bluuebunny/crossref_metadata_2025 huggingface dataset:
Experiments on Thermal Convection in Snow
Authors: Powers D. , Colbeck S.C. , O'Neill K.
Published: February 2017

Thermal convection is observed in snow and in a compact of water-saturated glass beads. While uncertainty in the permeability of the snow limits our ability to compare the observed and calculated onset of convection, agreement between the observed and calculated effects of convection on heat transfer in snow is good. Experimental results with glass beads agree with both the calculated onset of and heat transfer by convection. Attempts are made to assess the effects of convection on snow metamorphism. While much is still uncertain about the significance of thermal convection in snow, it is clear that the phenomenon does occur.
Estimation and Effects of Internal Accumulation on Five Glaciers in Alaska
Authors: Trabant D. C. , Mayo L. R.
Published: February 2017

Estimates of internal accumulation for five Alaskan glaciers in different climates suggests that internal accumulation occurs in glaciers throughout Alaska. A method of estimating the quantity of internal accumulation is based on the annual minimum temperature ?Γ the snow-firn interface. The consistency of the estimates suggests that valid approximations of internal accumulation can be made over large areas. The estimates have not been confirmed by measurements of mass change, but are believed to be accurate within 10 percent. Point balance quantities of internal accumulation range from 0.65 m a-1, which is 64% of the annual accumulation on McCall Glacier, to 0.12 m a-1, 7% of the annual accumulation on Wolverine Glacier. The amount of internal accumulation decreases rapidly from north to south in Alaska; however it should be expected worldwide where sub-freezing, porous firn is invaded by water.

Internal accumulation occurs in temperate glaciers in strongly maritime climates as well as glaciers in cold continental climates where it delays the onset of runoff, accelerates the process of converting snow into glacier ice, and explains the occurrence of temperate glaciers in permafrost areas.
Bidirectional Reflectance of Polar and Alpine Snow Surfaces
Authors: Kuhn M.
Published: February 2017

The reflectance distribution of polar and alpine snow was measured under various conditions at 450, 514, 750 and 1000 nm wavelength. A reflectance peak appears in the azimuth directions up to 60° to both sides of the solar azimuth, is more prominent at high zenith angles of incidence and of reflectance and is better developed in coarse than in fine-grained snow. Under natural conditions, when only hemispherical-directional reflectivity can be determined, the anisotropy is spread in the blue part of the spectrum where the diffuse component dominates global irradiance. Bidirectional reflectance of a laser beam at 514 nm over alpine snow is comparable to that at 1000 nm over polar snow.
Internal Freezing of Snow Cover due to its Own Cold Heat Sources (Abstract)
Authors: Yamada Y. , Ikarashi T.
Published: February 2017

This report discusses the one-dimensional freezing of dry snow/ wet snow systems for the condition first examined by Stefan: the problem of heat conduction with phase change. There are two systems of internal freezing: one is a closed system of temperature rise in a dry snow layer sandwiched between upper and lower wet snow layers; the other an open system of freezing of a thin wet layer provoked mainly by an upper dry snow layer facing the atmosphere at its surface. The latter negatively concerns the release of some avalanches, because the weak layers of surface avalanches in districts where the melt-freeze metamorphism prevails (as in the Horuriku district of Japan) may be the thin wet granular snow layers.

Numerical results are given for different conditions of internal freezing. A comparison with field observations reveals the fundamental aspect of this phenomenon and the possibility of avalanche release.
Transient creep of polycrystalline ice under uniaxial compression: an assessment of internal state variable models
Authors: Meyssonnier J. , Goubert A.
Published: February 2017

The efforts to develop damage models for ice cannot be disconnected from a better knowledge of the undamaged ice behaviour. In this respect the transient behaviour of polycrystalline ice still needs to be investigated. The present paper is a contribution to the development of rheological models which can be used in varying load situations. The deformation processes which should be the foundation of the models are described. The models of Le Gac and Duval (1980) and of Sunder and Wu (1989a) are tested against two uniaxial compression tests on isotropic granular ice under varying load. They fail to describe both primary creep and the response to increments/decrements of the applied load. A new model, based on a decomposition of the viscoplastic strain into two components which account separately for kinematic and isotropic hardening, is shown to give better results.
An electronic device for long-term snow wetness recording
Authors: Denoth A.
Published: February 2017

