bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of the ELA on Hardangerjøkulen, Norway, during the 1995/96 winter season using repeat-pass SAR coherence
Authors: Kelly R. E. J.
Published: November 2007

This paper demonstrates the utility of European Remote-sensing Satellite (ERS) synthetic aperture radar (SAR) interferometry for monitoring the transient snow-line (TSL) on Hardangerjøkulen ice cap, southern Norway, during the 1995/96 winter. The study shows how coherence information (an interferometry product) over the ice cap can be used to locate the TSL after the summer melt season. Spatial variations in coherence over the ice cap between successive ERS tandem-phase passes from summer to winter are related to surface and volume snow stability and surface ice stability. Temporal differences of coherence images between winter and summer are investigated using histogram analysis. A histogram threshold is found for the 1995/96 winter that can be used to identify the location of the TSL and then estimate the equilibrium-line altitude (ELA). The result shows good agreement (0.5%) with the field-estimated ELA from the Norwegian Water and Energy Administration. Themethod appears to be straightforward for this ice cap and it is envisaged that it could be a useful complementary method on other ice caps where repeat-pass SAR data are available.
Arctic sea-ice area and volume production:1996/97 versus 1997/98
Authors: Kwok Ron
Published: November 2007

The RADARSAT geophysical processor system (RGPS) produces measurements of ice motion and estimates of ice thickness using repeat synthetic aperture radar maps of the Arctic Ocean. From the RGPS products, we compute the net deformation and advection of the winter ice cover using the motion observations, and the seasonal ice area and volume production using the estimates of ice thickness. The results from the winters of 1996/97 and 1997/98 are compared. The second winter is of particular interest because it coincides with the Surface Heat Budget of the Arctic Ocean (SHEBA) field program. The character of the deformation of the ice cover from the two years is very different. Over a domain covering a large part of the western Arctic Ocean (~2.5 × 106 km2), the net divergence of that area during the 6 months of the first winter was 2.7% and for the second winter was 49.3%. In a subregion where the SHEBA camp was located, the net divergence was almost 38% compared to a net divergence of the same subregion of ~9% in 1996/97. The resulting deformation created a much larger volume of seasonal ice than in the earlier year. The net seasonal ice-volume production is 1.6 times (0.38 m vs 0.62 m) that of the first year. In addition to the larger divergence, this part of the ice cover advected a longer distance toward the Chukchi Sea over the same time-span. The total coverage of multi-year ice remained almost identical at ~2.08 × 106 km2, or 83% of the initial area of the domain. In this paper, we compare the behavior of the ice cover over the two winters and discuss these observations in the context of large-scale ice motion and atmospheric-pressure pattern.
Using aerial photography to study glacier changes in Norway
Authors: Andreassen Liss Marie , Elvehøy Hallgeir , Kjøllmoen Bjarne
Published: November 2007

The Norwegian Water Resources and Energy Administration has photographed glacial areas in Norway for several decades. Detailed maps or digital terrain models have been made for selected glaciers from vertical aerial photographs. Multiple models of seven glaciers have been used here to calculate glacier volume change during the time between mappings using the geodetic method. Analyses and results are presented and compared with traditional mass-balance measurements. We estimated uncertainties of ±1.3–2.7mw.e. for the geodetic method, and ±1.3 –3.5mw.e. for the traditional method. The discrepancies between the methods varied between 0.4 and 4.7 mw.e. All glaciers decreased in volume from the 1960s/70s to the 1990s, except Hardangerjøkulen. This glacier experienced a significant increase in volume: the geodetic and traditional methods showed net balance values of +6.8m and +9.4mw.e., respectively. Trollbergdalsbreen had the largest total volume loss: the geodetic and traditional methods showed net balance values of –12.3 and –16.8mw.e.
Motion of major ice-shelf fronts in Antarctica from slant-range analysis of radar altimeter data, 1978–98
Authors: Zwally H.J. , Beckley M.A. , Brenner A.C. , Giovinetto M.B.
Published: November 2007

Slant-range analysis of radar altimeter data from the Seasat, Geosat and European Remote-sensing Satellite (ERS-1 and -2) databases is used to determine barrier location at particular times, and estimate barrier motion (kma–1) for major Antarctic ice shelves. The analysis covers various multi-year intervals from 1978 to 1998, supplemented by barrier location maps produced elsewhere for 1977 and 1986. Barrier motion is estimated as the ratio between mean annual ice-shelf area change for a particular interval, and the length of the discharge periphery. This value is positive if the barrier location progresses seaward, or negative if the barrier location regresses (break-back). Either positive or negative values are lower-limit estimates because the method does not detect relatively small area changes due to calving or surge events. The findings are discussed in the context of the three ice shelves that lie in large embayments (the Filchner–Ronne, Amery and Ross Ice Shelves), and marginal ice shelves characterized by relatively short distances between main segments of grounding line and barrier (those in the Dronning Maud Land sector between 010.1°W and 032.5°E, and the West and Shackleton Ice Shelves). The ice shelves included in the study account for approximately three-quarters of the total ice-shelf area of Antarctica, and discharge approximately two-thirds of the total grounded ice area.
Temperature variability at Siple Dome, West Antarctica, derived from ECMWF re-analyses, SSM/I and SMMR brightness temperatures and AWS records
Authors: Das Sarah B. , Alley Richard B. , Reusch David B. , Shuman Christopher A.
Published: November 2007

