bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spatial variations in heat at the base of the Antarctic ice sheet from analysis of the thermal regime above subglacial lakes
Authors: Siegert Martin J. , Dowdeswell Julian A. 
Published: September 2017

AbstractAntarctic subglacial lakes provide аn important boundary condition for thermal analysis of the ice sheet in that the basal ice temperature over lakes may be assumed to be at the pressure-melting point. We have used a one-dimensional vertical heat-transfer equation to determine theoretical temperature values for the ice-sheet base above 77 subglacial lakes identified from airborne radio-echo-sounding data covering 50% of Antarctica. Variations in our temperature results to below the pressure-melting temperature over lakes are due to either our estimate of the geothermal heat flux or a neglect of heat derived from (a) internal ice deformation and (b) basal sliding, in the thermal model. Our results indicate that, when the geothermal heat flux is set at 54 m W m−2, the ice-sheet base above 70% of the known Antarctic subglacial lakes is calculated to be at the pressure-melting value. These lakes are located mainly around Dome C, Ridge B and Vostok station. For the ice sheet above subglacial lakes located hundreds of kilometres from the ice divide, using the same thermal model, loss of heat due to vertical advection is calculated to be relatively high. In such regions, if the ice-sheet base is at the pressure-melting point, heat lost due to vertical advection must be supplemented by heat from other sources. For the three lakes beneath Terre Adélie and George V Land, for instance, the basal thermal gradient calculated to produce pressure melting at the ice-sheet base is equivalent to 1.5–2 times the value obtained when 54 m W m−2 of geothermal heat is used as the sole basal thermal component. We suggest that, as distance from the ice divide increases, so too does the amount of heat due to internal ice deformation and basal sliding. Moreover, by considering the ice-sheet basal thermal characteristics above subglacial lakes which lie on the same ice flowline, we demonstrate empirically that the heat due to these horizontal ice-motion terms varies pseudo-exponentially with distance from the ice divide. The location along a flowline where a rapid increase in the basal heat gradient is calculated may correspond to the onset of large-scale basal sliding.
Geometry, motion and mass balance of Dyer Plateau, Antarctica
Authors: Raymond Charles , Weertman Bruce , Thompson Lonnie , Mosley-Thompson Ellen , Peel David , Mulvaney Rob 
Published: September 2017

AbstractGeodetic surveying and ground-based radar profiling were used to determine geometry and surface motion of the ice sheet on the Dyer Plateau, Antarctica, in the vicinity of an ice-core site on a local dome. Vertical strain measurements in the core hole constrain the depth profile of vertical velocity. These geophysical measurements are used to analyze the profiles of density and annual layer thickness measured on the ice core to estimate the current mass balance of the ice column and the past history of accumulation rate. Consideration of horizontal and vertical mass-flow divergence shows that the profiles of density and vertical velocity are not fully consistent with steady state. Mean density of the firn layer appears to be increasing, which leads to the deduction of a small rate of mass increase (≈ 0.02 m a− 1 ice-equivalent thickness). Over the last 200a there has been a gradual increase in accumulation rate from about 0.46 m a− 1 to 0.54 m a− 1 ice-equivalent thickness in recent time.
Stress-induced foliation in the terminus of Variegated Glacier, Alaska, U.S.A, formed during the 1982–83 surge
Authors: Tad Pfeffer W. 
Published: September 2017

AbstractTwo sets of bubble-density foliation, one vertical and transverse to flow and one vertical and parallel to flow, developed in the ice of the terminus lobe of Variegated Glacier, Alaska, during the passage of the final stages of the 1982–83 surge in June and July 1983. The transverse foliation is ubiquitous in the lower part of the terminus lobe affected by the surge, but disappears abruptly over a 5 m wide zone at the boundary of the surge, as delineated by measurable strain rates during the surge. The ice at the outermost limit of the transverse foliation experienced compressive total strains no greater than −0.10 (logarithmic), but did briefly experience high strain rates, corresponding to compressive stresses in excess of 0.6 MPa. It is concluded that the transverse foliation was developed as a consequence of high compressive stress rather than high total strains. Experimental evidence of Kamb indicates that bubble migration occurs under similar tectonic conditions to those in the terminus lobe during the surge, and that bubble migration is related to stress rather than finite deformation or rotation of pre-existing structures. Bubble migration may be the mechanism by which the transverse foliation developed during the passage of the surge front.
Climate sensitivity of glaciers in southern Norway: application of an energy-balance model to Nigardsbreen, Hellstugubreen and Alfotbreen
Authors: Oerlemans J. 
Published: September 2017

