scholarly journals Basal conditions at Engabreen, Norway, inferred from surface measurements and inverse modelling

2018 ◽  
Vol 64 (246) ◽  
pp. 555-567
Author(s):  
ANNE M. SOLGAARD ◽  
ALEXANDRA MESSERLI ◽  
THOMAS SCHELLENBERGER ◽  
CHRISTINE S. HVIDBERG ◽  
ASLAK GRINSTED ◽  
...  
Keyword(s):  
Surface Velocity ◽  
Direct Access ◽  
Outlet Glacier ◽  
Basal Friction ◽  
Ice Cap ◽  
Basal Sliding ◽  
Hydrological System ◽  
Surface Measurements ◽  
Essential Input ◽  
Strain Heating

ABSTRACTEngabreen is an outlet glacier of the Svartisen Ice Cap located in Northern Norway. It is a unique glacier due to the Svartisen Subglacial Laboratory which allows direct access to the glacier bed. In this study, we combine both sub- and supraglacial observations with ice-flow modelling in order to investigate conditions at the bed of Engabreen both spatially and temporally. We use the full-Stokes model Elmer/Ice and satellite-based surface-velocity maps from 2010 and 2014 to infer patterns of basal friction. Direct measurements of basal sliding and deformation of lower layers of the ice are used to adjust the ice viscosity and provide essential input to the setup of our model and influence the interpretation of the results. We find a clear seasonal cycle in the subglacial conditions at the higher elevation region of the study area and discuss this in relation to the subglacial hydrological system. Our results also reveal an area with an overdeepening where basal friction is significantly lower than elsewhere on the glacier all year round. We attribute this to either water pooling at the base, or saturated sediments and increased strain heating at this location which softens the ice further.

scholarly journals Observation of recent surges of Vatnajökull, Iceland, by means of ERS SAR interferometry

2003 ◽  
Vol 37 ◽  
pp. 69-76 ◽  
Author(s):  
Andrea Fischer ◽  
Helmut Rott ◽  
Helgi Björnsson
Keyword(s):  
Sar Interferometry ◽  
Surface Velocity ◽  
Outlet Glacier ◽  
Ice Cap ◽  
Glacier Terminus ◽  
New Information ◽  
Basal Sliding ◽  
Velocity Changes ◽  
Extended Area ◽  
In The Beginning

AbstractRecent surges of two outlet glaciers of the Vatnajökull ice cap, Iceland, were observed using European Remote-sensing Satellite (ERS) synthetic aperture radar (SAR) tandem interferograms from12 different dates between December 1995 and January 2000. ERS SAR interferometry provided new information on the temporal and spatial variations in surface velocity during surges, after fieldwork became impossible. The area affected by the surge and therefore by increased basal sliding was delineated. Themigration of flow divides on the ice cap during a surge was described. At Sylgjujökull, a western outlet glacier covering an area of 175 km2, the fully developed surge and its abating phase were studied. Over a period of 4 2 years after December 1995, the ice motion decreased steadily, with initially the highest velocities and subsequently the most pronounced decrease in velocity at the glacier terminus. The surge of Dyngjujökull, a northern outlet glacier covering an area of 1040 km2, reached its maximum in 1999/2000. Slow acceleration over an area of about 200 km2 was first observed between March 1996 and January 1997. The interferogram from January 1999 shows a well-developed surge area, covering 210 km2. This area more than doubled by January 2000, with maximum velocities reaching >7 md–1. Between January 1997 and January 2000, the flow divide between Dyngju- and Skeiðararjökull shifted 16 km to the south. The investigations indicate that a surge cycle on these glaciers spans several years, with slowly increasing motion over an extended area in the beginning, and more pronounced velocity changes during the active surge phase lasting 1–2 years.

scholarly journals Flow dynamics and iceberg calving rates of Devon Ice Cap, Nunavut, Canada

