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

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

Automated summary

The study implemented full CO2 ice cloud microphysics into the LMD Mars Global Climate Model and conducted global simulations. The model results compared well with observations, reproducing the seasonal and latitudinal variability of CO2 ice clouds. The study also quantified the contribution of these clouds to surface CO2 ice deposits.