CR4.4

Water flowing under ice sheets and glaciers can have a strong influence on ice dynamics, particularly through pressure changes, suggesting that a comprehensive ice-sheet model should include the effect of basal hydrology. Modeling subglacial hydrology remains a challenge however, mainly due to the range of spatial and temporal scales involved - from subglacial channels to vast subglacial lakes. Additionally, subglacial drainage networks dynamically evolve over time. To address some of these challenges, we have developed an Adaptive Mesh Refinement (AMR) model based on the Chombo software framework. We extend the model proposed by Sommers and others, (2018) with a few changes to accommodate the transition from unresolved to resolved elements. We handle the strong non-linearities present in the equations by resorting to an efficient non-linear Full Approximation Scheme (FAS-MG) algorithm.  We outline the details of the algorithm and present convergence analysis results demonstrating its effectiveness. Additionally, we present results validating our approach, using test cases from the SHMIP inter-comparison project (2018). We finish by discussing ongoing work related to the coupling of SUHMO with the BISICLES AMR ice-sheet model.

How to cite: Felden, A., Martin, D., and Ng, E.: SUHMO: an AMR SUbglacial Hydrology MOdel, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1236, https://doi.org/10.5194/egusphere-egu22-1236, 2022.

In recent years, temperatures over the Greenland ice sheet have been rising, leading to an increase in surface melt. This increase however can not be reduced to a simple number. Throughout the recent years we have seen some extreme melt seasons with melt extending over the whole surface of the ice sheet (2012) or melt seasons of lower amplitudes but with a longer duration (2010). The effect of those variations on the subglacial system and hence on ice dynamic are poorly understood and are still mainly deduced from studies based on mountain glaciers.

Here we apply the Ice-sheet and Sea-level System Model (ISSM) to a synthetic glacier with a geometry similar to a Greenland ice sheet land terminating glacier. The forcing is designed such that it allows to investigate different characteristics of the melt season such as its length or intensity. Subglacial hydrology and ice dynamics are coupled within ISSM allowing to study the response of the system in terms of subglacial water pressure and the final impact on ice dynamics. Of particular interest is the evolution of the distribution of the efficient and inefficient component of the subglacial drainage system which directly impacts the water pressure evolution at the base of the glacier. We note that the initiation of the melt season and the intensity of the melt at this period is a crucial parameter when studying the dynamic response of the glacier to different melt season characteristics.

From those results, we can infer a more precise evolution of the dynamics of land terminating glaciers that are heavily driven by their subglacial drainage system. We also highlight which changes in the melt season pattern would be the most damageable for glacier stability in the future.

How to cite: de Fleurian, B., Davy, R., and Langebroek, P. M.: Is longer or warmer melt season a more important driver of ice dynamics., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4926, https://doi.org/10.5194/egusphere-egu22-4926, 2022.

Subglacial flow routing, which simply routs water down the Shreve potential (i.e. the hydraulic potential assuming that water pressure equals a fraction of ice overburden pressure), is probably the most widely used subglacial drainage model.  It is easy to use via many ready-to-use implementations, does not suffer from numerical issues, such as non-convergence, and is fast.

Here I present two improvements to this venerable model: (1) the spatial uncertainties in the input fields (surface and bed topography, water input) and the model parameter (fraction between water pressure and floation pressure) are taken into account by representing them as Gaussian random fields. This a allows to assess the impact of these uncertainties by running Monte Carlo simulations.  (2) the dependence of the ice melting point on the pressure (leading to the so-called supercooling effects) can lead to deflections in the flow directions where descending flow-paths are favoured over ascending ones.  I present a framework on how this effect can be taken into account in subglacial flow routing.

To showcase the improved model, I present results from applying it to catchments of Antarctica which show that the sizes of sub-catchments can have uncertainties of several order of magnitudes.  This demonstrates the need of using such an improved model to make predictions of fluxes, including their uncertainties, into subglacial lakes or at the grounding line.

How to cite: Werder, M.: Subglacial water flow routing v2.0: taking uncertainties and supercooling effects into account, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12718, https://doi.org/10.5194/egusphere-egu22-12718, 2022.

Emission of CH₄ and CO₂ was recently discovered at the western margin of the Greenlandic Ice Sheet (GrIS) ^1,2. While knowledge on both carbon sources, extent and magnitude of these emissions are still very limited, the previous studies indicate that a primary driver for emission is degassing of dissolved and pressurized gases in the meltwater as it reaches the glacial margin. In this way we suggest that glacial hydrology plays a key role in regulating emission on both temporal and spatial scale.

In our studies of subglacial CH₄ and CO₂ emissions we have so far observed that the seasonal variations in meltwater discharge is correlated to both the magnitude of gas concentrations as well as timing of emissions^3,4. We propose that the seasonal variations in the connectivity of subglacial channels to both 1) pockets of sediment with CH₄ and CO₂ production from both anaerobic and aerobic biological processes and 2) supraglacial meltwater via englacial conduits could be a mechanism, which could explain the overall temporal and seasonal patterns of gas concentrations observed at the glacial margin.

We hypothesize that by observing hydrological and geochemical processes at the margin together with CH₄ and CO₂ in high frequency over the melt season it can be inferred how subglacial hydrological processes regulate biogeochemical and carbon turnover processes. Knowledge on these mechanism and processes are important for future upscaling CH₄ and CO₂ emission to seasonal periods and larger spatial scales through modeling as well as the assessment of the potential importance of subglacial carbon emissions to the climate system.