AbstractThree glaciers in southern Norway, with very different mass-balance characteristics, are studied with an energy-balance model of the ice/snow surface. The model simulates the observed mass-balance profiles in a satisfactory way, and can thus be used with some confidence in a study of climate sensitivity. Calculated changes in equilibrium-line altitude for a 1 K temperature increase are 110, 108 and 135 m for Nigardsbreen, Hellstugubreen and Alfotbreen, respectively. The corresponding changes in mass balance, averaged over the entire glacier area, are −0.88, −0.715 and −1.11 m year−1 (water equivalent).Runs with an ice-flow model for Nigardsbreen, to which calculated mass-balance profiles arc imposed, predict that the front will advance by 3 km for a 1 K cooling, and will retreat by as much as 6.5 km for a 1 K warming. The response to a 10% increase in precipitation would be a 2 km advance of the snout, whereas a 4 km retreat is predicted for a 10% decrease. This large sensitivity (as compared to many other glaciers) is to a large extent due to the geometry of Nigardsbreen.
Hintereisferner, Austria: mass-balance reconstruction and numerical modelling of the historical length variations
Authors: Greuell W. 
Published: September 2017

AbstractIn this study, an attempt is made to simulate the historical length variations of Hintereisferner (Austria). Key elements in the model are: (1) reconstruction of the local climate (Vent); (2) multiple linear regression analysis between measured climate and mass-balance variations; (3) reconstruction of the mass balance with (1) and (2); (4) a one-dimensional numerical ice-flow model forced by the mass-balance history. Progress relative to the previous, similar studies can be summarized as two points: (1) the mass balance is reconstructed more carefully, and (2) sensitivity experiments show that uncertainties and assumptions in the flow model hardly affect the response of the glacier to climate fluctuations with a time-scale of the order of (or larger than) the response time of the glacier (around 100 year).In the regression equation, mass-balance fluctuations are determined by the temperature during the ablation season and the annual precipitation. Cloudiness has no effect. The local temperature can be reconstructed as far back as 1755 with the temperature series from Basel, but the reconstruction of precipitation is unreliable prior to 1900. Mainly due to this point, a simulation starting in 1755 failed completely. However, in a run starting in 1894, the total retreat since then (2.2 km) was reproduced within 10%.
Flow-law hypotheses for ice-sheet modeling
Authors: Alley Richard B. 
Published: September 2017

AbstractIce-flow modeling requires a flow law relating strain rates to stresses in situ, but a flow law cannot be measured directly in ice sheets. Microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain-size and c-axis fabric. These microscopic processes can be inferred from observations of the physical properties, and the flow law can then be estimated from the microscopic processes.A review of available literature shows that this approach can be imperfectly successful. Interior regions of large ice sheets probably have depth-varying flow-law “constants”, with the stress exponent, n, for power-law creep less than 3 in upper regions and equal to 3 only in deep ice; n probably equals 3 through most of the thickness of ice shelves and ice streams.
Calculated variations of annual ice ablation at the margin of the Greenland ice sheet, West Greenland, 1961–90
Authors: Braithwaite Roger J. , Olesen Ole Β. , HøJmark Thomsen Henrik 
Published: September 2017

