bluuebunny/crossref_metadata_2025_split huggingface dataset:
A mesoscopic approach for modelling texture evolution of polar ice including recrystallization phenomena
Authors: Gödert Günter 
Published: November 2007

AbstractA material model for the simulation of anisotropic behaviour due to texture development in polar ice is presented. Emphasis is laid on the strain-induced texture development and its relaxation due to rotation recrystallization and grain boundary migration in the low-velocity regime. The model is based on two scales (mesoscopic approach). Kinematics, balance equations and constitutive assumptions are defined with respect to the grain level (mesoscale). Slip-system behaviour is assumed to be Newtonian. Recrystallization and grain boundary migration are taken into account via a diffusion-type evolution of the crystallites orientation. Due to the inextensibility of the ice crystallites along their c axes, the Sachs–Reuss assumption is adopted. Volume averaging yields associated macroscopic relations, where the internal structure is represented by a second-order structure tensor. The proposed approach is illustrated by applying it to initially isotropic material under homogeneous deformation, giving results qualitatively in agreement with experimental evidence. Finally, it is shown that the proposed model is, under some simplifying conditions, directly related to phenomenological internal variable models (e.g. Morland and Staroszczyk, 1998).
Using internal layers from the Greenland ice sheet, identified from radio-echo sounding data, with numerical models
Authors: Baldwin Duncan J. , Bamber Jonathan L. , Payne Antony J. , Layberry Russel L. 
Published: November 2007

AbstractSpatially extensive internal layers have been traced in airborne radio-echo sounding (RES) data collected over Greenland during the late 1990s. By linking internal layers within individual flight-lines at crossover points, it is possible to identify spatially continuous layers that are interpreted as isochronous surfaces. Several of the survey lines pass over the GRIP core site, and this allows us to use the published GRIP age–depth relationship to accurately date these surfaces. Two layers, with ages of 3891 and 6956 years BP, have been traced over a large part of North Greenland. Accurately dated and spatially continuous isochrones are valuable for both assimilation within, and verification of, numerical models. For example, comparison of isochronous surfaces from a numerical simulation with those layers observed in RES data can be used to inform the choice of parameters (e.g. rheology) and climate history used to force a numerical model. To demonstrate the potential of the RES data, two layers for North Greenland were used to determine palaeo-accumulation rates. The inversion from layer depth to accumulation rate requires a three-dimensional velocity field. This velocity field is constructed by combining a two-dimensional balance-velocity field with an assumed vertical structure for the horizontal velocity. The isochronous-layer derived accumulation rates were compared with the Bales and others (2001) rates. A larger east–west gradient was found across the central ice divide for the derived accumulation rate, suggesting a trend in the Holocene accumulation rates for this region. The layers were also compared with isochronous surfaces derived from simulations of a three-dimensional thermodynamic ice-sheet model. Using the isochronous-layer derived accumulation rates to force the model improved the match between modelled and observed layers.
Radio-frequency attenuation beneath Siple Dome,West Antarctica, from wide-angle and profiling radar observations
Authors: Winebrenner Dale P. , Smith Benjamin E. , Catania Ginny A. , Conway Howard B. , Raymond Charles F. 
Published: November 2007

AbstractKnowledge of the spatial distribution of bed lubrication regimes, i.e. frozen vs wet conditions, is crucial for understanding ice-sheet flow. Radar sounding can probe differing reflectivities between wet and frozen beds, but is limited by uncertainty in attenuation within the ice of bed echoes. Here we present two methods to estimate attenuation: (1) wide-angle radar sounding, in which source and receiver locations are varied so as to vary propagation path length, and thus echo amplitude; and (2) profiling, inwhich similar variations are obtained by sounding through varying ice thicknesses (assuming constant bed reflectivity). Siple Dome, West Antarctica, provides unusually favorable circumstances for application of these methods: the bed beneath Siple Dome is flat and uniform in its radar reflectivity, while ice thickness varies by several hundred meters. Wide-angle data 4 km from the summit yield an estimate for characteristic attenuation length of 124 m (35 dB km–1 loss), whereas profiling yields an estimate of 168 m.The difference between estimates is modest compared to the range of attenuation lengths reported in the literature. It may nonetheless prove informative by bounding effects of two ice properties to which the methods respond differently: (1) wide-angle sounding sampled relatively warm (lossy) ice beneath the summit, whereas the profiling method sampled relatively cold ice beneath the flanks as well; and (2) strain-induced crystal orientation fabrics and resulting dielectric anisotropy in the ice would vary from summit to flank, and may influence wide-angle sounding more strongly than profiling.
Observation of recent surges of Vatnajökull, Iceland, by means of ERS SAR interferometry
Authors: Fischer Andrea , Rott Helmut , Björnsson Helgi 
Published: November 2007

