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Curvature vorticity and shear vorticity are examined in an attempt to elucidate the physical mechanisms behind tropical cyclone (TC) intensity change. A particular emphasis is placed on the role of the shear to curvature vorticity conversion term throughout this study. Curvature vorticity and shear vorticity budgets are calculated from the equations given by Bell and Keyser (2003) using data from a case study of TC Ivan (2004) simulated using the Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) fifth generation non-hydrostatic Mesoscale Model (MM5). The first portion of this analysis focuses on describing the spatial and temporal evolution of each of the relevant terms. Given the use of an Earth relative reference frame, the terms studied in this analysis are limited to the vertical advection term, vorticity conversion term, stretching term, and tilting term. Storm centered azimuthal spectral analyses are relied upon to elucidate the primary scales at which each term is organizing at. Following this, layer averages of the areal-average of the magnitude of each term and the areal-average of each term within a 0.5° radius of the TC center are used to perform an intercomparison between tendency terms as well as for the purpose of relating each to changes in minimum sea level pressure. Although correlation coefficients are as high as 0.52, the computations are unable to directly link one single term in either the curvature vorticity tendency or shear vorticity tendency equation to the intensity change of the TC. To further examine the role of shear to curvature vorticity conversions in intensity change, hindcasts of selected storms from the 2004-2006 Atlantic hurricane season simulated using the Weather Research and Forecast system for hurricane prediction (HWRF) are examined to determine if a correlation exists between the shear to curvature vorticity conversion term and intensity change. Specifically, this portion of the study focuses on the initial hour of the hindcast given the interest in using calculated values of the vorticity conversion term as an operational product for predicting intensity change. Areal-averages and spectral analysis are used to compute a single value used for correlation with intensity change. Prior to performing this analysis, an evaluation of the physical consistency of the initial vortex used in the HWRF hindcasts with best track data (Jarvinen et al., 1984) is performed using wind-pressure relationships. While the wind-pressure relationship for the initial vortex is in reasonable agreement with best track observations for TCs with wind speeds below 50 m/s, a low bias in the minimum sea level pressure of the vortex is noted for maximum wind speeds above this threshold. Additionally, the HWRF initial vortex undergoes gradient adjustment during the first 6 hours of the forecast that becomes increasingly larger with increasing wind speeds. The results of the correlation show a more robust relationship between vorticity conversions and current intensity (R²<0.77) rather than for intensity change (R²<0.1). Although reasons for why the diagnostic relation exists are unclear, plausible explanations include the initialization scheme, spatial resolution of the model, and the parameterizations used.
A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science.
Includes bibliographical references.
T. N. Krishnamurti, Professor Directing Thesis; Robert Hart, Committee Member; Paul Ruscher, Committee Member.
Florida State University
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