Investigating firn structure and density in the accumulation area of the Grosser Aletschgletscher using ground-penetrating radar

Abstract. The role of firn structure and density in geodetic glacier mass balance estimation has been constrained, with studies in alpine conditions primarily relying on models. Our research focuses on understanding the firn structures, density, and accumulation history in the Grosser Aletschgletscher accumulation area, using field methods mainly involving ground-penetrating radar (GPR) as a geophysical tool and glaciological methods such as snow pits, snow cores, firn cores, and isotope analysis. We characterise the firn structure and determine the spatial firn density–depth profiles by estimating electromagnetic wave velocities using the GPR-based common mid-point (CMP) method. This is done by identifying reflection hyperbolae using semblance analysis of the CMP data set. Three density–depth profiles, up to 37 m depth, were obtained at various locations within the glacier accumulation area. The Ligtenberg (LIG) and Kuipers Munnekee (KM) firn compaction models were selected from the community firn models (CFMs) to evaluate how well the model results matched the observations. These models were predominantly adjusted to fit the estimated 1-D firn density profiles from CMP measurements by optimising model coefficients based on regional Alpine climatic conditions, rather than the conventional method of tuning to the firn core density profiles. Further, a method is introduced to estimate accumulation history by chronologically identifying GPR-derived internal reflection horizons (IRHs) as annual firn layers, by comparing the estimated snow water equivalent (SWE) within each IRH to SWE from long-term point mass balance measurements available at the accumulation area of the glacier. We investigated the spatial distribution of the firn density and the glacier's accumulation history over the past 10–14 years (2010–2023) using a 1.8 km GPR transect, supported by CMP-derived density–depth profiles. Furthermore, our findings emphasise the importance of direct measurements, such as snow cores, firn cores, and isotope samples, in identifying the previous end-of-summer horizon. In this study, we demonstrate the potential of integrating GPR, direct measurements, and firn compaction models to monitor firn structures and density, ultimately enhancing glacier mass balance estimation in future research.