scholarly journals Thermal structure and basal sliding parametrisation at Pine Island Glacier – a 3-D full-Stokes model study

2015 ◽  
Vol 9 (2) ◽  
pp. 675-690 ◽  
Author(s):  
N. Wilkens ◽  
J. Behrens ◽  
T. Kleiner ◽  
D. Rippin ◽  
M. Rückamp ◽  
...  
Keyword(s):  
Flow Structure ◽  
Thermal Structure ◽  
Complex Surface ◽  
Surface Flow ◽  
Surface Velocity ◽  
Model Study ◽  
Basal Sliding ◽  
Modelling Studies ◽  
Global Sea Level ◽  
The Antarctic

Abstract. Pine Island Glacier is one of the fastest changing glaciers of the Antarctic Ice Sheet and therefore of scientific interest. The glacier holds enough ice to raise the global sea level significantly (~ 0.5 m) when fully melted. The question addressed by numerous modelling studies of the glacier focuses on whether the observed changes are a start of an uncontrolled and accelerating retreat. The movement of the glacier is, in the fast-flowing areas, dominated by basal motion. In modelling studies the parametrisation of the basal motion is therefore crucial. Inversion methods are commonly applied to reproduce the complex surface flow structure of Pine Island Glacier by using information of the observed surface velocity field to constrain, among other things, basal sliding. We introduce two different approaches of combining a physical parameter, the basal roughness, with basal sliding parametrisations. This way basal sliding is again connected closer to its original formulation. We show that the basal roughness is an important and helpful parameter to consider and that many features of the flow structure can be reproduced with these approaches.

scholarly journals Thermal structure and basal sliding parametrisation at Pine Island Glacier – a 3-D full-Stokes model study

2014 ◽  
Vol 8 (5) ◽  
pp. 4913-4957
Author(s):  
N. Wilkens ◽  
J. Behrens ◽  
T. Kleiner ◽  
D. Rippin ◽  
M. Rückamp ◽  
...  
Keyword(s):  
Flow Structure ◽  
Thermal Structure ◽  
Complex Surface ◽  
Surface Flow ◽  
Surface Velocity ◽  
Model Study ◽  
Basal Sliding ◽  
Modelling Studies ◽  
Global Sea Level ◽  
The Antarctic

Abstract. Pine Island Glacier is one of the fastest changing glaciers in the Antarctic Ice Sheet and therefore in scientific focus. The glacier holds enough ice to raise global sea level significantly (∼0.5 m), when fully melted. The question addressed by numerous modelling studies of the glacier focuses on whether the observed changes are a start for an uncontrolled and accelerating retreat. The movement of the glacier is, in the fast flowing areas, dominated by basal motion. In modelling studies the parametrisation of the basal motion is therefore crucial. Inversion methods are commonly applied to reproduce the complex surface flow structure at Pine Island Glacier, which use information of the observed surface velocity field, to constrain basal sliding. We introduce two different approaches of combining a physical parameter, the basal roughness, with basal sliding parametrisations. This way basal sliding is connected again to its original formulation. We show that the basal roughness is an important and helpful parameter to consider and that many features of the flow structure could be reproduced with these approaches.

scholarly journals Changes in ice-shelf buttressing following the collapse of Larsen A Ice Shelf, Antarctica, and the resulting impact on tributaries

2016 ◽  
Vol 62 (235) ◽  
pp. 905-911 ◽  
Author(s):  
SAM ROYSTON ◽  
G. HILMAR GUDMUNDSSON
Keyword(s):  
Antarctic Peninsula ◽  
Surface Velocity ◽  
Ice Shelf ◽  
Mass Redistribution ◽  
Grounding Line ◽  
Modelling Studies ◽  
Speed Up ◽  
Stringent Test ◽  
The Antarctic ◽  
The Impact