Here, we will present data that couples meltwater discharge to measurements of dissolved CH₄ and gaseous CH₄ and CO₂ as well as campaign measurements of water geochemistry and its isotopic composition. Preliminary data shows that CH₄ and CO₂ export display a clear diurnal signal in response to variations in the composition of melt water discharge. EC measurements and isotopic composition of melt water show a dominance of surface meltwater to subglacial meltwater, but clear diurnal trends in the mixing between these two water sources can be deduced from both isotope and elemental geochemistry of the meltwater.

1. Christiansen, J. R. & Jørgensen, C. J. First observation of direct methane emission to the atmosphere from the subglacial domain of the Greenland Ice Sheet. Sci. Rep. 8, 16623 (2018).

2. Lamarche-Gagnon, G. et al. Greenland melt drives continuous export of methane from its bed. Nature 73–77 (2018). doi:10.1038/s41586-018-0800-0

3. Jørgensen, C. J., Mønster, J., Fuglsang, K. & Christiansen, J. R. Continuous methane concentration measurements at the Greenland Ice Sheet-atmosphere interface using a low-cost low-power metal oxide sensor system. Atmos. Meas. Tech. Discuss. 13, 3319–3328 (2020).

4. Christiansen, J. R., Röckmann, T., Popa, M. E., Sapart, C. J. & Jørgensen, C. J. Carbon emissions from the edge of the Greenland Ice sheet reveal subglacial processes of methane and carbon dioxide turnover. J. Geophys. Res. Biogeosciences 1–13 (2021). doi:10.1029/2021jg006308

How to cite: Jørgensen, C. J., Sapper, S. E., Blunier, T., Gkinis, V., and Christiansen, J. R.: Temporal relationship between meltwater discharge and CH4 and CO2 emissions from the Greenland Ice Sheet., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9202, https://doi.org/10.5194/egusphere-egu22-9202, 2022.

The englacial and subglacial drainage system exerts key controls on glacier dynamics.  However, due to its inaccessibility, it is still only poorly understood and more detailed observations are important, particularly to validate and tune physical models describing its dynamics.

By creating artificial glacier moulins - boreholes connected to the subglacial drainage system and supplied with water from surface streams - we present a novel method to monitor the evolution of englacial hydrological systems with high temporal resolution.  Here, we use artificial moulins as representations for vertical, pressurised, englacial R-channels.  From tracer and pressure measurements we derive time series of the hydraulic gradient, discharge, flow speed and channel cross-sectional area.  Using these, we compute the Darcy-Weisbach friction factor, obtaining values which increase from 0.1 to 13 within five days of channel evolution (corresponding to a Manning friction factor of 0.03 to 0.3 s m^-1/3).

Furthermore, we simulate the growth of the channel cross-sectional area using different temperature gradients.  The comparison to our measurements largely supports the common assumption that the temperature follows the pressure melting point.  The deviations from this behaviour are analysed using various heat transfer parameterisations to assess their applicability.

Finally, we discuss how artificial moulins could be combined with glacier-wide tracer experiments to constrain parameters of subglacial drainage more precisely. The presented approach allows to accurately quantify the englacial transit time of the tracer and thus, in turn, to quantify the subglacial transit time; something which has not been achieved to date.

How to cite: Pohle, A., Werder, M. A., Gräff, D., and Farinotti, D.: Using artificial moulins to characterise englacial R-channels, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8644, https://doi.org/10.5194/egusphere-egu22-8644, 2022.

Simulating the hydrological systems underneath ice sheets and glaciers is important for estimating the freshwater flux into the ocean as well as inferring the characteristics of the hydrological system and its impact on ice sheet dynamics. In particular, simulations of the subglacial hydrological system in high temporal and spatial resolution and coupled to ice sheet models are needed to investigate the formation of ice streams. In order to be able to run simulations efficiently, both codes need to be parallelised. To this end, we present our approach for a parallelised version of the confined-unconfined aquifer system (CUAS) model (Beyer et al., 2018) that was established as a python code. CUAS is simulating an effective porous medium layer, in which the transmissivity indicates if the flow is channelised. Transmissivity is evolving by melt, creep and cavity opening. A fully implicit finite difference scheme is used for the hydraulic head while an explicit Euler time step is used for the transmissivity. 

The new CUAS-MPI version is written in C++ and instrumented for performance measurements. The parallelisation is done with MPI, where we take advantage of PETSc data structures and linear equation system solvers. The code has been designed to be coupled to the Ice Sheet and Sea-level system Model (ISSM) using preCICE (precice.org). 

Pumping tests that are widely used in applied groundwater hydrology are performed to test the model implementation including the boundary conditions and to compare with the analytical solutions. We further present test applications to the Greenland Ice Sheet, with the major focus on performance, rather than on characteristics of the hydrological system.

How to cite: Kleiner, T., Fischler, Y., Emunds, R., Oestreich, L., Bischof, C., Schmiedel, J., Sayag, R., and Humbert, A.: CUAS-MPI - A parallelised version of the confined-unconfined aquifer system model applied to the Greenland Ice Sheet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7234, https://doi.org/10.5194/egusphere-egu22-7234, 2022.