Water content, water distribution and variations with time of these quantities in the natural snow cover are important parameters for microwave remote-sensing studies, snow stability investigations and snow hydrology studies. The most promising method for snow wetness determination is the measurement of the dielectric constant at radio frequencies. Recent developments of electronic devices for long-term recording of snow wetness are reported. The measurement system consists of two parts: the tuning and display unit with the battery pack and the flat capacitive sensor. The system is controlled by a built-in low-power microcomputer, the operation frequency for the sensor is 20.00 MHz. Measurement data are displayed on an LCD and are also stored in a RAM; data transfer for a PC for further data processing is possible by a built-in V24 interface. Up to four individual sensors can be operated simultaneously. Field measurements of long-term variations in snow wetness during diurnal cycles and registrations of penetrating meltwater or rain-water waves with the formation of water shock fronts are presented.
Estimating snow avalanche runout distances from topographic data
Authors: Fujisawa K. , Tsunaki R. , Kamiishi I.
Published: February 2017

To protect rural areas and roads from disasters caused by snow avalanches, avalanche hazard maps are being developed. In Japan, when determining snow avalanche runout distances, empirical values are normally used. These assume that the line-of-sight angle at the starting point of an avalanche, viewed from its runout point, is less than 18° for surface-layer avalanches and less than 24° for full-depth layer avalanches. This rule of thumb ignores topographic features of the slope carrying the large mass of snow. We have made analyses to find a method that can obtain the avalanche runout distance, taking into consideration the longitudinal profiles of the sites, using regression analyses on data from 66 avalanches. The line-of-sight angle can be expressed sufficiently by functions, such as line-of-sight angle between the starting point of an avalanche and a point on its course of flow 10° in gradient, an angle at the starting point of an avalanche, and the curvature of an approximate quadratic curve of topography. Based on statistics for the above relationship, we have established four different ranking categories to classify the degree of hazard. If the starting points of avalanches are known, hazardous areas can automatically be ranked on a basis of topographic data.
A two-dimensional computational model of turbulent atmospheric surface flows with drifting snow
Authors: Liston G.E. , Brown R.L. , Dent J.D.
Published: February 2017

A physically based computational model of drifting snow in two-dimensional terrain is developed. The model considers the case where wind speeds are low enough to neglect the transport of particles from the saltation layer into the turbulent flow field. The model has two distinct parts, one describing the turbulent airflow, and a second describing the mass-transport process and resulting snow-accumulation patterns produced by saltation transport. The turbulent-flow model consists of a general solution of the time-averaged, two-dimensional Navier-Stokes equations, where the k-∊ turbulence model is used to close the system of equations. The turbulent-flow model is coupled to a saltation model to compute the time evolution of the surface wind fields and snowdrift formation in the vicinity of a solid fence. Modeled wind fields and snow-accumulation profiles are similar to published field and experimental data.
Passive-microwave remote sensing of snow
Authors: Koike Tosio , Suhama Tomoyuki
Published: February 2017

The radiative-transfer theory and the Rayleigh scattering model were used to obtain the microwave brightness temperatures from dry snowpacks on the ground, which was modelled by a scattering dielectric layer and an underlying homogeneous half-space. The total radio brightness at the radiometer was the sum of the direct and diffuse radiation field intensities. The result of the model application was in good agreement with the observational data, which were obtained through the airborne experiments for the MOS-1 Verification Program around Asahikawa, Hokkaido, Japan, in February 1988.
Numerical simulation of fluctuations of Hintereisferner, Ötztal Alps, since AD 1850
Authors: Schlosser Elisabeth
Published: February 2017

A one-dimensional ice flow model was adapted to reconstruct fluctuations of Hintereisferner, Ötztal Alps, since the last postglacial maximum in the mid-19th century. Both front positions and longitudinal ice thickness profiles were considered.

As forcing function the specific mass balance was used. The model was calibrated with the period 1953–91, because since 1953 the mass balance has been determined directly. For the period before 1953, as a first step so-called dendro-mass balances (derived from tree rings) were used. Then the mass balance was also parameterized as a function of temperature and precipitation from adjacent climate stations. With both forcings Hintereisferner could be reconstructed back to about 1850 with an accuracy that lies within the accuracy of the model (200 m grid-point distance).
Surface reflectance of Hintereisferner, Austria, from Landsat 5 TM imagery
Authors: Koelemeijer Robert , Oerlemans Johannes , Tjemkes Stephen
Published: February 2017

