Dynamics and Predictability of the Intraseasonal Disturbances
Chang, Jen-Cheng
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https://hdl.handle.net/2142/72453
Description
Title
Dynamics and Predictability of the Intraseasonal Disturbances
Author(s)
Chang, Jen-Cheng
Issue Date
1993
Doctoral Committee Chair(s)
Mak, Mankin
Department of Study
Atmospheric Sciences
Discipline
Atmospheric Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Atmospheric Science
Abstract
We are interested in establishing how the instability properties of the background circulation might be related to the observed intraseasonal variability. We focus on the intraseasonal variability in two Northern Hemispheric winters with distinctly different background conditions. The daily 500-mb global streamfunction fields are used. The year-to-year variations of the intraseasonal variability are closely related to the variations of the corresponding seasonal-mean flow.
A normal-mode instability analysis of a linear barotropic model reveals, however, that the seasonal-mean basic flow of the season with a stronger intraseasonal variability is in fact barotropically more stable than that of the weaker season. A DEOF (Dynamical-Empirical Orthogonal Function) analysis shows a fairly white spectrum in each season, implying a weak direct dynamical relation between the observed intraseasonal variability and the individual normal modes. Furthermore, the first few DEOFs are either barotropically neutral or weakly stable normal modes, but are not the more unstable modes.
A kinetic energy budget analysis confirms that the barotropic conversion process is one of the two major processes to generate the observed intraseasonal variability in NH wintertime. An optimal-mode analysis shows that about 40% of the observed intraseasonal disturbances can be potentially intensified through the non-modal instability process. The fraction of the intraseasonal kinetic energy of those growing modes has indeed increased to about 60% after 1 day. The result supports the conjecture that the non-modal barotropic instability of the seasonal-mean basic flow is important to the generation of the intraseasonal variability.
We also explore the feasibility of using a barotropic model to predict the intraseasonal flow. The predictability of the linear model is good for at most 6 days. Under certain favorable initial conditions, the model simulations can be useful for up to 10 days. The modest performance is due to the lack of other important dynamical/physical processes in the model. The initial uncertainty is only a minor issue. The high-frequency eddy forcing is essential to the simulations.
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