Momentum exchange due to internal waves and wakes generated by flow past topography in the atmosphere and lakes

Topographic features on the earth's surface with length scales of 5-100 km have a significant effect on the general circulation of the atmosphere. This is accomplished by the drag that they exert on the atmosphere through the generation of internal gravity waves that may propagate to great heights, through hydraulic transitions in low level airstreams across mountain ranges, and through the formation of wakes. The satisfactory representation of these processes in general circulation models is important for both weather and climate forecasting. This may be accomplished by characterising the small-scale topography by suitable statistical parameters, and (conceptually) replacing it by simpler topography that has the same values of these parameters, and for which the dynamics have been studied and are reasonably well known. The drag of this topography on the atmospheric flow may then be estimated for modelling purposes. The extension of these concepts to flow past topography in lakes is discussed. As a beginning, this is done by examining the properties of stratified flow past obstacles on an inclined plane surface, simulating part of the bottom of a typical lake. The three dynamical processes listed above for the atmosphere are again relevant, with conspicuous asymmetries present because of the sloping terrain.