Understanding the distribution and evolution of surface meltwater on the Antarctic Ice Sheets is vital in enabling us to predict how the ice sheet will respond to a warming climate. The majority of Antarctic surface meltwater studies have typically been limited by either spatial or temporal scale. We have overcome these limitations by using a fully automated method to map surface meltwater across the entire Antarctic continent between 2006 and 2021 at a monthly temporal resolution. Furthermore, by accounting for variability in both cloud cover and satellite image coverage, we have generated the first consistent and continuous multi-year time series of Antarctic-wide surface meltwater to date. Here, we present results from analysis of this dataset, including long-term trends in surface meltwater extent, comparison between surface meltwater area and modelled melt, and associations between surface meltwater area and climatic factors. Regression analysis shows strong correlations between surface meltwater area and modelled snowmelt around the ice sheet margin, increasing our confidence in regional climate models to predict future melt conditions. Synoptic scale climate regimes, such as the Southern Annular Mode, exert a strong controlling factor on surface meltwater area totals on an annual basis. However, regional climatic processes and melt-albedo feedbacks can have strong second-order influences on localised melt rates, resulting in high variability in meltwater coverage, especially in parts of East Antarctica. This multi-year dataset offers the opportunity to explore surface meltwater evolution at local, catchment and continental scales, and will be of widespread use in understanding the operation of surface hydrological systems.

How to cite: Tuckett, P., Ely, J., Sole, A., Livingstone, S., Lea, J., and Jones, J.: Monthly Antarctic-wide surface meltwater evolution between 2006 and 2021, and its links to climate, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2884, https://doi.org/10.5194/egusphere-egu22-2884, 2022.

The growth of shallow, low-permeability ice slabs in Greenland’s firn is known to increase surface meltwater runoff by hindering vertical percolation. However, the partitioning of meltwater between local impoundment, downslope runoff, and drainage to the ice sheet bed is still poorly constrained. Northwest Greenland is a particularly interesting study area for understanding the role of englacial hydrology because ice-penetrating radar surveys have identified coexisting ice slabs and firn aquifers in this region. These results suggest that ice slabs may not necessarily preclude local firn water storage. However, the mechanism that would allow these two distinct facies to develop together is unclear.

               To examine the relationship between firn aquifers and ice slabs in Northwest Greenland, we analyzed six-years of NASA Operation IceBridge radar data between 2011 and 2017. These observations show that isolated, short-lived water pockets frequently develop beneath the ice slabs and over time refreeze to form kilometer-scale ellipsoidal buried ice masses. These ice blobs covered ~14% of the 1176 radar line-kms flown in 2017 and analysis of Landsat imagery between 2000 and 2016 shows they are spatially correlated with visible runoff in supraglacial lakes, streams, or massive slush swamps. High-resolution optical satellite imagery also shows that surface crevassing is widespread in this region and that many of the ice blobs are associated with moulins or lake drainage events in the 2012 melt season. This suggests that ice blobs form where fractures create high permeability pathways through the ice slab through which surface meltwater can drain into the relict firn. In the short term, this process impounds water and heat in the upper 30 meters of the ice column, reducing surface runoff and limiting the immediate impact of surface melt on local ice dynamics. However, on longer timescales, this efficient filling of pore space beneath supraglacial flow paths may lead to more efficient surface hydrology and full ice thickness hydrofracture at high elevations. 

How to cite: Culberg, R., Chu, W., and Schroeder, D.: Shallow Fracture Buffers High Elevation Runoff in Northwest Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3004, https://doi.org/10.5194/egusphere-egu22-3004, 2022.

Surface meltwater is becoming an increasing driver for ice shelf disintegration and consequent mass loss from the AIS. In this regard, monitoring surface melt over Antarctic ice shelves can enhance our understanding of their stability. Earth Observation (EO) satellites provide decadal records of land dynamics over Antarctica, and have been applied in surface melt monitoring. Thereby, they hold a potential to monitor the spatiotemporal evolution of surface melt over the entire AIS. Among the wealth of EO satellites, scatterometer and radiometer observations are most frequently used for surface melt detection, followed by SAR and optical data. Most studies used observations from a single satellite to study surface melt, while specific sensor characteristics (e.g., spatial resolution, overpass time, penetration depth) largely influence the potential for detecting surface melt. Therefore, we compare differences in melt detection between radiometer, scatterometer, SAR and optical sensors to assess the opportunities and challenges in observing surface melt for different EO satellites. We apply state-of-the-art melt detection algorithms to radiometer (Special Sensor Microwave Imager/Sounder, SMMIS), scatterometer (Advanced Scatterometer, ASCAT), Synthetic Aperture Radar (SAR; Sentinel-1), and optical (Moderate Resolution Imaging Spectroradiometer, MODIS) data over the Larsen B+C and Amery Ice Shelves for the 2015-2020 melt seasons. We construct melt timeseries and spatial maps using the melt detection algorithms. and intercompare the spatiotemporal patterns of detected melt. Finally, we compare areas with different melt patterns with auxiliary data sets (i.e., Regional Atmospheric Climate Model (RACMO2), Digital Elevation Models (DEM), high resolution optical imagery). Our results show that the largest differences in detected melt between the EO satellites can be linked to physical properties of the surface, sensor properties and atmospheric conditions. Over the blue ice areas, MODIS indicates more surface melt than the other sensors, as they miss blue ice areas due to either a coarse spatial resolution or the applied detection algorithms. However, over the ice shelves, MODIS detects significantly less surface melt, which can be attributed to the very high cloud obstruction frequency over AIS. Based on this intercomparison, we discuss the opportunities and challenges for melt detection across the AIS regarding the choice of different sensors and the chosen melt detection algorithms. We conclude that merging observations from different satellites (e.g. using machine learning) would further strengthen our knowledge on the presence of surface melt across the AIS, since this combines the strengths of specific sensors based on their sensor characteristics and the area of interest.