ABSTRACTThe dominant mass-loss process on the Antarctic Peninsula has been ice-shelf collapse, including the Larsen A Ice Shelf in early 1995. Following this collapse, there was rapid speed up and thinning of its tributary glaciers. We model the impact of this ice-shelf collapse on upstream tributaries, and compare with observations using new datasets of surface velocity and ice thickness. Using a two-horizontal-dimension shallow shelf approximation model, we are able to replicate the observed large increase in surface velocity that occurred within Drygalski Glacier, Antarctic Peninsula. The model results show an instantaneous twofold increase in flux across the grounding line, caused solely from the reduction in backstress through ice shelf removal. This demonstrates the importance of ice-shelf buttressing for flow upstream of the grounding line and highlights the need to explicitly include lateral stresses when modelling real-world settings. We hypothesise that further increases in velocity and flux observed since the ice-shelf collapse result from transient mass redistribution effects. Reproducing these effects poses the next, more stringent test of glacier and ice-sheet modelling studies.

scholarly journals Basal Sliding and Conditions at the Glacier Bed as Revealed by Bore-hole Photography

1978 ◽  
Vol 20 (84) ◽  
pp. 469-508 ◽  
Author(s):  
H. F. Engelhardt ◽  
W. D. Harrison ◽  
Barclay Kamb
Keyword(s):  
Flow Direction ◽  
Water Pressure ◽  
Surface Flow ◽  
Water Levels ◽  
Surface Velocity ◽  
Internal Motions ◽  
Basal Sliding ◽  
Bed Roughness ◽  
Atmospheric Factors ◽  
Basal Shear

AbstractBore-hole photography demonstrates that the glacier bed was reached by cable-tool drilling in five bore holes in Blue Glacier, Washington. Basal sliding velocities measured by bore-hole photography, and confirmed by inclinometry, range from 0.3 to 3.0 cm/d and average 1.0 cm/d, much less than half the surface velocity of 15 cm/d. Sliding directions deviate up to 30° from the surface flow direction. Marked lateral and time variations in sliding velocity occur. The glacier bed consists of bedrock overlain by a ≈ 10 cm layer ofactive subsole drift, which intervenes between bedrock and ice sole and is actively involved in the sliding process. It forms a mechanically and visibly distinct layer, partially to completely ice-free, beneath the zone of debris-laden ice at the base of the glacier. Internal motions in the subsole drift include rolling of clasts caught between bedrock and moving ice. The largest sliding velocities occur in places where a basal gap, of width up to a few centimeters, intervenes between ice sole and subsole drift. The gap may result from ice—bed separation due to pressurization of the bed by bore-hole water. Water levels in bore holes reaching the bed drop to the bottom when good hydraulic connection is established with sub-glacial conduits; the water pressure in the conduits is essentially atmospheric. Factors responsible for the generally low sliding velocities are high bed roughness due to subsole drift, partial support of basal shear stress by rock friction, and minimal basal cavitation because of low water pressure in subglacial conduits. The observed basal conditions do not closely correspond to those assumed in existing theories of sliding.

scholarly journals Thermal structure of Dome Fuji and east Dronning Maud Land, Antarctica, simulated by a three-dimensional ice-sheet model

2004 ◽  
Vol 39 ◽  
pp. 433-438 ◽  
Author(s):  
Fuyuki Saito ◽  
Ayako Abe-Ouchi
Keyword(s):  
Thermal Structure ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Glacial Cycles ◽  
Geothermal Heat ◽  
Modelling Studies ◽  
Dronning Maud Land ◽  
Pressure Melting ◽  
The Antarctic ◽  
Good Agreement

AbstractThree-dimensional structures of temperature focused on Dome Fuji and east Dronning Maud Land, Antarctica, simulated in a three-dimensional shallow ice model, are reported. With a geothermal heat flux of 54.6 mWm–2, as used in several modelling studies of the Antarctic ice sheet, and an enhancement factor of 1.3, which is smaller than in previous studies, the model result taking into account the glacial cycles is in good agreement with the borehole temperature and surface topography at Dome Fuji. The basal temperature at Dome Fuji must be at or very close to the pressure-melting point. The simulated amplitude of basal temperature through glacial/interglacial cycles is <1 K.

scholarly journals Surface-velocity and strain-rate variations at the glacier Austre Okstindbreen, Okstindan, Norway, 1976–95