We produced four independent temperature time series derived from different sensors for the Siple Dome region of West Antarctica to investigate seasonal to interannual temperature variability over the last 20 years. We use data from automatic weather station air-temperature records (1997–99), European Centre for Medium-range Weather Forecasts surface temperature from the 15 year re-analyses (ERA-15, 1979–93), and emissivity-corrected brightness temperatures from the Special Sensor Microwave/Imager (1987–99) and the Scanning Multichannel Microwave Radiometer (1978–87). Each technique has limitations and errors, but all respond to temperature, and all agree in the large patterns of temperature variability over time. Our results show that there is high seasonal to interannual variability in both mean temperature and variance in the Siple Dome region during the study period. In particular, fluctuations in seasonal to interannual temperature variance occur on an approximately 5 year cycle and correlate with variations in the Southern Oscillation Index.
Modeled seasonal variations of firn density induced by steady-state surface air-temperature cycle
Authors: Jun Li , Zwally H. Jay
Published: November 2007

Seasonal variations of firn density in ice-sheet firn layers have been attributed to variations in deposition processes or other processes within the upper firn. A recent high-resolution (mm-scale) density profile, measured along a 181 m core from Antarctica, showed small-scale density variations with a clear seasonal cycle that apparently was not related to seasonal variations in deposition or known near-surface processes (Gerland and others, 1999). A recent model of surface elevation changes (Zwally and Li, in press) produced a seasonal variation in firn densification, and explained the seasonal surface elevation changes observed by satellite radar altimeters. In this study, we apply our one-dimensional time-dependent numerical model of firn densification that includes a temperature-dependent formulation of firn densification based on laboratory measurements of grain growth. The model is driven by a steady-state seasonal surface temperature and a constant accumulation rate appropriate for the measured Antarctic ice core. The modeled seasonal variations in firn density show that the layers of snow deposited during spring to mid-summer that have the highest temperature history compress to the highest density, and the layers deposited during later summer to autumn that have the lowest temperature history compress to the lowest density. The initial amplitude of the seasonal difference of about 0.13 reduces to about 0.09 in 5 years and asymptotically to 0.0 at greater depth, which is consistent with the core measurements.
Optical characteristics of cryoconite (surface dust) on glaciers: the relationship between light absorbency and the property of organic matter contained in the cryoconite
Authors: Takeuchi Nozomu
Published: November 2007

Optical characteristics of the cryoconite collected from nine glaciers in the Himalaya, Tibet and the Arctic (Canada and Svalbard) were analyzed. The spectral light reflectance (visible region) of the cryoconite on the six glaciers in the Arctic and the Himalayawas generally low, indicating high light absorbency (dark coloration) of the cryoconite. In contrast, the spectral reflectances of the cryoconite on the three glaciers in Tibet were significantly higher than on the other glaciers. There was no significant difference in the spectral reflectance of mineral particles contained in the cryoconite between the Tibetan and the other glaciers, indicating that the difference in the albedo of the cryoconites is not due to the mineral particles, but due to organic matter contained in the cryoconite. Chemical analysis of the organic matter in the cryoconites revealed that the light absorbency of cryoconites is due to the amount of humic substances, which are dark-colored organic substances, the residue of bacterial decomposition of organic matter. The cryoconite of the three glaciers in Tibet contained significantly smaller amounts of humic substances than that of the other glaciers, probably due to different biological or chemical conditions. Results show that the formation of the humic substances in the cryoconite affects its optical characteristics, and possibly affects the surface albedo of the glaciers.
The impact of cloud cover on the net radiation budget of the Greenland ice sheet
Authors: Cawkwell F. G. L. , Bamber J. L.
Published: November 2007

Energy-balancemodels driven by radiation and turbulent heat fluxes have been widely applied to predicting the response of the Greenland ice sheet to climate change. However, a lack of knowledge of the temporal and spatial distribution of cloud amount and type has necessitated the use of parameterizations or statistical models of cloud cover. This deficiency results in large uncertainties in both shortwave and longwave radiation fluxes. Stereo-matching of nadir and forward view AlongTrack Scanning Radiometer-2 (ATSR-2) image pairs has been shown to be a reliable method of retrieving cloud top height, and further cloud properties can be derived from thermal imagery allowing classification into cloud type. A 1 year cloud record for a transect across southern Greenland derived from stereo-matching is presented here, and comparisons are made with climate re-analysis data and ground observations. The cloud-cover data were used in a simple radiative transfer model, and the impact of clouds on the net radiation fluxes was found to be considerable. Different cloud scenarios produced up to 40 Wm–2 difference in net radiation balance. In the ablation zone, where the albedo is lower and most variable, the sensitivity to cloud-cover fraction was less marked, but the higher spatial resolution of the ATSR-2 cloud record was reflected by a much more varied trend in radiation balance. Whether the net radiation balance increases or decreases with increased cloud cover was found to be a function of the cloud amount and type and also the surface albedo. The sensitivity of the model to a ±5% change in cloud amount was found to be comparable to a 1 K change in temperature. This clearly demonstrates the importance of reliable, quantitative cloud data in mass-balance and other glaciological studies.
Mapping of a hydrological ice-sheet drainage basin on the West Greenland ice-sheet margin from ERS-1/-2 SAR interferometry, ice-radar measurement and modelling
Authors: Ahlstrøm A. P. , Bøggild C. Egede , Mohr J. J. , Reeh N. , Christensen E. Lintz , Olesen O. B. , Keller K.
Published: November 2007

