Ablation and dynamics of four ice cliffs on the partially debris-covered glacier tongue of Kanderfirn, Swiss Alps

Debris-covered glaciers influence the regional hydrological cycle by modulating glacier melt processes. One important control on melt variability on debris-covered glaciers are ice cliffs, which have been identified as melt hotspots, exhibiting ablation rates that far exceed those of the surrounding debris-covered area. As a result, they contribute disproportionately to total glacier mass loss. However, their dynamics and contribution to overall ablation have been quantified for only a few glaciers, mainly in the Himalaya. Quantifying and parameterizing ice cliff dynamics, including modelling frameworks, is needed to reliably project the future evolution of debris-covered glaciers in different mountain regions, as well as their water supply.

The aim of this study is to quantify ice cliff melt and backwasting rates in alpine settings, to generate a reference dataset for ice cliff model applications and to assess their relative contribution to total ablation. Ablation and dynamics of four ice cliffs were measured between 26 August and 19 September 2025 at the Kanderfirn. The Kanderfirn, a valley glacier in the Swiss Alps, was selected for this study as the tongue comprises both debris-covered and debris-free areas and, thus, enables the study of different ablation processes at the same site. Four ice cliffs representing the four cardinal orientations were selected to assess differences in melt and backwasting rates related to the ice cliff orientation, which are discussed in the literature. In total, eight stakes were installed at the four ice cliffs, with one stake drilled into the bare ice face of each cliff and a corresponding stake placed in the debris-covered area immediately above each cliff, to quantify the vertical, tangential and sub-debris melt rates as well as horizontal backwasting rates. Eleven additional stakes were installed at sites with varying debris thickness. Moreover, repeated UAV surveys were carried out to generate digital elevation models for the investigation of the geometric evolution of the ice cliffs over a one-month period. Finally, a surface energy-balance model was applied to model ice cliff and sub-debris ablation using the UAV-based digital elevation model and meteorological data from on- and off-glacier weather stations.

The in-situ results show clear contrasts in melt rates between ice cliff faces and the surrounding debris-covered area, as well as variability among ice cliffs with different orientations, including differences in backwasting rates. Two separate estimates of apparent vertical melt rate are derived from tangential melt measurements as well as from backwasting and sub-debris melt, combined with the slope angle. The close agreement between both results indicates consistent field measurements. The measurements and modelling provide valuable insights into the ablation and dynamics of ice cliffs on an Alpine glacier.