Landsat 5 Thematic Mapper observations are used to derive the horizontal distribution of reflectance over Hintereisferner in Austria. To analyze temporal evolution of surface reflectance, three different dates are considered. The retrieval method adopts the linear relation between the planetary and surface reflectance, which follows directly from the principles of invariance. Constants in this linear relation are determined using a simple two-stream radiation model for each of the three days under consideration. From a comparison with observations the retrieved albedo is shown to compare favorably with ground observations.
Snow grain-size determination from Landsat imagery over Terre Adélie, Antarctica
Authors: Bourdelles Barbara , Fily Michel
Published: February 2017

A Landsat Thematic Mapper scene over Terre Adélie, Antarctica, is used to estimate the snow grain-size. New calibration coefficients are computed for the TM scene, using sea-water reflectance. Topographic effects are corrected with the visible band TM2. Atmospheric effects are taken into account. The snow theoretical reflectance is calculated with the Wiscombe and Warren (1980) model. Estimated snow grain-sizes are similar to those given in the literature.
Multifrequency and polarimetric SAR observations on alpine glaciers
Authors: Rott Helmut , Davis Robert E.
Published: February 2017

The glacier-covered test site Rofental in the Austrian Alps was surveyed by the AIRSAR of the NASA/Jet Propulsion Laboratory in August 1989 and June 1991. A few days after the AIRSAR survey in 1989 Landsat TM and SPOT images were acquired, which provide the opportunity to compare visible and radar data. Field activities during the SAR surveys included measurements of dielectrical, structural, and surface roughness properties and the deployment of corner reflectors. After polarimetric and radiometric calibration of the SAR data, polarimetric signatures of the main target types, snow, glacier ice and moraines, were derived. Pronounced effects of surface roughness were observed at C- and L-band for artificially roughened snow surfaces. Parallel AIRSAR swaths of the 1991 survey provide the basis for studies of incidence angle effects. Due to the increasing penetration depth, the unpolarized backscattering components of the wet snow areas increase from the C- to the P-band. The multifrequency backscattering properties of the various surfaces offer the best means for target discrimination.
Ice-sheet surging and ice-stream formation
Authors: Fowler A. C. , Johnson C.
Published: February 2017

A simplified model of ice-sheet behaviour is described. It combines the assumptions of rapid ice flow, high viscous activation energy and realistic sediment-based sliding dynamics to form a non-linear diffusion-type equation which can display relaxation oscillations analogous to those of surging glaciers, and which may be relevant to large-scale surges of the Hudson Strait and Cabot Strait ice streams of the Laurentide ice sheet.

When the physics of this model is applied to a laterally extensive unidirectional ice flow, such as that in the Siple Coast of Antarctica, an appropriate mechanism may exist for the spontaneous generation of ice streams.
On elevation models as input for mass-balance calculations of the Greenland ice sheet
Authors: van de Wal R. S. W. , Ekholm S.
Published: February 2017

In this paper the elevation model for the Greenland ice sheet based upon radio-echo-sounding flights of the Technical University of Denmark (TUD) (Letréguilly and others, 1991) are compared with the satellite-altimetry model (Tscherning and others, 1993) improved with airborne-laser and radar altimetry (IA model). Although the general hypsometry of both data sets is rather similar, differences seem to be large at individual points along the ice margin. Over the entire ice sheet, the difference between the IA model and the TUD model is 33 m with a root-mean-square error of 112 m. Differential GPS measurements collected in the ice-marginal zone near Søndre Strømfjord show that the IA model is more accurate than the TUD model. The latter data set underestimates the elevation by approximately 150 m in the ice-marginal zone near Søndre Strømfjord.

Calculation of the ablation with an energy-balance model and with a degree-day model points to a 20% decrease in the ablation if the IA model is used. Not only does this show the sensitivity of ablation calculations to the orographic input but it also indicates that the ablation calculated by the models used nowadays is relatively overestimated.
An anisotropic flow law for ice-sheet ice and its implications
Authors: Azuma Nobuhiko , Goto-Azuma Kumiko
Published: February 2017