AbstractAnnual ice ablation has been measured at three locations at the margin of the Greenland ice sheet: Nordbogletscher (for 6 years: 1977–78 to 1982– 83), Qamanârssûp sermia (8 years: 1979–80 to 1986–87) and at Paakitsup Akuliarusersua (7 years: 1982–83 to the present). As the data sets cover different periods, it is difficult to compare ablation directly between the three sites. However, measured series at each site can be extended over the last 30 years (1961–90) by simulations using climatic data, and the extended data series can be compared. The pattern of calculated ablation variations at the three locations is remarkably similar with no sign of any trend towards increased ablation in recent years. There was generally low ablation from the mid-1970s to the mid-1980s that may explain the recent thickening of the ablation area which has been detected by satellite radar altimetry. Ablation varies substantially from year to year, e.g. with a standard deviation of the order of ±0.5 m water a−1, and any future monitoring programme must detect trends of increasing ablation against this background of natural variations.
The Western Barents Sea and the Svalbard archipelago 18 000 years ago – a finite-difference computer model reconstruction
Authors: Isaksson Elisabeth 
Published: September 2017

AbstractA finite-difference computer model was used to reconstruct a steady-state ice sheet over the western Barents Sea and the Svalbard archipelago at the last glacial maximum 18 000 BP. Flowlines were reconstructed from the ice margin to the ice dome based on information about glacial geology, bedrock topography, basal thermal regime and the present precipitation pattern. The modelling result suggests a 1700 m high marine ice dome east of Spitsbergen and a 1350 m high terrestrial dome over northwest Spitsbergen. Over most of the terrestrial areas, the bed is frozen while in the marine areas the bed is thawed. This is in agreement with both the terrestrial and the marine glacial geology records. The height of the terrestrial dome implies that the ice sheet was thin and left several of the higher mountain peaks as nunataks. Three major ice streams were reconstructed in the most pronounced over-deepened marine channels. This modelling experiment did not allow any major ice streams in the fjords of Spitsbergen during steady-state conditions.
Thermal conductivity of porous ice in hailstone shells
Authors: Zheng Guoguang , List Roland 
Published: September 2017

AbstractThe thermal conductivity and diffusivity of porous ice accreted on spherical and spheroidal hailstone models were measured over a density range of 620–915 kg m−3. By scanning the evolving surface temperature distributions during cooling in a cold airflow the thermal conductivity was varied in iterative fashion until the internal heat flux produced the correct surface temperature distribution. The results indicate a linear dependence of the thermal conductivity, ki, and diffusivity, αi, on density. For example, lowering the density by 10% lowers ki by 15%. Within the range of cloud conditions, the density variations affect the thermal parameters more than temperature does. The results also indicate a continuous decrease of the thermal conductivity from bulk ice via consolidated porous ice to loosely packed snow.
Temperate ice permeability, stability of water veins and percolation of internal meltwater
Authors: Lliboutry L. 
Published: September 2017

AbstractIn temperate glacier ice, in situ, besides water veins, there are water lenses, on grain boundaries more or less perpendicular to the direction of maximum pressure p1 (at the grain scale). Geometry of veins is developed. Grains are modelled as equal tetrakaidecahedra. The stress and temperature fields around a vein at a smaller, microscopic scale are estimated and the water discharge by a Vein is calculated. The time-derivative of the cross-sectional area S of a vein is governed neither by energy dissipation in the water nor by plasticity, but by capillarity effects and salinity. A “vasodilator threshold” pd for water pressure pw in the veins is defined. Normally, Pw < Pd, then S has a stable value, the same for any orientation of the vein, and the microscopic temperature is uniform. The coefficient of permeability is proportional to (Pd-pw)−4, and thus a true Darcy law does not hold. As an application, the percolation of internal meltwater is studied; in an upper boundary layer about 2 m thick this meltwater flows upwards, because in the bulk of the glacier pw is very close to P1, whereas it is zero at the surface. When, exceptionally, pw > pd, S increases irreversibly. Whether it leads to the formation of “worm-holes” is discussed.
Diagnosing the imbalance of Glaciar Santa Rosa, Cordillera Raura, Peru
Authors: Ames Alcides , Hastenrath Stefan 
Published: September 2017

