Repository logo

Browsing by Author "Schubert, Wayne, advisor"

Now showing 1 - 3 of 3
  • Thumbnail Image
    ItemOpen Access
    Global omega equation: derivation and application to tropical cyclogenesis in the north Atlantic Ocean
    (Colorado State University. Libraries, 2012) Dostalek, John F., author; Schubert, Wayne, advisor; DeMaria, Mark, advisor; Estep, Don, committee member; Johnson, Richard, committee member; Vonder Haar, Tom, committee member
    The quasi-geostrophic omega equation has been used extensively to examine the large-scale vertical velocity patterns of atmospheric systems. It is derived from the quasi-geostrophic equations, a balanced set of equations based on the partitioning of the horizontal wind into a geostrophic and an ageostrophic component. Its use is limited to higher latitudes, however, as the geostrophic balance is undefined at the equator. In order to derive an omega equation which can be used at low latitudes, a new balanced set of equations is developed. Three key steps are used in the formulation. First, the horizontal wind is decomposed into a nondivergent and an irrotational component. Second, the Coriolis parameter is assumed to be slowly varying, such that it may be moved in and out of horizontal derivative operators as necessary to simplify the derivation. Finally, the mass field is formulated from the nondivergent wind field. The resulting balanced set of equations and the omega equation derived from them are valid over the whole sphere. In addition, they take a similar form to the quasi-geostrophic equations. The global omega equation is applied to the problem of tropical cyclogenesis in the Atlantic Ocean. The omega fields are used to compare those disturbances that eventually undergo cyclogenesis with those that dissipate. Composite analysis is employed and, in order to account for the different regional behavior of tropical cyclogenesis, the Atlantic is divided into three subbasins: the Tropics, the Subtropics, and the Gulf of Mexico. It is found that the large-scale omega is not strong enough to account for the magnitude of vertical velocities found in tropical cyclones, but acts to provide a favorable environment for convection to develop. The greatest difference between the developing composite and dissipating composite is seen in the Tropics, where the large-scale ascent at low levels on the leading edge of the disturbance due to frictional forcing in the developing composite is significantly greater than the ascent at the leading edge of the dissipating disturbance. The other two subbasins do not exhibit such large statistical differences, but examining the omega fields and the dominant forcing terms do lend insight into the physical differences between those distubances which develop and those that do not. As an additional application, the 850-hPa omega is used as a predictor in an operational tropical cyclogenesis probability product. Overall, the inclusion of the omega field improves the performance of the product, as measured in terms of the Brier skill score. Due to a difficulty in interpreting how the linear discriminant analysis handles the omega field however, it may be that the large-scale omega may be of more value in the genesis product's screening step than in its prediction step.
  • Thumbnail Image
    ItemOpen Access
    Tropical cyclone evolution via internal asymmetric dynamics
    (Colorado State University. Libraries, 2008) Hendricks, Eric A., author; Schubert, Wayne, advisor
    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.
  • Thumbnail Image
    ItemOpen Access
    Tropical cyclone inner core structure and intensity change
    (Colorado State University. Libraries, 2011) Musgrave, Kate D., author; Schubert, Wayne, advisor; Davis, Christopher, advisor; Johnson, Richard, committee member; Thompson, David, committee member; Kirby, Michael, committee member
    This dissertation focuses on two projects that examine aspects of the relationship between tropical cyclone (TC) storm-scale dynamics and intensity. TC intensity change is a forecast challenge combining influences from the large-scale environment, the underlying ocean state, and the storm-scale dynamics within the TC. In particular structures and processes involving the TC eye are observed to have an impact on current and future intensity. The first project examines observations of TC eyes from aircraft reconnaissance flown into Atlantic basin TCs over the period 1989-2008. Relationships between TC eye diameter and type and intensity and intensity change are investigated. Consistent with previous studies, eye diameter does not display a direct relationship with intensity. Smaller eye diameters are observed at all intensities, though both the most and least intense TCs with eyes have smaller average eye diameters. Smaller eyes also have the largest variability in intensity change. Larger eyes show smaller ranges for intensity change, and the largest eyes tend to maintain or weaken in intensity. TCs with eyes reported had higher intensification rates and higher probabilities of undergoing rapid intensification. The second project takes a theoretical approach to examining the TC response to the location of the convection within the vortex structure using the balanced vortex model. An annular ring of heating is placed along an idealized axisymmetric vortex. The largest increase in intensity is produced when the heating is placed within the radius of maximum winds. Intensification still occurs at a lessened rate when the heating is contained within the vorticity skirt, and when the heating is outside the vorticity skirt the vortex does not intensify. The strength of the vortex increases in all cases, though less so than the intensity when the heating is within the radius of maximum winds.