Preprints
https://doi.org/10.5194/tc-2021-180
https://doi.org/10.5194/tc-2021-180

  30 Jun 2021

30 Jun 2021

Review status: this preprint is currently under review for the journal TC.

Modelling surface temperature and radiation budget of snow-covered complex terrain

Alvaro Robledano1,2, Ghislain Picard1, Laurent Arnaud1, Fanny Larue1, and Inès Ollivier1 Alvaro Robledano et al.
  • 1Univ. Grenoble Alpes, CNRS, Institut des Géosciences de l’Environnement (IGE), UMR 5001, Grenoble, F-38041, France
  • 2Univ. Grenoble Alpes, Université de Toulouse, Météo-France, CNRS, CNRM, Centre d’Etudes de la Neige, 38000 Grenoble, France

Abstract. The surface temperature controls the temporal evolution of the snowpack playing a key role in many physical processes such as metamorphism, snowmelt, etc. It shows large spatial variations in mountainous areas because the surface energy budget is affected by specific radiative processes that occur due to the topography, such as the modulation of the irradiance by the local slope, the shadows and the re-illumination of the surface from surrounding slopes. These topographic effects are often neglected in large scale models considering the surface as flat and smooth. Here we aim at estimating the surface temperature and the energy budget of snow-covered complex terrains, in order to evaluate the relative importance of the different processes that control the spatial variations. For this, a modelling chain is implemented to derive surface temperature in a kilometre-wide area from local radiometric and meteorological measurements at a single station. The main component is the Rough Surface Ray-Tracing (RSRT) model, based on a photon transport Monte Carlo algorithm to quantify the incident and reflected radiation on every facet of a mesh, describing the snow-covered surface. RSRT is coupled to a surface scheme in order to estimate the complete energy budget from which the surface temperature is solved. To assess the modelling chain performance, we use in situ measurements of surface temperature and satellite thermal observations (TIRS sensor aboard Landsat-8) in the Col du Lautaret area, in the French Alps. The satellite images are corrected from atmospheric effects with a single-channel algorithm. The results of the simulation show (i) an agreement between the simulated and observed surface temperature at the station for a diurnal cycle in winter within 0.3 °C; (ii) the spatial variations of surface temperature are on the order of 5 to 10 °C between opposed slope orientations and are well represented by the model; (iii) the importance of the considered topographic effects is up to 1 °C, the most important being the modulation of solar irradiance by the topography, followed by altitudinal variations in air temperature, long-wave thermal emission from surrounding terrain, spectral dependence of snow albedo, and absorption enhancement due to multiple bounces of photons in steep terrain. These results show the necessity of considering the topography to correctly assess the energy budget and the surface temperature of snow-covered complex terrain.

Alvaro Robledano et al.

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Alvaro Robledano et al.

Alvaro Robledano et al.

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Short summary
Topography controls the surface temperature of snow-covered, mountainous areas. We developed a modelling chain that uses ray-tracing methods to quantify the impact of a few topographic effects on snow surface temperature at high spatial resolution. Its large spatial and temporal variations are correctly simulated over a 50 km2 area in the French Alps, and our results show that excluding a single topographic effect results in cooling (or warming) effects on the order of 1 °C.