  The impact of gravity waves (GW) on diurnal tides and the global circulation
in the middle/upper atmosphere of Mars is investigated using a General
Circulation Model (GCM). We have implemented a stochastic parameterization of
non-orographic GW into the Laboratoire de M\'et\'eorologie Dynamique (LMD) Mars
GCM (LMD-MGCM) following an innovative approach. The source is assumed to be
located above typical convective cells ($\sim$ 250 Pa) and the effect of GW on
the circulation and predicted thermal structure above 1 Pa ($\sim$ 50 km) is
analyzed. We focus on the comparison between model simulations and observations
by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter during
Martian Year 29. MCS data provide the only systematic measurements of the
Martian mesosphere up to 80 km to date. The primary effect of GW is to damp the
thermal tides by reducing the diurnal oscillation of the meridional and zonal
winds. The GW drag reaches magnitudes of the order of 1 m/s/sol above 10$^{-2}$
Pa in the northern hemisphere winter solstice and produces major changes in the
zonal wind field (from tens to hundreds of m/s), while the impact on the
temperature field is relatively moderate (10-20K). It suggests that GW induced
alteration of the meridional flow is the main responsible for the simulated
temperature variation. The results also show that with the GW scheme included,
the maximum day-night temperature difference due to the diurnal tide is around
10K, and the peak of the tide is shifted toward lower altitudes, in better
agreement with MCS observations.
