GRAVITY-WAVE DRAG IN 3-DIMENSIONAL ATMOSPHERIC MODELS OF MARS

Gravity waves are disturbances whose restoring force is buoyancy. In t he atmosphere they are made possible by the variation of potential tem perature with height. They can be excited by various mechanisms includ ing flow over orography. The momentum carried by these disturbances ca n exert significant drag forces on the mean flow in the terrestrial me sosphere. One parameterization of this effect, the Lindzen gravity wav e drag scheme, is inserted into two simplified general circulation mod els (GCMs) of the Martian atmosphere, and its effects are described. T he influence of large-amplitude, planetary-scale topography on wave br eaking altitudes is discussed. We show that above 60 km in altitude, g ravity wave drag dominates the momentum balance of the Martian atmosph ere on the largest scales. This indirectly alters the thermal state of the atmosphere through the thermal wind relation. Model winds at this level are consistent with other, simpler studies, as well as with Ear th-based spectroscopic observations. Below 60 km, the effects of plane tary wave-mean flow interaction, as well as heating associated with Ha dley circulation descent, are significant in determining the wind stre ngth. Eddy diffusion coefficients derived from the Lindzen parameteriz ation above 50 km are consistent with values implied from chemical mod eling of the Martian middle atmosphere, as well as with simpler models of gravity wave drag.