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The bulk crystal orientation in ice influences the flow of glaciers and ice streams. The ice c-axes fabric is most reliably derived from ice cores. Because these are sparse, the spatial and vertical distribution of the fabric in the Greenland and Antarctic ice sheets is largely unknown. In recent years, methods have been developed to determine fabric characteristics from polarimetric radar measurements. The aim of this paper is to present an improved method to infer the horizontal fabric asymmetry by precisely determining the travel-time difference using co-polarised phase-sensitive radar data. We applied this method to six radar measurements from the East Greenland Ice-core Project (EastGRIP) drill site on Greenland's largest ice stream to give a proof of concept by comparing the results with the horizontal asymmetry of the bulk crystal anisotropy derived from the ice core. This comparison shows an excellent agreement, which is a large improvement compared to previously used methods. Our approach is particularly useful for determining the vertical profile of the fabric asymmetry in higher resolution and over larger depths than was achievable with previous methods, especially in regions with strong asymmetry.

Ice shelves play a key role in the stability of the Antarctic Ice Sheet due to their buttressing effect. A loss of buttressing as a result of increased basal melting or ice shelf disintegration will lead to increased ice discharge. Some ice shelves exhibit channels at the base that are not yet fully understood. In this study, we present in situ melt rates of a channel which is up to 330 m high and located in the southern Filchner Ice Shelf. Maximum observed melt rates are 2 m yr−1. Melt rates inside the channel decrease in the direction of ice flow and turn to freezing ∼55 km downstream of the grounding line. While closer to the grounding line melt rates are higher within the channel than outside, this relationship reverses further downstream. Comparing the modeled evolution of this channel under present-day climate conditions over 250 years with its present geometry reveals a mismatch. Melt rates twice as large as the present-day values are required to fit the observed geometry. In contrast, forcing the model with present-day melt rates results in a closure of the channel, which contradicts observations. The ice shelf experiences strong tidal variability in vertical strain rates at the measured site, and discrete pulses of increased melting occurred throughout the measurement period. The type of melt channel in this study diminishes in height with distance from the grounding line and is hence not a destabilizing factor for ice shelves.

Over the past 60 years, scientists have strived to understand the past, present and future of the Antarctic Ice Sheet. One of the key components of this research has been the mapping of Antarctic bed topography and ice thickness parameters that are crucial for modelling ice flow and hence for predicting future ice loss and ensuing sea level rise. Supported by the Scientific Committee on Antarctic Research (SCAR), the Bedmap3 Action Group aims not only to produce new gridded maps of ice thickness and bed topography for the international scientific community, but also to standardize and make available all the geophysical survey data points used in producing the Bedmap gridded products. Here, we document the survey data used in the latest iteration, Bedmap3, incorporating and adding to all of the datasets previously used for Bedmap1 and Bedmap2, including ice-bed, surface and thickness point data from all Antarctic geophysical campaigns since the 1950s. More specifically, we describe the processes used to standardize and make these and future survey and gridded datasets accessible under the ‘Findable, Accessible, Interoperable and Reusable’ (FAIR) data principles. With the goals to make the gridding process reproducible and to allow scientists to re-use the data freely for their own analysis, we introduce the new SCAR Bedmap Data Portal (bedmap.scar.org, last access: 18 October 2022) created to provide unprecedented open access to these important datasets, through a user-friendly webmap interface. We believe that this data release will be a valuable asset to Antarctic research and will greatly extend the life cycle of the data held within it. Data are available from the UK Polar Data Centre: https://data.bas.ac.uk.

