bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics and potential climatic significance of “miniature ice caps” (crest- and cornice-type low-altitude ice archives)
Authors: Haeberli Wilfried , Frauenfelder Regula , Kääb Andreas , Wagner Stephan
Published: November 2007

Long-term ice-core records of Alpine glaciers are usually taken from cold-firn areas at high altitudes, as on Mont Blanc and Monte Rosa. Perennial ice bodies at lower altitudes, however, also bear information about the past. Recent findings from the remains of such ice (the Oetztal iceman found in Austria; wooden bows at Lötschen Pass, Switzerland) clearly indicate the hitherto little-recognized fact that small, more-or-less static perennial ice bodies which are cold and frozen to the underlying bedrock may contain very old ice and, hence, important palaeoclimatic information about warm periods with minimum ice extent in the Alps. Since autumn 1998, investigations have been initiated on a crest-type location or “miniature ice cap” at Piz Murtèl, Engadine, Swiss Alps. First results from shallow drilling, temperature data-logging, geodetic surveying, visual observation, finite-element modelling of simplified basic two-dimensional configurations and comparison to earlier measurements at similar sites provide promising perspectives concerning a little-studied phenomenon with considerable scientific-environmental research potential. Specific characteristics of the investigated site, and probably of many other comparable mountain sites, are: cold ice (about –4°C at 10 m depth), no basal sliding, small mass turnover, striking lack of a firn zone, accumulation mainly by superimposed ice, and direct access to old layers (centuries, millennia?) at the ice/bedrock interface.
Genesis of dewatering structures and its implications for melt-out till identification
Authors: Carlson Anders E.
Published: November 2007

Dewatering structures are a common feature used to identify melt-out till, and the lack of such structures in till could preclude deposition by melt-out. To assess the conditions under which melt-out till can be deposited without forming dewatering structures, I use geotechnical data and a quasi-two-dimensional model of geothermal melt-out. Critical discharge determined from geotechnical data suggests that low-hydraulic-conductivity till can transport up to 1.3 m3water a–1m–2without forming dewatering structures, which is two to three orders of magnitude greater than the volume of meltwater produced at the base of glaciers. The model indicates that debris content of the ice and the ability of the till to drain govern effective pressure during melt-out. If the drainage system is poorly developed or the till comes from debris-poor ice, effective pressure is below zero, the condition under which dewatering structures could form. However, till from relatively debris-rich ice (>40% debris) with a well-developed drainage system (channels every 10 m) can dewater without forming dewatering structures. This suggests that the lack of dewatering structures in till does not necessarily imply deposition by lodgement or deforming bed.
Strain-induced phase changes within cold basal ice from Taylor Glacier, Antarctica, indicated by textural and gas analyses
Authors: Samyn Denis , Fitzsimons Sean J. , Lorrain Reginald D.
Published: November 2007

This paper reports detailed textural and gas measurements conducted in cold basal ice (–17°C from the margin of Taylor Glacier, an outlet glacier of the East Antarctic ice sheet. The analyzed samples were retrieved from a basal ice sequence excavated at the end of a subglacial tunnel dug near the glacier snout. The basal sequence exhibits two contrasting ice facies, defined as the englacial and stratified facies. On the one hand, analysis of ice crystal textures from the basal ice sequence provides evidence for localized ductile deformation, especially within the stratified facies where significant dynamic recrystallization was detected. On the other hand, high-resolution gas analyses reveal that strong changes in gas composition occurred at the structural interfaces of the stratified facies. These gas composition changes are typical of melting–refreezing processes but are not associated with any significant loss of gas volume. Given the specific subglacial thermal conditions at the margin of Taylor Glacier, we interpret this phenomenon as resulting from microscopic phase changes involving selective gas redistribution through the pre-melt phase. It is argued that such processes may play an important role in the post-genetic geochemical evolution of cold debris-laden ice and may be enhanced through intense strain conditions.
Using remote-sensing data to determine equilibrium-line altitude and mass-balance time series: validation on three French glaciers, 1994–2002
Authors: Rabatel Antoine , Dedieu Jean-Pierre , Vincent Christian
Published: November 2007

Alpine glaciers are very sensitive to climate fluctuations, and their mass balance can be used as an indicator of regional-scale climate change. Here, we present a method to calculate glacier mass balance using remote-sensing data. Snowline measurements from remotely sensed images recorded at the end of the hydrological year provide an effective proxy of the equilibrium line. Mass balance can be deduced from the equilibrium-line altitude (ELA) variations. Three well-documented glaciers in the French Alps, where the mass balance is measured at ground level with a stake network, were selected to assess the accuracy of the method over the 1994–2002 period (eight mass-balance cycles). Results obtained by ground measurements and remote sensing are compared and show excellent correlation (r2 > 0.89), both for the ELA and for the mass balance, indicating that the remote-sensing method can be applied to glaciers where no ground data exist, on the scale of a mountain range or a given climatic area. The main differences can be attributed to discrepancies between the dates of image acquisition and field measurements. Cloud cover and recent snowfalls constitute the main restrictions of the image-based method.
Identifying microstructural deformation mechanisms in snow using discrete-element modeling
Authors: Johnson Jerome B. , Hopkins Mark A.
Published: November 2007

