Date of Award
Doctor of Philosophy in Oceanography
This Ph.D. dissertation presents a process-oriented study focusing on the coherent large eddies, commonly know as roll vortices (rolls), in the hurricane boundary layer (HBL). We develop a new methodology for explicit representation of the vertical fluxes induced by rolls in the hurricane model. In this method a two-dimensional high- resolution roll-resolving model SRM (Single-Grid Roll-Resolving Model) is embedded at multiple horizontal grid points in the hurricane model. Such numerical design explicitly resolves the two-way interactions between the small-scale rolls and the large-scale hurricane flow. The dynamics of rolls and their impacts on the hurricane structure and intensity are investigated through a series of numerical experiments, conducted with the SRM embedded either in an axisymmetric HBL model (Chapters 1 and 2) or the three- dimensional full-physics hurricane model COAMPS-TC (Chapter 3).
Chapter 1 focuses on rolls in the linear phase. The effects of mean wind and stratification in the HBL on the characteristics of rolls are investigated. We identify two important factors associated with the HBL mean wind that affect the characteristics of rolls. The dynamical HBL height affects the wavelength of rolls, and the magnitude of the mean wind shear affects the growth rate of rolls. The mixed layer height in the stratification profile is another important factor affecting the characteristics of rolls. Provided the mean wind profiles are the same, the rolls have a larger growth rate under a higher mixed layer, and these rolls can trigger internal wave beams that are more inclined from the vertical direction and reach into a higher level.
Chapter 2 focuses on the properties of rolls in the equilibrium state, including their structures, vertical momentum fluxes and their effects on the HBL mean wind. We find that the mixed layer height is important in affecting the magnitude of the rolls and the structure of the internal waves triggered in the stably stratified layer above. The cross-roll momentum flux is dependent on the mean wind shear, but the along-roll momentum flux is not. Therefore, there is no physical basis for applying the K-theory to represent the roll-induced momentum fluxes. The rolls induce more significant changes in the mean radial wind than in the mean azimuthal wind. It is found that rolls affect the mean radial wind by redistributing the azimuthal momentum vertically in the HBL.
Chapter 3 focuses the effects of rolls on the development of a stationary and axisymmetric hurricane. It is found that the roll-induced wind changes in the HBL lead to the changes in the structure and intensity of the entire hurricane. The roll-induced vertical transport of the azimuthal momentum flux is primarily responsible for these changes. By enhancing the vertical momentum exchange, the rolls trigger a chain of dynamical responses within the HBL, increasing the mass convergence and inducing a more active deep eyewall convection, which leads to the enhanced hurricane intensity.
Gao, Kun, "Dynamics and Impacts of Roll Vortices in the Hurricane Boundary Layer" (2015). Open Access Dissertations. Paper 408.