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https://hdl.handle.net/2142/19647
Description
Title
Dynamics of storm tracks
Author(s)
Lee, Woo-Jin
Issue Date
1994
Doctoral Committee Chair(s)
Mak, Mankin
Department of Study
Atmospheric Science
Discipline
Atmospheric Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Atmospheric Science
Language
eng
Abstract
The main objective of this study is to explore the dynamics of storm tracks in the context of a three-layer hemispheric linear-balanced model. We establish first how the instability properties of the relevant background circulation might be related to the observed storm tracks. We examine next how the topography could influence the local cyclogenesis. Finally, we investigate how the model storm tracks are organized in a nonlinear model simulation. We focus on the behavior of the synoptic-scale disturbances in the 1982-83 boreal winter, a season with a strong El Nino event and a pronounced zonally asymmetric mean state in the extratropical atmosphere.
The linear instability analysis of the observed winter mean state reveals that the basic state is unstable with the presence of a reasonable surface friction. In spite of the large zonal asymmetry in the basic state, some degree of order among the large number of unstable modes is found with a cluster analysis. The structures of the PAC (North Pacific) and ATL (North Atlantic) modes are remarkably similar to the observed storm tracks in the North Pacific and the North Atlantic, respectively. Local friction experiments show that the PAC modes are local modes, and that the ATL modes are global modes. Pulse asymptote experiments show that the background flows in the North Pacific and the North Atlantic are found to be absolutely stable, but convectively unstable. The local energetics and the evolution of the adjoint of the PAC mode show that the PAC mode is maintained by a combination of weak absolute instability off the coast of Asia and strong convective instability over the eastern Pacific. Transient development of small amplitude disturbances is systematically investigated in terms of an optimal mode analysis.
Linear instability analysis with the actual orography in the Northern Hemisphere reveals that the topographically-induced stationary waves give rise to unstable normal modes which have large amplitudes in northwestern Pacific and northwestern Atlantic. The local cyclogenesis in both storm track regions is due in part to the topographically-induced stationary waves downstream of the Tibetan Plateau. The effect of topography on the local cyclogenesis in the Pacific storm track region is found to be cyclogenetic.
The organization of model storm tracks by equilibrated disturbances is examined by the statistical/dynamical analysis of the long record of the model simulations. Well-defined wave packets are found in the equilibrated disturbances in the upper model level. The analysis reveals that both convective and absolute instability in the basic flow are stabilized through the nonlinear process. The equilibrated forced-dissipative nonlinear flow becomes weakly unstable with respect to modal perturbation, and is still quite unstable with respect to non-modal perturbation. The release of non-modal instability is largely responsible for the organization of the model storm tracks. (Abstract shortened by UMI.)
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