The Filchner-Ronne sector of Antarctica contains a number of deep-bedded ice streams and glaciers potentially vulnerable to the Marine Ice Sheet Instability (MISI). Previous work has shown that, in a warming climate, the ocean circulation in the cavity underneath the Filchner-Ronne Ice Shelf (FRIS) could switch from its present cold state to an alternate warm mode, in which intrusions of Circumpolar Deep Water (CDW) cause high basal melt rates near the deep grounding lines of potentially vulnerable glaciers. However, less work has been done on modeling the response of the ice sheet and ice shelf system to such a mode switch in the cavity circulation. Here, we use the Ice-sheet and Sea-level System Model (ISSM) to simulate the response of the Filchner-Ronne sector of Antarctica over multi-centennial timescales to changes in basal melt rate caused by a mode switch in the cavity circulation. We force ISSM with both melt rates directly calculated by the cavity-resolving Finite-Element Sea ice-Ocean Model (FESOM) and with parameterized melt rate forcing derived from CMIP6 global models. We find that parameterized melt rates in high-emissions scenarios cause rapid grounding line retreat at almost all of the major glaciers and ice streams feeding the FRIS beginning in the 22nd century, followed by ice shelf collapse and rapid sea level rise in the 23rd. Using FESOM simulated melt rates the destabilization of the FRIS sector proceeds more slowly. During the 22nd century retreat is concentrated in specific ice streams, reflecting the more heterogeneous distribution of melt rate in the ocean model as opposed to the parameterized forcing. In the 23rd century retreat becomes more widespread, culminating in ice shelf collapse and rapid sea level rise in the 24th and 25th centuries.

Mass changes of the polar Ice Sheets and their contribution to global mean sea level is an essential climate variable, critical for adaptation planning and important to understand the earth system in a warming climate. To ensure long term continuation of ice elevation and change records ESA proposed the Copernicus Polar Ice and Snow Topography Altimeter mission (CRISTAL) to be launched in 2027. CRISTAL, for the first time, will carry a dual-frequency altimeter in Ku and Ka Band to monitor changes in the height of ice sheets and glaciers and thickness of and snow on sea ice. Therefore, with the upcoming CRISTAL mission it is absolutely necessary to focus on sensor specific characteristics and its impact on elevation change records beforehand. To tackle these questions, we determine surface elevation change rates with a focus on the last decade of laser and radar altimetry observations using ICESat2, CryoSat-2, Sentinel-3 and SARAL/Altika. We investigate ice sheet wide and regional differences of elevation change rates derived from the four missions in the period from 2019 to 2022 with focus on different sensor characteristics (Laser/Ku/Ka Band), different acquisition/processing strategies ((P)LRM, SAR, SARIn), retracking (TFMRA,ICE1,ICE2) and orbit geometry (81.5°/88°).Our findings show that trend estimates of individual missions diverge most strongly at the margins and areas of complex topography but also in the flat interior of ice sheets and are highly dependent on the choice of retracker. We show that in order to determine volume changes as consistently as possible across missions, a uniform processing is required that includes backscatter/leading edge correction to suppress the influence of time-varying radar penetration due to changes in volume scattering.

The existence of ice-shelf basal channels has a significant impact on both buttressing ability and basal melting of ice shelves in Antarctica. Although they can provide a unique perspective of processes for the mass transfer from grounded ice sheet to floating ice shelf, their origination and evolution are still not fully understood.Here we used airborne and ground-based radar to investigate the subglacial channel features at the grounding line (GL). We determined the geometry of five channels (two of which are characterized for the first time) in a set of flow-perpendicular radar profiles, also perpendicular to the channels. We found that the evolution from grounded subglacial channels to the ice-shelf basal channels mainly goes through three stages: (1) The grounded subglacial channels appear 4 to 5 km upstream of the GL and their incision into the ice sheet increases while approaching the GL; (2) as the subglacial channels extend into the grounding zone (1 to 2 km downstream of GL), their inner walls started melting, also they keep their roof-top features; (3) on the shelf interaction with the ocean, surface accumulation and ice dynamics further lead to flattening and widening, progressively turning them into generally known ice-shelf basal channels.Additionally to radar observations, we investigated the role of subglacial hydrology with two modelling approach (subglacial water flux with CUAS-MPI and water routing with CiDRE). A comparison shows that most channel locations in the radar profiles match with areas of higher subglacial water presence, consequently implying that subglacial water flux could mainly be responsible for maintaining the presence of subglacial channels.Based on the already earlier proposed relation that the presence of a sub-ice shelf basal channel is linked to a corresponding channel at the GL, we identify one now active channel at the GL to be related to one which was earlier until 59 years ago. This indicates that basal channels and consequently basal water flux across the GL can change at least on the scale of centuries. Our observed reactivation of a subglacial channel confirms the suitability of basal channels in ice shelves to be used as proxies of past subglacial hydrological activities and other potentially larger events.