How to cite: de Roda Husman, S., Hu, Z., Lhermitte, S., Wouters, B., and Kuipers Munneke, P.: Earth Observation for Surface Melt Monitoring over Antarctic Ice Shelves: Opportunities and Challenges, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5463, https://doi.org/10.5194/egusphere-egu22-5463, 2022.

One of the consequences of rising temperatures observed in Northeast Greenland during the last decades is the increase in surface melting. Meltwater runoff can reach the bed and influence the ice flow regime through changes in basal conditions. Yet, depending on the drainage system, meltwater increase can have opposite effects on basal sliding and there is no consensus yet on the mechanisms that govern this hydrological switch. Here we present some advances in understanding the effects of surface meltwater percolation on ice dynamics at the Northeast Greenland Ice Stream (NEGIS) by investigating surface melt-basal sliding interactions at various temporal scales. To this end, we make use of a fully coupled model approach, comprising the finite-element ice-flow model Elmer/Ice and the hydrological scheme GlaDS. The latter is capable of representing the water drainage at all levels of the ice column. High resolution daily surface mass balance reconstructions available for the years 2014-2018 and simulated by the surface energy balance model COSIPY are used to force the ice-flow model. After first sensitivity tests aiming to explore the parameters of the ice-flow-hydrology system model, a fully coupled simulation for the period 2014-2018 is performed. This advanced modelling framework allows us to tackle the response of the NEGIS ice flow to seasonal and multi-annual variations in surface melt through changes in the drainage system.

How to cite: Tabone, I., Fürst, J., and Mölg, T.: Investigating the influence of increased meltwater runoff on basal sliding in Northeast Greenland., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2197, https://doi.org/10.5194/egusphere-egu22-2197, 2022.

Mass loss from the Greenland Ice Sheet (GrIS) is predicted to contribute up to 10 cm to global sea level rise by 2100. This mass loss is due to both increased meltwater production and therefore increased runoff, which is occurring at higher elevations on the ice sheet, as well as ice-dynamical feedback processes such as supraglacial lakes (SGLs) draining rapidly to the ice sheet bed; enhancing basal sliding. Therefore, the specific processes through which SGLs drain has an important control on mass loss from the GrIS. This highlights the need for high-resolution, integrated datasets that provide a comprehensive view of supraglacial hydrological networks, including SGL drainage events, on the GrIS.

Here, we compare SGL characteristics and drainage dynamics of a southwestern sector of the GrIS throughout both an extreme high melt season (2019) and an extreme low melt season (2018). SGLs are delineated throughout both the summer seasons from Sentinel-2 and Landsat 8 optical imagery using threshold-based normalized difference water index (NDWI) methods followed by extensive manual enhancement to increase accuracy. SGL depths are calculated using a radiative transfer model and individual lake volume is determined. The resulting meltwater maps have a spatial resolution of 10 to 30 m and have a temporal resolution of weekly to fortnightly. The following SGL characteristics are determined: i) area; ii) volume; iii) elevation; iv) ice surface slope; and v) solidity. SGL drainage dynamics are analysed by tracking lakes through the duration of each melt season and determining if a lake drains rapidly by hydrofracture, slowly drains via channel incision and overflow, or does not drain and instead refreezes at the end of the melt season. To do this, we use the Fully Automated supraglacial lake area and volume tracking at enhanced resolution (FASTER) algorithm, developed by Williamson et al. (2018).

As temperatures continue to increase, the frequency of high melt years like 2019 will also increase. As such, it ever more important to understand supraglacial meltwater characteristics and dynamics in high melt seasons, especially compared to years with limited melt.

How to cite: Glen, E., Banwell, A., Maddalena, J., Leeson, A., Corr, D., and McMillan, M.: A comparison of supraglacial lake characteristics and drainage dynamics in Southwest Greenland between an extreme high, and an extreme low, melt season, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7675, https://doi.org/10.5194/egusphere-egu22-7675, 2022.

Water-saturated firn layers, or firn aquifers, have recently been identified from satellite microwave image time-series data in some western Antarctic Peninsula ice shelves. Subsequent field work on the Wilkins Ice Shelf (this work) and Müller Ice Shelf (MacDonell et al., 2021) has proven the existence of these perennial ice shelf firn aquifers. Most of the aquifer areas are maintained by seasonal meltwater recharge. Brine aquifers have been known for many decades in certain slow-moving ice shelves with shorter or absent melt seasons (e.g., McMurdo Ice Shelf). We present both satellite evidence of meltwater firn aquifers in several areas of the Antarctic Peninsula, and radar profile evidence consistent with extensive brine infiltration in the Abbott, Nickerson, and Shackleton Ice Shelves. These latter ice shelves had been previously identified as likely sites of widespread brine infiltration (Cook et al., 2018).

