Troposphere-to-mesosphere microphysics of carbon dioxide ice clouds in a Mars Global Climate Model

International audienceWe have implemented full CO 2 ice cloud microphysics into the LMD Mars Global Climate31Model (MGCM) and we have conducted the first global simulations. The microphysical model32implementation follows the modal scheme used for water ice cloud microphysics in the MGCM,33but includes specific aspects that need to be accounted for when dealing with CO2 ice clouds.34These include nucleation of CO 2 on water ice crystals and CO 2 condensation theory adapted for35the Martian conditions. The model results are compared to available observations globally, and36separately for polar regions and equatorial mesosphere. The observed seasonal and latitudinal37variability of the CO2 ice clouds is in general reproduced. The polar regions are covered by38CO 2 ice clouds during the winter as observed. Instead of forming only in the lowest 10-15 km39of the atmosphere, they extend up to several tens of kilometers above the surface in the model,40dictated by the modeled temperature structure. We have also quantified the contribution of the41cloud microphysics to the surface CO2 ice deposits. Snowfall from these clouds contributes up42to 10% of the atmosphere-surface ice flux in the polar regions in our simulations, in the range43that has been indirectly deduced from observations. In the mesosphere, notable amounts of CO 244ice clouds form only when water ice crystals are used as condensation nuclei in addition to45dust particles, and their spatial distribution is in agreement with observations. The mesospheric46temperature structure, dominated by tides, dictates the longitudinal and seasonal distribution of47these clouds. The seasonal and local time variations of the clouds are not fully reproduced by the48model. There is a long pause in CO2 ice cloud formation in the model around the aphelion season,49but clouds have been observed during this period, although with a lower apparition frequency.50Modeled mesospheric clouds form mainly during the night and in the morning, whereas during51the daytime, when most of the cloud observations have been made, the model rarely predicts52clouds. These discrepancies could be explained by the strong dependence of the cloud formation53process on mesospheric temperatures that are themselves challenging to reproduce and sensitive54to the MGCM processes and parameters. The rare possibilities for nighttime observations might55also bias the observational climatologies towards daytime detections. Future developments of56the model consist in the inclusion of a possible exogenous condensation nucleus source in the57mesosphere and the radiative eect of CO 2 ice cloud