Christophe Ogier

Last Name

Ogier

First Name

Christophe

ORCID

0000-0002-5526-6071

Organisational unit

09599 - Farinotti, Daniel / Farinotti, Daniel

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Publications 1 - 10 of 11

Assimilating near-real-time mass balance stake readings into a model ensemble using a particle filter
Landmann, Johannes Marian; Künsch, Hans Rudolf; Huss, Matthias; et al. (2021)
The Cryosphere
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 20 min. 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 0.12 m water equivalent per day (mw.e.d−1) and of more than 5 mw.e. in 81 d. 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 0.24 mw.e.
AIRETH 2.0 – a revamped helicopter-borne GPR for glaciological applications
Farinotti, Daniel; Moser, Raphael; Anhorn, Barthelemy; et al. (2024)
EGUsphere
Towards a better understanding of uncommon glacier outburst floods
Ogier, Christophe (2024)
Exploring englacial hydrology with surface nuclear magnetic resonance
Gabriel, Laura; Hertrich, Marian; Moser, Raphael; et al. (2024)
EGUsphere
The amount and distribution of liquid water inside a glacier are relevant for its dynamics, related natural hazards or for sediment transport. Experimentally investigating the glacier's hydrology is challenging because of restricted accessibility, investigation depth, material properties, and environmental factors. In addition, the subglacial drainage network is highly dynamic and undergoes diurnal and seasonal changes. This contribution investigates the application of surface nuclear magnetic resonance (SNMR) to characterize the liquid water distribution within Swiss Alpine glaciers. Analogous to magnetic resonance imaging (MRI) in medicine, SNMR utilizes an oscillating magnetic field to excite nuclear spins of hydrogen atoms within water molecules. The subsequent spin relaxation is then analyzed, providing insights into the probed material. In simpler terms, this process allows us to directly detect liquid water in ice and gain information on its spatial distribution. We conducted a first SNMR field survey on Rhonegletscher in the summer of 2023. During this survey, we tested various measurement configurations, including separate-loop measurements and the application of noise-compensation loops. The latter proved crucial for subsequent data processing. After carefully optimizing the processing scheme, we extracted SNMR signals in several recordings despite the poor signal-to-noise ratio. The results were compared to 1D forward-modelled data, suggesting that the average water content in the survey area lay between 0.7 and 1.2 %. In addition, we could show that a homogenous water distribution over the entire ice column cannot explain the observed data and that we need to consider more complex subsurface models including at least one additional water layer. Specifically, our ongoing research aims to identify which configurations of the subglacial water distribution (e.g., homogenous water distribution vs layered water-ice structure resulting from an englacial water channel) are distinguishable experimentally. Moreover, the study seeks to optimize measurement design and data processing methodologies to acquire information more efficiently, and effectively handle the expected low signal-to-noise ratios. In future field campaigns, we intend to deploy SNMR for selected glaciological case studies within the Swiss Alps. A primary focus will be on efficiently detecting water pockets that may pose a potential risk of downstream flooding upon rupture. Similarly, we want to investigate the extent to which we can distinguish cold from temperate ice.
Definition, formation and rupture mechanisms of water pockets in alpine glaciers: Insights from an updated inventory for the Swiss Alps
Ogier, Christophe; Fischer, Mauro; Werder, Mauro; et al. (2025)
Journal of Glaciology
The term 'water pocket' describes invisible en- and subglacial water reservoirs that can cause sudden glacial outburst floods. However, there is currently no consensus on its definition and the formation and rupture mechanisms of water pockets remain poorly understood. This study aims to understand the mechanisms behind water pocket outburst floods (WPOFs) from alpine glaciers by analyzing their spatial and temporal distribution, pre-event meteorological conditions and the glacio-geomorphic features of the glaciers from which the floods originate. To this end, we updated an inventory of known WPOFs in the Swiss Alps to 91 events from 37 individual glaciers. Most WPOFs occurred between June and September, likely linked to meltwater input. Meteorological data indicate anomalously high temperatures during the days preceding most events and heavy precipitation on 25% of days for which WPOFs occur, indicating that water pockets typically rupture during periods of high water input. We propose four mechanisms of water pocket formation: temporary subglacial channel blockage (which is the mechanism suggested most often for our inventory), hydraulic barriers, water-filled crevasses and accumulation of liquid water behind barriers of cold ice (thermal barriers). Overall, our analysis highlights the challenge of understanding WPOFs due to the subsurface nature of water pockets, emphasizing the need for field-based research to improve their detection and monitoring.
Subglacial cavity collapses on Swiss glaciers: Spatiotemporal distribution and mass loss contribution
Hösli, Leo; Ogier, Christophe; Bauder, Andreas; et al. (2025)
Journal of Glaciology
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 $19.8\times 10<^>6\,\text{m}<^>3$ 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 d-1 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.
Drainage of an ice-dammed lake through a supraglacial stream: hydraulics and thermodynamics
Ogier, Christophe; Werder, Mauro; Huss, Matthias; et al. (2021)
The Cryosphere
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.
Ground penetrating radar in temperate ice: englacial water inclusions as limiting factor for data interpretation
Ogier, Christophe; van Manen, Dirk-Jan; Maurer, Hansruedi; et al. (2023)
Journal of Glaciology
Exploring englacial hydrology with surface nuclear magnetic resonance
Gabriel, Laura; Hertrich, Marian; Ogier, Christophe; et al. (2023)
Abstract Volume 21st Swiss Geoscience Meeting
Stick-slip imaging through the GPR phase: Turning temperate ice 'noise' into signal
Aichele, Johannes; Ogier, Christophe; Anhorn, Barthélémy (2024)
EGUsphere
Ground Penetrating Radar (GPR) is a major tool to investigate, map and monitor polar ice sheets and alpine glaciers. Alpine glaciers are often composed of temperate ice, which has significantly different backscatter properties from cold ice. Radar attenuation is much stronger in temperate ice than in cold ice, because the radar signal encounters strong scattering in temperate ice. A major candidate for this scattering is the presence of liquid water inclusions, which are much smaller than the radar wavelength. The large contrast between water and ice dielectric permittivity would explain the diffuse radar scattering in temperate ice. Indeed, recent numerical modelling of the radar signal in temperate ice confirmed the contribution of liquid water inclusions on the scattering of the radar signal (Ogier, 2023). Here, we investigate if the strong scattering caused by liquid water inclusions, which is usually treated as noise, can be in fact exploited to unravel dynamic processes inside the glacier. This strong scattering results in large radar phase variations in space, which remain constant over short timescales (hours - days), during which the glacial water content remains constant. During that timescale, however, the mountain glacier might experience sudden internal deformation due to intermittent sliding at the glacier base, also called glacier stick-slip. This deformation might be resolved using difference imaging and the spatio-temporal properties of the radar phase. We numerically model radar wave propagation throughout temperate ice (i.e. with the presence of liquid water inclusions) before and after an idealized glacier deformation and show, that through phase difference imaging the internal movement of the sub-wavelength scatterers can be mapped. Finally, we discuss how this novel type of monitoring could be applied in the field, which is planned for spring 2024.

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Christophe Ogier

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