The hydrofracture-driven disintegration of the Wilkins Ice Shelf in February-March of 2008, and subsequent rapid calving events extending into the winter season, justify a closer look at the relative potential for fresh-water aquifers, brine aquifers, and surface melt ponds for inducing hydrofracture in ice shelves. The destructive impact of surface or near-surface meltwater on floating ice is now well-established and is implicated in the loss of the Larsen A and Larsen B ice shelves, and rapid late-stage disintegrations of several tabular icebergs. Brine-aquifer-induced disintegration was suspected for the Wilkins breakup (Scambos et al., 2009), but now appears to be related to the effects of a freshwater system. A question remains regarding the vulnerability of ice shelves with significant brine infiltration in an aquifer.

We will present field measurements from the Wilkins Ice Shelf and discuss the relative hydrofracturing potential of fresh water and brines under various scenarios pertinent to ice shelf stability. The potential for future expansion of fresh-water aquifers under warming coastal conditions, and the characteristics of a hypothetical transitioning from a cold brine aquifer to a fresh-water aquifer will be discussed.

Cook, S., Galton-Fenzi, B.K., Ligtenberg, S.R. and Coleman, R., 2018. Brief communication: widespread potential for seawater infiltration on Antarctic ice shelves. The Cryosphere, 12(12), 3853-3859, doi: 10.5194/tc-12-3853-2018.

MacDonell, S., Fernandoy, F., Villar, P. and Hammann, A., 2021. Stratigraphic analysis of firn cores from an antarctic ice shelf firn aquifer. Water, 13(5), 731, doi:10.3390/w13050731.

Scambos, T., Fricker, H.A., Liu, C.C., Bohlander, J., Fastook, J., Sargent, A., Massom, R. and Wu, A.M., 2009. Ice shelf disintegration by plate bending and hydro-fracture: Satellite observations and model results of the 2008 Wilkins ice shelf break-ups. Earth and Planetary Science Letters, 280(1-4), 51-60, doi:10.1016/j.epsl.2008.12.027.

How to cite: Scambos, T., Miller, J., Culberg, R., Shuman, C., Montgomery, L., Miège, C., Brogioni, M., and Long, D.: Antarctic Ice Shelf Aquifers:Characteristics and Potential Contributions to Ice Shelf Loss, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-820, https://doi.org/10.5194/egusphere-egu22-820, 2022.

On land-terminating regions of ice sheets, large and transient changes in surface motion are not expected outside of summer due to the lack of concurrent melt forcing.  Here, we document the dynamic response to a cascading lake drainage that occurred during winter in Greenland using a high-resolution DInSAR timeseries (6-day) acquired from Sentinel-1 and optical imagery from Landsat-8 and Sentinel-2. A total of fifteen supraglacial lakes and several smaller supraglacial water features were identified to have drained during the event resulting in a velocity wave that propagates from the inland regions to the margin. Along the wave path, speeds increase up to three times pre-drainage velocities as the wave passes. Bifurcation of the velocity wave during the event implies at least two distinct subglacial flood pathways develop which drain from the margin over 100 km apart. By tracking the wavefront, we estimate the wave velocity through the event which we infer to be similar to the drainage velocity. Wave speeds of between 0.04 and 0.28 m s^-1 suggests the subglacial flood propagates mainly through an inefficient drainage system. Using temporally overlapping portions of two DInSAR velocity maps which have an accuracy of greater than 0.1 m yr^-1, we decompose the signal and demonstrate some of the motion is a result of surface uplift which constrains locations of likely flow pathways. Overall, our results demonstrate a sustained dynamic response to melt forcing can occur in the absence of surface melt. This indicates melt-induced ice motion changes are not limited to summer and transient winter dynamics might commonly occur as stored meltwater is released.

How to cite: Maier, N., Andersen, J., Mouginot, J., Gimbert, F., and Gagliardini, O.: Cascading supraglacial drainage observed to cause large-scale acceleration and uplift during winter in Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10584, https://doi.org/10.5194/egusphere-egu22-10584, 2022.

The ESA 4DGreenland project has the objective of performing an integrated assessment of Greenland’s hydrology through maximizing the use of Earth observation (EO) data. However, as meltwater is activated on the surface of the Greenland ice sheet and percolates into the firn, the firn water storage and delayed runoff are unobservable from EO, and we must resort to modeling to quantify this component. 

The DTU-firn model has previously been used to quantify the change in firn air volume as a leading component of altimetric mass balance estimates and was initially tuned using firn core density observations. Here, we revisit the fundamentals of the model and perform model inversion using as many observational datasets as possible. This observational data ranges from direct measurements of firn compaction derived from “coffee-can” experiments, over traditional observation of densities and temperature from firn cores to regional observation of firn stratigraphy from airborne radar surveys. This large diversity in the data sources ensures the best possible constraints for capturing as many aspects as possible of the firn dynamics.  

One important model feature of the DTU-firn model is its ability to resolve individual precipitation events as model layers. This feature promotes the capability of retaining water and shows promising results in line with the in-situ observation. The updated DTU-firn model is therefore used within the 4DGreenland project to provide updated estimates of meltwater retention and delayed runoff as a function of the available water at the surface of the firn. Combined with results obtained by the HIRHAM- and MAR-firn models, it also enables better quantification of firn model uncertainties. Having built confidence in the model we can treat the retention and delayed runoff as a function of the available water at the surface of the firn. This then gives the possibility of using the observed surface melt, also acquired within 4DGreenland from satellite microwave data, in the assessments of Greenland’s hydrology and thereby increasing the useability of EO data.