2005 ◽  
Vol 51 (173) ◽  
pp. 219-230 ◽  
Author(s):  
David O. Burgess ◽  
Martin J. Sharp ◽  
Douglas W.F. Mair ◽  
Julian A. Dowdeswell ◽  
Toby J. Benham
Keyword(s):  
Flow Regimes ◽  
Flow Dynamics ◽  
Surface Velocity ◽  
Ice Flow ◽  
Eastern Margin ◽  
Ice Cap ◽  
Close Relationship ◽  
Basal Sliding ◽  
Devon Ice Cap ◽  
Iceberg Calving

AbstractThe surface velocity field of Devon Ice Cap, Nunavut, Canada, was mapped using interferometric synthetic aperture radar (InSAR). Ascending European Remote-sensing Satellite 1 and 2 (ERS-1/-2) tandem mode data were used for the western and southeast sectors, and 3 day repeat pass ERS-1 imagery for the northeast sector. Speckle-tracking procedures were used with RADARSAT 1 imagery to obtain surface velocities over the terminus of Belcher Glacier (a major calving front) where decorrelation between ERS data occurred. The InSAR data highlight a significant contrast in ice-flow dynamics between the east and west sides of the ice cap. Ice movement west of the main north–south divide is dominated by relatively uniform ‘sheet’ flow, but three fast-flowing outlet glaciers that extend 14–23km beyond the ice-cap margin also drain this region. Several outlet glaciers that extend up to 60 km inland from the eastern margin drain the eastern side of the ice cap. The dominant ice-flow regimes were classified based on the relationship between the driving stress (averaged over a length scale of ten ice thicknesses) and the ratio of surface velocity to ice thickness. The mapped distribution of flow regimes appears to depict the spatial extent of basal sliding across the ice cap. This is supported by a close relationship between the occurrence of flow stripes on the ice surface and flow regimes where basal sliding was found to be an important component of the glacier motion. Iceberg calving rates were computed using measured surface velocities and ice thicknesses derived from airborne radio-echo sounding. The volume of ice calved between 1960 and 1999 was estimated to be 20.5 ± 4.7 km3 (or 0.57 km3 a–1). Approximately 89% of this loss occurred along the eastern margin. The largest single source is Belcher Glacier, which accounts for ~50% of the total amount of ice calved.

scholarly journals Simulating the roles of crevasse routing of surface water and basal friction on the surge evolution of Basin 3, Austfonna ice-cap

2017 ◽  
Author(s):  
Yongmei Gong ◽  
Thomas Zwinger ◽  
Jan Åström ◽  
Bas Altena ◽  
Thomas Schellenberger ◽  
...  
Keyword(s):  
Control Method ◽  
Numerical Models ◽  
Frictional Heating ◽  
Element Model ◽  
Surface Velocity ◽  
Basal Friction ◽  
Ice Cap ◽  
Input Surface ◽  
Melt Water ◽  
Surface Melt

Abstract. The marine-terminating outlet in Basin 3, Austfonna ice-cap has been accelerating since the mid-1990s. Step-wise multiannual acceleration associated with seasonal summer speed-up events was observed before the outlet enters the basin-wide surge in autumn 2012. We use multiple numerical models to explore hydrologic activation mechanisms for the surge behavior. A continuum ice dynamic model is used to invert basal friction coefficient distributions using the control method and observed surface velocity data between April 2012 and July 2014. This provides an input to a discrete element model capable of simulating individual crevasses, with the aim of finding locations where summer melt water enters the glacier and reaches the bed. The possible flow paths of input surface melt water at the glacier bed and basal melt water are calculated according to the gradient of the hydraulic potential. The inverted friction coefficients show the unplugging of the stagnant ice front and expansion of low friction regions before the surge reaches its peak velocity in January 2013. Crevasse distribution reflects to a high degree the basal friction pattern. The melt water reaches the bed through the crevasses located above the margins of the sub-glacial valley and the basal melt that is generated mainly by frictional heating flows either to the fast flowing units or potentially gets accumulated in an over-deepened region. Based on these results, the mechanisms facilitated by basal melt water production, surface melt water and crevasse opening, for the surge in Basin 3 are discussed.

scholarly journals Investigating changes in basal conditions of Variegated Glacier prior to and during its 1982–1983 surge

