12th Conference on Middle Atmosphere

Wednesday, 6 November 2002: 3:45 PM
A mechanism of enhanced temperature fluctuations by gravity waves in critical layers
Tatiana L. Erukhimova, Texas A&M University, College Station, TX; and V. Y. Trakhtengerts
Mesoscale fluctuations of atmospheric temperature are usually attributed to gravity wave activity. Quite large temperature perturbations (of the order of 10-15 K) are believed to trigger the formation of solid particles in Polar Stratospheric Clouds in the Arctic polar region. At lower stratospheric heights such perturbations can be difficult to explain with linear models of gravity waves.

We have shown that wave-induced distortion of atmospheric parameters by gravity waves propagating in shear wind flows can be resonantly enhanced near the critical levels, where the horizontal phase velocity of the wave is equal to the velocity of the background mean flow. The resulting amplitude of the perturbation is nonlinear with respect to the wave amplitude and can be several times larger than predicted by linear theory. The amplitude and time scale of temperature oscillations are found for two different types of gravity wave activity: (i) a quasi-monochromatic gravity wave of constant amplitude, when the flow pattern near critical layer is determined by nonlinearity, and viscosity effects are small; and (ii) a non-stationary wave field of a gravity wave envelope with fairly large spread of phase velocities and a noise-like spectrum, when the motion of a given volume element takes the form of a diffusion in the vertical direction. It is shown that the resonant effect of a critical layer leads to considerable temperature disturbances (about 10-15 K) with small wave amplitudes typical for lower stratospheric heights. This effect could be relevant to the explanation of the existence of lower temperatures than synoptically predicted in the Arctic polar region.

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