A new flow law for anisotropic polycrystalline ice is presented. The strain-rate tensor is related by a geometrical factor tensor (G) to the stress tensor. The G factor tensor can be obtained front the c-axis fabric data and stress condition. This new flow law describes well the direction-dependent mechanical properties of anisotropic ice which cannot be demonstrated by Glen’s flow law. For example, the new flow law can explain the fact that a strong single-maximum fabric ice, such as Dye 3 Wisconsin ice, can deform several times faster than isotropic ice under horizontal shear but can hardly deform under vertical or horizontal normal stress. We also show that at a deeper part of an ice sheet, where a single-maximum fabric develops, a positive vertical strain rate can be produced with only a horizontal shear stress.
Ice-sheet flow features and rheological parameters derived from precise altimetric topography
Authors: Rémy F. , Ritz C. , Brisset L.
Published: February 2017

For the first time high-quality coverage of the ERS-1 radar altimeter provides a very accurate surface topographic map covering 80% of the Antarctic ice sheet that can contribute significantly to glaciological studies such as ice-sheet flow modelling. The topography allows estimation of the ice-flow direction, the balance velocity and the basal shear stress. A relationship between shear stress, basal temperature and a parameter related to strain rate helps in mapping the behaviour anomalies of these parameters. Longitudinal stress, sliding, bedrock topography and variation in the pre-exponential factor of the flow law are found to play a major role in the ice-flow pattern. This relation can also be used to estimate rheological parameters: the Glen exponentnis found to be 1 forT< −10°C and 3–4 for higher temperatures, whereQis found to be 70 kJ mol−1.
Isostatic postglacial rebound over Fennoscandia with a self-gravitating spherical visco-elastic Earth model
Authors: Le Meur E.
Published: February 2017

Accounting for isostasy in glaciological models has always been a necessity but these models mostly use very simple parameterizations (Le Meur and Huybrechts, 1996). The need for more realistic isostatic parameterizations rapidly became apparent, especially in the treatment of bedrock-sensitive issues such as the grounding-line migration (Huybrechts, 1990a, b). To this end, a rather sophisticated Earth model, avoiding most of the common assumptions, has been developed and is presented here. The two key groups of parameters, to which the model is most sensitive, are the Earth properties and the rheological law used for the mantle. The aim of this paper is first to justify the use of Maxwell rheology for the mantle and then to tune the most sensitive Earth parameter, namely the mantle viscosity, in order to match the numerous present-day uplift data over Fennoscandia. So, in the first instance, a short review of the different available rheologies is presented and discussed. The visco-elastic theory, as well as the mathematical background used in the present model, is also briefly sketched. Secondly, a glacial scenario over Fennoscandia served as an input for the model in a calibration test on the mantle-viscosity values. The degree of agreement of the model outputs with the present-day measurements gives a reference set of Green functions, to which one can reasonably refer when modelling the isostatic response over areas where such a control is not possible (Le Meur and Huybrechts, 1996). Finally, a closer look to the time-dependent surface displacements will confirm the ability for the model to reproduce correctly the main postglacial rebound characteristic features.
Dynamic/thermodynamic simulations of Laurentide ice-sheet instability
Authors: Greve R. , MacAyeal D. R.
Published: February 2017

A crucial element of several leading theories of Laurentide ice-sheet instability (i.e. Heinrich events and advance/retreat cycles of the southern margin) is the evolution of melting conditions at the subglacial bed. Despite the great importance basal-temperature conditions play in these theories, relatively little has been done to test their physical plausibility. We therefore undertake a numerical model study of the ice-sheet temperature field along an important transect which extends from the lobate southern margin of the Laurentide ice sheet to the iceberg-calving from at the terminus of Hudson Strait. Our experiments illustrate the influence of important aspects of ice-sheet thermodynamics on ice-sheet instability, including horizontal advection and the development of an internal temperate-ice reservoir. Free oscillations of the basal temperature and ice thickness in Hudson Strait are possible under a restricted range of parameters elucidated by the model. These free oscillations may provide a basis for understanding ice-sheet instability, e.g. Heinrich events, with time-scales in the range of 103–104 a.
The Response of Areal Snow Cover to Climate Change in a Snowmelt—Runoff Model
Authors: Rango A.
Published: February 2017