2011 ◽  
Vol 5 (3) ◽  
pp. 659-672 ◽  
Author(s):  
M. Jay-Allemand ◽  
F. Gillet-Chaulet ◽  
O. Gagliardini ◽  
M. Nodet
Keyword(s):  
Flow Line ◽  
Stokes Problem ◽  
Water Pressure ◽  
Drainage System ◽  
Surface Velocity ◽  
Parameter Distribution ◽  
Basal Friction ◽  
Friction Parameter ◽  
Basal Sliding ◽  
Regular Increase

Abstract. Variegated Glacier (Alaska) is known to surge periodically after a sufficient amount of cumulative mass balance is reached, but this observation is difficult to link with changes in the basal conditions. Here, using a 10-yr dataset, consisting of surface topography and surface velocity observations along a flow line for 25 dates, we have reconstructed the evolution of the basal conditions prior to and during the 1982–1983 surge. The model solves the full-Stokes problem along the central flow line using the finite element method. For the 25 dates of the dataset, the basal friction parameter distribution is inferred using the inverse method proposed by Arthern and Gudmundsson (2010). This method is here slightly modified by incorporating a regularisation term in the cost function to avoid short wavelength changes in the friction parameter. Our results indicate that dramatic changes in the basal conditions occurred between 1973 to 1983. Prior to the surge, periodic changes can be observed between winter and summer, with a regular increase of the sliding from 1973 to 1982. During the surge, the basal friction decreased dramatically and an area of very low friction moved from the upper part of the glacier to its terminus. Using a more complex friction law, these changes in basal sliding are then interpreted in terms of basal water pressure. Our results support that dramatic changes took place in the subglacial drainage system of Variegated Glacier, moving from a relatively efficient drainage system prior to the surge to an inefficient one during the surge. By reconstructing the water pressure evolution at the base of the glacier it is possible to propose a scenario for the hydrological history leading to the occurrence of a surge.

scholarly journals Automated detection of basal icequakes and discrimination from surface crevassing

2019 ◽  
Vol 60 (79) ◽  
pp. 167-181 ◽  
Author(s):  
Thomas S. Hudson ◽  
Jonathan Smith ◽  
Alex M. Brisbourne ◽  
Robert S. White
Keyword(s):  
Seismic Energy ◽  
Automated Detection ◽  
Ice Streams ◽  
Glacier Surface ◽  
Ice Dynamics ◽  
Outlet Glacier ◽  
Ice Cap ◽  
Basal Sliding ◽  
Dynamics Models ◽  
Critical Process

ABSTRACTIcequakes at or near the bed of a glacier have the potential to allow us to investigate the interaction of ice with the underlying till or bedrock. Understanding this interaction is important for studying basal sliding of glaciers and ice streams, a critical process in ice dynamics models used to constrain future sea-level rise projections. However, seismic observations on glaciers can be dominated by seismic energy from surface crevassing. We present a method of automatically detecting basal icequakes and discriminating them from surface crevassing, comparing this method to a commonly used spectrum-based method of detecting icequakes. We use data from Skeidararjökull, an outlet glacier of the Vatnajökull Ice Cap, South-East Iceland, to demonstrate that our method outperforms the commonly used spectrum-based method. Our method detects a higher number of basal icequakes, has a lower rate of incorrectly identifying crevassing as basal icequakes and detects an additional, spatially independent basal icequake cluster. We also show independently that the icequakes do not originate from near the glacier surface. We conclude that the method described here is more effective than currently implemented methods for detecting and discriminating basal icequakes from surface crevassing.

scholarly journals Linking surface hydrology to flow regimes and patterns of velocity variability on Devon Ice Cap, Nunavut

2015 ◽  
Vol 61 (226) ◽  
pp. 387-399 ◽  
Author(s):  
Faye R. Wyatt ◽  
Martin J. Sharp
Keyword(s):  
Drainage System ◽  
System Structure ◽  
Surface Velocity ◽  
Point Distribution ◽  
Drainage Systems ◽  
Ice Cap ◽  
Basal Sliding ◽  
Landsat 7 ◽  
Devon Ice Cap ◽  
Glacier Flow

