Spatial distribution of surface albedo at the Forni Glacier (Stelvio National Park, Central Italian Alps)

Abstract

The albedo is a fundamental component of the surface energy balance of glaciers, determining the amount of net solar radiation available for melt. However, its temporal and spatial distribution on the glacier surface is often overlooked, and in melt models constant values of ice and snow albedo are assumed for the whole glacier, leading to inaccurate estimations of glacier melt. In this study, we retrieve the albedo on the surface of the Forni Glacier, Stelvio National Park, Central Italian Alps, using four satellite images from the Landsat 7 ETM+ sensors at 30m resolution, and investigate its spatial distribution during the ablation season between 2011 and 2013. The retrieval method includes correction steps for all processes that influence the relationship between the satellite signal and the albedo: i) radiometric calibration, ii) atmospheric correction using the 6S radiative transfer code, iii) correction for local topographic effects, iv) correction for the anisotropy of the reflected radiation over the hemisphere, and v) narrowband to broadband conversion. In spite of the uncertainties introduced by each of these steps (0.02, 0.01, 0.04, 0.05 and 0.01, respectively), the Landsat-retrieved albedo values have a very good correspondence with field measurements from the permanent Automatic Weather Station (AWS) located over the eastern part of the ablation tongue (±0.01 difference). While meteorological conditions, including snowfalls and rainfalls, exert a short lasting influence on the albedo, under conditions typical of the ablation season the albedo values for the ablation tongue are those of debris-rich ice, confirming the ongoing darkening phenomena previously observed in literature.