The hydrological ice-sheet basin draining into the Tasersiaq lake, West Greenland (66°13’ N, 50°30’W), was delineated, first using standard digital elevation models (DEMs) for ice-sheet surface and bedrock, and subsequently using a new high-resolution dataset, with a surface DEM derived from repeat-track interferometric synthetic aperture radar (SAR) and a bedrock topography derived from an airborne 60 MHz ice-penetrating radar. The extent of the delineation was calculated from a water-pressure potential as a function of the ice-sheet surface and bedrock elevations and a hydraulic factor k describing the relative importance of the potential of the ice overburden pressure compared to the bedrock topography. Themeltwater run-off for the basin delineations was modelled with an energy-balance model calibrated with observed ice-sheet ablation and compared to a 25 year time series of measured basin run-off. The standard DEMs were found to be inadequate for delineation purposes, whereas delineations from high-resolution data were found to be very sensitive to changes in k in a non-linear way, causing a factor 5 change of basin area, corresponding to a doubling of the modelled runoff. The 50% standard deviation of the measured basin run-off could thus be explained by small year-to-year variations of the k-factor.
Scattering of VHF radio waves from within the top 700 m of the Antarctic ice sheet and its relation to the depositional environment: a case-study along the Syowa–Mizuho–Dome Fuji traverse
Authors: Fujita Shuji , Maeno Hideo , Furukawa Teruo , Matsuoka Kenichi
Published: November 2007

Radio-wave scattering is a convenient method to image the properties of large internal regions of ice sheets. We used a ground-based radar system with short pulses of 60 and 179MHz frequencies to scatter off internal strata within 100–700 m of the surface in the ice sheet of East Antarctica. Data were examined along an 1150 kmlong traverse line that was approximately along the ice flowline from inland of Dome Fuji station to the coast. The scattered waves are from strata, and the dominant cause of the scattering was changes in dielectric permittivity across the strata. Therefore, density fluctuations primarily cause the scattering, although variations in ice-crystal fabrics and acidity could also have effects. The power scattered from the same depths varied by > 15 dB from one location to another. These variations correlate with the accumulation rate, changes in the surface slope, and subglacial bedrock undulations. Variations of the scattered power suggest that density contrasts in the strata are highly variable depending on these interdependent local conditions. The distribution of strata along the route allowed estimates of the ice-flow trajectories to depths of about 250 m.
50 years of ice-front changes between the Adélie and Banzare Coasts, East Antarctica
Authors: Frezzotti Massimo , Polizzi Marco
Published: November 2007

Ice-front change may well be a sensitive indicator of regional climate change. We studied the coastal sector of Wilkes Land, East Antarctica, along the Adélie and Banzare Coasts, extending from Buchanan Bay (67°05’ S, 144°30’ E) to Porpoise Bay (67°S, 128°E). The glaciers in this area drain the northern part of Dome C (area 270 000 km2). A comparison of maps, photographs and satellite images, dated several years apart, led to an estimation of the fluctuations of 18 ice fronts over the 50 years 1947–97 .The area of the floating glaciers in 1963 was 3035 km2, and in 1989, 2785 km2. The main glaciers in the area are Zélée, Astrolabe, du Français, Commandant Charcot and Pourquoi Pas for the Adélie Coast, and Dibble, May, Sandford and Frost Glaciers for the Clarie and Banzare Coasts. Most of the floating glaciers have shown cyclical behaviour without a marked trend, but a general reduction since 1947. The reduction in the area of floating glaciers since the 1950s may be linked to changes in ice–ocean interaction, as noted for the floating glaciers of the George V Coast and the Cape Adare area, and sea-ice extent. The calving behaviour of the main glacier tongues is characterized by an accumulation of icebergs projecting from the coast to form iceberg tongues, held in place by grounding and joined together by annual or perennial fast ice.
Crevasse ages on the northern margin of Ice Stream C, West Antarctica
Authors: Smith B. E. , Lord N. E. , Bentley C. R.
Published: November 2007

