Abstract
The internal gravity wave drag formulations developed by Palmer et al. and Lindzen have been included in a diurnally varying model of the middle atmosphere, which extends from the surface to 100 km. The combined impact of these formulations was to improve further the agreement with observation of the model mean zonal wind and temperature distributions. In particular a 20 K warming of the lower stratosphere of the winter hemisphere was obtained, which considerably rectified the previous cold bias.
The Palmer et al. scheme is based on orographically excited gravity waves, and it is possible to follow the induced gravity wave stress from the surface to the lower thermosphere in the model. Results for a number of geographical regions show quite diverse vertical profiles, with the wave stress being usually absorbed in the lower stratosphere and mesosphere. However, in some cases absorption only occurred in the mesosphere. Marked diurnal and semi-diurnal temporal fluctuations were obtained in the wave stress at high levels, forced by the tidal variations at levels where the wave stress was partially absorbed. Thus gravity wave-tidal interactions were readily simulated in the model. The major limitation of this scheme was found to be the lack of wave drag generated over the extensive oceanic areas, particularly in the southern hemisphere.
The Lindzen scheme only identifies a breaking height for the upwards propagating gravity waves, usually in the upper mesosphere, and thus no information is obtainable on the wave stress profile at lower levels. This scheme does have the advantage of operating at all points of the Earth's surface, but at a given model point it was found to be highly intermittent temporally. The breaking height was extremely sensitive to the zonal wind profile. The diurnal variation of this wind profile, owing to tidal influences, was extremely high in the mesosphere and lower thermosphere of the model. This resulted in a corresponding variation in the gravity wave induced drag, indicating that gravity wave-tidal interactions were also simulated with the Lindzen scheme.
