Abstract
A re-analysis is presented here of a 10-year mass balance series at Findelengletscher, a temperate mountain glacier in Switzerland. Calculating glacier-wide mass balance from the set of glaciological point balance observations using conventional approaches, such as the profile or contour method, resulted in significant deviations from the reference value given by the geodetic mass change over a five-year period. This is attributed to the sparsity of observations at high elevations and to the inability of the evaluation schemes to adequately estimate accumulation in unmeasured areas. However, measurements of winter mass balance were available for large parts of the study period from snow probings and density pits. Complementary surveys by helicopter-borne ground-penetrating radar (GPR) were conducted in three consecutive years. The complete set of seasonal observations was assimilated using a distributed mass balance model. This model-based extrapolation revealed a substantial mass loss at Findelengletscher of -0.43m w.e. a^-1 between 2004 and 2014, while the loss was less pronounced for its former tributary, Adlergletscher (-0.30m w.e. a^-1). For both glaciers, the resulting time series were within the uncertainty bounds of the geodetic mass change. We show that the model benefited strongly from the ability to integrate seasonal observations. If no winter mass balance measurements were available and snow cover was represented by a linear precipitation gradient, the geodetic mass balance was not matched. If winter balance measurements by snow probings and snow density pits were taken into account, the model performance was substantially improved but still showed a significant bias relative to the geodetic mass change. Thus the excellent agreement of the model-based extrapolation with the geodetic mass change was owed to an adequate representation of winter accumulation distribution by means of extensive GPR measurements.
Highlights
Glacier mass balance is being monitored around the world to investigate and understand glacier response to climatic change (WGMS, 2013; Zemp et al, 2015)
We suggest that a correct reproduction of glacier-wide surface mass balance requires an evaluation scheme to resolve the accumulation distribution
We demonstrate that integrating snow depth measurements can dramatically improve the quality of the calculated glacier-wide surface mass balance
Summary
Glacier mass balance is being monitored around the world to investigate and understand glacier response to climatic change (WGMS, 2013; Zemp et al, 2015). When surveyed at the end of the hydrological year, ablation can be derived from measurements at stakes while snow pits provide accumulation (Østrem and Brugman, 1991) Extrapolating these point observations by means of the contour line method (Østrem and Brugman, 1991) or the profile method (Escher-Vetter et al, 2009), allows glacier-wide mass changes to be calculated in a straightforward manner. These approaches require a dense spatial coverage of observations that can only be achieved through maintenance of extensive networks of stakes and snow pits. This can be achieved through modeling approaches that explicitly compute ablation and accumulation and allow for a process-based integration of measurements (Hock and Holmgren, 2005; Machguth et al, 2006b; Reijmer and Hock, 2008; Huss et al, 2009; van Pelt and Kohler, 2015)
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