A dynamic model of dry snow deformation is developed using a discrete-element technique to identify microstructural deformation mechanisms and simulate creep densification processes. The model employs grain-scale force models, explicit geometric representations of individual ice grains, and snow microstructure using assemblies of grains. Ice grains are randomly oriented cylinders of random length with hemispherical ends. Particle contacts are detected using a novel and efficient method based on the dilation operation in mathematical morphology. Grain-scale ice interaction algorithms, based on observed snow and ice microscale behavior, are developed and implemented in the model. These processes include grain contact sintering, grain boundary sliding and rotation at contacts, and grain contact deformation in tension, compression, shear, torsion and bending. Grain-scale contact force algorithms are temperature- and rate-dependent, with both elastic and viscous components. Grain bonds rupture when elastic stresses exceed ice tensile or shear strengths, after which intergranular friction and particle rearrangement control deformation until the snow compacts to its critical density. Simulations of creep settlement using 1000-grain model snow samples indicate the bulk viscosity of snow is controlled by the grain contact viscosity and area, grain packing and the increased number of frozen bonds that form during settlement. A linear relationship between contact viscosity and bulk snow viscosity at any specified density indicates that the linear model parameters can be accurately scaled, allowing simulations to be conducted for a broad range of dynamic and viscous creep deformation problems.
Global variations of local asymmetry in glacier altitude: separation of north–south and east–west components
Authors: Evans Ian S. , Cox Nicholas J.
Published: November 2007

North–south and east–west differences in firn-line altitude, equilibrium-line altitude or middle altitudes of glaciers can be separated by regression on the cosine and sine of glacier aspect (accumulation area azimuth). Allowing for regional trends in altitude, the north–south differences expected from radiation and shade effects can be reliably quantified from World Glacier Inventory (WGI) data. The north–south differences are greater in sunnier climates, mid-latitudes and steeper relief. Local altitude differences between north- and south-facing glaciers are commonly 70–320 m. Such asymmetry is near-universal, although weak in the Arctic and tropics. East–west contrasts are less, and found mainly in the tropics and areas most exposed to strong winds. Altitude, latitude, glacier gradient and height range, calculable from most of the WGI data, are potential controls on the degree of north–south contrast, as well as surrogates for climatic controls (temperature, precipitation, radiation and cloudiness). An asymmetric sine–cosine power model is developed to describe the variation of north–south contrast with latitude. Multiple regression over 51 regions shows altitude and latitude to be the strongest controls of this contrast. Aspect–altitude analysis for former glaciers provides new evidence of cloudiness.
Analysis of meteorological data and the surface energy balance of McCall Glacier, Alaska, USA
Authors: Klok E.J. , Nolan M. , Van Den Broeke M.R.
Published: November 2007

We report on analysis of meteorological data for the period 27 May–20 August 2004, from two automatic weather stations on McCall Glacier, Alaska, USA, aimed at studying the relationship between climate and ablation. One station is located on a mountain ridge and the other in the ablation area where we also analyzed the energy balance. The weather station on the glacier measured an average temperature of 5.3°C (at 2 m height above surface) and wind speed of 3.1 m s−1 (at 3 m height). A sonic height ranger and ablation stakes indicate a specific mass balance of –1.94 ± 0.09 m w.e between 15 June and 20 August. The specific mass balance calculated from the surface energy balance, –2.06 ± 0.18 m w.e., is in close correspondence to this. The latter is the sum of 0.12 m w.e. of snowfall, 0.003 m w.e. of deposition and –2.18 m w.e. of melt. Net radiation contributes 74% of the melt energy. Compared to ablation measurements in the early 1970s, summer ablation was large. This increase is explained by a combination of a relatively higher net radiation, a lower albedo and larger turbulent heat fluxes that led to more energy being available for melting. No single meteorological variable can be isolated as being the principal reason for the high ablation, however. The lower ice albedo (0.19) is possibly due to ash deposits from forest fires.
The effect of particles on dynamic recrystallization and fabric development of granular ice during creep
Authors: Song Min , Baker Ian , Cole David M.
Published: November 2007

The mechanical behavior and microstructural evolution of laboratory-prepared, particle-free fresh-water ice and ice with 1 wt.% (~0.43 vol.%) silt-sized particles were investigated under creep with a stress level of 1.45 MPa at −10°C. The particles were present both within the grains and along the grain boundaries. The creep rates of specimens with particles were always higher than those of particle-free ice. Dynamic recrystallization occurred for both sets of specimens, with new grains nucleating along grain boundaries in the early stages of creep. The ice with particles showed a higher nucleation rate. This resulted in a smaller average grain-size for the ice with particles after a given creep strain. Fabric studies indicated that ice with particles showed a more random orientation of c axes after creep to ~10% strain than the particle-free ice.
Fabric and texture at Siple Dome, Antarctica
Authors: Diprinzio C.L. , Wilen L.A. , Alley R.B. , Fitzpatrick J.J. , Spencer M.K. , Gow A.J.
Published: November 2007