AbstractFor Glaciar Santa Rosa, in the Cordillera Raura, Peru, the extent and volume are determined for 1980 and 1961 from topographic mapping, and for two undated earlier stages LME and MAX from moraines. The regime of the glacier in terms of ice flow and mass balance is evaluated for the 1977–83 period. More than half of the water discharge from the glacier is not renewed by precipitation but supplied by the ice thinning. The rate of surface lowering of 2 m a−1 liquid-water equivalent corresponds to an energy amount for melting of 22 W m−2. Sensitivity analyses of Glaciar Yanamarey in the nearby Cordillera Blanca show that such a decrease of energy available for melting could be produced by a cloudiness increase of one-tenth, an air temperature decrease of more than 2°C, a decrease in specific humidity of 1 g kg−1, or some combination of heat-budget processes. Such changes in the atmospheric environment would be required to stabilize the glacier at its recent volume. As another indication of the recent imbalance, the maximum volume flux is found about 100 m below the equilibrium-line altitude. Under continuation of recent climatic conditions, the glacier may disappear within a few decades. These inferences coorroborate a recent assessment for the neighboring Cordillera Blanca.
Structural evolution of Variegated Glacier, Alaska, U.S.A., since 1948
Authors: Lawson Wendy 
Published: September 2017

AbstractАn excellent photographic database, comprising 12 sets of vertical aerial photographs taken between 1948 and 1993, has enabled the structural glaciology of surge-type Variegated Glacier, Alaska, U.S.A., tо be analyzed for a period that spans two surge cycles and includes the effects of three surges. The glacier-wide patterns of crevasse development at the end of each of these surges, which occurred in 1947–48, 1964–65 and 1982–83, were remarkably similar, despite differences in surge intensities and extents. At the end of each surge a three-fold longitudinal zonation of crevasse orientation characterised that part of the glacier that had been affected by surge activity. This zonation comprises: (i) transverse crevasses in a narrow upper zone, (ii) superimposed longitudinal and transverse crevasses in an extensive middle zone, and (iii) longitudinal crevasses in a narrow lower zone. The main difference between patterns produced by successive surges was small variations in the extent оf the three zones. During quiescence, the most intense crevasse development occurs in the upper glacier in the latter part of quiescence, and is a function of increasingly steep down-glacier velocity gradients developing in the buildup to the impending surge. Some crevassing also develops in the central part of the glacier in mid-quiescence that is not related to the surge nature of the glacier, but to normal flow processes over bedrock. There is no structural evidence to indicate that any of the tributaries of Variegated Glacier surge. This observation suggests that the bulb-like loops in the medial moraines at Variegated Glacier form during quiescence as a result of the steady flow of the tributaries into the stagnant trunk glacier, rather than as a result of tributary surges.
Avalanche forecasting — an expert system approach
Authors: Schweizer Jürg , Föhn Paul M. B. 
Published: September 2017

AbstractAvalanche forecasting for a given region is still a difficult task involving great responsibility. Any tools assisting the expert in the decision-making process are welcome. However, an efficient and successful tool should meet the needs of the forecaster. With this in mind, two models, were developed using a commercially available software: CYBERTEK-COGENSYSTM, a judgment processor for inductive decision-making–a principally data-based expert system. Using weather, snow and snow-cover data as input parameters, the models evaluate for a region the degree of avalanche hazard, the aspect and altitude of the most dangerous slopes. The output result is based on the snow-cover stability. The new models were developed and have been tested in the Davos region (Swiss Alps) for several years. To rate the models, their output is compared to the a posteriori verified hazard. the first model is purely data-based. Compared to other statistical models, the differences are: more input information about the snow cover from snow profiles and Rutschblock tests, the specific method to search for similar situations, the concise output result and the knowledge base that includes the verified degree of avalanche hazard. The performance is about 60%. The second, more-refined model, is both data- and rule-based. It tries to model the decision-making process of a pragmatic expert and has a performance of about 70%, which is comparable to the accuracy of the public warning.
Visible and near-infrared reflectivity during the ablation period on Peyto Glacier, Alberta, Canada
Authors: Cutler Paul M. , Scott Munro D. 
Published: September 2017

