Type of Document	Thesis
Author	Evans, Allen Clark
URN	etd-06282006-174844
Title	Dynamics of the Wind Field Expansion Associated with Extratropically Transitioning Tropical Cyclones
Degree	Master of Science
Department	Meteorology, Department of
Advisory Committee	Advisor Name Title Robert Hart Committee Chair Philip Cunningham Committee Member T.N. Krishnamurti Committee Member
Keywords	Cyclones Hurricanes Tropical Storms Extratropical Transition Wind Field Expansion
Date of Defense	2006-06-27
Availability	unrestricted
Abstract Extratropical transition, or ET, can be characterized by the transformation of an initially symmetric, warm-core tropical cyclone into an initially cold-core, asymmetric extratropical cyclone. As a consequence of undergoing transition, changes in the synoptic and dynamic characteristics of the cyclone are realized. Of particular note is the wind field evolution, one of the aspects of ET that has seen little research into its causes. Previous informal theories toward understanding the wind field evolution based upon key meteorological conservation principles do not accurately account for its observed evolution, while formal studies into other aspects of the ET process (e.g. Ritchie et al. 2001, Jones et al. 2003) have only mentioned its existence or its resultant impacts. This study attempts to bridge this gap by analyzing the physical and dynamical mechanisms involved with both the expansion of the wind field and outward movement of the radial wind maximum during the transition process. One ET case, North Atlantic Tropical Cyclone Bonnie (1998), is modeled using the Pennsylvania State University/NCAR Mesoscale Model version 5 (MM5; Dudhia 1993) at 12km horizontal resolution. The evolution of the cyclone within the model output is found to be an accurate measure of reality when compared to the observed track and dynamical evolution of the cyclone. Analysis of the model output shows that the expansion of the wind field is brought about by the net import of absolute angular momentum from a midlatitude trough of low pressure along descending isentropic trajectories in the western semicircle of the cyclone. Export of absolute angular momentum in the outward branch of the secondary circulation in the eastern semicircle of the cyclone partially negates but does not balance the import to the west; thus a net import of momentum into the cyclone is seen. Redistribution of momentum within the cyclone is accomplished through vertical pressure torques. The overall evolution is found to be consistent with that for a developing extratropical cyclone as shown by Johnson and Downey (1976). Net cooling (warming) inside (outside) of the radial wind maximum is shown to lead to the outward movement of this feature via a hydrostatic response in the radial height gradient, a response opposite to that seen with eyewall contraction (Shapiro and Willoughby 1982) yet consistent with the transition into a cold-core vortex. The observed results are used to formulate a conceptual model for the evolution of the wind field during ET. Implications toward the wind field evolution with other post-ET structural evolutions, such as warm seclusion cyclones and those that remain cold core yet strengthen (e.g. Hart et al. 2006), are drawn in conjunction with the observed results. Related concepts of vertical wind shear and cyclone size are discussed as natural outgrowths of the wind field expansion process.
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