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Numerical Simulations of Gravity Currents in Uniform Shear Flows

Chaing ChenMesoscale Atmospheric Processes Branch, Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland

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Abstract

The purpose of this study is to examine the effect of wind shear on gravity currents in a neutral atmosphere by using a two-dimensional, nonhydrostatic primitive equation model. The depth of the gravity current is found to be directly related to the sign and the magnitude of the shear; a flow with a positive wind shear produces a gravity current of greater depth than that with a negative shear. As the positive wind shear (i.e., positive ∂u/∂z) increases, the gravity current becomes unstable. For sufficiently large positive shear, the gravity current displays a diffuse structure with two distinct gravity current heads. It is found that enhanced eddy mixing, triggered by the presence of a reversed (rear to front) flow in the prefrontal environment, is the source of this phenomenon. From a vorticity budget analysis, it is found that the rear-to-front flow is less efficient in “ventilating” or “removing” vorticity generated at the leading edge of the gravity current. Therefore, the accumulation of vorticity leads to the development of enhanced eddy circulations that subsequently destroy the gravity current head.

Abstract

The purpose of this study is to examine the effect of wind shear on gravity currents in a neutral atmosphere by using a two-dimensional, nonhydrostatic primitive equation model. The depth of the gravity current is found to be directly related to the sign and the magnitude of the shear; a flow with a positive wind shear produces a gravity current of greater depth than that with a negative shear. As the positive wind shear (i.e., positive ∂u/∂z) increases, the gravity current becomes unstable. For sufficiently large positive shear, the gravity current displays a diffuse structure with two distinct gravity current heads. It is found that enhanced eddy mixing, triggered by the presence of a reversed (rear to front) flow in the prefrontal environment, is the source of this phenomenon. From a vorticity budget analysis, it is found that the rear-to-front flow is less efficient in “ventilating” or “removing” vorticity generated at the leading edge of the gravity current. Therefore, the accumulation of vorticity leads to the development of enhanced eddy circulations that subsequently destroy the gravity current head.

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