In the 1997/98 austral summer field season, we conducted a ground-penetrating radar survey on the northern shear margin of Ice Stream C, West Antarctica. The radar data were used to identify features near the surface of the ice, including internal layers and buried crevasses. The survey was intended to determine the variation in the age of buried crevasses along the ice stream. A procedure was developed by which the accumulation rate and the age of buried crevasses can be estimated based on radar records, firn-core measurements and the assumption that the crevasses were once open to the surface. With this method we were able to determine the age of buried crevasses with a standard error of 15–20%. We discuss our new results in conjunction with those of Retzlaff and Bentley (1993) on the southern margin of the ice stream. Typical crevasse ages were found to range from 120 to 200 years, although crevasses in a few areas were significantly younger. The youngest crevasses are at the extreme upstream end of the survey, but the next youngest were found midway along the ice stream. Crevasses upstream and downstream are older, with ages 40–80 years greater than those in the middle. Crevasses on the northern shear margin of tributary C2 were 30–50 years older than those on the southern margin. These patterns of crevassing suggest that variability in shear margin response to changes in ice-stream flow played an essential role in determining the time at which crevassing became inactive.
An image-enhanced DEM of the Greenland ice sheet
Authors: Scambos Ted A. , Haran Terry
Published: November 2007

We have assembled an elevation grid for the Greenland ice sheet using a combination of the best current digital elevation model (DEM) (Bamber and others, 2000a, 2001) and 44 Advanced Very High Resolution Radiometer satellite images acquired in spring 1997. The images are used to quantitatively enhance the representation of surface undulations through photoclinometry. Gridcell spacing of the new DEM is 625 m. To validate the new DEM, we compared profiles extracted from it and the Bamber and others DEM with airborne laser altimetry profiles collected in the 1990s by the Airborne Topographic Mapper (Krabill and others, 1995). The image-enhanced DEM has a greatly improved representation of decameter-relief surface features <15 km in lateral extent, and reduces the mean elevation error in regions having these features by 20–50%. Root-mean-squared errors are typically 7–15m in the Bamber DEM, and 4–10m after image enhancement. However, the photoclinometry process adds some noise. In very smooth portions of the ice sheet where decameter undulationsare absent, the photoclinometry process caused a slight increase in the rms error, from ~1 min the Bamber and others DEM to ∼2.5 min the image-enhanced DEM. The image-enhanced DEM will be useful for inferring accumulation-rate variations over the undulation field, or for improving maps of bedrock elevation through inversion of surface elevation, for example. We briefly explore the preliminary steps of this latter application.
Ice-sheet velocity mapping: a combined interferometric and speckle-tracking approach
Authors: Joughin Ian
Published: November 2007

The first and second RADARSAT Antarctic Mapping Missions (AMM-1 and -2) have now acquired interferometric synthetic aperture radar (SAR) overmuch of the ice sheet. The RADARSAT 24 day repeat cycle is nearly ideal for measuring slow ice motion (e.g. <100ma–1), but application of SAR interferometry is limited in faster-moving areas. With a 1day repeat period, ERS-1/-2 tandem SAR data are much better matched to fast motion, but are not always available. Fortunately, several authors have demonstrated the ability to measure velocity in fast-moving areas by tracking SAR speckle from image to image, which works well even in the absence of visible features. While these estimates have intrinsically lower resolution and poorer accuracy than direct phase measurements, they serve well in areas where there are no data suitable for conventional interferometry. This paper describes algorithms I have developed for merging interferometric and speckle-tracking data from multiple swaths to form a single seamless mosaic of velocity. At each point in the mosaic, all the available data are combined to produce estimates of the velocity and the associated error. This technique is demonstrated using RADARSAT data collected over Lambert Glacier, Antarctica, during AMM-1 and -2.
Reflectance of Antarctica from 3 to 5 μm: discrimination of surface snow and cloud properties
Authors: Calvin Wendy M. , Milman Margaret , Kieffer Hugh H.
Published: November 2007

Current techniques of cloud discrimination in polar regions, ice surface temperature measurement, sea-ice and snowfield extent mapping often rely on data acquired in the region from 3 to 5 mm. The Advanced Very High Resolution Radiometer (AVHRR) and the recently launched Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on Terra have spectral bands in this region used for these purposes. Approaches often consider the radiance value in this spectral range in terms of a single equivalent brightness temperature. However, this spectral region contains contributions from both solar-reflected and thermal-emitted radiance, and a water ice reflectance peak at 3.7 μm can be highly variable and a sensitive indicator of grain-size in icy particles either in clouds or as surface snow. In December 1992 the Galileo spacecraft, on its way to Jupiter, flew by and acquired images of Antarctica that included spectral coverage in 408 channels in the wavelength range 1–5 μm with the Near Infrared Mapping Spectrometer (NIMS). The NIMS spectra provide a basis for the separation of the reflected and emitted components in this wavelength region. This separation then allows the examination of the observed variation of the reflected component with respect to cloud and surface ice properties. This analysis may help refine current algorithms for cloud discrimination in AVHRR and MODIS using channels from 3 to 5 μm.
Synthetic aperture radar detection of the snowline on Commonwealth and Howard Glaciers, Taylor Valley, Antarctica
Authors: Bardel Patrick , Fountain Andrew G. , Hall Dorothy K. , Kwok Ron
Published: November 2007

