Eyewall Buoyancy and Intensity Change in Hurricane Rita (2005)

Wednesday, 20 April 2016: 5:30 PM
Ponce de Leon A (The Condado Hilton Plaza)
Annette M. Foerster, University of Hawaii at Manoa, Honolulu, HI; and M. M. Bell

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. It 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. An indirect retrieval approach will be presented, which allows estimation of the three-dimensional buoyancy and pressure perturbations in the entire eyewall region using aircraft radar observations.

A detailed analysis of the structure and buoyancy of the eyewall convection in Hurricane Rita (2005) will be presented. First, the performance of the thermodynamic retrieval approach will be assessed using a Weather Research and Forecasting (WRF) simulation. Second, the retrieval will be applied to airborne observations from the Hurricane Rainband and Intensity Change Experiment (RAINEX) field campaign. The results will be used to analyze the convective forcing in the eyewall during periods of intensification and enhanced vertical wind shear.

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