Simulating optical top-of-atmosphere radiance satellite images over snow-covered rugged terrain

Maxim Lamare, Marie Dumont, Ghislain Picard, Fanny Larue, François Tuzet, Clément Delcourt, Laurent Arnaud

Research output: Contribution to JournalArticleAcademicpeer-review

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.
Original languageEnglish
Article number3995
Pages (from-to)3995-4020
JournalThe Cryosphere
Volume14
Issue number11
DOIs
Publication statusPublished - 14 Nov 2020

Funding

Financial support. This research has been supported by the Agence Nationale de la Recherche (grant no. EBONI ANR-16-CE01-006), the Centre National d’Etudes Spatiales (APR CNES MIOSOTIS grant), and the Fondation BNP Paribas (APT project). Acknowledgements. CNRM CEN and IGE are part of Labex OSUG@2020 (investissement d’avenir – ANR10 LABX56). This work has been carried out using the resources made available by the Copernicus Research and User Support (RUS) service (http: //rus-copernicus.eu, last access: 12 November 2020), funded by the European Commission, managed by the European Space Agency, and operated by CS SI and its partners. The authors are grateful to Lautaret staff and staff at Station Alpine Joseph Fourier (SAJF) for instrument maintenance and for supporting the in situ experiments.

FundersFunder number
APR CNES
Agence
European Commission
European Space Agency
Agence Nationale de la RechercheEBONI ANR-16-CE01-006
Centre National d’Etudes Spatiales
Fondation BNP Paribas

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