Synthetic aperture radar (SAR) images of Taylor Valley, Antarctica, were acquired in January 1999 in coordination with ground-based measurements to assess SAR detection of the snowline on dry polar glaciers. We expected significant penetration of the radar wave resulting in an offset of the SAR-detected snowline relative to the true snowline. Results indicated no detectable displacement of the SAR snowline. Snow depths of 15 cm over ice can be detected on the imagery. We hypothesize that the optical depth of thin snowpacks is enhanced by reflection and refraction of the radar beam by internal snow layers. The enhanced optical depth increases the volume scattering, and thereby enhances backscatter sufficiently to be detected by the SAR. Consequently, SAR imagery may be used directly to image the position of transient snowlines in dry polar regions.
Experimental study on the micro-fabric of frozen sediment using triaxial deformation and computerized tomography
Authors: Jilin Qi , Yuanming Lai , Yibing Pu
Published: November 2007

The micro-fabric of frozen sediment was studied using a triaxial deformation machine with computerized tomography (CT) to describe a quantitative relationship between microstructure and mechanical behaviour at different temperatures and confining pressures. Micro-fabric changes were described by CT images and CTvalues. Relationships between CT values and the stress–strain curves were analyzed. It was shown that CTvalues can describe the deformation quantitatively.
Spatial and temporal scales of anisotropic effects in ice-sheet flow
Authors: Thorsteinsson Throstur , Waddington Edwin D. , Fletcher Raymond C.
Published: November 2007

Along most particle paths in polar ice sheets, ice experiences a slowly changing local deviatoric-stress pattern, and develops a fabric characteristic of its current stress state, in which there is generally a correspondence between non-zero strain-rate components and non-zero deviatoric-stress components. Where the stress pattern changes rapidly in special transition zones, fabric may evolve more slowly than the local stress field, and unusual or unexpected deformation patterns can result. The degree to which fabric tracks the local stress is determined by the relative characteristic times for changes in stress, given by the transition-zone width and ice velocity, and for changes in crystal orientation, given, in the absence of recrystallization, by the inverse of the local strain rate due to the principal stress. Recrystallization can significantly reduce the time-scale of fabric adjustment. We examine transition zones where ice (a) enters ice-stream margins, (b) is overrun by a migrating divide, and (c) flows through a strong saddle. Stress and fabric tend to be significantly misaligned in ice-stream margins and in flow through a saddle. When stresses that are markedly different from in situ stresses are applied to ice specimens during creep tests, deformation may be difficult to interpret.
A new bedrock and surface elevation dataset for modelling the Greenland ice sheet
Authors: Bamber Jonathan L. , Baldwin Duncan J. , Gogineni S. Prasad
Published: November 2007

A new digital elevation model of the surface of the Greenland ice sheet and surrounding rock outcrops has been produced from a comprehensive suite of satellite and airborne remote-sensing and cartographic datasets. The surface model has been regridded to a resolution of 5 km, and combined with a new ice-thickness grid derived from ice-penetrating radar data collected in the 1970s and 1990s. A further dataset, the International Bathymetric Chart of the Arctic Ocean, was used to extend the bed elevations to include the continental shelf. The new bed topography was compared with a previous version used for ice-sheet modelling. Near the margins of the ice sheet and, in particular, in the vicinity of small-scale features associated with outlet glaciers and rapid ice motion, significant differences were noted. This was highlighted by a detailed comparison of the bed topography around the northeast Greenland ice stream.
Ice-core insights into the flow and shut-down of Ice Stream C, West Antarctica
Authors: Voigt Donald E. , Alley Richard B. , Anandakrishnan Sridhar , Spencer Matthew K.
Published: November 2007

Vigorous flow of central regions of Ice Stream C, West Antarctica, near the UpC camp ended about the year 1830, based on analysis of a firn and ice core taken at the camp. Ice-stream flow was characterized by repeated fracturing and healing, probably subsurface, especially near the onset of streaming flow. High longitudinal stresses caused fracturing, recrystallization of the ice and elongation of bubbles, and enhanced densification rates of high-density firn indicating power-law-creep behavior.
Comparison of modelled and observed responses of a glacier snowpack to ground-penetrating radar
Authors: Kohler Jack , Moore John C. , Isaksson Elisabeth
Published: November 2007

The upper 10 m of the firn of a Svalbard glacier is imaged along the centre line using a 500 MHz ground-penetrating radar, and a 10 m firn core taken along the profile. Complex reflection coefficients are calculated from the high-resolution capacitance and conductance measurements made on the snow core. The reflection coefficient depth series is converted to the time domain and convolved with model radar monopulses to synthesize traces that compare well with radar traces recorded near the ice core. Differences are probably due to cm-scale physical and chemical inhomogeneities that are smoothed when imaged by the radar beam, which integrates information over areas that are of the same order of magnitude as the depth to the layer.
A dislocation-based analysis of the creep of granular ice: preliminary experiments and modeling
Authors: Cole David M.
Published: November 2007

