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The Effect of Barotropic Shear on Upper-Level Induced Cyclogenesis: Semigeostrophic and Primitive Equation Numerical Simulations

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  • 1 Institute for Atmospheric Science, ETH, Zurich, Switzerland
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Abstract

Idealized numerical experiments within the frameworks of semigeostrophic and primitive equation dynamics were performed to study the effect of barotropic shear on idealized upper-level induced cyclogenesis. Localized finite-amplitude potential temperature anomalies were used as initial perturbations, and the atmosphere was considered as a dry frictionless fluid of uniform quasigeostrophic potential vorticity on an f-plane.

It is demonstrated that the main features of the numerical simulations are in essence unaffected by the choice of the dynamical framework. They comprise, for instance, the development of elongated cold fronts under anticyclonically sheared conditions, a “T-bone” shaped frontal palette in the unsheared case (cf. Shapiro and Keyser), and a Bergen-type occlusion process in the simulations with cyclonic shear. This confirms the profound dynamical influence of lateral shear in the background environment upon the resulting surface cyclone and frontal structures (and the accompanying evolutions at upper levels) that has been found in previous normal-mode experiments. This sensitivity is shown to be related to the different orientation of the additional deformation field associated with the background shear. The differences between surface cold and warm fronts are analyzed in more detail using a combined Eulerian and Lagrangian approach.

Consideration is also given to the shortcomings of the present approach and to a possible strategy for further idealized model investigations.

Corresponding author address: Dr. Heini Wernli, Institute for Atmospheric Science, ETH Hönggerberg, CH-8093 Zurich, Switzerland.

Email: wernli@atmos.umnw.ethz.ch

Abstract

Idealized numerical experiments within the frameworks of semigeostrophic and primitive equation dynamics were performed to study the effect of barotropic shear on idealized upper-level induced cyclogenesis. Localized finite-amplitude potential temperature anomalies were used as initial perturbations, and the atmosphere was considered as a dry frictionless fluid of uniform quasigeostrophic potential vorticity on an f-plane.

It is demonstrated that the main features of the numerical simulations are in essence unaffected by the choice of the dynamical framework. They comprise, for instance, the development of elongated cold fronts under anticyclonically sheared conditions, a “T-bone” shaped frontal palette in the unsheared case (cf. Shapiro and Keyser), and a Bergen-type occlusion process in the simulations with cyclonic shear. This confirms the profound dynamical influence of lateral shear in the background environment upon the resulting surface cyclone and frontal structures (and the accompanying evolutions at upper levels) that has been found in previous normal-mode experiments. This sensitivity is shown to be related to the different orientation of the additional deformation field associated with the background shear. The differences between surface cold and warm fronts are analyzed in more detail using a combined Eulerian and Lagrangian approach.

Consideration is also given to the shortcomings of the present approach and to a possible strategy for further idealized model investigations.

Corresponding author address: Dr. Heini Wernli, Institute for Atmospheric Science, ETH Hönggerberg, CH-8093 Zurich, Switzerland.

Email: wernli@atmos.umnw.ethz.ch

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