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.
Numerical investigations of the slow-down of Whillans Ice Stream, West Antarctica: is it shutting down like Ice Stream C?
Authors: Bougamont Marion , Tulaczyk Slawek , Joughin Ian 
Published: November 2007

AbstractWe investigate whether the recent slow-down of Whillans Ice Stream (WIS), West Antarctica, may lead to its complete stoppage in the near future, using a numerical model. Basal resistance to ice-stream motion is represented by a continuous till layer whose strength changes in response to basal melting and freezing. We implement a basal drainage system, which acts to hinder ice-stream stoppage through supply of extra water to those parts of the bed that are experiencing freezing. The ice module is a standard flowline model with parameterized key out-of-plane effects (Raymond, 1996). The most important result of our modeling effort is that we never obtained a slow-down that would not be followed by a complete stoppage within <100 years. WIS slow-down can be avoided in the model if large basal water-input rates are assumed, to satisfy basal freezing. In comparison, the tested perturbations in WIS width and mass balance had a relatively small effect on the tendency of the simulated ice stream to slow down. These results underscore the need for more quantitative constraints on the efficiency of sub-ice-stream water drainage. We conjecture that the present-day slow-down of WIS will evolve to shut-down in the next few decades, unless an addition of basal water prevents freeze-on-driven bed strengthening.
Numerical simulation of three-dimensional velocity fields in pressurized and non-pressurized Nye channels
Authors: Bates Paul D. , Siegert Martin J. , Lee Victoria , Hubbard Bryn P. , Nienow Peter W. 
Published: November 2007

AbstractChannels incised into bedrock, or Nye channels, often form an important component of subglacial drainage at temperate glaciers, and their structure exerts control over patterns and rates of (a) channel erosion, (b) water flow-velocity and (c) water pressure. The latter, in turn, exerts a strong control over basal traction and, thus, ice dynamics. In order to investigate these controls, it is necessary to quantify detailed flow processes in subglacial Nye channels. However, it is effectively impossible to acquire such measurements from fully pressurized, subglacial channels. To solve this problem, we here apply a three-dimensional, finite-volume solution of the Reynolds averaged Navier– Stokes (RANS) equations with a one-equation mixing-length turbulence closure to simulate flow in a 3 m long section of an active Nye channel located in the immediate foreground of Glacier de Tsanfleuron, Switzerland. Numerical model output permits high-resolution visualization of water flow through the channel reach, and enables evaluation of the experimental manipulation of the pressure field adopted across the overlying ice lid. This yields an increased theoretical understanding of the hydraulic behaviour of Nye channels, and, in the future, of their effect on glacier drainage, geomorphology and ice dynamics.
Numerical investigation of the effects of temporal variations in basal lubrication on englacial strain-rate distribution
Authors: Sugiyama Shin , Gudmundsson G. Hilmar , Helbing Jakob 
Published: November 2007

AbstractThe effects of spatial and temporal variations in basal lubrication on the englacial strain rate and surface velocity distribution are investigated with a numerical ice-flow model. General aspects of the solutions are compared to measurements made on Lauteraargletscher, Switzerland, in 2001, that showed diurnal fluctuations in both surface velocity and englacial vertical strain. We find that spatial gradients in basal lubrication can set up variations in the deviatoric stress field that increases with distance to the bed and has a maximum value near the glacier surface. This stress field produces a significant strain rate near the surface. The temporal evolution of a slippery zone is identified as a possible cause of the observed diurnal variations in the vertical strain rate. Although general aspects of the measurements can be explained in this way, the calculated vertical strain rates are too small, suggesting that the modeled effective viscosity values using Glen’s flow law are too large near the surface.
Tidal bending of glaciers: a linear viscoelastic approach
Authors: Reeh Niels , Christensen Erik Lintz , Mayer Christoph , Olesen Ole B. 
Published: November 2007

AbstractIn theoretical treatments of tidal bending of floating glaciers, the glacier is usually modelled as an elastic beam with uniform thickness, resting on an elastic foundation. With a few exceptions, values of the elastic (Young’s) modulus E of ice derived from tidal deflection records of floating glaciers are in the range 0.9–3 GPa. It has therefore been suggested that the elastic-beam model with a single value of E ≈ 1GPa adequately describes tidal bending of glaciers. In contrast, laboratory experiments with ice give E = 9.3 GPa, i.e. 3–10 times higher than the glacier-derived values. This suggests that ice creep may have a significant influence on tidal bending of glaciers. Moreover, detailed tidal-deflection and tilt data from Nioghalvfjerdsfjorden glacier, northeast Greenland, cannot be explained by elastic-beam theory. We present a theory of tidal bending of glaciers based on linear viscoelastic-beam theory. A four-element, linear viscoelastic model for glacier ice with a reasonable choice of model parameters can explain the observed tidal flexure data. Implications of the viscoelastic response of glaciers to tidal forcing are discussed briefly.
Comparison between computed balance velocities and GPS measurements in the Lambert Glacier basin, East Antarctica
Authors: Testut Laurent , Hurd Rachael , Coleman Richard , Rémy Frédérique , Legrésy Benoît 
Published: November 2007