How to cite: Simonsen, S. B., Hansen, N., Otosaka, I., Vandecrux, B., and Sandberg Sørensen, L.: Assessing firn water storage from a multi-data firn-model inversion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4909, https://doi.org/10.5194/egusphere-egu22-4909, 2022.

Most of the Greenland ice sheet (∼78% to 92%) is underlain by porous snow and firn, into which meltwater can percolate and refreeze, or run off. However, the fate of meltwater in firn areas is poorly constrained. We identified the ice sheet’s annual visible runoff limits by mapping surface hydrological features in >25,000 Landsat satellite scenes. Between 1985–1992 and 2013–2020, the visible runoff limits along the west and north margins rose by 58–329 metres, expanding the visible runoff area by ~29%. Estimates using two different regional climate models suggest that the enlarged area has contributed from 190–264 Gt to 320–491 Gt of runoff since 1985, equivalent to as much as ∼10% of recent annual runoff from the west and north margins. However, the spread highlights that runoff processes in the percolation zone are a source of considerable uncertainty among the major models. We demonstrate that sustained excess melting since the 1990s has provided favourable conditions for anomalous near-surface firn densification. Much of the expanded visible runoff area is underlain by relatively impermeable and persistent ice slabs that have previously been identified by airborne radar campaigns. These slabs have recently enabled sustained runoff from higher elevations even in cooler summers. Our findings highlight that lateral runoff over densified near-surface firn is pervasive in several sectors of the ice sheet and therefore must be incorporated into future runoff projections.

How to cite: Tedstone, A. and Machguth, H.: Increasing surface runoff from Greenland's firn areas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2296, https://doi.org/10.5194/egusphere-egu22-2296, 2022.

Mapping the ice bed interface with radar is challenging in many alpine glaciers where the ice is temperate, and in-ice absorption is high. It is also difficult in selected regions of polar ice sheets such as near grounding zones and in ice streams where clutter and rough beds increase incoherent volume scattering. The lack of information for the ice geometry impedes our process understanding, e.g., basal sliding (requires knowledge about the basal roughness) and the routing of subglacial water flow (requires knowledge on basal smoothness). The lack of observations to constrain variations in ice thickness on the sub-kilometre scale is thus still a bottleneck to confidently predict ice dynamics and expected rates of sea-level rise.

A recent development in radioglaciology, namely the application of phase-coherent polarimetric radar, provides an excellent opportunity to overcome these limitations. Radar polarimetry has made significant strides in the last few years to constrain internal ice structure and their impact on the deformation of ice sheets, including the reconstruction of ice micro-structure parameters previously obtained from ice cores. Here, we suggest that the ice-bed interface can be identified in characteristic patterns of the polarimetric coherence phase. This new metric provides information in areas where the backscattered power amplitude does not show any signatures of the ice-bed interface. We provide examples for this across a wide range of glaciological settings, including cold (Colle Gnifetti, Switzerland) and temperate (Hintereisferner, Austria) alpine glaciers, thin grounding zones (Ekström Ice Shelf, East Antarctica) and thick ice domes (Dome C, East Antarctica). If this holds, then the ice thickness mapping in challenging glaciological settings should preferably be done using a quad-polarimetric acquisition geometry. For ground-based surveys, this can be done using an autonomous ice rover, for which we provide a proof-of-concept study on the Ekström Ice Shelf in Antarctica.

How to cite: Ershadi, M. R., Drews, R., Koch, I., Oraschewski, F., Prinz, R., Martin, C., and Eisen, O.: Mapping the bed in challenging radar environments on alpine glaciers and ice sheets using radar polarimetry, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9915, https://doi.org/10.5194/egusphere-egu22-9915, 2022.

Subglacial bedforms such as mega-scale glacial lineations and drumlins are commonly thought to form during active ice flow. They are often present in deglaciated areas with various elongation ratios, consisting of different materials , information which led to the development of different formation theories. However, these exposed examples were formed not only by subglacial processes during glaciation but also altered by processes during and after deglaciation. Here, we analyse in-situ properties and topography beneath Rutford Ice Steam, a fast flowing ice stream in West Antarctica to evaluate current theoretical models of bedform formation. We present a combination of seismic and radar data, including high-resolution 3D radar topography covering the upstream end of a bedform. Data acquisition and processing of the high-resolution 3D radar dataset result in a horizontal resolution of 24 m along- and across-track and a vertical resolution of 12 m. Using seismic acoustic impedance and calibrated radar reflectivity subglacial properties of the bedforms as well as the surrounding area are identified.

A depression around the upstream end of a 360 m wide, 50 m high and more than 13 km long bedform was observed for the first time analysing the high-resolution 3D radar topography.  The depression consists of a deepening up to 45 m deep and 360 m wide and is seen to extend around 10.5 km downstream. Radar reflectivity reveals that the material the depression is excavated into at least partly consists of low porosity material. Radar reflectivity and seismic acoustic impedance  along the bedform imply a stiffer upstream end which softens along flow. The subglacial topography and properties give evidence that the bedform and the depression are formed by a combination of  erosional and depositional processes. Both processes are likely interlinked, as implied by the comparable volume of the moat and the bedform at the upstream end of the bedform. Based on these observations we support or reject common bedform formation theories beneath Rutford Ice Stream.

