Horizontal vorticity redistribution and vortex alignment in developing and mature tropical cyclones

The three-dimensional redistribution of vorticity within a vortex is examined here in the context of tropical cyclone (TC) structure and intensity change. Aspects of the horizontal vorticity mixing dynamics are first presented in a novel analysis of high temporal resolution wind fields derived from airborne dual-Doppler observations of Hurricane Olivia (1994). Seven consecutive composites of Olivia's wind field with 30-min time resolution depict a weakening storm undergoing substantial structural changes.; The problem of vortex alignment (and the attendant three-dimensional redistribution of vorticity) is then re-examined in an effort to further understand the underlying dynamics of TC-like vortices tilted by vertical shear. The study is motivated in part by the analysis of Hurricane Olivia. Olivia's asymmetric evolution in the presence of increasing environmental vertical shear is consistent with that predicted by existing “vortex in shear” theories. These theories, however, are based on a nonlinear interpretation of unforced vortex alignment originally developed to explain the emergence of vertically-coherent vortex structures in geostrophic turbulence. For small to moderate vortex tilts, a simpler and more insightful linear model for unforced vortex alignment is presented. This model provides the basis for a deeper understanding of the dynamics of rapidly-rotating, vertically-sheared vortices.; The linear model is formally valid as long as the tilted vortex can be meaningfully represented through a wave, mean-flow decomposition. This is typically true if the vortex cores at upper and lower levels overlap. The validity of the linear model is tested for a range of vortex tilts using a quasi-geostrophic model in both its complete and linear, equivalent-barotropic forms.; The vertical alignment dynamics in the aforementioned small to moderate tilt regime is accurately captured by linear vortex Rossby wave processes. For internal Rossby deformation radii larger than the horizontal scale of the tilted vortex, an azimuthal wavenumber one near-discrete vortex Rossby wave, or quasi-mode, exists. The quasi-mode is characterized by its steady cyclonic propagation, long lifetime, and resistance to differential rotation, behaving much like a discrete vortex Rossby wave. The quasi-mode traps disturbance energy causing the vortex to precess and thus prevents alignment. For internal deformation radii smaller than the horizontal vortex scale, the quasi-mode disappears into the continuous spectrum of vortex Rossby waves which promote complete alignment by irreversibly (but linearly) redistributing potential vorticity (PV).; The linear alignment theory is extended to stronger vortices in the Asymmetric Balance system with results similar to those for geostrophic vortices. In addition to providing new insight into the asymptotic dynamics of vortex merger in three dimensions, these results also are believed to have relevance to the problem of tropical cyclogenesis. Cyclogenesis initiated through the merger of low-level convectively-generated positive PV within a weak incipient vortex is captured by quasi-linear dynamics. A potential dynamical barrier to TC development in which the quasi-mode frustrates vertical alignment can be identified using the linear alignment theory in this case.