Preferred c-axis orientations are present in the firn at Siple Dome, West Antarctica, and recrystallization begins as shallow as 200 m depth in ice below –20°C, based on digital analysis of c-axis fabrics, grain-sizes and other characteristics of 52 vertical thin sections prepared in the field from the kilometer-long Siple Dome ice core. The shallowest section analyzed, from 22 m, shows clustering of c axes toward the vertical. By 200 m depth, girdle fabric and other features of recrystallized ice are evident in layers (or regions), separated by layers (regions) of typically finer-grained ice lacking evidence of recrystallization. Ice from about 700–780m depth, which was deposited during the last ice age, is especially fine-grained, with strongly vertical c axes, but deeper ice shows much larger crystals and strong evidence of recrystallization. Azimuthal asymmetry of some c-axis fabrics, trends in grain-size, and other indicators reveal additional information on processes and history of ice flow at Siple Dome.
Creep of granular ice with and without dispersed particles
Authors: Song Min , Cole David M. , Baker Ian
Published: November 2007

The effects of silt-sized particles (average diameter of 50 μm) on the compressive creep of polycrystalline ice have been studied at stress levels from 0.1 to 1.45 MPa and temperatures of –12ºC and –10°C. Dislocation densities during creep have been estimated using a dislocation-based model of anelasticity. The results indicate that at low concentrations (up to 4wt.%), particles increase the minimum creep rate. Power-law behavior with an exponent of 3 was observed for both particle-free ice and ice with 1 wt.% particles when the stress was >0.3 MPa. In contrast, linear behavior was observed when the stress was <0.3 MPa. Calculations show that the linear behavior is associated with a constant dislocation density, and the power-law behavior is associated with increasing dislocation densities with increasing stress.
Kinematics of Spruce Creek rock glacier, Colorado, USA
Authors: Leonard Eric M. , Staab Peter , Weaver Stephen G.
Published: November 2007

A 14 year kinematic survey of Spruce Creek rock glacier, Colorado, USA, provides information on rates and controls of surface strain. Steel-tape measurements of differential movement yield data of cm-scale accuracy, sufficient to assess strain patterns over small portions of the very slowly deforming rock glacier. Flow rates are typically <10cma–1, and measured strain rates range from 1.0 × 10–5a–1 to 1.5 × 10–3a–1. The primary control on longitudinal strain is changing surface slope, with extending flow occurring in areas of down-valley increase in slope, and compressing flow in areas of down-valley slope decrease. Relatively high strain rates are associated with higher flow velocities and with the impingement of faster-flowing up-valley portions of the rock glacier on the slower-moving lower portion. Overall strain rates decreased through the study period, probably as a result of a slowing of the upper part of the rock glacier. Transverse ridges are associated with longitudinal shortening, caused by either slope changes or impingement of faster-moving sections of the rock glacier on slower-moving sections, but transverse ridges do not occur in every area of strong longitudinal shortening.
Recumbent folding of divide arches in response to unsteady ice-divide migration
Authors: Jacobson H. Paul , Waddington Edwin D.
Published: November 2007

Arches in stratigraphic layers directly under a flow divide (Raymond bumps) are predicted by models of steady ice-sheet flow, and have been observed in several ice domes. Here, we model the evolution of these layers when a formerly stationary divide migrates rapidly to a new position, then again becomes stationary, leaving the arched layers in a flank position. As they are then carried downstream with the flow, these abandoned arches can develop into recumbent folds. These folds can occur over a wide range of divide migration speeds. The shearing flow that produces these recumbent folds also distributes the folded layers over a wide distance downstream from the original divide location. If the divide offset is abrupt, ‘pre-cores’, or material lines comprising core-relative isochrones, can be used to quickly identify portions of an abandoned Raymond bump that would be overturned at any future ice-core site downstream. If, as appears to be the case in Greenland, the divide is never stable long enough to produce a mature arch, folds of this type would not occur. The most likely place to find such folds might be the flank of an ice ridge bounded by unsteady ice streams.
Flow dynamics and iceberg calving rates of Devon Ice Cap, Nunavut, Canada
Authors: Burgess David O. , Sharp Martin J. , Mair Douglas W.F. , Dowdeswell Julian A. , Benham Toby J.
Published: November 2007