1997 ◽  
Vol 24 ◽  
pp. 320-325
Author(s):  
Frank M. Jacobsen ◽  
Wilfred H. Theakstone ◽  
N. Tvis Knudsen
Keyword(s):  
Cross Section ◽  
Surface Flow ◽  
Horizontal Component ◽  
Equilibrium Line ◽  
Surface Velocity ◽  
Large Area ◽  
Equilibrium Line Altitude ◽  
Basal Sliding ◽  
Internal Deformation ◽  
The Mean

Long-term observations of surface velocities and strain rates at the Norwegian glacier Austre Okstindbreen revealed both temporal and spatial variations. During a period of 6 years, the amount of ice passing through a cross-section slightly below the mean equilibrium-line altitude (1250 m) was some 30% less than the amount which accumulated above the equilibrium line. The mean horizontal component of surface velocity at the centre of the cross-section was of the order of 45–50 m a−1, whilst the thinner marginal ice moved less rapidly. At an altitude of about 1230–1200 m, surface velocities generally increased as the ice entered a steep icefall. In the lower part of the icefall, mean surface velocities again were of the order of 50 m a−1. From there, a general decrease down-glacier was evident, and longitudinal compression along the curving centre line of flow was accompanied by lateral extension. The contribution of internal deformation to surface flow at the lower part of the glacier, which was less than 150 m thick, is likely to have been relatively small, and between-year variations of the horizontal component of surface flow which affected a large area probably were a response to changes of basal sliding rates, reflecting variations of mass balance and water availability.

scholarly journals Basal Sliding and Conditions at the Glacier Bed as Revealed by Bore-hole Photography

1978 ◽  
Vol 20 (84) ◽  
pp. 469-508 ◽  
Author(s):  
H. F. Engelhardt ◽  
W. D. Harrison ◽  
Barclay Kamb
Keyword(s):  
Flow Direction ◽  
Water Pressure ◽  
Surface Flow ◽  
Water Levels ◽  
Surface Velocity ◽  
Internal Motions ◽  
Basal Sliding ◽  
Bed Roughness ◽  
Atmospheric Factors ◽  
Basal Shear

AbstractBore-hole photography demonstrates that the glacier bed was reached by cable-tool drilling in five bore holes in Blue Glacier, Washington. Basal sliding velocities measured by bore-hole photography, and confirmed by inclinometry, range from 0.3 to 3.0 cm/d and average 1.0 cm/d, much less than half the surface velocity of 15 cm/d. Sliding directions deviate up to 30° from the surface flow direction. Marked lateral and time variations in sliding velocity occur. The glacier bed consists of bedrock overlain by a ≈ 10 cm layer of active subsole drift, which intervenes between bedrock and ice sole and is actively involved in the sliding process. It forms a mechanically and visibly distinct layer, partially to completely ice-free, beneath the zone of debris-laden ice at the base of the glacier. Internal motions in the subsole drift include rolling of clasts caught between bedrock and moving ice. The largest sliding velocities occur in places where a basal gap, of width up to a few centimeters, intervenes between ice sole and subsole drift. The gap may result from ice—bed separation due to pressurization of the bed by bore-hole water. Water levels in bore holes reaching the bed drop to the bottom when good hydraulic connection is established with sub-glacial conduits; the water pressure in the conduits is essentially atmospheric. Factors responsible for the generally low sliding velocities are high bed roughness due to subsole drift, partial support of basal shear stress by rock friction, and minimal basal cavitation because of low water pressure in subglacial conduits. The observed basal conditions do not closely correspond to those assumed in existing theories of sliding.

