Mean Fields Induced by Local Gravity-Wave Forcing in the Middle Atmosphere

Abstract

We examine the role of geostrophic adjustment in the middle atmosphere for given wave packet forcing by a three-dimensional hydrostatic model. The problem is solved directly so that solutions are expressed as convolution integrals of the Green's function and the external forcing. It is shown that the induced fields consist of two different kinds of modes. One, produced by forcing vorticity only, is steady quasi-geostrophic flow;, this is restricted to the forcing region. The other, produced by both forcing vorticity and forcing divergence, is oscillatory, and is in the form of gravity waves propagating out of the forcing region plus inertial oscillations in the forcing region. The scales and amplitudes of the induced gravity waves are determined by the forcing. For a typical example, a 200 km ? 200 km gravity wave packet of momentum flux 0.5 N m?2 absorbed in a layer of 5 km thickness centered near 18 km altitude, the gravity waves spread to a larger region ? 1000 km ? 1000 km at a level 7 km above the forcing region. At that level the horizontal and vertical wavelengths of the induced waves are about 300 km and 7 km respectively, and the momentum flux is substantially reduced to 10?4 N m?2. The wave parameters of such induced modes are consistent with the wave source parameters used in some general circulation models for the middle atmosphere. The results suggest that geostrophic adjustment processes may play an important role in specifying the gravity wave spectrum in the middle atmosphere.