The surface velocity field of Devon Ice Cap, Nunavut, Canada, was mapped using interferometric synthetic aperture radar (InSAR). Ascending European Remote-sensing Satellite 1 and 2 (ERS-1/-2) tandem mode data were used for the western and southeast sectors, and 3 day repeat pass ERS-1 imagery for the northeast sector. Speckle-tracking procedures were used with RADARSAT 1 imagery to obtain surface velocities over the terminus of Belcher Glacier (a major calving front) where decorrelation between ERS data occurred. The InSAR data highlight a significant contrast in ice-flow dynamics between the east and west sides of the ice cap. Ice movement west of the main north–south divide is dominated by relatively uniform ‘sheet’ flow, but three fast-flowing outlet glaciers that extend 14–23km beyond the ice-cap margin also drain this region. Several outlet glaciers that extend up to 60 km inland from the eastern margin drain the eastern side of the ice cap. The dominant ice-flow regimes were classified based on the relationship between the driving stress (averaged over a length scale of ten ice thicknesses) and the ratio of surface velocity to ice thickness. The mapped distribution of flow regimes appears to depict the spatial extent of basal sliding across the ice cap. This is supported by a close relationship between the occurrence of flow stripes on the ice surface and flow regimes where basal sliding was found to be an important component of the glacier motion. Iceberg calving rates were computed using measured surface velocities and ice thicknesses derived from airborne radio-echo sounding. The volume of ice calved between 1960 and 1999 was estimated to be 20.5 ± 4.7 km3 (or 0.57 km3 a–1). Approximately 89% of this loss occurred along the eastern margin. The largest single source is Belcher Glacier, which accounts for ~50% of the total amount of ice calved.
Exact solutions and verification of numerical models for isothermal ice sheets
Authors: Bueler Ed , Lingle Craig S. , Kallen-Brown Jed A. , Covey David N. , Bowman Latrice N.
Published: November 2007

Comparison of numerically computed solutions to exact (analytical) time-dependent solutions, when possible, is superior to intercomparison as a technique for verification of numerical models. At least two sources of such exact solutions exist for the isothermal shallow ice-sheet equation: similarity solutions and solutions with ‘compensatory accumulation’. In this paper, we derive new similarity solutions with non-zero accumulation. We also derive exact solutions with (i) sinusoidal-in-time accumulation and (ii) basal sliding. A specific test suite based on these solutions is proposed and used to verify a standard explicit finite-difference method. This numerical scheme is shown to reliably track the position of a moving margin while being characterized by relatively large thickness errors near the margin. The difficulty of approximating the margin essentially explains the rate of global convergence of the numerical method. A transformed version of the ice-sheet equation eliminates the singularity of the margin shape and greatly accelerates the convergence. We also use an exact solution to verify an often-used numerical approximation for basal sliding and we discuss improvements of existing benchmarks.
Retreat of Glaciar Tyndall, Patagonia, over the last half-century
Authors: Raymond Charles , Neumann Thomas A. , Rignot Eric , Echelmeyer Keith , Rivera Andrés , Casassa Gino
Published: November 2007

We report measurements of ice surface elevation, ice thickness and surface area for Glaciar Tyndall, Patagonia, made in 1999–2002. The measurements, together with previously published observations, show acceleration over the last few decades of the rates of thinning and retreat of the main calving front. The acceleration of shrinkage appears to be driven by a combination of climate and feedback processes, the dominant feedback being increased melting associated with lowering of the glacier surface (elevation feedback). The melting capacity in the main terminus lake is now too small to be a major factor accelerating the retreat. The glacier bed has low slope and remains below the elevation of the lake spillway for >14 km upstream from the 2000 calving front, indicating the potential for extensive retreat under the influence of strong elevation feedback and increasing interaction with the lake as it enlarges.
Ice-elevation changes of Glaciar Chico, southern Patagonia, using ASTER DEMs, aerial photographs and GPS data
Authors: Rivera Andrés , Casassa Gino , Bamber Jonathan , Kääb Andreas
Published: November 2007

Hielo Patagónico Sur (HPS; southern Patagonia icefield) is the largest temperate ice mass at mid-latitudes in the Southern Hemisphere. With few exceptions, the glaciers in this region have been retreating during the last 50 years. Based on field data, vertical aerial photographs and satellite images, ice-elevation changes since 1975 on Glaciar Chico, one of the main tongues of HPS, are presented. A maximum ice thinning of 5.4 ± 0.55 m a-1 was observed at the glacier front between 1975 and 1997. Global positioning system (GPS) data were used in the accumulation area of the glacier to infer a thinning rate of 1.9 ± 0.14 m a-1 between 1998 and 2001. This thinning rate is three times higher than the snow accumulation rate estimated for that part of the glacier. A mean net glacier mass balance of -0.29 ± 0.097 km3 w.e. a-1 was estimated between 1975 and 2001. Climate data suggest an increase in temperature and a reduction in precipitation during most of the 20th century in the vicinity of HPS. Although these climate changes are the primary explanation for the observed ice-elevation changes of the glacier, ice-dynamics effects are also believed to play an important role.
Validation of AVHRR- and MODIS-derived albedos of snow and ice surfaces by means of helicopter measurements
Authors: Greuell Wouter , Oerlemans Johannes
Published: November 2007