The cryosphere is represented in some hydrological models by the arcal extent of snow cover, a variable that has been operationally available in recent years through remote sensing. In particular, the snowmelt runoff model (SRM) requires the remotely sensed snow-cover extent as a major input variable. The SRM is well-suited for simulating the hydrological response of a basin to hypothetical climate change because it is a non-calibrated model. In order to run the SRM in a climate-change mode, the response of the areal snow cover to a change in climate is critical, and must be calculated as a function of elevation, precipitation, temperature, and snow-water equivalent. For the snowmelt-runoff season, the effect of climate change on conditions in the winter months has a major influence. In a warmer climate, winter may experience more rain vs snow events, and more periods of winter snowmelt that reduce the snow water equivalent present in the basin at the beginning of spring snow melt. As a result, the spring snowmelt runoff under conditions of climate warming will be affected not only by different temperatures and precipitation, but also by a different snow cover with a changed depletion rate. A new radiation-based version of the SRM is under development that will also take changes in cloudiness and humidity into account, making climate-change studies of the cryosphere even more physically based.
Cryospheric data for model validations: requirements and status
Authors: Barry R. G.
Published: February 2017

The general status of cryospheric datasets required in climate model studies is reviewed. Datasets are necessary as boundary conditions, and for validation purposes. The former application is decreasing as cryospheric variables are increasingly being derived prognostically. By contrast, the scope of cryospheric parameters that can be validated is expanding. Cryospheric datasets suitable for validation studies are reported and areas where data are lacking are identified.
Modelling the change in structure and mechanical properties in dry-snow densification to ice
Authors: Golubev Vladimir N. , Frolov Anatoly D.
Published: February 2017

The regular packing of spheres or polyhedrons of various shapes linked by rigid bonds is presented and discussed as a model of snow structure. Basic structural parameters of this model are: the coordination number and introduced dimensionless factors of friability and rigidity. The snow densification is described as successive changes of these parameters. Use of the model allows us to relate the density increase from ~130 to ~320, ~550, ~700, ~820 and 917 kg m−3, while the coordination number of the structure increases accordingly from 3 (friable hexagonal) to 4 (tetrahedral), 6 (cubic), 8, 10, 12 (dense hexagonal). These structural changes are in good agreement with the critical densities established in experimental studies of snow densification and the physical properties of snow. It is shown that the model presented allows us to estimate the mechanical properties of ice-porous media: Young’s modulus, Poisson’s ratio and strength.
A constant-speed penetrometer for high-resolution snow stratigraphy
Authors: Schneebeli Martin , Johnson Jerome B.
Published: February 2017

A new constant-speed penetrometer for field and laboratory measurement has been developed. The initially independent work of SFISAR and CRREL has been brought together, and a portable field device is now in an advanced stage of testing. The new penetrometer has high rigidity and a high-resolution large dynamic range force sensor. It uses a much smaller sensing head (5 mm) than previous designs and has a constant-speed drive. With this construction, the penetration resistance of very fine layers and the influence of the bonding strength between snow grains can be more accurately determined than is possible with the rammsonde or Pandalp. Artificial foam layers as thin as 2 mm and thin layers in snow have been detected by the penetrometer. Thin snow layers detected from penetration-resistance profiles have been correlated to fine layering as determined from plane-section microphotographs of samples taken adjacent to the profile. The instrument’s measurements are highly repeatable and the lack of subjective decisions when operating the penetrometer makes the penetration resistance a quantitative measure of snow stratigraphy.
Cavities and the effective pressure between abrading clasts and the bedrock
Authors: Hindmarsh R. C. A.
Published: February 2017

The mechanical configuration of cavities associated with clasts is discussed, and how this affects the water pressure underneath the clast. The range of permissible water pressure is determined by flow parallel to the bed rather than by flow towards the bed. The actual pressure is controlled by hydraulic connections to areas away from the clast. The degree of cavitation provides an additional degree of freedom which adjusts to ensure that horizontal and vertical forces respect the Coulomb friction condition.
The origin of till sequences by subglacial sediment deformation beneath mid-latitude ice sheets
Authors: Boulton G. S.
Published: February 2017

A theory of erosion and deposition as a consequence of subglacial sediment deformation over beds of unlithified sediment is reviewed and applied to large-scale till sequences formed on the southern flanks of the North American and British and European ice sheets during the last glacial cycle. The distribution of till thickness, till lithology in relation to source materials and intra-till erosion surfaces along a flowline in the Michigan lobe of the North American ice sheet are shown to be compatible with the deformational theory but not with other modes of till genesis. It is then demonstrated, in the case of the British ice sheet, how the assumption of a deformational origin for tills can be used to infer time-dependent patterns of ice-sheet dynamic behaviour. By reference to an example from the Netherlands, it is argued that many till sequences interpreted as melt-out tills are more likely to have formed by subglacial sediment deformation.
Observations of tunnel channels in glacial sediments with shallow land-based seismic reflection
Authors: Pugin A. , Pullan S. E. , Sharpe D. R.
Published: February 2017