How to cite: Schlegel, R., Brisbourne, A., Murray, T., Booth, A., Smith, A., Clark, R., and King, E.: Properties and High-Resolution Topography of Subglacial Bedforms Beneath a West Antarctic Ice Stream, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11291, https://doi.org/10.5194/egusphere-egu22-11291, 2022.

The dynamics of soft-bedded glacial sliding over water-saturated tills are poorly constrained and difficult to realistically capture in large scale models. While experiments characterise till as a plastic material with a pressure dependent yield stress, large scale models rely on a viscous or power-law description of the subglacial environment to efficiently constrain the basal sliding rate of the ice. Further, the subglacial water pressure may fluctuate on annual to daily timescales, leading to transient adjustment of the till.

Here, we construct a continuum two-phase model of coupled fluid and solid flows, using Darcy flow to describe the movement of water through the pore space of the till that deforms according to a granular-inspired rheology. After calibrating our model against steady-state experiments, we force the model with a fluctuating effective pressure at the ice-till interface to infer the resulting relationships between basal traction, solid fraction, rate of deformation, and till flux. Shear dilation introduces internal pressure variations and transient dilatant strengthening emerges, leading to hysteretic behaviour in low-permeability materials.

Our model predicts a time-dependent effective sliding law, with permeability-dependent lag between changes in effective pressure and the sliding speed. This deviation from traditional steady-state sliding laws may play an important role in a wide range of transient ice-sheet phenomena, from glacier surges to the tidal response of ice streams. 

How to cite: Warburton, K., Hewitt, D., and Neufeld, J.: A time-dependent sliding law for granular till, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11677, https://doi.org/10.5194/egusphere-egu22-11677, 2022.

An understanding of subglacial processes are a vital component of ice-sheet models for sea level rise prediction as the use of different sliding laws can result in very different outcomes. In particular, the West Antarctic ice streams, are potentially unstable, and are underlain by soft (unconsolidated) beds, which have rarely been studied. Innovative in situ wireless subglacial experiments and web connected RTK GPS data from Iceland have shown that stick-slick motion can occur at different time scales throughout the whole year, and this allowed the quantification of different sedimentary processes. We investigate the results from four soft bedded glaciers. We compare the similarities and differences; and in particular describe the relationship with subglacial hydrological processes and temperature rise. We discuss the implications for ice sheet models and reconstructions of Quaternary sedimentary processes.

How to cite: Hart, J., Martinez, K., Baurley, N., and Robson, B.: Quantifying subglacial soft bed sedimentary processes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1801, https://doi.org/10.5194/egusphere-egu22-1801, 2022.

Enhanced ice velocity around the margins of the Greenland Ice Sheet is facilitated by the presence and pressure of subglacial meltwater. However, annual and longer-term velocity may be moderated by hydrological regulation, which reduces late melt-season and winter ice velocities following summers with relatively high surface melting and associated meltwater flux to the bed. While detail is lacking, hydrological regulation is likely driven by variations in the efficiency of subglacial drainage pathways and by associated variations in post-melt-season water retention at the glacier bed. Spatial variations in these processes may be viewed in terms of domains of differing degrees of hydrological connection to large subglacial meltwater channels. To date, a ‘well connected’ domain and an ‘isolated’ domain have been characterized, and an intermediate ‘weakly connected’ domain proposed. Generally, changes in the extent and/or pressurisation of the isolated domain are proposed as the driver of hydrological regulation. Yet, identifying the precise nature and timing of hydrological regulation, as well as the role of the weakly-connected domain, remain elusive.

             Here, we investigate the nature of the onset of hydrological regulation through a field experiment ~30 km from the terminus of Sermeq Kujalleq/Store Glacier, a fast-moving Greenlandic outlet glacier. We recorded simultaneous high-resolution time series of surface meltwater discharge, surface ice velocity and subglacial water pressure in two boreholes drilled at different distances from a substantial moulin. Analysis of the magnitude and timing of diurnal cycles and longer-term trends in all four records reveals that initially, in July, one borehole intersected the isolated subglacial domain and the other a ‘weakly-connected’ domain, with the latter showing a gradual decline in water pressure through the summer melt season. Transition to a winter state over the period ~10^th – 20^th August was marked by (i) a decrease in surface melting; (ii) a decrease in the amplitude of diurnal water pressure cycles in both boreholes, and (iii) a decrease in surface velocity. This transition was accompanied by an almost instantaneous (<1 d) switch in the borehole hitherto intersecting the weakly-connected domain to the isolated domain, evidenced by a 180° phase shift in the timing of its diurnal water pressure cycle. After this transition, diurnal cycles in all records diminish and both subglacial water pressure and ice surface velocity increase gradually through the winter.

                We conclude that ice velocity at our study location is at least partly governed by water pressure within a weakly-connected subglacial drainage domain. Water pressure here declines gradually through the melt season but increases after transition to hydraulic isolation, a transition that occurs over a period of only some days in the autumn. At the scale represented by records from individual boreholes the transition can occur over just some hours.

How to cite: Hubbard, B., Doyle, S., Christoffersen, P., Chudley, T., and Law, R.: Borehole-based insights into the nature and timing of hydrological-regulation at a fast-moving Greenlandic outlet glacier, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12228, https://doi.org/10.5194/egusphere-egu22-12228, 2022.

