  We present the first general circulation model simulations that quantify and
reproduce patches of extremely cold air required for CO$_2$ condensation and
cloud formation in the Martian mesosphere. They are created by subgrid-scale
gravity waves (GWs) accounted for in the model with the interactively
implemented spectral parameterization. Distributions of GW-induced temperature
fluctuations and occurrences of supersaturation conditions are in a good
agreement with observations of high-altitude CO$_2$ ice clouds. Our study
confirms the key role of GWs in facilitating CO$_2$ cloud formation, discusses
their tidal modulation, and predicts clouds at altitudes higher than have been
observed to date.
