scholarly journals A confined–unconfined aquifer model for subglacial hydrology and its application to the North East Greenland Ice Stream

2017 ◽  
Author(s):  
Sebastian Beyer ◽  
Thomas Kleiner ◽  
Vadym Aizinger ◽  
Martin Rückamp ◽  
Angelika Humbert
Keyword(s):  
Computational Cost ◽  
Ice Sheet ◽  
Unconfined Aquifer ◽  
Ocean Modeling ◽  
Model Parameters ◽  
Efficient System ◽  
East Greenland ◽  
North East ◽  
The North ◽  
Ice Stream

Abstract. Subglacial hydrology plays an important role in the ice sheet dynamics as it determines the sliding velocity of ice sheets and also drives freshwater into the ocean. Modeling subglacial water has been a challenge for decades, and only recently new approaches have been developed such as representing subglacial channels and thin water sheets by separate layers of variable permeability. We extend this concept by modeling a confined and unconfined aquifer system (CUAS) in a single layer. The advantage of this formulation is that it prevents unphysical values of pressure at reasonable computational cost. We also performed sensitivity tests to investigate the effect of different model parameters. The strongest influence of model parameters was detected in terms governing the opening and closure of channels. Furthermore, we applied the model to the North East Greenland Ice Stream, where an efficient system independent of seasonal input was identified about 500 km downstream from the ice divide. Using the effective pressure from the hydrology model in the Ice Sheet System Model (ISSM) shows considerable improvements of modeled velocities in the coastal region.

scholarly journals A confined–unconfined aquifer model for subglacial hydrology and its application to the Northeast Greenland Ice Stream

2018 ◽  
Vol 12 (12) ◽  
pp. 3931-3947 ◽  
Author(s):  
Sebastian Beyer ◽  
Thomas Kleiner ◽  
Vadym Aizinger ◽  
Martin Rückamp ◽  
Angelika Humbert
Keyword(s):  
Computational Cost ◽  
Ice Sheet ◽  
Coastal Region ◽  
Single Layer ◽  
Unconfined Aquifer ◽  
Model Parameters ◽  
Efficient System ◽  
Aquifer System ◽  
Ice Stream ◽  
Ice Sheet Dynamics

Abstract. Subglacial hydrology plays an important role in ice sheet dynamics as it determines the sliding velocity. It also drives freshwater into the ocean, leading to undercutting of calving fronts by plumes. Modeling subglacial water has been a challenge for decades. Only recently have new approaches been developed such as representing subglacial channels and thin water sheets by separate layers of variable hydraulic conductivity. We extend this concept by modeling a confined–unconfined aquifer system (CUAS) in a single layer of an equivalent porous medium (EPM). The advantage of this formulation is that it prevents unphysical values of pressure at reasonable computational cost. We performed sensitivity tests to investigate the effect of different model parameters. The strongest influence of model parameters was detected in terms of governing the opening and closure of the system. Furthermore, we applied the model to the Northeast Greenland Ice Stream, where an efficient system independent of seasonal input was identified about 500 km downstream from the ice divide. Using the effective pressure from the hydrology model, the Ice Sheet System Model (ISSM) showed considerable improvements in modeled velocities in the coastal region.

scholarly journals The effect of the north-east ice stream on the Greenland ice sheet in changing climates

2007 ◽  
Vol 1 (1) ◽  
pp. 41-76 ◽  
Author(s):  
R. Greve ◽  
S. Otsu
Keyword(s):  
Large Scale ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Simulation Code ◽  
North East ◽  
The North ◽  
Ice Stream ◽  
Basal Sliding ◽  
Speed Up ◽  
Fast Flow

Abstract. The north-east Greenland ice stream (NEGIS) was discovered as a large fast-flow feature of the Greenland ice sheet by synthetic aperture radar (SAR) imaginary of the ERS-1 satellite. In this study, the NEGIS is implemented in the dynamic/thermodynamic, large-scale ice-sheet model SICOPOLIS (Simulation Code for POLythermal Ice Sheets). In the first step, we simulate the evolution of the ice sheet on a 10-km grid for the period from 250 ka ago until today, driven by a climatology reconstructed from a combination of present-day observations and GCM results for the past. We assume that the NEGIS area is characterized by enhanced basal sliding compared to the "normal", slowly-flowing areas of the ice sheet, and find that the misfit between simulated and observed ice thicknesses and surface velocities is minimized for a sliding enhancement by the factor three. In the second step, the consequences of the NEGIS, and also of surface-meltwater-induced acceleration of basal sliding, for the possible decay of the Greenland ice sheet in future warming climates are investigated. It is demonstrated that the ice sheet is generally very susceptible to global warming on time-scales of centuries and that surface-meltwater-induced acceleration of basal sliding can speed up the decay significantly, whereas the NEGIS is not likely to dynamically destabilize the ice sheet as a whole.