Measurements of the Free Surface Flow Structure Under an Impinging, Free Liquid Jet

1992 ◽  
Vol 114 (1) ◽  
pp. 79-84 ◽  
Author(s):  
J. Stevens ◽  
B. W. Webb
Keyword(s):  
Free Surface ◽  
Flow Structure ◽  
Liquid Layer ◽  
Free Surface Flow ◽  
Reynolds Numbers ◽  
Surface Flow ◽  
Surface Velocity ◽  
Liquid Jet ◽  
Radial Coordinate ◽  
Measured Velocity

The objective of this research was to characterize the flow structure under an impinging liquid jet striking a flat, normally oriented surface. The approach was the measurement of the free surface velocities of the jet prior to impingement and the surface velocities of the radially spreading liquid layer. A novel laser-Doppler velocimetry technique was used. The LDV system was configured such that the measurement volume would span the time-dependent fluctuations of the free surface, with the surface velocity being measured. The mean and fluctuating components of a single direction of the velocity vector were measured. It was found that the radial liquid layer data collapsed well over the range of jet Reynolds numbers 16,000 < Re < 47,000 if plotted in dimensionless coordinates, where the measured velocity was normalized by the average jet exit velocity and the radial coordinate was normalized by the nozzle diameter. Mean liquid layer depths were inferred from the velocity measurements by assuming a velocity profile across the layer, and were reported. Pre-impingement jet measurements suggest that the flow development is nearly complete two diameters from the nozzle exit.

Surface-velocity and strain-rate variations at the glacier Austre Okstindbreen, Okstindan, Norway, 1976–95

1997 ◽  
Vol 24 ◽  
pp. 320-325 ◽  
Author(s):  
Frank M. Jacobsen ◽  
Wilfred H. Theakstone ◽  
N. Tvis Knudsen
Keyword(s):  
Cross Section ◽  
Surface Flow ◽  
Horizontal Component ◽  
Equilibrium Line ◽  
Surface Velocity ◽  
Large Area ◽  
Equilibrium Line Altitude ◽  
Basal Sliding ◽  
Internal Deformation ◽  
The Mean

Long-term observations of surface velocities and strain rates at the Norwegian glacier Austre Okstindbreen revealed both temporal and spatial variations. During a period of 6 years, the amount of ice passing through a cross-section slightly below the mean equilibrium-line altitude (1250 m) was some 30% less than the amount which accumulated above the equilibrium line. The mean horizontal component of surface velocity at the centre of the cross-section was of the order of 45–50 m a−1, whilst the thinner marginal ice moved less rapidly. At an altitude of about 1230–1200 m, surface velocities generally increased as the ice entered a steep icefall. In the lower part of the icefall, mean surface velocities again were of the order of 50 m a−1. From there, a general decrease down-glacier was evident, and longitudinal compression along the curving centre line of flow was accompanied by lateral extension. The contribution of internal deformation to surface flow at the lower part of the glacier, which was less than 150 m thick, is likely to have been relatively small, and between-year variations of the horizontal component of surface flow which affected a large area probably were a response to changes of basal sliding rates, reflecting variations of mass balance and water availability.

On the thickness of the Antarctic ice, and its relations to that of the glacial epoch

2021 ◽  
pp. 1-8
Author(s):  
James CROLL ◽  
David SUGDEN
Keyword(s):  
Thermal Regime ◽  
Freezing Point ◽  
Ice Sheet ◽  
Heat Sources ◽  
Mean Velocity ◽  
Point Estimates ◽  
Glacial Epoch ◽  
Antarctic Continent ◽  
Global Sea Level ◽  
The Antarctic

ABSTRACT At a time when nobody has yet landed on the Antarctic continent (1879), this presentation and accompanying paper predicts the morphology, dynamics and thermal regime of the Antarctic ice sheet. Mathematical modelling of the ice sheet is based on the assumptions that the thickness of tabular icebergs reflects the average thickness of the ice at the margin and that the surface gradients are comparable to those of reconstructed former ice sheets in the Northern Hemisphere. The modelling shows that (a) ice is thickest near the centre at the South Pole and thins towards the margin; (b) the thickness at the pole is independent of the amount of snowfall at that place; and (c) the mean velocity at the margin, assuming a mean annual snowfall of two inches per year, is 400–500 feet per year. The thermal regime of the ice sheet is influenced by three heat sources – namely, the bed, the internal friction of ice flow and the atmosphere. The latter is the most significant and, since ice has a downwards as well as horizontal motion, this carries cold ice down into the ice sheet. Since the temperature at which ice melts is lowered by pressure at a rate of 0.0137 °F for every atmosphere of pressure (something known since 1784), much of the ice sheet and its base must be below the freezing point. Estimates of the thickness of ice at the centre depend closely on the surface gradients assumed and range between 3 and 24 miles. Such uncertainty is of concern since both the volume and gravitational attraction of the ice mass have an effect on global sea level. In order to improve our estimate of the volume of ice, we will have to wait 76 years for John Glen to develop a realistic flow law for ice.