The mass balance of the Greenland ice sheet is strongly influenced by the dynamics of its outlet glaciers. Therefore, it is of paramount importance to accurately and continuously monitor these glaciers, especially the variation of their frontal positions. A temporally comprehensive parameterization of glacier calving is essential to understand dynamic changes and to constrain ice sheet modelling. However, current calving front records are often limited in temporal resolution as they rely on manual delineation, which is laborious and not feasible with the increasing amount of satellite imagery available. In this contribution, we address this problem by applying an automated method to extract calving fronts from optical satellite imagery. The core of this workflow builds on recent advances in the field of deep learning while taking full advantage of multispectral input information. The performance of the method is evaluated using three independent validation datasets. Eventually, we apply the technique to Landsat-8 imagery. We generate 9243 calving front positions across 23 Greenland outlet glaciers from 2013 to 2021. Resulting time series resolve not only long-term and seasonal signals but also sub-seasonal patterns. We discuss the implications for glaciological studies and present a first application analysing the interaction between calving front variation and bedrock topography. Our method and derived results represent an important step towards the development of intelligent processing strategies for glacier monitoring, opening up new possibilities for studying and modelling the dynamics of Greenland outlet glaciers. Thus, these also contribute to advance the construction of a digital twin of the Greenland ice sheet, which will improve our understanding of its evolution and role within the Earth's climate system.

Over the past three decades, inversions for ice sheet basal drag have become commonplace in glaciological modeling. Such inversions require regularization to prevent over-fitting and ensure that the structure they recover is a robust inference from the observations, confidence which is required if they are to be used to draw conclusions about processes and properties of the ice base. While L-curve analysis can be used to select the optimal regularization level, the treatment of L-curve analysis in glaciological inverse modeling has been highly variable. Building on the history of glaciological inverse modeling, we demonstrate general best practices for regularizing glaciological inverse problems, using a domain in the Filchner-Ronne catchment of Antarctica as our test bed. We show a step by step approach to cost function normalization and L-curve analysis. We show that the optimal regularization level converges towards a finite non-zero limit in the continuous problem, associated with a best knowable basal drag field. We find that, when inversion results are judged by a metric that accounts for both the variance of the result and the quality of the fit, then they support nonlinear as opposed to linear sliding laws. We also find that geometry-based approximations for effective pressure degrade inversion performance, but that an actual hydrology model may marginally improve performance in some cases. Our results with 3D inversions suggest that the additional model complexity is not justified by the 2D nature of the velocity data, but we are hopeful that inversions of 3D models may be better situated to take advantage of new constraints in the future. We conclude with recommendations for best practices in glaciological inversions moving forward.

Ice shelves are large floating ice masses, that are formed when glaciers are becoming afloat at the margin of ice sheets. One dominating mass loss mechanism of ice shelves is calving, describing the detachment of icebergs at the front. Ice shelves stabilize inland ice glaciers due to buttressing. If the stabilizing effect of an ice shelf vanishes because of disintegration or thinning, the corresponding glacier accelerates resulting in sea level rise. To describe calving and disintegration of ice shelves, it is important to investigate fracture propagation in ice. A powerful method in fracture mechanics is the phase field method which is based on Griffith's theory. It approximates cracks in a diffuse manner by using a continuous scalar field. We propose a phase field fracture model for ice considering its characteristic material properties. The material behavior of ice depends on the considered time scales. On short time scales it behaves like a solid and while it acts like a fluid on long time scales, which classifies it as a viscoelastic material of Maxwell type. This has been verified by observations. The phase field method allows us to simulate typical fracture situations of ice shelves in Antarctica and Greenland.

