Patil et al. (2025) Investigating firn structure and density in the accumulation area of the Grosser Aletschgletscher using ground-penetrating radar

Identification

• Journal: ˜The œcryosphere
• Year: 2025
• Date: 2025-11-12
• Authors: Akash Patil, C. Mayer, Thorsten Seehaus, Alexander Raphael Groos, Andreas Bauder
• DOI: 10.5194/tc-19-5547-2025

Research Groups

• Institute of Geography, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
• Bavarian Academy of Sciences and Humanities, Geodesy and Glaciology, Munich, Germany
• Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, Switzerland
• Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Sion, Switzerland

Short Summary

This study investigates firn structure, density, and accumulation history in the Grosser Aletschgletscher using ground-penetrating radar (GPR) and glaciological methods, demonstrating the potential to enhance glacier mass balance estimations by providing spatially distributed firn properties and accumulation records over the past 10-14 years.

Objective

• To investigate firn structure, density distribution, and accumulation history in the Grosser Aletschgletscher accumulation area using ground-penetrating radar (GPR) and glaciological methods.
• To develop and apply a novel iterative method for identifying annual firn layers from GPR-derived internal reflection horizons (IRHs) by comparing with long-term point mass balance measurements.
• To test and calibrate firn compaction models (LIG and KM) against GPR-derived firn density profiles under regional Alpine climatic conditions.

Study Configuration

• Spatial Scale: Grosser Aletschgletscher accumulation area (Swiss Alps), including Jungfraufirn, Mönchsjoch, and Ewigschneefeld. A 1.8 km GPR transect was measured on Ewigschneefeld. Elevations ranged from approximately 3380 m to 3600 m above sea level.
• Temporal Scale: Field data were acquired during February-March and May 2024. The study traced accumulation history over the past 10-14 years (2010-2023). Firn models were spun up for 100 years, then forced with data from 2005-2015 repeatedly, and subsequently used daily input data for simulations.

Methodology and Data

• Models used: Ligtenberg (LIG) firn compaction model, Kuipers Munnekee (KM) firn compaction model, Community Firn Model (CFM) framework.
• Data sources:
  • Geophysical data: Ground-penetrating radar (GPR) data including a 1.8 km transect (using IDS monostatic 200 and 600 MHz dual-frequency system) and three Common Mid-Point (CMP) profiles (using PulseEkko bi-static 500 MHz system).
  • Glaciological data: Snow pits (up to 4 m deep), snow cores (up to 5.8 m deep), a firn core (3.8 m deep), and isotope analysis (δ18O and δD samples) from these cores and pits.
  • Meteorological data: Daily mean temperature from Jungfraujoch weather station (3580 m a.s.l.) and daily mean precipitation data from Grimsel research station (1952 m a.s.l.), scaled to the study area.
  • Reference data: Long-term point mass balance measurements (Snow Water Equivalent, SWE) from GLAMOS (Glacier Monitoring Switzerland) at Jungfraufirn (3390 m a.s.l.).

Main Results

• GPR-based CMP analysis successfully derived 1-D firn density-depth profiles up to 37 m depth, showing densities ranging from 320 kg/m³ to 850 kg/m³ across different locations.
• Spatial variability in firn densification was observed, with the pore close-off density (830 kg/m³) reached at approximately 20 m depth at lower elevations (Jungfraufirn) and 30 m depth at higher elevations (Mönchsjoch, Ewigschneefeld).
• An iterative method, comparing GPR-derived SWE with long-term point mass balance data, successfully identified up to 15 annual firn layers, tracing the accumulation history from 2010 to 2023.
• The extreme summer melt of 2022 resulted in complete ablation of the annual firn layer at lower elevations (Jungfraufirn) but partial survival at higher elevations (Mönchsjoch, Ewigschneefeld).
• Untuned LIG and KM firn compaction models underestimated shallow firn density and predicted pore close-off depths beyond 45 m.
• Tuning model coefficients based on CMP-derived density profiles significantly improved model fit, shifting the depth to firn density (550 kg/m³) to 4-7 m and pore close-off density to approximately 40-42 m. Higher degree-day factors for snow (DDF snow) further reduced pore close-off depth to 34-37 m.
• The 1.8 km GPR transect revealed the spatial distribution of firn density (400-810 kg/m³) and accumulation, indicating higher firn density at shallower depths in lower elevation parts of the transect.

Contributions

• First application of GPR-based Common Mid-Point (CMP) measurements to derive firn density-depth profiles in Alpine glacier conditions, providing detailed 1-D density distributions without reliance on firn cores.
• Introduction of a novel iterative method to identify annual firn layers from GPR-derived Internal Reflection Horizons (IRHs) by validating Snow Water Equivalent (SWE) against long-term point mass balance measurements.
• Demonstration of the potential to calibrate community firn compaction models (LIG, KM) using GPR-derived density profiles, rather than solely firn core data, for improved representation of Alpine firn densification processes.
• Investigation of spatial firn density and accumulation distribution along a 1.8 km GPR transect, highlighting the impact of elevation and melt/refreezing events on firn stratigraphy and densification in temperate glaciers.
• Emphasizes the importance of integrating geophysical, glaciological, and modeling approaches to reduce uncertainties in geodetically derived glacier mass balance estimates.

Funding

• Elitenetzwerk Bayern (grant no. IDP M3OCCA)
• Deutsche Forschungsgemeinschaft (DFG) (grant number, SE 3091/5-1 awarded to Thorsten Seehaus)

Citation

@article{Patil2025Investigating,
  author = {Patil, Akash and Mayer, C. and Seehaus, Thorsten and Groos, Alexander Raphael and Bauder, Andreas},
  title = {Investigating firn structure and density in the accumulation area of the Grosser Aletschgletscher using ground-penetrating radar},
  journal = {˜The œcryosphere},
  year = {2025},
  doi = {10.5194/tc-19-5547-2025},
  url = {https://doi.org/10.5194/tc-19-5547-2025}
}