Gravity-wave parameterization and the breakdown of Newton's Third Law

Parameterizations of gravity-wave drag (GWD) in general circulation models (GCMs) of the Earth's atmosphere are founded upon the conservation of wave pseudomomentum flux and explicitly satisfy Newton's Third Law in each vertical column of the model. However, these notions are valid only for horizontally uniform flow. A more general approach would see GWD parameterization founded upon the conservation of wave action. This would allow a conservation principle in situations of horizontally non-uniform flow. In this more general framework Newton's Third Law may be violated and the flow subjected to wave induced forces arising from wave dynamics that are conservative rather than dissipative in nature (Buhler and McIntyre 2004). In this study we reformulate a current parameterization of orographic GWD (Scinocca and Mcfarlane 2000) to allow horizontally non-uniform flow and to conserve wave action flux rather than pseudomometum flux. The impact of this new formulation is investigated by offline calculations and fully interactive GCM simulations.