The bulk crystal orientation in ice influences the flow of glaciers and ice streams. The ice c-axes fabric is most reliably derived from ice cores. Because these are sparse, the spatial and vertical distribution of the fabric in the Greenland and Antarctic ice sheets is largely unknown. In recent years, methods have been developed to determine fabric characteristics from radar measurements. The aim of this paper is to present an improved method to infer the horizontal fabric asymmetry by precisely determining the travel-time difference using co-polarised phase-sensitive radar data. We applied this method to six radar measurements from the EastGRIP drill site on Greenland's largest ice stream to give a proof-of-concept by comparing the results with the horizontal asymmetry of the bulk crystal anisotropy derived from the ice core. This comparison shows an almost perfect agreement, which is a large improvement compared to previously used methods. Our approach is particularly useful for determining the vertical profile of the fabric asymmetry in higher resolution and over larger depths than was achievable with previous methods, especially in regions with strong asymmetry.

The Northeast Greenland Ice Stream has recently seen significant change to its floating margins and has been identified as vulnerable to future climate warming. Inflow of warm Atlantic Intermediate Water (AIW) from the continental shelf has been observed in the vicinity of the Nioghalvfjerdsfjorden (79∘ N) Glacier calving front, but AIW penetration deep into the ice shelf cavity has not been observed directly. Here, we report temperature and salinity measurements from profiles in an epishelf lake, which provide the first direct evidence of AIW proximal to the grounding line of 79∘ N Glacier, over 50 km from the calving front. We also report evidence for partial un-grounding of the margin of 79∘ N Glacier taking place at the western end of the epishelf lake. Comparison of our measurements to those close to the calving front shows that AIW transits the cavity to reach the grounding line within a few months. The observations provide support for modelling studies that infer AIW-driven basal melt proximal to the grounding line and demonstrate that offshore oceanographic changes can be rapidly transmitted throughout the sub-ice-shelf cavity, with implications for near-future stability of the ice stream.

The largest floating tongue of Greenland’s ice sheet, Nioghalvfjerdsbrae, has so far been relatively stable with respect to areal retreat. Curiously, it experienced significant less thinning and ice flow acceleration than its neighbour Zacharias Isbrae. Draining more than 6% of the ice sheet, Nioghalvfjerdsbrae might become a large contributor to sea level rise in the future. Therefore, the stability of the floating tongue is a focus of this study. We employ a suite of observational methods to detect recent changes. We found that the calving style has changed at the southern part of the eastern calving front from normal tongue-type calving to a crack evolution initiated at frontal ice rises reaching 5-7km and progressing further upstream compared to 2010. The calving front area is further weakened by a substantial increase of a zone of fragments and open water at the tongue’s southern margin, leading to the formation of a narrow ice bridge. These geometric and mechanical changes are a precursor of instability of the floating tongue. We complement our study by numerical ice flow simulations to estimate the impact of future break-up or disintegration events on the ice discharge. These idealised scenarios reveal that a loss of the south-eastern area would lead to 1% of increase of ice discharge at the grounding line, while a sudden collapse of the frontal area (46% of the floating tongue area) will enhance the ice discharge by 8.3% due to loss in buttressing.Humbert, A., Helm, V., Neckel, N., Zeising, O., Rückamp, M., Khan, S. A., Loebel, E., Gross, D., Sondershaus, R., and Müller, R.: Precursor of disintegration of Greenland's largest floating ice tongue, The Cryosphere Discuss. [preprint], https://doi.org/10.5194/tc-2022-171, in review, 2022

The dynamic mass loss of ice sheets due to ice flow constitutes one of the biggest uncertainties in projections of ice-sheet evolution and sea-level rise. One central, understudied aspect of ice flow is how the bulk orientation of the ice-crystal lattice (fabric) translates to the mechanical anisotropy of ice. Here, we present a comprehensive analysis of the depth-averaged fabric distribution and corresponding directional flow enhancement factors covering a large area of the Northeast Greenland Ice Stream onset region. Our results are based on a combination of methods applied to extensive airborne and ground-based radar surveys, ice-core observations, and numerical ice-flow modelling. They show a strong spatial variability of the horizontal fabric anisotropy and a rapid crystal reorganization on the order of hundreds of years coinciding with the ice-stream geometry. The ice in parts of the ice stream is found to be up to one order of magnitude harder for pure-shear deformation in flow direction compared to isotropic ice, while the shear margins are potentially softened for horizontal simple shear by a factor of two. The high spatial resolution and large-scale coverage of our results is unprecedented and significantly contributes towards more accurate ice-flow models and a better understanding of ice-stream dynamics. Moreover, our estimated time scale of crystal reorganization and methodological approaches can potentially also be transferred to the investigation of ice on extraterrestrial bodies and other geologic materials.

