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Stephen E. Mudrick

Abstract

Fine-resolution, dry, inviscid, Boussinesq formulations of a quasi-geostrophic model and a primitive equations model are used in a study of frontogenesis. These three-dimensional models employ horizontal and vertical resolution on the order of 100 km and 1 km, respectively; an integration uses about 40 grid points in each horizontal direction and 20 in the vertical. The initial states consist of two baroclinic basic currents upon which are superimposed quasi-geostrophically balanced, small-amplitude perturbations corresponding to the most unstable mode in each case. In the second case, the wave grows by barotropic as well as by baroclinic processes.

The most rapid surface frontogenesis occurs where the synoptic-scale, quasi-geostrophic convergence contributes significantly to the pure deformational increase of the horizontal temperature gradient. In these integrations, this distribution favors formation of warm fronts. The frontal zones in the quasi-geostrophic and primitive equations models agree in structure with earlier theoretical solutions by Stone and Hoskins, respectively.

The horizontal deformation, as well as the “indirect” vertical circulation, is important in producing upper level frontogenesis. The two models generate similar patterns of vertical motion. A feedback mechanism relating the action of the horizontal deformation and the indirect circulation and leading to upper level frontogenesis is postulated.

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Stephen E. Mudrick

Abstract

A linear, quasi-geostrophic, channel model is used to study the instability of zonally independent jets on a midlatitude beta plane. The jets possess strong horizontal as well as vertical wind shear. The most unstable normal mode solution for a symmetric jet is compared to corresponding solutions for asymmetric jets, where the maximum winds are skewed toward the cyclonic side. A significant increase in the growth rate is found for the asymmetric jets, the rate increasing with the amount of skew. In addition, skewing the jet results in raising the maximum of the eddy momentum flux to the top of the channel, compared to maxima at lower levels for the symmetric jet solution. Thus, the use of more realistic, cyclonically skewed jets as basic states for instability studies may result in more realistic growth rates as well as in more realistic eddy flux patterns.

Results qualitatively similar to those of Brown (1969) are found in that a weak, secondary maximum in the growth rate curve shows up for short, shallow modes for some of the asymmetric jet solutions.

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