Investigating temporal changes in the Alpine firn density and compaction rate using repeat ground penetrating radar measurements at the accumulation areas of the Grosser Aletschgletscher

We present an application of ground-penetrating radar-based (GPR) common midpoint method (CMP) to quantify temporal changes in firn density and compaction rates, complemented by direct observations, such as firn cores at the accumulation area of the Grosser Aletschgletscher. We identify the last summer horizon and characterise the firn stratigraphy using firn core and isotope analysis. The comparison of the acquired firn core and the CMP-derived density-depth profile from the Ewigscheefeld shows similar density-depth variations. Our three CMP gather results illustrate the spatially varied depth to the pore close-off density (830 kg/m³), which is approximately 25 and 17 m at Ewigschneefeld and Jungfraufirn, respectively,  depicting the spatial variation in firn densification. Further, we identified 10-15 annual layers from the CMP-derived internal reflection horizons (IRHs) by comparing estimated snow water equivalent (SWE) with point mass-balance measurements. Temporal changes in firn density-depth profiles obtained from CMP data measured a year apart illustrate that certain identified annual layers at shallower depths are denser than deeper layers (100-150 kg/m³). Our results demonstrate that the influence of summer melts is a dominating process on Alpine firn densification, rather than the conventional densification driven by accumulated snow. We investigated the temporal changes in spatial firn stratigraphy from a 4.4 km long GPR profile by comparing it with a previously measured GPR transect from the same location. Our investigation exemplifies the possibility of quantifying firn densification and compaction rates using unique temporal GPR measurements in an Alpine glacier.