AbstractSupraglacial meltwater reaching a glacier bed can increase ice surface velocities via hydraulic jacking and enhanced basal sliding. However, the relationships between the structure of supraglacial drainage systems, sink-point distributions, glacier flow processes and the magnitude of interannual velocity variability are poorly understood. To explore the hypothesis that spatial variations in the rate and mechanisms of glacier flow are linked to variations in supraglacial drainage system structure and sink-point distribution across an ice cap, we mapped supraglacial drainage systems on Devon Ice Cap from Landsat-7 ETM+ imagery. Spatial patterns of surface velocity and interannual velocity variability were determined using gradient correlation applied to Landsat-7 ETM+ images. Velocity variability is greater in areas close to sink-point locations, presumably because hydrologically forced basal sliding and/or bed deformation are enhanced in such areas. The distribution and characteristics of supraglacial drainage systems may play an important role in determining the distribution of regions of basal sliding, highlighting the need for knowledge of the supraglacial drainage system structure and sink-point distribution to inform efforts to model the dynamic response of Arctic ice caps to future climate warming.

scholarly journals Sensitivity of basal conditions in an inverse model: Vestfonna Ice-Cap, Nordaustlandet/Svalbard

2012 ◽  
Vol 6 (1) ◽  
pp. 427-467 ◽  
Author(s):  
M. Schäfer ◽  
T. Zwinger ◽  
P. Christoffersen ◽  
F. Gillet-Chaulet ◽  
K. Laakso ◽  
...  
Keyword(s):  
Inverse Method ◽  
Velocity Fields ◽  
Flow Modeling ◽  
Velocity Data ◽  
Ice Flow ◽  
Outlet Glacier ◽  
Basal Friction ◽  
Ice Cap ◽  
Precise Knowledge ◽  
Temperature Errors

Abstract. The dynamics of Vestfonna ice-cap (Svalbard) is dominated by fast-flowing outlet glaciers. Its mass balance is poorly known and affected dynamically by these fast flowing outlet glaciers. Hence it is a challenging target for ice-flow modeling. Precise knowledge of the basal conditions and implementation of a good sliding law are crucial for the modeling of this ice-cap. Here we use the Full-Stokes finite element code Elmer/Ice to model the 3-D flow over the whole ice-cap. We use a Robin inverse method to infer the basal friction from the surface velocities observed in 1995. Our results illustrate the importance of the basal friction parameter in reproducing observed velocity fields. We also show the importance of having variable basal friction as given by the inverse method to reproduce the velocity fields of each outlet glacier – a simple parameterization of basal friction cannot give realistic velocities in a forward model. We study the robustness and sensitivity of this method with respect to different parameters (mesh characteristics, ice temperature, errors in topographic and velocity data). The uncertainty in the observational parameters and input data proved to be sufficiently small not to adversely affect the fidelity of the model.

scholarly journals Bayesian Inference of Ice Softness and Basal Sliding Parameters at Langjökull

2021 ◽  
Vol 9 ◽  
Author(s):  
Giri Gopalan ◽  
Birgir Hrafnkelsson ◽  
Guðfinna Aðalgeirsdóttir ◽  
Finnur Pálsson
Keyword(s):  
Statistical Models ◽  
Surface Velocity ◽  
Physical Parameters ◽  
Velocity Data ◽  
Ice Cap ◽  
Basal Sliding ◽  
Deformation Component ◽  
Ice Velocity ◽  
Spatially Varying ◽  
Sampling Approach

We develop Bayesian statistical models that are designed for the inference of ice softness and basal sliding parameters, important glaciological quantities. These models are applied to Langjökull, the second largest temperate ice cap in Iceland at about 900 squared kilometers in area. The models make use of a relationship between physical parameters and ice velocity as stipulated by a shallow ice approximation that is generally applicable to Langjökull. The posterior distribution for ice softness concentrates around 18.2 × 10−25s−1Pa−3; moreover, spatially varying basal sliding parameters are inferred allowing for the decomposition of velocity into a deformation component and a sliding component, with spatial variation consistent with previous studies. Bayesian computation is conducted with a Gibbs sampling approach. The paper serves as an example of statistical inference for ice softness and basal sliding parameters at temperate, shallow glaciers using surface velocity data.