AbstractComparisons between computed balance velocities, obtained from two different computing schemes, and global positioning system (GPS)-derived velocities were made in the Lambert Glacier basin region, East Antarctica. The two computing schemes used for the balance-velocity computations (a flowline (FL) scheme (Remy and Minster, 1993) and a finite-difference (BW) scheme (Budd and Warner, 1996; Fricker and others, 2000)) were first evaluated and compared. One of the key issues studied was the spatial resolution of the digital elevation model (DEM), representing the surface topography of the ice sheet, and the sensitivity of the balance velocities to the length of smoothing applied to the DEM. Comparison with the GPS velocities validated the two schemes to within 5–25% but showed the high sensitivity of the flowline method to the length scale of the smoothing. The finite-difference scheme was found to be robust to the chosen smoothing scale, but the balance-velocity values increased when a finer-resolution DEM was used. Both FL and BW computing schemes tended to overestimate the balance velocities in comparison with the GPS values; some of this discrepancy can be attributed to ice-sheet sliding.
Intra-annual and intra-seasonal flow dynamics of a High Arctic polythermal valley glacier
Authors: Bingham Robert G. , Nienow Peter W. , Sharp Martin J. 
Published: November 2007

AbstractMeasurements of surface dynamics on polythermal John Evans Glacier, Nunavut, Canada, over two winter periods and every 7–10 days throughout two melt seasons (June–July 2000, 2001) provide new insight into spatio-temporal patterns of High Arctic glacier dynamics. In the lower ablation zone, mean annual surface velocities are 10–21 m a–1, but peak velocities up to 50% higher are attained during late June/early July. In the upper ablation zone and lower accumulation zone, mean annual surface velocities are typically 10–18 m a–1, and peak velocities up to 40% higher occur during late July. In the upper accumulation zone, mean annual surface velocities are 2–9 m a–1, and motion in mid- to late July exceeds this by up to 10%. Rapid drainage of ponded supraglacial water in the upper ablation zone to an initially distributed subglacial drainage system in mid-June may force excess surface motion in the warm-based lower glacier. The data indicate that the duration of the velocity response may be related to the rate of channelization of the basal drainage, and the velocity response may be transmitted up-glacier by longitudinal coupling. An increase in surface velocities in the middle glacier in late July occurs in conjunction with the opening of two further moulins in the accumulation zone.
Snow-accumulation studies in Antarctica with ground-penetrating radar using 50, 100 and 800 MHz antenna frequencies
Authors: Sinisalo Anna , Grinsted Aslak , Moore John C. , Kärkäs Eija , Pettersson Rickard 
Published: November 2007

AbstractSnow radar profiles were measured in Dronning Maud Land, East Antarctica, in the vicinity of the Finnish research station Aboa during austral summer 1999/2000. The aim was to study the annual layering in the upper 50 m of the snowpack and to compare the results obtained by three radar antenna frequencies (50, 100 and 800 MHz). Intercomparison of the radar profiles measured by the three frequencies shows that some individua linternal layers are visible with different antennas. Sparse accumulation-rate data from stake measurements and snow pits are compared with layer depths. The comparison reveals a great deal of scatter due to the large interannual variability in accumulation patterns. Using the radar layers as isochrones together with a model of depth–density–radar-wave velocity allows the individual accumulation data to be integrated, and a better estimate of accumulation patterns is obtained. Using the radar layering seems to be a much better method of estimating accumulation rate in this region than using a short series of stake measurements, even in the absence of deep ice cores to directly date the radar layering.
Scattering of VHF radio waves from within an ice sheet containing the vertical-girdle-type ice fabric and anisotropic reflection boundaries
Authors: Fujita Shuji , Matsuoka Kenichi , Maeno Hideo , Furukawa Teruo 
Published: November 2007

