Recent studies have suggested that the radial location of eyewall convection relative to the radius of maximum wind is an important component of intensification efficiency. However, our understanding of the physical processes that determine the location and strength of eyewall convection is still incomplete. Ordinary tropical convection is generally forced by positive buoyancy, but rotational forces and a strengthening warm core play an progressively important role in organizing convection as a TC intensifies. Axisymmetric conceptual and numeric models of the eyewall are characterized by moist neutral ascent forced by boundary layer convergence. In contrast, three-dimensional models suggest that a significant fraction of eyewall convective elements may contain positive buoyancy, and that asymmetric forcing by vertical shearing flow and mesoscale vorticity anomalies may play an important role. Further clarification is needed to determine the role of buoyancy in TC intensity change throughout its life-cycle.

Buoyancy can not be observed directly, nor is it defined uniquely. Buoyancy can be deduced from simultaneous measurements of kinematic and thermodynamic fields, but high-resolution measurements of these fields in a tropical cyclone only exist along an aircraft track or a dropsonde profile. This study will present an indirect retrieval approach that provides an estimate of the thermodynamic structure and buoyancy throughout the entire eyewall region using aircraft radar observations. This novel thermodynamic retrieval derives buoyancy and perturbation pressures from a spline-based variational analysis that combines Doppler radar with aircraft and dropsonde observations. It is is specifically designed for strongly rotating systems, in which the balanced reference state is in both hydrostatic and gradient wind balance. An analysis of the structure and buoyancy of eyewall convection in Hurricane Rita (2005) will be presented, using data from the Hurricane Rainband and Intensity Experiment (RAINEX) field campaign.