scholarly journals Sensitivity of basal conditions in an inverse model: Vestfonna ice cap, Nordaustlandet/Svalbard

2012 ◽  
Vol 6 (4) ◽  
pp. 771-783 ◽  
Author(s):  
M. Schäfer ◽  
T. Zwinger ◽  
P. Christoffersen ◽  
F. Gillet-Chaulet ◽  
K. Laakso ◽  
...  
Keyword(s):  
Inverse Method ◽  
Velocity Fields ◽  
Flow Modeling ◽  
Velocity Data ◽  
Ice Flow ◽  
Outlet Glacier ◽  
Basal Friction ◽  
Ice Cap ◽  
Precise Knowledge ◽  
Temperature Errors

Abstract. The dynamics of Vestfonna ice cap (Svalbard) are dominated by fast-flowing outlet glaciers. Its mass balance is poorly known and affected dynamically by these fast-flowing outlet glaciers. Hence, it is a challenging target for ice flow modeling. Precise knowledge of the basal conditions and implementation of a good sliding law are crucial for the modeling of this ice cap. Here we use the full-Stokes finite element code Elmer/Ice to model the 3-D flow over the whole ice cap. We use a Robin inverse method to infer the basal friction from the surface velocities observed in 1995. Our results illustrate the importance of the basal friction parameter in reproducing observed velocity fields. We also show the importance of having variable basal friction as given by the inverse method to reproduce the velocity fields of each outlet glacier – a simple parametrization of basal friction cannot give realistic velocities in a forward model. We study the robustness and sensitivity of this method with respect to different parameters (mesh characteristics, ice temperature, errors in topographic and velocity data). The uncertainty in the observational parameters and input data proved to be sufficiently small as not to adversely affect the fidelity of the model.

scholarly journals Simulating the roles of crevasse routing of surface water and basal friction on the surge evolution of Basin 3, Austfonna ice cap

2018 ◽  
Vol 12 (5) ◽  
pp. 1563-1577 ◽  
Author(s):  
Yongmei Gong ◽  
Thomas Zwinger ◽  
Jan Åström ◽  
Bas Altena ◽  
Thomas Schellenberger ◽  
...  
Keyword(s):  
Control Method ◽  
Numerical Models ◽  
Frictional Heating ◽  
Element Model ◽  
Surface Velocity ◽  
Basal Friction ◽  
Ice Cap ◽  
Activation Mechanisms ◽  
Speed Up ◽  
High Degree

Abstract. The marine-terminating outlet in Basin 3, Austfonna ice cap, has been accelerating since the mid-1990s. Stepwise multi-annual acceleration associated with seasonal summer speed-up events was observed before the outlet entered the basin-wide surge in autumn 2012. We used multiple numerical models to explore hydrologic activation mechanisms for the surge behaviour. A continuum ice dynamic model was used to invert basal friction coefficient distributions using the control method and observed surface velocity data between April 2012 and July 2014. This has provided input to a discrete element model capable of simulating individual crevasses, with the aim of finding locations where meltwater entered the glacier during the summer and reached the bed. The possible flow paths of surface meltwater reaching the glacier bed as well as those of meltwater produced at the bed were calculated according to the gradient of the hydraulic potential. The inverted friction coefficients show the “unplugging” of the stagnant ice front and expansion of low-friction regions before the surge reached its peak velocity in January 2013. Crevasse distribution reflects the basal friction pattern to a high degree. The meltwater reaches the bed through the crevasses located above the margins of the subglacial valley and the basal melt that is generated mainly by frictional heating flows either to the fast-flowing units or potentially accumulates in an overdeepened region. Based on these results, the mechanisms facilitated by basal meltwater production, crevasse opening and the routing of meltwater to the bed are discussed for the surge in Basin 3.

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