International audience; Integrative and Comprehensive Understanding on Polar Environments (iCUPE) project developed 24 novel datasets utilizing in-situ observational capacities within the Arctic or remote sensing observations from ground or from space. The datasets covered atmospheric, cryospheric, marine, and terrestrial domains. This paper connects the iCUPE datasets to United Nations’ Sustainable Development Goals and showcases the use of selected datasets as knowledge provision services for policy- and decision-making actions. Inclusion of indigenous and societal knowledge into the data processing pipelines enables a feedback mechanism that facilitates data driven public services.

Curvilinear channels on the surface of an ice shelf indicate the presence of large channels at the base. Modelling studies have shown that where these surface expressions intersect the grounding line, they coincide with the likely outflow of subglacial water. An understanding of the initiation and the ice–ocean evolution of the basal channels is required to understand the present behaviour and future dynamics of ice sheets and ice shelves. Here, we present focused active seismic and radar surveys of a basal channel, ∼950 m wide and ∼200 m high, and its upstream continuation beneath Support Force Glacier, which feeds into the Filchner Ice Shelf, West Antarctica. Immediately seaward from the grounding line, below the basal channel, the seismic profiles show an ∼6.75 km long, 3.2 km wide and 200 m thick sedimentary sequence with chaotic to weakly stratified reflections we interpret as a grounding line fan deposited by a subglacial drainage channel directly upstream of the basal channel. Further downstream the seabed has a different character; it consists of harder, stratified consolidated sediments, deposited under different glaciological circumstances, or possibly bedrock. In contrast to the standard perception of a rapid change in ice shelf thickness just downstream of the grounding line, we find a flat topography of the ice shelf base with an almost constant ice thickness gradient along-flow, indicating only little basal melting, but an initial widening of the basal channel, which we ascribe to melting along its flanks. Our findings provide a detailed view of a more complex interaction between the ocean and subglacial hydrology to form basal channels in ice shelves.

Over the past two decades, ice loss from the Greenland ice sheet (GrIS) has increased owing to enhanced surface melting and ice discharge to the ocean1–5. Whether continuing increased ice loss will accelerate further, and by how much, remains contentious6–9. A main contributor to future ice loss is the Northeast Greenland Ice Stream (NEGIS), Greenland’s largest basin and a prominent feature of fast-flowing ice that reaches the interior of the GrIS10–12. Owing to its topographic setting, this sector is vulnerable to rapid retreat, leading to unstable conditions similar to those in the marine-based setting of ice streams in Antarctica13–20. Here we show that extensive speed-up and thinning triggered by frontal changes in 2012 have already propagated more than 200 km inland. We use unique global navigation satellite system (GNSS) observations, combined with surface elevation changes and surface speeds obtained from satellite data, to select the correct basal conditions to be used in ice flow numerical models, which we then use for future simulations. Our model results indicate that this marine-based sector alone will contribute 13.5–15.5 mm sea-level rise by 2100 (equivalent to the contribution of the entire ice sheet over the past 50 years) and will cause precipitous changes in the coming century. This study shows that measurements of subtle changes in the ice speed and elevation inland help to constrain numerical models of the future mass balance and higher-end projections show better agreement with observations.

Basal melt of ice shelves is a key factor governing discharge of ice from the Antarctic Ice Sheet as a result of its effects on buttressing. Here, we use radio echo sounding to determine the spatial variability of the basal melt rate of the southern Filchner Ice Shelf, Antarctica, along the inflow of Support Force Glacier. We find moderate melt rates with a maximum of 1.13 m a−1 about 50 km downstream of the grounding line. The variability of the melt rates over distances of a few kilometres is low (all but one < 0.15 m a−1 at < 2 km distance), indicating that measurements on coarse observational grids are able to yield a representative melt rate distribution. A comparison with remote-sensing-based melt rates revealed that, for the study area, large differences were due to inaccuracies in the estimation of vertical strain rates from remote sensing velocity fields. These inaccuracies can be overcome by using modern velocity fields.

