Using a Mars general circulation model (GCM), we investigate the characteristics of gravity waves in the atmosphere of Mars. Fluctuations due to internal gravity waves at tidal periods, such as diurnal, semidiurnal, and terdiurnal tides, are predominant in the lower atmosphere. In particular, the diurnal and semidiurnal tides having large zonal wave numbers (s > 10) are significant. Fluctuations associated with nontidal gravity wave components in the lower atmosphere are negligibly small except for the stationary orographic component. Nontidal gravity waves grow in amplitude in the middle atmosphere, while tidal components remain the dominant form of gravity waves. In the upper atmosphere (>80 km), nontidal gravity wave components and fluctuations due to the diurnal and semidiurnal tides having large zonal wave numbers are equally important. We estimate the global distributions of the gravity wave energy at various altitudes and attempt to define various sources. Our results show that most nontidal gravity wave energy is closely related with the diurnal amplitude except in the lower atmosphere where association with orography predominates. This suggests that nonorographic gravity waves are generated by the breaking and dissipation of the diurnal tide. On the other hand, nontidal gravity wave energy in the lower atmosphere peaks in mountainous regions and points to their source as being of orographic nature. We also enumerate the major differences between gravity waves on Mars and Earth as simulated by global models.
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