Primary and Secondary Eyewall Convective Forcing in Hurricane Rita (2005)

Tropical cyclone eyewall convection plays a critical role in 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 forcing mechanisms that determine the location and strength of eyewall convection is still incomplete. Axisymmetric conceptual and numerical 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 also play an important role.

This study will present an analysis of the convective forcing mechanisms observed during the primary and secondary eyewall formations of Hurricane Rita (2005) from the RAINEX/IFEX field campaign. A spline-based variational analysis technique called SAMURAI is used to combine Doppler radar data with Stepped Frequency Microwave Radiometer, flight level, and dropsonde observations. Improved retrievals of high-resolution winds and pressure gradients in the boundary layer are used to assess low-level forcing, and a novel thermodynamic retrieval is used to assess buoyancy and pressure perturbations in the free troposphere. The mesoscale analysis suggests that buoyancy, low-level supergradient winds, and vortex asymmetries all contribute to eyewall convective forcing, but the relative importance of different mechanisms may have evolved throughout Rita's life-cycle due to the changing dynamic and thermodynamic environment.