We describe the validation of surface albedos of snow and glacier ice as derived from Advanced Very High Resolution Radiometer (AVHRR) and MOderate Resolution Imaging Spectrometer (MODIS) satellite data. For this purpose we measured surface albedos from a helicopter over Vatnajokull, Iceland, and the Kangerlussuaq transect (western part of the Greenland ice sheet) in Thematic Mapper (TM) bands 2 and 4 and AVHRR bands 1 and 2, and converted these values to ‘measured albedos’ in three MODIS bands. Relative to other validation methods, our helicopter measurements have the advantages of larger spatial coverage and of (almost) direct measurements in satellite-sensor spectral bands. We found the smallest differences between the satellite-derived and helicopter albedos for the Kangerlussuaq transect: for AVHRR data a mean difference of 0.01 in both bands (with the satellite in near-nadir position) and for two MODIS images a mean difference of 0. 00-0.02 for bands 2 and 4, and 0.03 for band 1. For two AVHRR images of Vatnajokull, we found mean differences of up to 0.06. Differences are primarily due to errors in the satellite-derived albedos, which, in turn, are mainly caused by errors in the calibration coefficients of the satellite sensors and insufficient knowledge of the angular distribution of the radiation reflected by snow and ice. Satellite data obtained from view zenith angles larger than ~50-55° appeared to be unsuitable.
Use of the ELA as a practical method of monitoring glacier response to climate in New Zealand’s Southern Alps
Authors: Chinn Trevor J. , Heydenrych Clive , Salinger M. Jim
Published: November 2007

In lieu of direct glacier surface mass-balance measurements, equilibrium-line altitudes (ELAs) have been measured over a 28 year period at 50 selected glaciers distributed along the glacierized length of New Zealand’s Southern Alps. Analysis of the data shows that ELAs are a useful measurement of glacier response to annual climate fluctuations, although there is much variability in the degree of response between glaciers in any given year. Comparisons of individual glacier annual ELA with the mean for all annual ELAs of the Southern Alps show a large variation of individual glacier response, with coefficients of variation (r2) ranging from 0.53 to 0.90. The ELA data show detailed, but qualitative, annual mass-balance variations on both regional and individual glacier scales. The ELA record closely predicts glacier termini responses that follow after appropriate response time delays. The recorded variability in climate response for the Southern Alps suggests no single glacier is truly representative for detailed studies of glacier-climate relationships, and that a large number of ELA measurements may be as good an indicator of climate as a few mass-balance measurements. Given the appropriate mass-balance gradient, mass-balance values may be calculated for any of the monitored glaciers.
Debris transport in a temperate valley glacier: Haut Glacier d’Arolla, Valais, Switzerland
Authors: Goodsell B. , Hambrey M.J. , Glasser N.F.
Published: November 2007

This paper considers the role of structural glaciology in debris entrainment, transport and deposition at Haut Glacier d’Arolla, a temperate valley glacier in Valais, Switzerland. Sedimentological descriptions and cIast anaIysis have been used to identify reIationships between ice structure and debris transport. Relationships identified are (1) debris associated with crevasse traces, (2) the folding of rockfall material incorporated within primary stratification to form medial moraines and (3) dirt cones and engIaciaI debris Iayers associated with reactivated crevasse traces. A conceptuaI modeI is introduced to summarize the manner in which ice structures at Haut Glacier d’Arolla control entrainment and deposition of debris.
Borehole optical stratigraphy and neutron-scattering density measurements at Summit, Greenland
Authors: Hawley Robert L. , Morris Elizabeth M.
Published: November 2007

We have made side-by-side measurements in several boreholes at Summit, Greenland, using borehole optical stratigraphy (BOS) and neutron-scattering density logging techniques. The BOS logs show strong positive correlation at shallow depths with neutron-scattering logs taken in the same borehole. This supports the hypothesis that BOS detects changes in density. The positive correlation between returned brightness and density decreases with depth and finally becomes negative. We conclude this inversion of correlation is related to changes in densification regime from grain-boundary sliding to pressure sintering.
Ice microstructure and fabric: an up-to-date approach for measuring textures
Authors: Durand Gaël , Gagliardini O. , Thorsteinsson Throstur , Svensson Anders , Kipfstuhl Sepp , Dahl-Jensen Dorthe
Published: November 2007

Automatic c-axes analyzers have been developed over the past few years, leading to a large improvement in the data available for analysis of ice crystal texture. Such an increase in the quality and quantity of data allows for stricter statistical estimates. The current textural parameters, i.e. fabric (crystallographic orientations) and microstructure (grain-boundary networks), are presented. These parameters define the state of the polycrystal and give information about the deformation undergone by the ice. To reflect the findings from automatic measurements, some parameter definitions are updated and new parameters are proposed. Moreover, a MATLAB® toolbox has been developed to extract all the textural parameters. This toolbox, which can be downloaded online, is briefly described.
A simple model to estimate ice ablation under a thick debris layer
Authors: Haidong Han , Yongjing Ding , Shiyin Liu
Published: November 2007

This paper presents a simple model to estimate ice ablation under a thick supraglacial debris cover. The key method employed in the model is to establish a link between the debris heat flux and the debris temperature at a certain depth when the heat transfer in the debris is described by a diffusion process. Given surface temperature, debris thermal properties and relevant boundary conditions, the proposed model can estimate mean debris temperature at interfaces of different debris layers using an iterative procedure, and then the heat flux for ice ablation. The advantage of the proposed model is that it only requires a few parameters to conduct the modeling, which is simpler and more applicable than others. The case study on Koxkar glacier, west Tien Shan, China, shows, in general, that the proposed model gives good results for the prediction of debris temperatures, except for an apparent phase shift between modeled and observed values. We suggest that this error is mainly due to complex phase relations between debris temperature and debris heat flux. The modeled ablation rates at three experimental sites also show good results, using a direct comparison with observed data and an indirect comparison with a commonly used energy-balance model.
Measuring specific surface area of snow by near-infrared photography
Authors: Matzl Margret , Schneebeli Martin
Published: November 2007

