Abstract
Abstract. The amount of reflected energy by snow and ice plays a fundamental role in their melting processes. Different non-ice materials (carbonaceous particles, mineral dust (MD), microorganisms, algae, etc.) can decrease the reflectance of snow and ice promoting the melt. The object of this paper is to assess the capability of field and satellite (EO-1 Hyperion) hyperspectral data to characterize the impact of light-absorbing impurities (LAIs) on the surface reflectance of ice and snow of the Vadret da Morteratsch, a large valley glacier in the Swiss Alps. The spatial distribution of both narrow-band and broad-band indices derived from Hyperion was analyzed in relation to ice and snow impurities. In situ and laboratory reflectance spectra were acquired to characterize the optical properties of ice and cryoconite samples. The concentrations of elemental carbon (EC), organic carbon (OC) and levoglucosan were also determined to characterize the impurities found in cryoconite. Multi-wavelength absorbance spectra were measured to compare the optical properties of cryoconite samples and local moraine sediments. In situ reflectance spectra showed that the presence of impurities reduced ice reflectance in visible wavelengths by 80–90 %. Satellite data also showed the outcropping of dust during the melting season in the upper parts of the glacier, revealing that seasonal input of atmospheric dust can decrease the reflectance also in the accumulation zone of the glacier. The presence of EC and OC in cryoconite samples suggests a relevant role of carbonaceous and organic material in the darkening of the ablation zone. This darkening effect is added to that caused by fine debris from lateral moraines, which is assumed to represent a large fraction of cryoconite. Possible input of anthropogenic activity cannot be excluded and further research is needed to assess the role of human activities in the darkening process of glaciers observed in recent years.
Highlights
Mountain glaciers represent an important source of fresh water across the planet
Clean ice and dirty ice reflectance is almost flat, especially in visible wavelengths, whereas melt pond and wet cryoconite show a maximum at circa 560 nm
Results from field campaigns and satellite hyperspectral data show that impurities and cryoconite decrease ice spectral reflectance in visible wavelengths from 0.9 to 0.1 and 0.06 respectively, adding a consistent input of energy to the glacier radiative balance
Summary
Mountain glaciers represent an important source of fresh water across the planet. These resources are seriously threatened by global climate change (Immerzeel et al, 2010), and a widespread reduction of glacier extension has been observed in recent years (Oerlemans, 2005; Paul et al, 2004). Surface processes that promote glacier melting are driven by both climate (i.e., temperature and precipitation) and changes in albedo The latter is mainly influenced by the metamorphism of snow in the infrared part of the reflectance spectrum of snow and by the impurity content (such as dust, soot, ash, algae, etc.) in the visible domain (Painter et al, 2007). Di Mauro et al.: Impact of impurities and cryoconite on glaciers the cryosphere on a global (Flanner et al, 2007, 2009), regional (He et al, 2014; Lee et al, 2017; Painter et al, 2010; Sterle et al, 2013) and local scale (Oerlemans et al, 2009) suggest that LAI accumulation on snow and ice decreases the albedo, with consequences on the radiative and mass balances, both in mountain glaciers and ice sheets (Dumont et al, 2014; Gabbi et al, 2015; Wittmann et al, 2017)
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