A numerical study of barotropic instability of a zonally varying easterly jet.
Tupaz, Jesus Bonifacio
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The structure and behavior of barotropi cal ly unstable and stable wave disturbances in the vicinity of a zonally varying easterly jet are studied numerically with a linearized barotropic vorticity equation on a B-plane. The easterly jet is approximated by a Bickley jet with a slow zonal variation. The numerical results are also compared with a simple mechanistic analytical model using the local phase speed and growth rate concepts. The results are grossly similar in several respects to that expected from the parallel flow theory of barotropic instability, however, the resultant structure of the waves causes a spatial growth rate greater than predicted by the local growth rates computed with a parallel flow model. In the stable region, the structure leads to strong dynamic damping. When a uniform advective velocity is added to a variable mean flow, the differences between the behavior of the computed waves and that implied by the parallel flow theory are somewhat reduced. The waves remove kinetic energy from the mean flow and most of this energy is removed on the downwind side of the jet. The computed structure and behavior of the waves have a number of features that resemble those observed in the vicinity of the upper troposphere easterly jet during the summer monsoon.
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