Abstract. The monitoring of snow-covered surfaces on Earth is largely facilitated by the wealth of satellite data available, with increasing spatial
resolution and temporal coverage over the last few years. Yet to date, retrievals of snow physical properties still remain complicated in mountainous
areas, owing to the complex interactions of solar radiation with terrain features such as multiple scattering between slopes, exacerbated over
bright surfaces. Existing physically based models of solar radiation across rough scenes are either too complex and resource-demanding for the
implementation of systematic satellite image processing, not designed for highly reflective surfaces such as snow, or tied to a specific satellite
sensor. This study proposes a new formulation, combining a forward model of solar radiation over rugged terrain with dedicated snow optics into a
flexible multi-sensor tool that bridges a gap in the optical remote sensing of snow-covered surfaces in mountainous regions. The model presented here allows one to perform rapid calculations over large snow-covered areas. Good results are obtained even for extreme cases, such as steep shadowed
slopes or, on the contrary, strongly illuminated sun-facing slopes. Simulations of Sentinel-3 OLCI (Ocean and Land Colour Instrument) scenes performed over a mountainous region in the
French Alps allow us to reduce the bias by up to a factor of 6 in the visible wavelengths compared to methods that account for slope inclination
only. Furthermore, the study underlines the contribution of the individual fluxes to the total top-of-atmosphere radiance, highlighting the
importance of reflected radiation from surrounding slopes which, in midwinter after a recent snowfall (13 February 2018), accounts on average for
7 % of the signal at 400 nm and 16 % at 1020 nm (on 13 February 2018), as well as of coupled diffuse radiation scattered by the
neighbourhood, which contributes to 18 % at 400 nm and 4 % at 1020 nm. Given the importance of these contributions,
accounting for slopes and reflected radiation between terrain features is a requirement for improving the accuracy of satellite retrievals of snow
properties over snow-covered rugged terrain. The forward formulation presented here is the first step towards this goal, paving the way for future
retrievals.
Simulating Optical Top-Of-Atmosphere Radiance Satellite Images over Snow-Covered Rugged Terrain