Modelling Slope Microclimates in the Mars Planetary Climate Model

A large number of surface features (e.g., frost, gullies, slope streaks, recurring slope lineae) are observed on Martian slopes. Their activity is often associated with the specific microclimates on these slopes, which have been mostly studied with one-dimensional radiative balance models to date. We develop here a parameterization to simulate these microclimates in 3D Global Climate Models. We first demonstrate that any Martian slope can be thermally represented by a poleward or equatorward slope, i.e., the daily average, minimum, and maximum surface temperatures depend on the North-South component of the slope. Based on this observation, we implement here a subgrid-scale parameterization to represent slope microclimates (radiative fluxes, volatile condensation, ignoring slope winds for now) in the Mars Planetary Climate Model and validate it through comparisons with surface temperature measurements and frost detections on sloped terrains. With this new model, we show that slope microclimates do not have a significant impact on the seasonal CO$_2$ and H$_2$O cycles. Furthermore, short-scale slopes do not significantly impact the thermal state of the atmosphere. 91\% of the active gullies are found where our model predicts CO$_2$ frost, suggesting that their activity is related to processes involving CO$_2$ ice. However, the low thicknesses ($\leq$~tens of cm) predicted at mid-latitudes there rule out mechanisms involving large amounts ($\sim$ meters) of ice. This model opens the way to new studies on surface-atmosphere interactions in present and past climates.