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.
Highlights
The snow surface is rarely flat and smooth on Earth
We have investigated the relative importance of several topographic effects in a mountainous terrain in the French Alps
520 For this, we have first developed a chain of models to predict surface temperature, by combining existing radiative transfer models (RSRT, SBDART) and a new surface energy budget model (RoughSEB)
Summary
The snow surface is rarely flat and smooth on Earth. Undulations exist over a very large range of scales. At the decametre to kilometre scale range, the snow surface topography is mostly determined by the underlying soil or ice topography (Revuelto et al, 2018). Because of all these undulations, the surface temperature can vary by several Celsius degrees across a study area, even without significant differences in the near-surface meteorological forcing (wind, air temperature, humidity). Even if the literature for the smaller scales – that of the ripples, dunes, sastrugi and penitents – is usually distinct and scarcer, the principles apply to all the scales because the radiative transfers between faces are invariant by scale change. The exception is over long distances when the 35 atmospheric scattering and absorption effects due to the air present between the terrain faces become significant (Lamare et al, 2020)
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