Détermination de l'albédo des surfaces enneigées par télédétection : application à la reconstruction du bilan de masse du glacier de Saint Sorlin

Abstract : Albedo is defined as the ratio of reflected to incident radiation over the solar spectrum and is a key parameter in the surface energy balance of snow and ice. This parameter is highly variable both temporally and spatially; thus remote sensing is an ideally suited approach for the retrieval of albedo data.The albedo value depends on both physical properties of the target and the characteristics of the incident radiation. Furthermore, the physical parameters linked with the albedo concept vary in consideration with spectral and angular ranges. Measurements of bi-directional reflectance over natural snow have shown that the Lambertian hypothesis may lead to significant error when estimating albedo from remote sensing data.Detailed knowledge of the angular distribution of radiation reflected by snow allows for the development of a new method to retrieve albedo values for mountainous, snow/ice covered areas. This method takes into account multiple reflections on mountainous areas, anisotropy of radiation reflected by snow and ice, and albedo spectral variations with surface physical properties. The method is applied to visible and near-infrared terrestrial photographs (spatial resolution 10 m) and MODIS data (spatial resolution 250 m). The accuracy of the method is evaluated at ±10% on the retrieved albedo value using concurrent field measurements at theSaint Sorlin Glacier (Grandes Rousses, France) during the summers of 2008 and 2009. The method is used to retrieve albedo data for this glacier from 2000 to 2009. Results indicate that the albedo of the ablation area of the Saint Sorlin Glacier has not shown any decreasing trend over this decade, in opposition to results presented for the Morteratsch Glacier (Switzerland). In addition, the minimal value over the summer period of the whole glacier averaged albedo is highly correlated to the specific annual mass-balance.Albedo data from MODIS and terrestrial photographs are then assimilated into the snow model CROCUS. This assimilation allows for an estimation of the spatialized mass-balance of the Saint Sorlin Glacier over the five studied hydrological years. Root mean square error is evaluated to 0.5 m w.e. For this study, we have used mid-scale meteorological data from SAFRAN. A brief analysis of the contribution from the atmospheric fluxes to the surface energy balance shows that, for the time period considered in this study, the shortwave radiation budget is the main process determining the surface energy balance. Furthermore, variability in shortwave radiation budget explains the major part of the daily variability in surface energy balance.The methods developed in this work are readily applicable to other temperate glaciers. They allows spatialized mass-balance reconstruction on a decadal scale and lead to improved quantification of the physical processes controlling mass-balance in temperate glaciers
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Marie Dumont. Détermination de l'albédo des surfaces enneigées par télédétection : application à la reconstruction du bilan de masse du glacier de Saint Sorlin. Sciences de la Terre. Université Paris-Est, 2010. Français. ⟨NNT : 2010PEST1059⟩. ⟨tel-00582925⟩

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