Ice motion within the land-terminating ablation zone of west Greenland typically follows a seasonal pattern with fastest flow in the spring as surface meltwater first accesses the hydraulically inefficient subglacial drainage system, followed by a gradual reduction in ice motion over the summer as efficient subglacial channels evacuate water stored over wide swathes of the subglacial environment. Minimum speeds occur in autumn, when surface meltwater delivery to the bed ceases, then, over winter, ice flow gradually recovers before the cycle repeats once again. This understanding is principally derived from high temporal resolution field observations with limited spatial coverage. It is now, however, possible to measure these seasonal patterns in detail over large spatial scales, enabling basin-scale comparisons between meltwater supply, subglacial hydrology and ice motion.

We present near continuous time-series of ice motion for a ~60,000 km² portion of predominantly the land-terminating western margin of the Greenland Ice Sheet (with coverage extending up to 150 km from the margin in winter and 25 km in summer) between 2016 and 2022 at up to 12-day temporal resolution derived from Sentinel-1, Sentinel-2 and Landsat 8 imagery. We compare ice motion with surface melt magnitude and timing (derived from meteorological observations and a positive degree day model), theoretical subglacial water routing, and glaciological context (e.g. ice thickness).

Our results reveal clear spatial patterns in seasonal coupling between ice sheet hydrology and motion. Overall, the amplitude of the seasonal cycle of ice motion is generally greater closer to the ice margin as has been shown previously by field observations. There is, however, distinct variability between subglacial hydraulic catchments. Subglacial catchments whose principal drainage pathway has a high hydraulic potential gradient typically experience smaller peaks in summer ice motion, but more prominent autumn ice flow minima. Subglacial water flow in such catchments has more potential to create hydraulically efficient channels, which can both accommodate spikes in meltwater delivery with muted ice flow acceleration, and also reduce basal water pressure across a wider area of the ice bed as surface meltwater delivery declines.

As ice sheet surface mass balance becomes more negative and marginal ice thins fastest (where the ice is land-terminating), subglacial hydraulic gradients and subglacial discharge will both increase. Based on our findings, these changes may lead to smaller summer ice flow accelerations and bigger winter slow-downs, driving an overall decrease in mean annual ice motion. Our results also highlight the importance of careful selection of field sites for measuring ice motion and caution against scaling up from spatially limited observations.

How to cite: Sole, A., Davison, B., and Livingstone, S.: Spatial patterns in seasonal coupling between ice sheet hydrology and motion in west Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3812, https://doi.org/10.5194/egusphere-egu22-3812, 2022.

Proglacial measurements of subglacial sediment export by meltwater are commonly used to estimate glacial erosion rates. Such estimates generally assume that subglacial meltwater flow is the dominant agent driving export and that eroded sediment is rapidly conveyed through subglacial drainage networks to the glacier outlet in efficient channels with large and generally-unsatisfied transport capacities. However, understanding of sediment transport processes under glaciers is limited, especially in the thin, snout-marginal zones of retreating temperate Alpine glaciers.

A growing body of field and model-based research from such systems challenges the theory that eroded sediment is rapidly evacuated. Conversely, Alpine glaciers develop intense diurnal and seasonal discharge variation leading to highly variable sediment transport competence and the moderation of sediment export by associated cycles of alluviation. This has important consequences for the timescales over which sediment export can be used as a reliable proxy for glacial erosion, though the problem remains that very little is known about when and under what conditions a glacier margin is capable of evacuating the sediment supplied to it.

This study attempts to elucidate sediment transport mechanisms and timescales in the main snout-marginal subglacial channel of glacier d’Otemma, Switzerland, by tracking 324 cobble-sized particles tagged with 433 MHz active radio transponders over the mid-to-late 2021 melt season. Tagged particles were injected into the channel via a 48 m deep borehole and were then tracked over a 350 m subglacial reach and a 150 m proglacial reach with a mobile antenna and stationary antenna arrays.

Only 86 particles (27%) were exported in 2021. Preliminary analyses indicate that cobble-sized particles generally have extended residence times in snout marginal zones (weeks to months), with no clear effect of particle size, shape or density on overall transport velocities. Ongoing work involves the daily localisation of particles using a signal strength-based algorithm, providing a unique record of the down-glacier transport of coarse particles in a subglacial channel. Repeat measurements will follow in 2022.

How to cite: Jenkin, M., Mancini, D., Miesen, F., Hofmann, M., Hubbard, B., Herman, F., and Lane, S. N.: Subglacial export of coarse sediment from temperate Alpine glaciers by meltwater, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3854, https://doi.org/10.5194/egusphere-egu22-3854, 2022.

The Totten Glacier is the main conduit for ice leaving the Aurora Subglacial Basin in East Antarctica. During December 2018 and January 2019, we deployed a 12 station broadband seismic array near the grounding zone of the Totten Glacier. Here we report on subglacial sedimentary structure and seismic activity recorded by this network. Previous gravity studies indicated the possibility that erosion had removed most of the subglacial sediments in the region. We use the receiver function analysis to reveal that 100-200 meters of subglacial sediment remain near the grounding zone. We also will summarize a range of glacier generated seismic activity recorded by the array. We find significant occurrence of tidally modulated near surface crevasse related events as well as basal stick-slip seismic activity. We will provide an overview into both the temporal and spatial variability of seismic activity and discuss implications for fast flow in the region.

How to cite: Winberry, P.: Seismic activity and subglacial sedimentary structure near the grounding zone of the Totten Glacier, East Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4867, https://doi.org/10.5194/egusphere-egu22-4867, 2022.