Consistent comparison of full-Stokes and higher-order approximation in the central North East Greenland Ice Stream

2021 ◽  
Author(s):  
Martin Rückamp ◽  
Thomas Kleiner ◽  
Angelika Humbert
Keyword(s):  
Ice Sheet ◽  
Higher Order ◽  
Surface Velocity ◽  
Numerical Comparison ◽  
Geophysical Research ◽  
East Greenland ◽  
North East ◽  
Aspect Ratios ◽  
Ice Stream ◽  
Depth Analysis

<p>Modelling ice sheet flow is at best modelled using the full-Stokes (FS) equations. However, it rarely sees application even in recent years due to its high computational demand and problems in numerical convergence due to the thin aspect ratio of ice sheets. For this reason, the modelling community has relied on simplified mathematical models, such as the three-dimensional Higher-Order (HO) approximation which neglects horizontal gradients of vertical velocities and bridging effects. Here, we conduct an analysis of the difference in stresses and velocity fields solving the FS system of equation and using two different types of HO approximations equivalent to LMLa (known as Blatter-Pattyn type) and LTSML according to Hindmarsh (2004). Our intention was to avoid any bias from a difference in discretization or implementation, therefore we implemented it in a single ice sheet model to avoid effects arising from discretization and implementation. We selected a subset of the North East Greenland Ice Stream (NEGIS) as investigation area. As differences between FS and HO emerge in regions with steep bedrock gradients or high aspect ratios, we step-wise increase spatial resolution from 12.8 km down to 0.1 km. Our analysis reveals that surface velocity differences between the FS and HO solution emerge below 1km horizontal resolution and increase with resolution. Compared to the absolute ice flow velocity, the relative error between FS and HO remains small. We present an in-depth analysis, that reveals that different factors are affected by the approximation, such as basal drag and rheology</p><p>References:<br>Hindmarsh, R. C. A.: A numerical comparison of approximations to the Stokes equations used in ice sheet and glacier modeling, Journal of Geophysical Research: Earth Surface, 109, https://doi.org/10.1029/2003JF000065, 2004</p>

Marine geophysical evidence for former expansion and flow of the Greenland Ice Sheet across the north-east Greenland continental shelf

2009 ◽  
Vol 24 (3) ◽  
pp. 279-293 ◽  
Author(s):  
Jeffrey Evans ◽  
Colm Ó Cofaigh ◽  
Julian A. Dowdeswell ◽  
Peter Wadhams
Keyword(s):  
Continental Shelf ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
East Greenland ◽  
North East ◽  
The North

scholarly journals Calculated Patterns of Accumulation on the Greenland Ice Sheet

1967 ◽  
Vol 6 (48) ◽  
pp. 795-803 ◽  
Author(s):  
Steven J. Mock
Keyword(s):  
Mass Balance ◽  
Transition Zone ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
East Greenland ◽  
North East ◽  
The North ◽  
Regression Techniques ◽  
Independent Parameters ◽  
North Greenland

All available mean annual accumulation data on the Greenland ice sheet (excluding the Thule peninsula) have been collected and analyzed using multiple regression techniques to develop equations capable of predicting mean annual accumulation. The analysis was carried out for north Greenland, south Greenland, and for the transition zone between the two major regions. The resulting equations show that mean annual accumulation can be predicted from the independent parameters latitude, longitude and elevation. The patterns of accumulation are shown in a series of isohyetal (contours of accumulation in terms of water) maps. The major feature shown is a well defined asymmetry in accumulation; a pronounced east-slope maximum in south Greenland and an equally pronounced west-slope maximum in north Greenland. Poleward of lat. 69° N., isohyets decrease in elevation to the north. Mean annual accumulation ranges from >90 g./cm.2in south-east Greenland to <15 g./cm.2in north-east Greenland. A brief discussion of mass balance estimates of the Greenland ice sheet and of the relevance of this study to them is included.

scholarly journals Heat flux and the North East Greenland Ice Stream

2021 ◽  
Author(s):  
Paul D. Bons ◽  
Tamara de Riese ◽  
Steven Franke ◽  
Maria-Gema Llorens ◽  
Till Sachau ◽  
...  
Keyword(s):  
Heat Flux ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Geothermal Heat ◽  
Radiogenic Heat ◽  
East Greenland ◽  
North East ◽  
Ice Stream

