Modeling Slope Microclimates in the Mars Planetary Climate Model

Abstract

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, that is, 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 CO2 and H2O cycles on a global scale. Furthermore, short‐scale slopes (i.e., less than ∼1 km in length) do not significantly impact the thermal state of the atmosphere. Ninety‐one percent of the active gullies are found where our model predicts CO2 frost, suggesting that their activity is related to processes involving CO2 ice. However, the low thicknesses (≤tens of cm) predicted at mid‐latitudes rule out mechanisms involving large amounts (∼meters) of ice. This model opens the way to new studies on surface‐atmosphere interactions in present and past climates.

A Reappraisal of Subtropical Subsurface Water Ice Stability on Mars

“…Our model was validated by comparison with surface temperatures measured on sloped terrain and seasonal variations in water frost formation (Lange et al., 2023a). It turned out that our model could overestimate certain temperatures by 2 K on average, and up to 5 K on certain poleward‐facing slopes, depending on local terrain properties (thermal inertia, slope angle, azimuth).…”

“…We have added a sub-grid slope parameterization to simulate slope microclimates. This parametrization is detailed and compared to observations in a companion paper (Lange et al, 2023a) that is summarized in Text S1 and illustrated in Figure S1 in Supporting Information S1. In short: for each PCM mesh, we decompose the cell as a distribution of sloped terrains (defined by characteristic slopes) and a flat terrain.…”

Dry Downhill Particle Motion on Mars

“…In particular, the rover measurements also give here a grain size of about 100 μm (Atwood-Stone & McEwen 2013). There are other types of features, e.g., slope streaks, where the question also arises as to what triggers them and sets the inclination of the slope (Aharonson et al 2003;Schorghofer & King 2011;Heyer et al 2020;Dundas 2021;Lange et al 2023). Slopes seem notably smaller here.…”

Turbulent Fluxes and Evaporation/Sublimation Rates on Earth, Mars, Titan, and Exoplanets

“…Like MarsWRF, the LMD GCM (Colaïtis et al, 2013;Forget et al, 1999;Millour et al, 2017) is also typically run at relatively coarse resolution, and uses an overall approach similar to our model. To help mitigate the spatial resolution issue, a 1-D version of the LMD GCM also exists (Spiga & Forget, 2009), and sub-grid slope parameterizations have been developed (Lange et al, 2023). However, there are crucial differences in the atmospheric stability functions used, and while our model explicitly includes key humidity effects and calculates interfacial transport coefficients that depend on the local aerodynamic flow conditions as described in Section 2.1, the LMD GCM does not (see Section S5 in Colaïtis et al ( 2013)).…”