Estimating ice albedo from fine debris cover quantified by a semi-automatic method: the case study of Forni Glacier, Italian Alps

Abstract

Abstract. In spite of the quite abundant literature focusing on fine debris deposition over glacier accumulation areas, less attention has been paid to the glacier melting surface. Accordingly, we proposed a novel method based on semiautomatic image analysis to estimate ice albedo from fine debris coverage (d). Our procedure was tested on the surface of a wide Alpine valley glacier (the Forni Glacier, Italy), in summer 2011, 2012 and 2013, acquiring parallel data sets of in situ measurements of ice albedo and high-resolution surface images. Analysis of 51 images yielded d values ranging from 0.01 to 0.63 and albedo was found to vary from 0.06 to 0.32. The estimated d values are in a linear relation with the natural logarithm of measured ice albedo (R = −0.84). The robustness of our approach in evaluating d was analyzed through five sensitivity tests, and we found that it is largely replicable. On the Forni Glacier, we also quantified a mean debris coverage rate (C r ) equal to 6 g m −2 per day during the ablation season of 2013, thus supporting previous studies that describe ongoing darkening phenomena at Alpine debris-free glaciers surface. In addition to debris coverage, we also considered the impact of water (both from melt and rainfall) as a factor that tunes albedo: meltwater occurs during the central hours of the day, decreasing the albedo due to its lower reflectivity; instead, rainfall causes a subsequent mean daily albedo increase slightly higher than 20 %, although it is short-lasting (from 1 to 4 days).

Geomorphology of Mount Ararat/Ağri Daği (Ağri Daği Milli Parki, Eastern Anatolia, Turkey)

“…These glaciers are characterized by a thick (i.e. from our field investigations >20 cm) debris cover, which strongly influences surface melting and ice body evolution (Azzoni et al, 2016;Diolaiuti, D'Agata, Meazza, Zanutta, & Smiraglia, 2009;Østrem, 1959). Moreover, this thick supraglacial debris cover has historically prevented accurate mapping of the glacier boundary; in fact, without a field survey, on medium-resolution satellite imagery it is very difficult to detect the exact limits of the debris-covered tongues with respect to the rock debris nearby.…”

Glacier Facies Mapping Using a Machine-Learning Algorithm: The Parlung Zangbo Basin Case Study

“…Specifically, thick debris cover can slow the ice melting rates owing to the low thermal conductivity of debris, while thin debris cover can enhance the ablation rates of underlying ice as a result of the low albedo of debris [32][33][34]. Moreover, the occurrence of debris at the glacier surface is one of the most important factors driving albedo changes because it influences the features and evolution of glaciers [35]. It is necessary to estimate, therefore, the distribution of debris-covered glaciers to assess the effect of debris cover on ice ablation and to investigate the response of glaciers to climate forcing in terms of mass balance [32].Parlung Zangbo basin in the southeastern Tibetan Plateau is one of the critical regions in the world where a small amplitude of climate change may result in dramatic glacier variations [36].…”

Small scale spatial variability of bare-ice albedo at Jamtalferner, Austria

“…A number of studies attribute recent darkening of European glaciers to increased accumulation of mineral dust (e.g. Oerlemans 2 30 35 40 45 50 55 et al., , Azzoni et al, 2016 and black carbon (e.g. Painter et al, 2013, Gabbi et al, 2015.…”

“…Different methodological approaches have been used to address specific changes in the surface characteristics of the ablation zone as they relate to changes in albedo and energy absorption across the electromagnetic spectrum: Using both hyperspectral satellite data and in situ measurements, Di Mauro et al (2017) find that the presence of elemental and organic carbon leads to darkening of the ablation zone at Vadret da Morteratsch glacier (Switzerland) and discuss potential anthropogenic contributions. Azzoni et al (2016) use semi-automatic image analysis techniques on photos of the ice surface at Forni glacier (Italy) to quantify the amount of fine debris present on the surface and its effect on the albedo. They find an overall darkening due to increasing dust, as well as significant effects of melt water.…”