AbstractWe studied the scattering of radio waves off strata within the ice sheet at Mizuho station, Antarctica, to determine the most plausible scattering mechanisms at this location. We measured the effects of birefringence and anisotropic scattering boundaries on the return signal using the following set of experimental conditions: (1) co-polarization and cross-polarization antenna arrangements, (2) all orientations of the antenna system, (3) 60 and 179 MHz frequencies, and (4) pulse lengths of 150–1000 ns. Analyses of the propagated radio waves suggested that the signal is dominated by anisotropic scatter-ingboundaries at 179 MHz, but effects from birefringence also occurred. At depths of 250– 750 m, the scattering was stronger when the polarization plane was along the flowline. In contrast, at depths of about 900–1500 m, scattering was stronger when the polarization plane was perpendicular to the flowline. We suggest that the scattering below about 250 m is related to a layered ice stratum of crystal-orientation fabrics with different types or different cluster strengths. Although our study was at a single site, similar remote measurements over wider regions should provide valuable information about the physical structure of this vast ice sheet.
A 14 year mass-balance record of a blue-ice area in Antarctica
Authors: Sinisalo Anna , Moore John C. , Van De Wal Roderik S.W. , Bintanja Richard , Jonsson Stig 
Published: November 2007

AbstractAccumulation and ablation rates over an Antarctic blue-ice area spanning the 14year period 1988–2002 are presented. Data were obtained by direct stake measurements. Large spatial and temporal variations in the net balance were observed without any clear trend over the entire period. There are marginally significant increases in snow accumulation, and in ablation in the blue-ice area farthest from the equilibrium zone (both at the 95% confidence level). The snow/blue-ice transition zone shows no change over the entire period of observation,and the blue-ice area near the zone shows no change in ablation rate over the 14 year period. The mass-balance gradient in Scharffenbergbotnen may have increased during the period 1988–2002. However, the changes are small, especially when compared with the changes observed elsewhere in Antarctica even relatively close to the blue-ice area. This may indicate that the blue-ice areas are relatively stable to changes in accumulation rate, and possibly temperature.
Ground-water intrusions in a mine beneath Høganesbreen, Svalbard: assessing the possibility of evacuating water subglacially
Authors: Melvold Kjetil , Schuler Thomas , Lappegard Gaute 
Published: November 2007

AbstractEvacuation of the ground-water intruding into a coal mine beneath Høganesbreen, Svalbard, is difficult and expensive. To solve this problem, it was proposed that the mine be connected to the ice–bedrock interface. Pumping hot water from the mine should establish a flow path along the glacier bed where the ground-water would drain gravitationally. In this paper, we assess the requirements for maintaining such a drainage system in open-channel conditions. To obtain the bedrock topography, we determined the ice thickness by ground-penetrating radar and subtracted it from the surface elevation measured by global positioning system. A measured temperature profile at the site where the mine should connect to the glacier bed (140m depth) revealed that the basal ice is below the pressure-melting point. The locations of major subglacial conduits were estimated using a hydraulic-potential approach. We adopted a model oftime-dependent discharge through a Röthlisberger channel to calculate a set of scenarios using different flow-law parameters. Results of the simulations suggest that for the given conditions, water flow would be pressurized, thereby inhibiting the gravitational drainage of the mine.
The “Galloping Glacier” trots: decadal-scale speed oscillations within the quiescent phase
Authors: Nolan Matt 
Published: November 2007

AbstractA 28 year record of annual surface speeds has revealed oscillations with a period of roughly 12 years during the quiescent phase of Black Rapids Glacier, Alaska, U.S.A., the original “Galloping Glacier”. These oscillations are hypothesized to be the manifestation of slowly propagating waves of till failure and till healing, with at least the second cycle being initiated by the anomalous advance of a tributary glacier. Observations support the idea that such dynamics may have occurred, but are not conclusive. In the conceptual model describing the mechanisms of till failure and healing, temporal variations in longitudinal stress gradients are proposed to be more important in causing till failure than temporal variations in effective pressure.
How much do we really know about glacier surging?
Authors: Harrison William D. , Post Austin S. 
Published: November 2007

AbstractSome of the ideas about glacier surging are considered, mainly but not entirely in the light of observations of temperate glaciers in Alaska, U.S.A., made within the last 15 years. Climate has an influence on surge recurrence interval. Climate and weather also affect surge initiation, termination and magnitude. Regional studies lead to the speculation that subglacial “till” plays a key role in surging, and it has been found under all surge-type glaciers studied so far, including Black Rapids and Variegated Glaciers, Alaska. In most of the glaciers studied, till deformation processes dominate the motion in quiescence. The linked-cavity model of surge triggering and rapid motion is not consistent with these observations, but the limited coverage of the observations does not rule it out under parts of the glaciers studied. The till observations in Alaska raise old questions about the interaction between till and the hydraulic systems of temperate glaciers. The role of stored water, which observations show to be active even in winter on Black Rapids Glacier, is noted.