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.

Outlet glaciers in Greenland experience a combination of seasonal and climate-driven change. In this study, we investigate the temporal evolution of the ice front position at Kangiata Nunaata Sermia (KNS), southwest Greenland. At this outlet glacier, a seasonal ice tongue forms almost every year in winter and breaks up in spring. The motion of the ice-ocean interface is one key driver of ice dynamics. Hence, it is important to understand and reconstruct terminus fluctuations. Utilizing the increasing availability of remote sensing data and deep learning algorithms, we accurately and frequently map the ice/ocean transition from optical satellite imagery. How can we incorporate that geometrical information of frontal positions into ice sheet models? The ice flow counteracts the mass loss due to iceberg calving and frontal melt. All those processes result in moving ice fronts over time. The interaction between ice flow and ice discharge is one challenge in combining observed ice front positions and modelling. We present two ways to include the geometric information of ice front positions into the Ice-sheet and Sea-level System Model (ISSM). In a first approach, we engage a level-set method to incorporate the derived calving front positions into the ice sheet model. By prescribing observed front positions, we ignore any physical basis of ice flow dynamics. However, the level-set method allows us to solve a differential equation continuous in time and space. Since we have to deal with observational gaps of ice front positions, not all stress changes caused by changing ice front positions might be resolved by this approach. In the second approach, we discuss physical drivers of terminus changes in more detail. We show parametrizations of calving and frontal melt depending on stress thresholds and subglacial water fluxes to reproduce the observed temporal evolution of the ice front position. The parametrization due to calving leads to ice loss velocities that are one order of magnitude higher than the one due to ocean melt. In the end, we want to combine both approaches to come up with an accurate parameterization of ice loss due to calving and melting, which fit observations and incorporate the physics of glacier flow.

Projections of the contribution of the Greenland ice sheet to future sea-level rise include uncertainties primarily due to the imposed climate forcing and the initial state of the ice sheet model. Several state-of-the-art ice flow models are currently being employed on various grid resolutions to estimate future mass changes in the framework of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). Here we investigate the sensitivity to grid resolution of centennial sea-level contributions from the Greenland ice sheet and study the mechanism at play.We employ the finiteelement higher-order Ice-sheet and Sea-level System Model (ISSM) and conduct experiments with four different horizontal resolutions, namely 4, 2, 1 and 0.75 km. We run the simulation based on the ISMIP6 core climate forcing from the MIROC5 global circulation model (GCM) under the highemission Representative Concentration Pathway (RCP) 8.5 scenario and consider both atmospheric and oceanic forcing in full and separate scenarios. Under the full scenarios, finer simulations unveil up to approximately 5% more sea-level rise compared to the coarser resolution. The sensitivity depends on the magnitude of outlet glacier retreat, which is implemented as a series of retreat masks following the ISMIP6 protocol. Without imposed retreat under atmosphereonly forcing, the resolution dependency exhibits an opposite behaviour with approximately 5% more sea-level contribution in the coarser resolution. The sea-level contribution indicates a converging behaviour below a 1 km horizontal resolution. A driving mechanism for differences is the ability to resolve the bedrock topography, which highly controls ice discharge to the ocean. Additionally, thinning and acceleration emerge to propagate further inland in high resolution for many glaciers. A major response mechanism is sliding, with an enhanced feedback on the effective normal pressure at higher resolution leading to a larger increase in sliding speeds under scenarios with outlet glacier retreat., SCOPUS: ar.j, info:eu-repo/semantics/published

Basal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water. To date, basal motion represents one of the largest uncertainties in ice-flow models. It is that component of the total flow velocity that can change most rapidly and can, therefore, facilitate rapid variations in dynamic behaviour. In this study, we investigate the subglacial properties of the East Antarctic Ice Sheet by statistically analysing the roughness of the bed topography, inferred from radio-echo sounding measurements. We analyse two sets of roughness parameters, one derived in the spatial and the other in the spectral domain, with two roughness parameters each. This enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet. We further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet. We find that one of the roughness parameters, the Hurst exponent derived in the spatial domain, coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity.