The nature of the fall-off at lower stresses from power-law behavior to a lower-order stress dependency is of particular interest in glacier and ice-sheet modeling. Preliminary experiments show that the stress level at which the fall-off occurs is a function of the specimen’s dislocation density. The analysis employs a dislocation-based model of anelasticity that provides a quantitative relationship between the effective dislocation density and the area of hysteresis loops observed in cyclic loading experiments. Combining this technique with a staged creep experiment makes it possible to calculate the dislocation density as a function of strain, thereby supporting a quantitative dislocation analysis of the deformation process.Work on saline ice established that the threshold stress associated with power-law behavior increased as a result of prior straining, power-law behavior emerged when the effective dislocation density increased measurably during deformation, and approximately linear behavior was evident when the dislocation density remained relatively constant. Those findings motivated the experiments on fresh-water ice presented here. The preliminary experiments show that pre-straining increases the stress associated with the fall-off from power-law behavior, and the results are interpreted in the context of a dislocation-based constitutive model developed for sea ice.
Numerical modelling of historical front variations and dynamic response of Sofiyskiy glacier, Altai mountains, Russia
Authors: De Smedt Bert , Pattyn Frank
Published: November 2007

The recent fluctuation of the central Asian climate, and its effect on the region’s glaciers, is poorly known, largely because of a lack of knowledge of the dynamic behaviour of so-called summer-accumulation-type glaciers. In this study, a one-dimensional numerical glacier model is used to simulate the dynamic response of Sofiyskiy glacier, Altai mountains, Russia, to climate forcing. A successful simulation of the observed historical front variations was accomplished by dynamic calibration. This resulted in a reconstruction of the recent mass-balance history of the glacier, showing a distinct decline in surface mass balance in the second half of the 19th century, a slightly higher mass balance at the beginning of the 20th century, followed by a steady decline towards present conditions. The future response of Sofiyskiy glacier was projected for six 21st-century climate scenarios. Under a “no-change” scenario, the glacier will retreat > 2 km by 2100. If air temperature gradually rises by > 5°C during this century, the glacier will vanish around 2100. Basic response characteristics of Sofiyskiy glacier were determined. These indicate rather low mass-balance sensitivity to temperature change, but a strong front reaction due to geometric conditions.
Equifinality of basal ice facies from an Antarctic cold-based glacier
Authors: Sleewaegen Suzanne , Denis Samyn  , Fitzsimons Sean J. , Lorrain Reginald D.
Published: November 2007

Three debris-bearing ice facies were recognized at the base of Suess Glacier, a cold-based glacier damming a lake in Taylor Valley, South Victoria Land, Antarctica. These facies are termed “amber ice”,“solid facies” and “basal stratified facies”. This paper uses stable-isotope composition (δD and δ18O), gas content and gas composition (CO2, O2 and N2) to develop an understanding of the processes responsible for the formation of these facies. The basal ice is characterized by a striking difference in ice properties between the innermost end of a 25 m long tunnel dug 200 m upstream from the glacier front and the front itself. At the glacier front, co-isotopic data plot along a well-defined freezing slope (S = 5.6), whereas, inside the tunnel, the isotopic data offset from the freezing slope and from the local meteoric water-line (which has a slope of 8.2). CO2 concentrations rise from a minimum of about 1000 ppmv in the tunnel to about 220000ppmv at the front. Taken together, these characteristics strongly suggest an increasing contribution of liquid water in the formation of basal ice towards the glacier terminus. We therefore conclude that visually similar basal ice facies can have different origins.
Plane ice-sheet flow with evolving and recrystallizing fabric
Authors: Staroszczyk Ryszard
Published: November 2007

A plane, gravity-driven, steady flow of a polar ice sheet over a horizontal bedrock, with no-slip basal conditions, is considered. The ice is modelled as a linearly viscous, incompressible and anisotropic fluid, with evolving orthotropic fabric depending on local strain rates and deformations. For prescribed free-surface elevation and non-uniform temperature field, the ice velocities required to maintain the assumed geometry are calculated by using the finite-element method. The focus is on the mechanism of dynamic (migration) recrystallization occurring near the base ofan ice sheet and leading to significant weakening, and ultimately to the complete loss, of the anisotropic fabric developed in the upper part of the ice cap. The weakening of the fabric with increasing strain rate and temperature is modelled by means of a scaling function depending continuously on a single effective strain-rate invariant. The results of numerical calculations demonstrate the effect of the recrystallization process on the overall flow rate of the ice sheet, and normalized velocity and strain-rate depth profiles are compared for flows with and without recrystallization involved to illustrate the effect of this mechanism on the large-scale behaviour of polar ice sheets.
Studying the effects of strain heating on glacial flow within outlet glaciers from the Heimefrontfjella Range, Dronning Maud Land, Antarctica
Authors: Pohjola Veijo A. , Hedfors Jim
Published: November 2007

