The effect of spatial variations in unresolved topography on gravity wave drag in the Martian atmosphere

Recent three dimensional simulations of the Martian atmosphere have shown that gravity wave drag is a large, and possibly dominant, term in the momentum balance above 50 km. In previous simulations, a constant value was used for the topographic variance on scales smaller than the model grid. Spatial variations in this quantity are now taken into account, and the results described. The topographic variance has its highest amplitude in the Tharsis and Hellas regions. Correspondingly, relatively low breaking levels are found here. Longitudinal variations in breaking levels and hence accelerations are very large, as are zonal asymmetries in zonal wind u. These asymmetries are expected to play a role in planetary wave propagation in the Martian middle atmosphere. Momentum balances are found to be dependent on nonlinear coupling terms in the momentum equations due to the high zonal winds and small planetary radius. At the equinoxes two regimes exist. One is where breaking levels are very low: here accelerations and eddy diffusion coefficients resulting from the parameterization of gravity wave breaking in the model are also very low. The other regime is where breaking levels are higher, and winds are stronger. In this region, vertical mixing is also very weak. These results give further credence to the idea that breaking thermal tides cause the intense vertical diffusion that has been inferred from observations at the Martian equinox.