Transient Gravity Wave-Critical Layer Interaction. Part I: Convective Adjustment and the Mean Zonal Acceleration

Abstract

Abstract Numerical simulations of vertically-propagating gravity waves interacting with critical layers are presented. For nearly monochromatic wave events, the wave amplitude behavior and mean zonal acceleration agree substantially with the predictions of the semi-analytic models of Grimshaw in 1975, Dunkerton in 1981–82 and Coy in 1983. A mean zonal wind “shock,” or steep sheer zone, forms at the base of a convectively unstable critical layer in these cases. Because the semi-analytic model is based on the WKB approximation, the gravity wave, mean-flow interaction proceeds somewhat differently when this approximation is not accurate. For highly transient wave packets containing a broad frequency spectrum, momentum deposition and convective instability occurs over a much broader range of heights than predicted by the semi-analytic model. For nearly monochromatic waves, on the other hand, partial reflection from the internal mean flow shock is observed. The inviscid gravity wave critical layer is inherentl...