Christophe Ogier

Glacier collapse features, linked to subglacial cavities, are increasingly common on retreating Alpine glaciers. These features are hypothesized to result from glacier downwasting and subsurface ablation processes but the understanding regarding their distribution, formation, and contribution to glacier mass loss remains limited. We present a Swiss-wide inventory of 223 collapse features observed over the past 50 years, revealing a sharp increase in their occurrence since the early 2000s. Using high-resolution digital elevation models, we derive a relationship between collapse feature area and ice ablation, and estimate the Swiss-wide contribution of collapse features to glacier mass loss to be 19.8 × 10⁶ m³ of ice between 1971 and 2023. Based on extensive observations at Rhonegletscher, including surface displacement, ground penetrating radar and drone-based elevation models, we quantify subsurface ablation rates of up to 27 cm per day and provide a detailed description of the collapse processes. We propose that glacier downwasting, enhanced energy supply through subglacial conduits, and locally increased basal melt are key components to subglacial cavity growth. Our results highlight the importance of collapse features in the ongoing retreat of Alpine glaciers, stressing the need for further research to understand their formation and long-term implications for glacier dynamics under climate change.

Ground penetrating radar (GPR) has been extensively used in glaciology to infer glacier's ice thickness, liquid water content, water drainage pathways, and other properties. The interpretation of such GPR data is not always straightforward and for temperate glaciers, the signal is often affected by strong scattering and attenuation. It has often been suggested that such effects originate from englacial water inclusions, since water and ice have a large contrast in their di-electric permittivity. To investigate such effects quantitatively, we perform an extensive numerical modeling study of GPR signals. By exploring how different liquid water contents (LWC) and water-inclusions size affect the GPR signal, we show that their effects are much larger than the potential presence of a wet snowpack or a heterogeneous distribution of ice permittivity. In particularly, we show that the presence of such water inclusions is a necessary and sufficient condition for reproducing the typical characteristics of GPR data acquired in the field. Further, we find that for 25 MHz GPR antennas, a bulk LWC ≳ 0.2%, associated with decimeters-scale water inclusions already limits bedrock detectability for ice thicknesses ≳ 100 m. Since these values are typical for Alpine glaciers, they clarify why the quality of GPR data is often poor in such environments.

Short-term glacier variations can be important for water supplies or hydropower production, and glaciers are important indicators of climate change. This is why the interest in near-real-time mass balance nowcasting is considerable. Here, we address this interest and provide an evaluation of continuous observations of point mass balance based on online cameras transmitting images every 20min. The cameras were installed on three Swiss glaciers during summer 2019, provided 352 near-real-time point mass balances in total, and revealed melt rates of up to 0.12m water equivalent per day (mw.e.d-1) and of more than 5mw.e. in 81d. By means of a particle filter, these observations are assimilated into an ensemble of three TI (temperature index) and one simplified energy-balance mass balance models. State augmentation with model parameters is used to assign temporally varying weights to individual models. We analyze model performance over the observation period and find that the probability for a given model to be preferred by our procedure is 39% for an enhanced TI model, 24% for a simple TI model, 23%, for a simplified energy balance model, and 14% for a model employing both air temperature and potential solar irradiation. When compared to reference forecasts produced with both mean model parameters and parameters tuned on single mass balance observations, the particle filter performs about equally well on the daily scale but outperforms predictions of cumulative mass balance by 95%-96%. A leave-one-out cross-validation on the individual glaciers shows that the particle filter is also able to reproduce point observations at locations not used for model calibration. Indeed, the predicted mass balances is always within 9% of the observations. A comparison with glacier-wide annual mass balances involving additional measurements distributed over the entire glacier mostly shows very good agreement, with deviations of 0.02, 0.07, and 0.24mw.e.

The glacier-dammed Lac des Faverges, located on Glacier de la Plaine Morte (Swiss Alps), has drained annually as a glacier lake outburst flood since 2011. In 2018, the lake volume reached more than 2 × 10⁶ m³, and the resulting flood caused damage to the infrastructure downstream. In 2019, a supraglacial channel was dug to artificially initiate a surface lake drainage, thus limiting the lake water volume and the corresponding hazard. The peak in lake discharge was successfully reduced by over 90 % compared to 2018. We conducted extensive field measurements of the lake-channel system during the 48 d drainage event of 2019 to characterize its hydraulics and thermodynamics. The derived Darcy–Weisbach friction factor, which characterizes the water flow resistance in the channel, ranges from 0.17 to 0.48. This broad range emphasizes the factor's variability and questions the choice of a constant friction factor in glacio-hydrological models. For the Nusselt number, which relates the channel-wall melt to the water temperature, we show that the classic, empirical Dittus–Boelter equation with the standard coefficients does not adequately represent our measurements, and we propose a suitable pair of coefficients to fit our observations. This hints at the need to continue research into how heat transfer at the ice–water interface is described in the context of glacial hydraulics.