The specific surface area (SSA) is considered an essential microstructural parameter for the characterization of snow. Photography in the near-infrared (NIR) spectrum is sensitive to the SSA. We calculated the snow reflectance from calibrated NIR images of snow-pit walls and measured the SSA of samples obtained at the same locations. This new method is used to map the snow stratigraphy. The correlation between reflectance and SSA was found to be 90%. Calibrated NIR photography allows quantitative determination of SSA and its spatial variation in a snow profile in two dimensions within an uncertainty of 15%. In an image covering 0.5–1.0 m2, even layers of 1mm thickness can be documented and measured. Spatial maps of SSA are an important tool in initializing and validating physical and chemical models of the snowpack.
Optical properties of snow in backscatter
Authors: Kaasalainen S. , Kaasalainen M. , Mielonen T. , Suomalainen J. , Peltoniemi J.I. , Näränen J.
Published: November 2007

We present an overview and the first systematic results on the backscatter properties of snow samples with different grain properties. During a 2 year study we investigated the effect of the apparent characteristics of the samples on the intensity enhancement in the direction of exact backscatter (retroreflection), using a specific instrument designed for backscatter measurements. We observed that a sharp peak in intensity (the hot spot) occurs for most types of snow, which is not observable with traditional goniometers. The key factors in the peak properties are the temperature (related to the changes in grain structure) and grain shape and size. These results form the basis of a larger backscatter data archive which is being applied in the systematic study and exploitation of ‘hot spots’ in remote sensing of natural targets, as well as in the development of airborne laser intensity measurement.
Snow saltation threshold measurements in a drifting-snow wind tunnel
Authors: Clifton Andrew , Rüedi Jean-Daniel , Lehning Michael
Published: November 2007

Wind tunnel measurements of snowdrift in a turbulent, logarithmic velocity boundary layer have been made in Davos, Switzerland, using natural snow. Regression analysis gives the drift threshold friction velocity (u*t), assuming an exponential drift profile and a simple drift to friction velocity relationship. Measurements over 15 snow covers show that u*t is influenced more by snow density and particle size than by ambient temperature and humidity, and varies from 0.27 to 0.69 ms–1. Schmidt’s threshold algorithm and a modified version used in SNOWPACK (a snow-cover model) agree well with observations if small bond sizes are assumed. Using particle hydraulic diameters, obtained from image processing, Bagnold’s threshold parameter is 0.18. Roughness lengths (z0) vary between snow covers but are constant until the start of drift. Threshold roughness lengths are proportional to . The influence of macroscopic objects on the roughness length is shown by the lower values measured over the smooth and flat snow surface of the wind tunnel (0.04 ≤ z0 ≤ 0.13 mm), compared to field measurements. Mean drifting-snow grain sizes for mainly new and partly decomposed snow are 100–175 μm, and independent of surface particle size.
Past and future mass balance of ‘Ka Roimata o Hine Hukatere’ Franz Josef Glacier, New Zealand
Authors: Anderson Brian , Lawson Wendy , Owens Ian , Goodsell Becky
Published: November 2007

Despite their relatively small total ice volume, mid-latitude valley glaciers are expected to make a significant contribution to global sea-level rise over the next century due to the sensitivity of their mass-balance systems to small changes in climate. Here we use a degree-day model to reconstruct the past century of mass-balance variation at ‘Ka Roimata o Hine Hukatere’ Franz Josef Glacier, New Zealand, and to predict how mass balance may change over the next century. Analysis of the relationship between temperature, precipitation and mass balance indicates that temperature is a stronger control than precipitation on the mass balance of Franz Josef Glacier. The glacier’s mass balance, relative to its 1986 geometry, has decreased at a mean annual rate of 0.02m a–1w.e. between 1894 and 2005. We compare this reduction to observations of terminus advance and retreat, of which Franz Josef Glacier has the best record in the Southern Hemisphere. For the years 2000–05 the relative mass balance ranged from –0.75 to +1.50m a–1w.e., with 2000/01 the only year showing a negative mass balance. In a regionally downscaled Intergovernmental Panel on Climate Change mean warming scenario, the annual relative mass balance will continue to decrease at 0.02m a–1w.e. through the next century.
Glacier mass balances (1993–2001), Taylor Valley, McMurdo Dry Valleys, Antarctica
Authors: Fountain Andrew G. , Nylen Thomas H. , MacClune Karen L. , Dana Gayle L.
Published: November 2007