Seeding metrics for image velocimetry performances in rivers

2021 ◽  
Author(s):  
Silvano Fortunato Dal Sasso ◽  
Alonso Pizarro ◽  
Sophie Pearce ◽  
Ian Maddock ◽  
Matthew T. Perks ◽  
...  
Keyword(s):  
Spatial Distribution ◽  
Optical Sensors ◽  
Large Scale ◽  
Surface Flow ◽  
Surface Velocity ◽  
Seeding Density ◽  
Earth System ◽  
Science Data ◽  
Image Velocimetry ◽  
Hydraulic Conditions

&lt;p&gt;Optical sensors coupled with image velocimetry techniques are becoming popular for river monitoring applications. In this context, new opportunities and challenges are growing for the research community aimed to: i) define standardized practices and methodologies; and ii) overcome some recognized uncertainty at the field scale. At this regard, the accuracy of image velocimetry techniques strongly depends on the occurrence and distribution of visible features on the water surface in consecutive frames. In a natural environment, the amount, spatial distribution and visibility of natural features on river surface are continuously challenging because of environmental factors and hydraulic conditions. The dimensionless seeding distribution index (SDI), recently introduced by Pizarro et al., 2020a,b and Dal Sasso et al., 2020, represents a metric based on seeding density and spatial distribution of tracers for identifying the best frame window (FW) during video footage. In this work, a methodology based on the SDI index was applied to different study cases with the Large Scale Particle Image Velocimetry (LSPIV) technique. Videos adopted are taken from the repository recently created by the COST Action Harmonious, which includes 13 case study across Europe and beyond for image velocimetry applications (Perks et al., 2020). The optimal frame window selection is based on two criteria: i) the maximization of the number of frames and ii) the minimization of SDI index. This methodology allowed an error reduction between 20 and 39% respect to the entire video configuration. This novel idea appears suitable for performing image velocimetry in natural settings where environmental and hydraulic conditions are extremely challenging and particularly useful for real-time observations from fixed river-gauged stations where an extended number of frames are usually recorded and analyzed.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References &lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Dal Sasso S.F., Pizarro A., Manfreda S.,&amp;#160;Metrics for the Quantification of Seeding Characteristics to Enhance Image Velocimetry Performance in Rivers.&amp;#160;Remote Sensing,&amp;#160;12, 1789 (doi: 10.3390/rs12111789), 2020.&lt;/p&gt;&lt;p&gt;Perks M. T., Dal Sasso S. F., Hauet A., Jamieson E., Le Coz J., Pearce S., &amp;#8230;Manfreda S, Towards harmonisation of image velocimetry techniques for river surface velocity observations. Earth System Science Data, https://doi.org/10.5194/essd-12-1545-2020, 12(3), 1545 &amp;#8211; 1559, 2020.&lt;/p&gt;&lt;p&gt;Pizarro A., Dal Sasso S.F., Manfreda S.,&amp;#160;Refining image-velocimetry performances for streamflow monitoring: Seeding metrics to errors minimisation,&amp;#160;Hydrological Processes, (doi: 10.1002/hyp.13919), 1-9, 2020.&lt;/p&gt;&lt;p&gt;Pizarro A., Dal Sasso S.F., Perks M. and Manfreda S., Identifying the optimal spatial distribution of tracers for optical sensing of stream surface flow, Hydrology and Earth System Sciences, 24, 5173&amp;#8211;5185, (10.5194/hess-24-5173-2020), 2020.&lt;/p&gt;

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