AbstractModels for calculating glacier mass balance are sensitive to surface reflectivity variation. Fieldwork carried out on Peyto Glacier, Alberta, Canada, contributes to the data set available for ice-reflectivity parameterization in such models. Hemispherical reflectivity in the visible and near-infrared parts of the solar spectrum was obtained for rock, snow and three contrasting glacier surfaces to examine temporal and spatial variations. Glacier-ice near-infrared reflectivity displays only minor spatial variation (0.12–0.17) in comparison with the visible range (0.23–0.40), the latter being influenced primarily by surface impurity content. Surface roughness is of minor importance compared with impurities. Temporal variation of reflectivity was weak at all glacier-ice and rock locations; slight variations observed were due to changes in either solar zenith angle or cloud amount. Snow reflectivity displayed pronounced diurnal asymmetry and a larger response to cloud cover. The minimal temporal variation in glacier-ice reflectivity simplifies its parameterization. This behaviour is additionally useful for satellite-based measurements of the reflectivity field on larger glaciers, as images obtained within a 6 h window centred on solar noon are likely to yield values which are within 2–3% of daily mean values.
First results from a pre-operational system for automatic detection and recognition of seismic signals associated with avalanches
Authors: Leprettre Benoit J. P. , Navarre Jean-Pierre , Taillefer Alain 
Published: September 2017

AbstractA system for automatic, reliable, semi-instantaneous estimation of avalanche activity is presented in an attempt to check deterministic models and improve the surveillance of risk zones. The principle is the seismic detection of avalanches. Two experimental sites equipped with standard seismological equipment are described. Avalanche seismic signals are recorded, as well as many extraneous signals of natural or human origin. After several years of a posteriori identification of the signals, we are now able to set up criteria for the automatic recognition of non-avalanche signals. We have therefore developed an automatic analysis/decision system to discriminate between avalanche signals and others. This system worked satisfactorily in pre-operational conditions during the winter of 1995-96. The results are presented and compared with other data related to avalanche activity. Although there is still room for improvement, our system seems to be able to estimate avalanche activity better than human visual observations.
Tidewater calving
Authors: Van Der Veen С.J. 
Published: September 2017

AbstractData from Columbia Glacier are used to identify processes that control calving from a temperate tidewater glacier and to re-evaluate models that have been proposed to describe iceberg calving. Since 1981, Columbia Glacier has been retreating rapidly, with an almost seven-fold increase in calving rate from the mid-1970s to 1993. At the same time, the speed of the glacier increased almost as much, so that the actual rate of retreat increased more slowly. According to the commonly accepted model, the calving rate is linearly related to the water depth at the terminus, with retreat of the glacier snout into deeper water, leading to larger calving rates and accelerated retreat. The Columbia Glacier data show that the calving rate is not simply linked to observed quantities such as water depth or stretching rate near the terminus. During the retreat, the thickness at the terminus appears to be linearly correlated with the water depth; at the terminus, the thickness in excess of flotation remained at about 50 m. This suggests that retreat may be initiated when the terminal thickness becomes too small, with the rate of retreat controlled by the rate at which the snout is thinning and by the basal slope. The implication is that the rapid retreat of Columbia Glacier (and other comparable tidewater glaciers) is not the result of an increase in calving as the glacier retreated into deeper water. Instead, the retreat was initiated and maintained by thinning of the glacier. For Columbia Glacier, the continued thinning is probably associated with the increase in glacier speed and retreat may be expected to continue as long as these large speeds are maintained. It is not clear what mechanism may be responsible for the speed-up but the most likely candidate is a change in basal conditions or subglacial drainage. Consequently, the behavior of tidewater glaciers may be controlled by processes acting at the glacier bed rather than by what happens at the glacier terminus.