Abstract
An inertio-gravity wave critical layer is defined as the region in which the conventional WKB-type dispersion relation is mathematically invalid (Yamanaka and Tanaka, 1984b). This layer is bounded by a turning level at which wavefront parallels the basic isopycnic surface and by a level inside which geostrophic adjustment for the wave momentum can hardly take place. The valve-like critical 'level' absorption (Grimshaw, 1975) is explained by an anisotropic wavefront revolution due to the baroclinicity of the basic field. The induced zonal-mean field, correct to the second order of wave amplitude, resembles an inertial oscillation or a symmetric isopycnical motion. The net absorption rate of the critical 'layer', which coincides with that of the non-inertial waves (Booker and Bretherton, 1967), can be explained by a resonant energy conversion from the wave to the zonal-mean inertial oscillation. The geostrophic flow deceleration results from a kind of resonant redistribution of mean kinetic energy so as to maintain the zonal-mean isopycnical motion; this leads to a final equilibrium state in which Richardson number of the mean zonal flow in the critical layer is unity. Although the non-inertial approximation holds outside the layer with a basic Richardson number larger than unity, above-mentioned features are important to discuss the gravity-wave stress and quasi-horizontal diffusion in the strato-sphere.