Mass balances were measured on four glaciers in Taylor Valley, Antarctica, from 1993 to 2001. We used a piecewise linear regression, which provided an objective assessment of error, to estimate the mass balance with elevation. Missing measurements were estimated from linear regressions between points and showed a significant improvement over other methods. Unlike temperate glaciers the accumulation zone of these polar glaciers accumulates mass in summer and winter and the ablation zone loses mass in both seasons. A strong spatial trend of smaller mass-balance values with distance inland (r2 = 0.80) reflects a climatic gradient to warmer air temperatures, faster wind speeds and less precipitation. Annual and seasonal mass-balance values range only several tens of millimeters in magnitude and no temporal trend is evident. The glaciers of Taylor Valley, and probably the entire McMurdo Dry Valleys, are in equilibrium with the current climate, and contrast with glacier trends elsewhere on the Antarctic Peninsula and in temperate latitudes.
Sea-ice and North Atlantic climate response to CO2-induced warming and cooling conditions
Authors: Nazarenko Larissa , Tausnev Nickolai , Hansen James
Published: November 2007

Using a global climate model coupled with an ocean and a sea-ice model, we compare the effects of doubling CO2 and halving CO2 on sea-ice cover and connections with the atmosphere and ocean. An overall warming in the 2 × CO2 experiment causes reduction of sea-ice extent by 15%, with maximum decrease in summer and autumn, consistent with observed seasonal sea-ice changes. The intensification of the Northern Hemisphere circulation is reflected in the positive phase of the Arctic Oscillation (AO), associated with higher-than-normal surface pressure south of about 50° N and lower-than-normal surface pressure over the high northern latitudes. Strengthening the polar cell causes enhancement of westerlies around the Arctic perimeter during winter. Cooling, in the 0.5 × CO2 experiment, leads to thicker and more extensive sea ice. In the Southern Hemisphere, the increase in ice-covered area (28%) dominates the ice-thickness increase (5%) due to open ocean to the north. In the Northern Hemisphere, sea-ice cover increases by only 8% due to the enclosed land/sea configuration, but sea ice becomes much thicker (108%). Substantial weakening of the polar cell due to increase in sea-level pressure over polar latitudes leads to a negative trend of the winter AO index. The model reproduces large year-to-year variability under both cooling and warming conditions.
Developing a bubble number-density paleoclimatic indicator for glacier ice
Authors: Spencer M.K. , Alley R.B. , Fitzpatrick J.J.
Published: November 2007

Past accumulation rate can be estimated from the measured number-density of bubbles in an ice core and the reconstructed paleotemperature, using a new technique. Density increase and grain growth in polar firn are both controlled by temperature and accumulation rate, and the integrated effects are recorded in the number-density of bubbles as the firn changes to ice. An empirical model of these processes, optimized to fit published data on recently formed bubbles, reconstructs accumulation rates using recent temperatures with an uncertainty of 41% (P < 0.05). For modern sites considered here, no statistically significant trend exists between mean annual temperature and the ratio of bubble number-density to grain number-density at the time of pore close-off; optimum modeled accumulation-rate estimates require an eventual ~2.02 ± 0.08 (P < 0.05) bubbles per close-off grain. Bubble number-density in the GRIP (Greenland) ice core is qualitatively consistent with independent estimates for a combined temperature decrease and accumulation-rate increase there during the last 5 kyr.
Scale-dependent roughness of a glacier surface: implications for radar backscatter and aerodynamic roughness modelling
Authors: Rees W.G. , Arnold N.S.
Published: November 2007

The roughness of the snow-free surface of the glacier midre Lovénbreen, Svalbard, has been investigated on scales between 1 mm and 300 m. It is shown that the roughness is reasonably well described by scale-free (fractal) models for scales longer than a few metres and shorter than about 100 mm. However, there is a break in the behaviour between these scales which can be characterized by a definite scale length of 70–500 mm and a root-mean-square height variation between around 6 and 70 mm. The aerodynamic roughness length contributed by these features is estimated to be 0.3–1.5 mm. Features on this scale are consistent with the observed microwave backscattering properties of the glacier.
Late-20th-century changes in glacier extent in the Caucasus Mountains, Russia/Georgia
Authors: Stokes Chris R. , Gurney Stephen D. , Shahgedanova Maria , Popovnin Victor
Published: November 2007

Glaciers occupy an area of ~1600km2 in the Caucasus Mountains. There is widespread evidence of retreat since the Little Ice Age, but an up-to-date regional assessment of glacier change is lacking. In this paper, satellite imagery (Landsat Thematic Mapper and Enhanced Thematic Mapper Plus) is used to obtain the terminus position of 113 glaciers in the central Caucasus in 1985 and 2000, using a manual delineation process based on a false-colour composite (bands 5, 4, 3). Measurements reveal that 94% of the glaciers have retreated, 4% exhibited no overall change and 2% advanced. The mean retreat rate equates to ~8ma–1, and maximum retreat rates approach ~38 m a–1. The largest (>10 km2) glaciers retreated twice as much (~12ma–1) as the smallest (<1 km2) glaciers (~6ma–1), and glaciers at lower elevations generally retreated greater distances. Supraglacial debris cover has increased in association with glacier retreat, and the surface area of bare ice has reduced by ~10% between 1985 and 2000. Results are compared to declassified Corona imagery from the 1960s and 1970s and detailed field measurements and mass-balance data for Djankuat glacier, central Caucasus. It is concluded that the decrease in glacier area appears to be primarily driven by increasing temperatures since the 1970s and especially since the mid-1990s. Continued retreat could lead to considerable changes in glacier runoff, with implications for regional water resources.
Singular spectrum analysis and envelope detection: methods of enhancing the utility of ground-penetrating radar data
Authors: Moore John C. , Grinsted Aslak
Published: November 2007

