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  • Author or Editor: Todd D. Ringler x
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Todd D. Ringler
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Kerry H. Cook

Abstract

The forcing of stationary waves by the earth’s large-scale orography is studied using a nonlinear stationary wave model based on the quasigeostrophic equations. The manner in which wind speed, meridional temperature gradient, Ekman pumping parameter, linear damping, orographic shape, and meridional wind structure affect the validity of the linearized equations is examined and the nonlinear response is investigated.

A critical mountain height that separates the linear from the nonlinear regime is defined based on the linear quasigeostrophic potential temperature equation applied at the surface. The largest critical heights (those responses in which nonlinearity is least important) are obtained when the surface damping is weak or nonexistent. Also, relative maximums in mountain critical heights are obtained when the ratio of surface wind to surface wind shear does not vary in the meridional direction. These critical height results are validated using the fully nonlinear stationary wave model.

The nonlinearly balanced response to imposed orography is diagnosed at the surface and aloft. The nonlinear effects of eddy wind/orography interaction and nonlinear advection are found to be important only in the vicinity of the orography. The structure of the nonlinear response at the surface is found to be robust and is characterized (in the Northern Hemisphere) by a high and low situated to the northwest and southeast, respectively, of the mountain center. This orientation of the surface response leads to a stationary wave train that propagates preferentially toward the equator.

The system is sensitive enough to both the surface wind and meridional temperature gradient that the observed seasonal variations in the zonal mean circulation will significantly alter the character of the response. As the meridional temperature gradient decreases, the relative importance of nonlinearity increases while the amplitude of the response at the upper levels decreases. Therefore, this model indicates that summertime mechanically forced stationary waves should be weaker, but more nonlinear, than their wintertime counterparts.

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Todd D. Ringler
and
Kerry H. Cook

Abstract

Idealized simulations of the atmosphere’s stationary response to the Rockies, Tibetan Plateau, and the Greenland Ice Sheet are made using a nonlinear, quasigeostrophic model and are compared to observations. Observational data indicate low-level heating (cooling) occurs above the Rockies and Tibet in the summer (winter). Low-level cooling is found above Greenland in both seasons. The atmosphere responds to both diabatic heating (termed thermal forcing) and low-level flow being obstructed by the mountain’s presence (termed mechanical forcing).

The response to thermal and mechanical forcing together can be very different from the response to either forcing individually. The presence of modest low-level heating or cooling (±1.5 K day−1) causes significant changes to the mechanical forcing and, thereby, to the stationary wave response. For example, while the nonlinear response to mechanical forcing and low-level heating is characterized by a cyclone over the orography, the response to mechanical forcing and low-level cooling consists of an anticyclone over the orography. These differences cannot be fully explained using linear theory. The presence of heating (cooling) tends to reduce (amplify) both the mechanical forcing and the far-field stationary wave response. In addition, the presence of low-level heating or cooling lowers the critical mountain height below which the response is essentially linear;including nonlinear temperature advection at the surface is especially important for obtaining an accurate response.

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