Imaging spectroscopy to assess the composition of ice surface materials and their impact on glacier mass balance

Abstract

Glacier surfaces are not only composed of ice or snow but are heterogeneous mixtures of different materials. The occurrence and dynamics of light-absorbing impurities affect ice surface characteristics and strongly influence glacier melt processes. However, our understanding of the spatial distribution of impurities and their impact on ice surface characteristics and the glacier's energy budget is still limited. We use imaging spectroscopy in combination with in-situ experiments to assess the composition of ice surface materials and their respective impact on surface albedo and glacier melt rates. Spectroscopy data were acquired in August 2013 using the Airborne Prism EXperiment (APEX) imaging spectrometer and were used to map the abundances of six predominant surface materials on Glacier de la Plaine Morte, Swiss Alps. A pixel-based classification revealed that about 10% of the ice surface is covered with snow, water or debris. The remaining 90% of the surface can be divided into three types of glacier ice, namely~7% dirty ice,~43% pure ice and~39% bright ice. Spatially distributed spectral albedo derived from APEX reflectance data in combination with in-situ multi-angular spectroscopic measurements was used to analyse albedo patterns present on the glacier surface. About 85% of all pixels exhibit a low albedo between 0.1 and 0.4 (mean albedo 0.29 ± 0.12), indicating that Glacier de la Plaine Morte is covered with a significant amount of light-absorbing impurities, resulting in a strong ice-albedo feedback during the ablation season. Using a pixel-based albedo map instead of a constant albedo for ice (0.34) as input for a mass balance model revealed that the glacier-wide total ablation remained similar (10% difference). However, the large local variations in mass balance can only be reproduced using the pixel-based albedo derived from APEX, emphasizing the need to quantify spatial albedo differences as an important input for glacier mass balance models.

Reconstructing the mass balance of Brewster Glacier, New Zealand, using MODIS-derived glacier-wide albedo

“…The changing albedo of glaciers and ice sheets is currently a topic of huge interest, with a particular focus on the Greenland Ice Sheet (Mernild et al, 2014;Dumont et al, 2014) and western European glaciers (Gabbi et al, 2015;Naegeli et al, 2015). Although the present study did not explore trends in the albedo sequence in detail (e.g.…”

Mass balance observations and reconstruction for Batysh Sook Glacier, Tien Shan, from 2004 to 2016

“…In fact, the overall mass change is determined mainly by the energy balance at the glacier surface, which is responsible for the direct reaction of the glacier to a certain change in climate. Therefore, the different reactions of glaciers in close vicinity are often related to other influencing factors such as: a) their different hypsometry (Kuhn et al, 1985), b) the influence of changing shortwave net radiation, which is controlled by the diverse characteristics of the albedo related to dirt or debris materials on the glacier surface, and varying fresh snowfalls during the summer months (Fujita and Ageta, 2000;Fujita, 2008aFujita, , 2008bLi et al, 2011;Naegeli et al, 2015), or c) a strong increase in the freezing level as discussed for Urumqihe Glacier No. 1 by Wang et al (2014).…”

Cross-Comparison of Albedo Products for Glacier Surfaces Derived from Airborne and Satellite (Sentinel-2 and Landsat 8) Optical Data

“…Orthorectified, atmospherically and topographically corrected HemisphericalConical-Reflectance-Factor (HCRF) data were obtained from the at-sensor radiances using the PARGE (Parametric Geocorrection) [43] and the ATCOR-4 software [44]. For more information about the processing of the APEX datasets used in this study, refer to [31]. …”

“…Imaging spectroscopy provides a quasi-continuous spectral and high spatial sampling that is optimal to improve the knowledge of processes acting at the ice-atmosphere interface and to derive glacier surface albedo [31]. This data is usually only available on-demand and thus rather cost-and time-expensive.…”

“…We deliberately excluded the MODIS glacier albedo products because of their coarse ground-sampling distance of 500 m, which is not suitable for mountain glaciers in steep terrain. We apply the albedo retrieval approach as developed for the Airborne Prism EXperiment (APEX) imaging spectrometer, including a Bidirectional Reflectance Distribution Function (BRDF) correction scheme for material-specific anisotropy characteristics, as benchmark algorithm to the aforementioned satellite data [31]. Our particular research objectives are fourfold: (i) assessing the impact of spectral and spatial sampling resolution with use of systematically resampled Landsat 8 and Sentinel-2 data based on APEX; (ii) evaluating the impact of environmental factors (snow fall, temperature) on albedo products; (iii) comparing albedo maps derived from the benchmark APEX algorithm with products derived from published narrow-to-broadband conversion formulae by Knap et al [29] and Liang [30]; and (iv) quantifying anisotropy effects on glacier surface albedo estimates.…”