Ice shelves play a key role in the stability of the Antarctic Ice Sheet due to their buttressing effect. A loss of buttressing as a result of increased basal melting or ice shelf disintegration will lead to increased ice discharge. Some ice shelves exhibit channels at the base that are not yet fully understood. In this study, we present in-situ melt rates of a channel which is up to 330 m high and located at the southern Filchner Ice Shelf. Maximum observed melt rates are 2.3 m a−1. Melt rates decline inside the channel along flow and turn into freezing 55 km downstream of the grounding line. While closer to the grounding line melt rates are higher within the channel than outside, this reverses further downstream. Comparing the evolution of this channel under present-day climate conditions over 250 years with its present geometry reveals a mismatch. This mismatch indicates melt rates two times higher were necessary over the past 250 years to form today's channel geometry. In contrast, forcing the model with present-day melt rates results in a closure of the channel, which contradicts observations. Time series of melt rate measurements show strong tidally-induced variability in vertical strain-rates. We found no evidence of seasonality, but discrete pulses of increased melting occurred throughout the measurement period. The type of melt channel in this study diminishes with distance from the grounding line and are hence not a destabilizing factor for ice shelves.

Accurately modelling the contribution of Greenland and Antarctica to sea level rise requires solving partial differential equations at a high spatial resolution. In this paper, we discuss the scaling of the Ice-sheet and Sea-level System Model (ISSM) applied to the Greenland Ice Sheet with horizontal grid resolutions varying between 10 and 0.25 km. The model setup used as benchmark problem comprises a variety of modules with different levels of complexity and computational demands. The core builds the so-called stress balance module, which uses the higher-order approximation (or Blatter–Pattyn) of the Stokes equations, including free surface and ice-front evolution as well as thermodynamics in form of an enthalpy balance, and a mesh of linear prismatic finite elements, to compute the ice flow. We develop a detailed user-oriented, yet low-overhead, performance instrumentation tailored to the requirements of Earth system models and run scaling tests up to 6144 Message Passing Interface (MPI) processes. The results show that the computation of the Greenland model scales overall well up to 3072 MPI processes but is eventually slowed down by matrix assembly, the output handling and lower-dimensional problems that employ lower numbers of unknowns per MPI process. We also discuss improvements of the scaling and identify further improvements needed for climate research. The instrumented version of ISSM thus not only identifies potential performance bottlenecks that were not present at lower core counts but also provides the capability to continually monitor the performance of ISSM code basis. This is of long-term significance as the overall performance of ISSM model depends on the subtle interplay between algorithms, their implementation, underlying libraries, compilers, runtime systems and hardware characteristics, all of which are in a constant state of flux. We believe that future large-scale high-performance computing (HPC) systems will continue to employ the MPI-based programming paradigm on the road to exascale. Our scaling study pertains to a particular modelling setup available within ISSM and does not address accelerator techniques such as the use of vector units or GPUs. However, with 6144 MPI processes, we identified issues that need to be addressed in order to improve the ability of the ISSM code base to take advantage of upcoming systems that will require scaling to even higher numbers of MPI processes.

Full-Stokes (FS) ice sheet models provide the most sophisticated formulation of ice sheet flow. However, its applicability is often limited due to its high computational demand and its owing numerical challenges. To balance computational demand and accuracy, the so-called Blatter-Pattyn (BP) stress regime is frequently used. Here, we explore the dynamic consequences caused by solving FS and the BP stress regime applied to the central part of the North East Greenland Ice Stream (NEGIS). To ensure a consistent comparison, we use one single ice sheet model to run the simulations under identical numerical conditions. A sensitivity study to grid resolution reveals that velocity differences between the FS and BP solution emerge below ~1 km horizontal resolution and continuously increases with resolution. Generally, BP produces higher surface velocities than FS, at a resolution of 0.1 km up to 5.8 % on average. In an extreme case, estimated ice discharge rates are up to 8 % overestimated by BP; in a rather classical case, BP reveals up to 3 % more ice discharge. Based on these minor model disagreements and given other large uncertainties in ice sheet projections, we conclude that the use of FS seems not an urgent issue and takes a secondary role in narrowing uncertainties of current sea-level projections. However, the englacial advection schemes from both stress regimes indicate severe impacts on internal layers of ice sheets.