The role of inner-core and boundary layer dynamics on tropical cyclone structure and intensification
December 01, 2017
Christopher Slocum
Committee: Wayne Schubert (Advisor), Mark DeMaria (Co-Advisor, NOAA), David Randall, Russ Schumacher, Michael Fiorino (NOAA), Michael Kirby (Mathematics)
Abstract
Inner-core and boundary layer dynamics are vital to the tropical cyclone life cycle. This study makes use of analytical solutions and numerical models to glean insight into the role of certain processes involved with the incipient, rapidly intensifying, and eyewall replacement stages. We develop a simplified, axisymmetric, one-layer, analytical model of tropical cyclone intensification. This model makes the wave–vortex approximation, an assumption to the kinetic energy of the system, which limits its use to flows with small Froude numbers. However, we gain the ability to solve time-evolving, analytical solutions with a mass sink after we filter the inertia-gravity waves. With this model, we gain insight into why tropical cyclones have long incubation periods prior to rapid intensification, how potential vorticity towers develop without frictional effects, and how a storm evolves in time through the maximum tangential velocity, total energy space. We also compare the results from the model based on the balance relation arising from the wave–vortex approximation to gradient balance to understand the limits of the model's applicability. We find that the model based on the wave–vortex approximation is best suited to fluids with flow speeds indicative of the external vertical normal mode in which case the change in fluid depth is small.To understand what influences the radial location of the mass sink associated with the eyewall convection in the tropical cyclone, we turn to boundary layer dynamics. Motivated by abrupt jumps observed in flight-level aircraft reconnaissance data in the horizontal wind fields of Hurricanes Allen (1980) and Hugo (1989), we use an axisymmetric, f-plane slab boundary layer model with a prescribed pressure forcing in conjunction with two local, steady-state models to assess the potential role of shocks in the tropical cyclone boundary layer. With this series of models, we examine the formation of shock-like structures arising from the nonlinear terms in the radial momentum equation. We also assess conditions necessary for the development of shock-like features in the tropical cyclone boundary layer through varying the intensity and radial extent of the winds in the initial vortex. We find that the local models adequately represent the boundary layer winds for weak vorticity and show the boundary layer updraft to be outside the radius of maximum wind. However, in strong, small vortices, we see that shock-like structures readily develop in the region where the hyperbolic nature of the boundary layer dominates, which places the Ekman pumping in the region of high inertial stability. We also find that this development can be hindered by the formation of a secondary inflow maximum in vortices with a large wind field. We will show that the formation of this secondary inflow maximum precedes the formation of a secondary eyewall.