A one-dimensional numerical thermodynamic model is used to study the effects of strain heating on temperature profiles along the flowline of two outlet glaciers in Dronning Maud Land, Antarctica, flowing down a steep escarpment. Measurements of ice surface velocities on the glaciers show higher speeds than surface speeds calculated using Glen’s flow law. These calculations are based on ice-temperature distributions excluding strain heating in the general heat equation. The incorporation of strain heating in the general heat equation produces higher ice temperatures, and calculated ice surface speeds that are close to the measured values. It is found that relatively short-scale temporal and spatial steps in basal topography are enough to drive the ice flow into a positive feedback loop as long as the bedrock step produces a stress that overcomes the advection of cool ice from the surface. In this case, where surface temperatures are –25°C, stresses of 0.4 MPa are enough to drive the base of the ice to the melting point within 102 years.
Investigating the ratio of basal drag and driving stress in relation to bedrock topography during a melt season on Storglaciären, Sweden, using force-budget analysis
Authors: Hedfors Jim , Peyaud Vincent , Pohjola Veijio A. , Jansson Peter , Pettersson Rickard
Published: November 2007

We apply the force-budget technique using the isothermal block-flow model, on Storglaciaren, Sweden, to investigate the ratio between basal drag and driving stress in relation to a bedrock ridge in the bed topography during a peak melt season. The input data consist of glacier surface velocities collected using differential global positioning system surveying of a stake net and geometry from previous radar soundings and digitized ice surface maps. The study focuses on the effects of transverse bedrock ridges upon basal stress conditions. The pattern of the calculated ratio of basal drag and driving stress shows a rhythmical position of relatively high and low basal drags on the stoss and lee sides, respectively, of the bedrock thresholds. One of the zones of low basal drag corresponds to the location where the highest basal sliding rate has been measured previously by borehode deformation studies. This zone also aligns with the area where the drainage system is suggested to change from englacial to subglacial.
Seasonal variation of snow-surface elevation in North Greenland as modeled and detected by satellite radar altimetry
Authors: Li Jun , Zwally H. Jay , Cornejo Helen , Yi Donghui
Published: November 2007

Comparison of the distribution of seasonal variations in surface elevation derived from a firn-densification–elevation model with observed variations derived from ERS-1/-2 satellite radar altimetry shows close similarity in the patterns of the amplitude of the variations over the North Greenland ice sheet. The amplitudes of the seasonal variations decrease from west to east and from south to north, determined by the accumulation rate and the surface-temperature distribution pattern. Several methods of estimating the amplitude of the seasonal variation in the observations are compared, including the use of a three-frequency sinusoidal function derived from the modeled seasonal variation that is asymmetric. The resulting correlation coefficient between the observed amplitude, estimated with the three-frequency function, and the modeled amplitude is 0.66 and the slope is 0.7. Residual differences may be caused by interannual variability in accumulation and temperature and other approximations in the model.
Dynamics and mass balance of the ice-sheet/ice-shelf regime at Nivlisen, Antarctica, as derived from a coupled three-dimensional numerical flow model
Authors: Paschke Birgit , Lange Manfred A.
Published: November 2007

The aim of this study is to derive basic ice dynamics for Nivlisen, a relatively small ice shelf in Dronning Maud Land, East Antarctica, and its drainage area. Therefore, the adaptation of an existing three-dimensional numerical flow model (Sandhäger, 2000) was undertaken, which had been applied previously to Ekstromisen and its drainage system. Compared to that region, Nivlisen and its drainage area represent a significantly more complicated geometrical situation. This has consequently large impacts on the flow regime, i.e. several modifications are necessary and reasonable. The diagnostic modelling is carried out in order to derive the current ice dynamics. The resulting ice-flow velocities seem to be too small compared to typical values for such glaciers. Therefore, the basal temperature at some gridpoints was set to pressure-melting point to obtain more realistic results. Furthermore, the use of a spatially varying parameter to describe basal sliding is advisable. Presenting the numerical results, the focus lies on the modelled ice-velocity field. The mass-balance quantities are calculated from the simulated velocity distribution and the underlying ice-geometry models derived from different datasets. The diagnostic results will serve as the basis for future prognostic modelling studies.
Elucidating changes in the degree of tracer dispersion in a subglacial channel
Authors: Schuler Thomas , Fischer Urs H.
Published: November 2007

Tracer injections into a subglacial channel at Unteraargletscher, Switzerland, were repeated at intervals of about 2 hours over two diurnal discharge cycles in August and September 2000. Records of dye concentration reveal a pronounced hysteresis in the velocity–dispersion relationship, thereby indicating alterations in the drainage system. Theoretical considerations for Röthlisberger channels suggest an evolution of the conduit cross-section in response to a diurnally varying discharge. We studied the relation between conduit cross-section and tracer dispersion with numerical tracer experiments. The velocity field for steady flow through a given conduit geometry is calculated using a commercial flow solver. Tracer transport is represented by a scalar volume which is advected by the velocity field. Experiments were conducted for several scenarios by varying flow velocity, conduit geometry and conduit roughness. Results show only a weak dependence of dispersion on conduit size. In contrast, changes in roughness of the conduit walls reveal a strong effect on tracer dispersion. Therefore, to explain the observed hysteresis in the velocity–dispersion relationship, we suggest that the evolution of a subglacial flow path might involve changes in roughness.
Estimation of water content in a temperate glacier from radar and seismic sounding data
Authors: Benjumea Beatriz , Macheret Yury Ya. , Navarro Francisco J. , Teixidό Teresa
Published: November 2007