We present a novel method of improving signal-to-noise ratio in radargrams. The method uses singular spectrum analysis (SSA) to separate each individual radar trace into orthogonal components. The components that explain most of the original trace variance contain mainly physically meaningful signal, while the components with little variance tend to be noise. Adding the largest-magnitude components together until the sum of components accounts for the variance above the noise level (typically 60–80%) of the original trace variance results in a much cleaner radargram with more easily seen internal features than in traditionally filtered data. The radargrams can be further enhanced by envelope-detecting the SSA-filtered data, as this measure of instantaneous energy minimizes the deleterious effects of innumerable phase changes at dielectric boundaries. Subsequent incoherent stacking results in far more structured radargrams than are achieved with traditionally processed radar data and amplitude stacking.
Ice-sheet modelling characteristics in sea-level-based temperature reconstructions over the last glacial cycle
Authors: Wilschut Frank , Bintanja Richard , Van De Wal Roderik S.W.
Published: November 2007

A widely used method for investigating palaeotemperatures is to analyze local proxy records (e.g. ice cores or deep-sea sediment cores). The interpretation of these records is often not straightforward, and global or hemispheric means cannot be deduced from local estimates because of large spatial variability. Using a different approach, temperature changes over the last glacial cycle can be estimated from sea-level observations by applying an inverse method to an ice-sheet model. In order to understand the underlying physical mechanisms, we used a 1-D ice-sheet model and a 3-D coupled thermodynamic ice-sheet–ice-shelf–bedrock model to investigate the importance of several physical processes for the inverse temperature reconstructions. Results show that (i) temperature reconstructions are sensitive to the employed formulation of mass balance, (ii) excluding thermodynamics in the ice sheet leads to a smaller temperature amplitude in the reconstruction and (iii) hysteresis in the non-linear relation between sea level and temperature occurs as a consequence of ice redistribution in the process of merging and separation of ice sheets. The ice redistribution does not occur if the geometry does not support the formation of a relatively flat dome, which tends to be preserved in warming conditions.
Nivlisen, an Antarctic ice shelf in Dronning Maud Land: geodetic–glaciological results from a combined analysis of ice thickness, ice surface height and ice-flow observations
Authors: Horwath Martin , Dietrich Reinhard , Baessler Michael , Nixdorf Uwe , Steinhage Daniel , Fritzsche Diedrich , Damm Volkmar , Reitmayr Gernot
Published: November 2007

Extensive observations on Nivlisen, an ice shelf on Antarctica’s Atlantic coast, are analyzed and combined to obtain a new description of its complex glaciological regime. We generate models of ice thickness (primarily from ground-penetrating radar), ellipsoidal ice surface height (primarily from ERS-1 satellite altimetry), freeboard height (by utilizing precise sea surface information) and ice-flow velocity (from ERS-1/-2 SAR interferometry and GPS measurements). Accuracy assessments are included. Exploiting the hydrostatic equilibrium relation, we infer the ‘apparent air layer thickness’ as a useful measure for a glacier’s density deviation from a pure ice body. This parameter exhibits a distinct spatial variation (ranging from ≈2 to ≈16m) which we attribute to the transition from an ablation area to an accumulation area. We compute mass-flux and mass-balance parameters on a local and areally integrated scale. The combined effect of bottom mass balance and temporal change averaged over an essential part of Nivlisen is –654 ± 170 kg m–2 a–1, which suggests bottom melting processes dominate. We discuss our results in view of temporal ice-mass changes (including remarks on historical observations), basal processes, near-surface processes and ice-flow dynamical features. The question of temporal changes remains open from the data at hand, and we recommend further observations and analyses for its solution.
Towards a field test for fracture propagation propensity in weak snowpack layers
Authors: Gauthier Dave , Jamieson Bruce
Published: November 2007

Slab avalanche release requires fracture initiation and propagation in a weak snowpack layer. While field tests of weak-layer strength are useful for fracture initiation, the challenge remains to find a verified field test for fracture propagation. We introduce the two current versions of a field test for fracture propagation propensity, and report results of testing conducted in the Columbia Mountains of British Columbia, Canada, during the winter of 2005. By extending the column of a stability test approximately 3 m in the downslope direction, the test method allows for the development of a flexural wave in the slab, and thereby maintains the contribution of this wave and the associated weak-layer collapse to the fracture process. Fracture lengths collected on a day and location where the propagation propensity of the snowpack was locally high show a bimodal distribution, with approximately 50% of observed fractures similar to those collected in stable snowpacks, and approximately 50% with much longer fracture lengths.