Re-Examining Tropical Cyclone Radial Structure From Flight-Level Aircraft Observations: Implications For Vortex Resiliency And Intensification

Abstract

The importance of the radial structure on two aspects of the tropical cyclone (TC) intensity change problem is addressed: the vortex resiliency to ambient vertical wind shear and rapid vortex intensification. Theoretical studies, based on vortex Rossby wave (VRW) dynamics, have established that the degree of vortex broadness in the near-core region beyond the radius of maximum wind (RMW) determines the realignment or tilting over of a TC vortex in vertically sheared environments. The sensitivity of the initial specification of idealized vortices demonstrated by numerical simulations brings into question how well the "true" nature of TC radial structure is represented by some commonly used idealized vortices. Although the importance of the initial radial structure on the rate of vortex intensification is not well established theoretically, the issue of whether unique aspects of the swirling wind structure are observed prior to rapid intensification events is also addressed. The primary circulation of TCs is re-examined by utilizing flight-level observations collected from Atlantic and eastern Pacific storms during 1977-2001, in which several hundred radial profiles of azimuthal-mean tangential wind and relative vorticity are constructed from over five thousand flight leg segments. This comprehensive analysis principally reaffirms that real TC structure is characterized by a relatively slow tangential wind decrease beyond the RMW and a monotonically decreasing skirt of significant cyclonic relative vorticity. These characteristics, however, are found to be conspicuously absent in some idealized vortices frequently used in theoretical studies of TC evolution. Secondly, an investigation of the relationship between the initial tangential wind structure and the future intensification rate reveals that no unique characteristics exist just prior to rapid intensification events. The restricted range observed in the tangential wind parameters for a limited number of cases, however, suggest that necessary conditions may exist for rapid intensification. Although the radial structure appears to be critical for the vortex resiliency, the sole importance of the initial swirling wind structure on the intensification remains inconclusive.