Radio-wave velocity measurements in temperate and polythermal glaciers, combined with dielectric mixture formulae by Looyenga or Paren, have been used during the last decade to estimate the water content in temperate ice. We have used a similar mixture formula by Riznichenko, but based on elastic properties of the material, to estimate the water content from seismic velocity data. To compare the suitability of the two methods, we have used seismic and radar data from a temperate glacier on an Antarctic island. The estimated water contents are within 0.4–2.3% (average 1.2 ±0.6%) when radio-wave velocities are used, and within 0.9–3.2% (average 2.2±0.9%) when seismic velocities are used. These results are similar to those directly measured from ice cores and to those estimated from radar data on other temperate glaciers. The water-content estimates from seismic data are higher than those from radar data, which we attribute to the different behaviour of seismic and radar velocities as functions of density. Near-surface conditions (ice–firn conditions, presence of crevasses, etc.) have a strong influence on the propagation of elastic and electromagnetic waves, and thus on the accuracy of the velocity determinations and water-content estimates, and so should not be disregarded.
Strain-rate formulation of ice fabric evolution
Authors: Morland Leslie W. , Staroszczyk Ryszard
Published: November 2007

Reorientation of individual crystal-glide planes as isotropic surface ice is deformed during its passage to depth in an ice sheet, lattice rotation, creates a fabric and associated anisotropy. A simple macroscopic description is that these material glide planes are rotated towards planes normal to an axis of compression, and away from planes normal to an axis of extension, inducing an instantaneous orthotropic viscous response with reflexional symmetries in the planes orthogonal to the current principal stretch axes. An orthotropic viscous law is presented for the strain rate expressed in terms of the deviatoric stress, the deformation, and three structure tensors based on the principal stretch axes. This anisotropic relation is expressed in terms of a single fabric response function in addition to the isotropic ice viscosity. The predicted responses in uniaxial compression and simple shear are determined. While the uniaxial response yields an explicit relation between the axial strain rate and stress, it is found that the shear response is governed by three, complicated, coupled relations between the shear strain rate and three deviatoricstress components. The new result derived here is the solution of this system: an explicit relation between the shear strain rate and shear stress. Correlation of these relations with idealized uniaxial and shear responses is then used to determine the required fabric function in the model law.
Science goals for a Mars polar-cap subsurface mission: optical approaches for investigations of inclusions in ice
Authors: Carsey Frank , Mogensen Claus T. , Behar Alberto , Engelhardt Hermann , Lane Arthur L.
Published: November 2007

The Mars polar caps are highly interesting features of Mars and have received much recent attention, with new and exciting data on morphology, basal units and layered outcroppings. We have examined the climatological, glaciological and geological issues associated with a subsurface exploration of the Mars North Polar Cap and have determined that a fine-scale optical examination of ice in a borehole, to study the stratigraphy, geochemistry and geochronology of the ice, is feasible. This information will enable reconstruction of the development of the cap as well as understanding the properties of its ice. We present visible imagery taken of dust inclusions in archived Greenland ice cores as well as in situ images of accreted lithologic inclusions in West Antarctica, and we argue for use of this kind of data for Mars climate reconstruction as has been successful with Greenland and Antarctic ice-core analysis.
A description of crevasse formation using continuum damage mechanics
Authors: Pralong Antoine , Funk Martin , Lüthi Martin P.
Published: November 2007

Continuum damage mechanics describes the progressive deterioration of material subjected to loading. Jointly used with a level-set method, it proves to be a promising approach to computing the interface motion of a damaged material. For polycrystalline ice, a local isotropic damage evolution law (generalized Kachanow’s law) applied to Glen’s flow law allows the description of tertiary creep and facilitates the modeling of crevasse opening using a failure criterion based on damage accumulation. The use of a level-set method permits the description, in a continuum approach, of the motion of a fractured glacier surface. Using these methods, a model is developed. The ability of this model to describe phenomena connected to crevasse opening is presented. The rupture of a large ice block from a hanging glacier is computed and analyzed. The regular acceleration of such an unstable ice block prior to its collapse is calculated and compared to the acceleration function obtained from observations. A good agreement between the two acceleration functions was found.
A surface motion survey of Black Rapids Glacier, Alaska, U.S.A.
Authors: Fatland Dennis R. , Lingle Craig S. , Truffer Martin
Published: November 2007

We describe a derivation of surface velocities and associated errors for Black Rapids Glacier, Alaska, U.S.A., using single-orbital-path synthetic aperture radar interferometry (InSAR). The technique described is adapted to small temperate glaciers with complex flow patterns. We also describe a motion anomaly, apparent in the InSAR phase signal, that persisted on Black Rapids Glacier for at least 78 days during winter 1991/92 and recurred in 1996. This anomaly is interpreted using a basal hydrology hypothesis in which a hydraulic head is maintained at the glacier bed at close to the overburden pressure. This permits a cumulative influx of 1.6 × 106 m3 of water under the glacier, a sort of shallow subglacial lake, that migrates downstream at an average rate of 30 m d− 1 over 78 days. The motion anomaly is speculated to be an unsuccessful bid for surge initiation.