Tropical cyclone evolution via internal asymmetric dynamics
Date
2008
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Abstract
This dissertation advances our understanding by which tropical cyclones (TCs) evolve solely due to internal dynamics, in the absence of large-scale environmental factors and surface fluxes, using a hierarchy of numerical model simulations, diagnostics and observations. In the first part, the role of inner-core (eye and eyewall) transport and mixing processes in TC structure and evolution is examined, and in the second part, some asymmetric dynamics of tropical cyclone evolution are studied: spontaneous inertia-gravity wave radiation from active TC cores and an observational case study of the role of vortical hot towers in tropical transition. The role of two-dimensional transport and mixing in TC structure and intensity change is quantified. First, the mixing properties of idealized hurricane-like vortices are assessed using the effective diffusivity diagnostic. Both monotonic and dynamically unstable vortices are considered. For generic deformations to monotonic vortices, axisymmetrization induces potential vorticity (PV) wave breaking outside the radius of maximum wind, forming a finite radial length surf zone characterized by chaotic mixing. Although on a much smaller scale, this surf zone is analogous to the surf zone outside the wintertime stratospheric polar vortex. For unstable rings, during barotropic instability both the inner and outer breaking PV waves create horizontal mixing regions. For thin ring breakdowns, the entire inner-core becomes a strong mixing region and passive tracers can be transported quickly over large horizontal distances. For thick ring breakdowns, an asymmetric partial barrier region may remain intact at the hurricane tangential jet, with mixing regions on each side where the waves break. The inner, breaking PV wave is quite effective at mixing passive tracers between the eye and eyewall; with a monotonic low-level equivalent potential temperature radial profile, these results support the hurricane super-intensity mechanism. Next, a systematic study of inner-core PV mixing resulting from unstable vortex breakdowns is conducted. After verifying linear theory, the instabilities are followed into their nonlinear regime and the resultant end states are assessed for 170 different PV rings, covering a wide spectrum of real hurricanes.
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Subject
gravity waves
hurricines
potential vorticity
tropical cyclones
atmospheric sciences