<p>The prominent North East Greenland Ice Stream (NEGIS) is an exceptionally large ice stream in the Greenland Ice sheet. It is over 500 km long, originates almost at the central ice divide, and contributes significantly to overall ice drainage from the Greenland Ice sheet. Surface velocities in the inland part of the ice stream are several times higher inside NEGIS than in the adjacent ice sheet. Modelling NEGIS is still a challenge as it remains unclear what actually causes and controls the ice stream.</p><p>An elevated geothermal heat flux is one of the factors that are being considered to trigger or drive the fast flow inside NEGIS. Unfortunately, the geothermal heat flux below NEGIS and its upstream area is poorly constrained and estimates vary from close to the global average for continental crust (ca. 60 mW/m<sup>2</sup>) to values as high as almost 1000 mW/m<sup>2</sup>. The latter would cause about 10 cm/yr of melting at the base of the ice sheet.</p><p>We present a brief survey of global geothermal heat flux data, especially from known hotspots, such as Iceland and Yellowstone. Heat fluxes in these areas that are known to be among the hottest on Earth rarely, if ever, exceed 300 mW/m<sup>2</sup>. A plume hotspot or its trail can therefore not cause heat fluxes at the high end of the suggested range. Other potential factors, such as hydrothermal fluid flow and radiogenic heat, also cannot raise the heat flux significantly. We conclude that the heat flux at NEGIS is very unlikely to exceed 100-150 mW/m<sup>2</sup>, and future modelling studies on NEGIS should thus be mindful of implementing realistic geothermal heat flux values. If NEGIS is not the result of an exceptionally high heat flux, we are left with the exciting challenge to find the true trigger of this fascinating structure.</p>

scholarly journals Calculated Patterns of Accumulation on the Greenland Ice Sheet

1967 ◽  
Vol 6 (48) ◽  
pp. 795-803 ◽  
Author(s):  
Steven J. Mock
Keyword(s):  
Mass Balance ◽  
Transition Zone ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
East Greenland ◽  
North East ◽  
The North ◽  
Regression Techniques ◽  
Independent Parameters ◽  
North Greenland

All available mean annual accumulation data on the Greenland ice sheet (excluding the Thule peninsula) have been collected and analyzed using multiple regression techniques to develop equations capable of predicting mean annual accumulation. The analysis was carried out for north Greenland, south Greenland, and for the transition zone between the two major regions. The resulting equations show that mean annual accumulation can be predicted from the independent parameters latitude, longitude and elevation. The patterns of accumulation are shown in a series of isohyetal (contours of accumulation in terms of water) maps. The major feature shown is a well defined asymmetry in accumulation; a pronounced east-slope maximum in south Greenland and an equally pronounced west-slope maximum in north Greenland. Poleward of lat. 69° N., isohyets decrease in elevation to the north. Mean annual accumulation ranges from >90 g./cm.2 in south-east Greenland to <15 g./cm.2 in north-east Greenland. A brief discussion of mass balance estimates of the Greenland ice sheet and of the relevance of this study to them is included.

scholarly journals Upstream flow effects revealed in the EastGRIP ice-core using a Monte Carlo inversion of a 2D ice-flow model

2021 ◽  
Author(s):  
Tamara Annina Gerber ◽  
Christine Hvidberg ◽  
Aslak Grinsted ◽  
Daniela Jansen ◽  
Steven Franke ◽  
...  
Keyword(s):  
Ice Core ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Source Area ◽  
Model Parameters ◽  
Ice Flow ◽  
East Greenland ◽  
Ice Stream ◽  
Flow Effects ◽  
Upstream Flow

&lt;p&gt;The North East Greenland ice-stream (NEGIS) is the largest active ice-stream on the Greenland ice-sheet and is a crucial contributor to the ice-sheet mass balance. To investigate the ice-stream dynamics and to gain information about the past climate, a deep ice-core is drilled in the upstream part of the NEGIS, termed the East Greenland ice-core project (EastGRIP). Upstream flow effects introduce non-climatic bias in ice-cores and are particularly strong at EastGRIP due to high ice-flow velocities and the location in an ice-stream on the eastern flank of the Greenland ice-sheet. Understanding and ultimately correcting for such effects requires information on the source area and the local atmospheric conditions at the time of ice deposition. We use a two-dimensional Dansgaard-Johnsen model to simulate ice-flow along three approximated flow-lines between the summit of the ice-sheet and EastGRIP. Model parameters are determined using a Monte Carlo inversion by minimizing the misfit between modeled isochrones and isochrones observed in radio-echo-sounding images. We calculate backward-in-time particle trajectories to determine the source area of ice found in the EastGRIP core today and present estimates of surface elevation and past accumulation-rates at the deposition site. The thinning function and accumulated strain obtained from the modeled velocity field provide useful information on the deformation history in the EastGRIP ice. Our results indicate that increased accumulation in the upstream area is predominantly responsible for the constant annual layer thickness observed in the upper part of the ice column at EastGRIP. Inverted model parameters suggest that the imprint of basal melting and sliding is present in large parts along the flow profiles and that most internal ice deformation happens close to the bedrock. The results of this study can act as a basis for applying upstream corrections to a variety of ice-core measurements, and the model parameters can be useful constraints for more sophisticated modeling approaches in the future.&amp;#160;&lt;/p&gt;

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