A regional hydrogeological study conducted by the Geological Survey of Canada acquired 35 line-km of 12-fold seismic reflection profiles on or adjacent to the Oak Ridges moraine, north of Toronto, Ontario, Canada. The three-dimensional geometry provided by these data aids in understanding the erosional and depositional processes that occurred beneath the Laurentide ice sheet during the late stages of glaciation. The seismic sections indicate large infilled channels in the subsurface which are interpreted as tunnel channels eroded by large, subglacial meltwater discharges. Two seismic profiles from different areas of the moraine show channel-cutting events of different ages and different types of infilling.
Greenland under changing climates: sensitivity experiments with a new three-dimensional ice-sheet model
Authors: Fabre Adeline , Letréguilly Anne , Ritz Catherine , Mangeney Anne
Published: February 2017

A new three-dimensional, time-dependent ice-sheet model, including the calculation of the coupled temperature and velocity fields, isostatic adjustment of the bedrock and a mass-balance parameterization, was used to reconstruct the evolution of the Greenland ice sheet in response to a climate history derived from the oxygen-18 measured in the GRIP ice core. Steady-state experiments were done to test the sensitivity of the model, first to variations of poorly known parameters, secondly to different climates. These experiments show that the modelled ice sheet is not very sensitive to variations in the geothermal heat flux, but very sensitive to changes in the accumulation.
Arctic and Antarctic precipitation simulations produced by the NCAR community climate models
Authors: Bromwich David H. , Chen Biao , Tzeng Ren-Yow
Published: February 2017

Precipitation predictions from globai-climate models (GCMs) for the ice-covered Arctic Ocean and the ice sheets of Antarctica are among the most important aspects of the inferred response of the polar areas to climate change. It is generally recognized that the atmospheric hydrologic cycle, which includes precipitation as a key part, is one of the components of the climate system that GCMs do not handle particularly well.

The present-day atmospheric-moisture budget poleward of 70° latitude in both hemispheres, as represented by two versions of the NCAR (U.S. National Center for Atmospheric Research) community climate model (CCM1 and CCM2), is compared with observational analyses. The quantities examined on the seasonal and annual timescales are precipitation, evaporation/sublimation and atmospheric poleward moisture transport. The results are discussed in terms of the physiographic and climatic characteristics of both polar regions and how the particular models handle moisture transport: CCM1 uses the positive-moisture fixer and CCM2 the semi- Lagrangian transport. A particularly important test both for models and for observations is the degree to which the independently determined moisture-budget quantities actually balance. Deficiencies of both observations and models are discussed.
Spectral bi-directional reflectance of snow and glacier ice measured in Dronning Maud Land, Antarctica
Authors: Winther Jan-Gunnar
Published: February 2017

Visible and near-infrared spectral reflectances of snow and superimposed ice were measured in Dronning Maud Land, Antarctica, during the 1992-93 austral summer. Spectral-reflectance curves of both snow and superimposed ice remain high ( > 80%) in the visible region. A pronounced decrease in reflectance appears in the near-infrared, especially for superimposed ice. Superimposed ice with a 1 cm thick surface layer of ice-bound snow crystals had a considerably higher reflectance than superimposed ice containing only a few snow crystals. Furthermore, these data prove that snow and superimposed ice reflect solar radiation specularly and suggest that the anisotropy strengthens with increasing wavelengths. Integrated in-situ reflectances corresponding to Landsat TM bands 1-4 show that TM band 1 is least affected, whereas TM band 4 is most affected by anisotropy. Furthermore, the anisotropy increases with increasing off-nadir viewing angles up to an angle corresponding to 90°-θs(θs= solar elevation). For a 15° off-nadir sensor-observation angle, the average snow reflectance for TM bands 1--4 is about 10% higher than at nadir. Similarly, the apparent reflectance can be more than 50% higher than the nadir reflectance for larger observation angles. Consequently, if satellite-derived reflectances are going to be considered asabsolutevalues, a topographic-correction model is needed to correct for the effects of anisotropy.
Changes and surface features of the Larsen Ice Shelf, Antarctica, derived from Landsat and Kosmos mosaics
Authors: Skvarca Pedro
Published: February 2017

Two uncontrolled mosaics were assembled at an ~1 : 1000 000 scale, covering for the first time almost the entire Larsen Ice Shelf area, using Landsat TM images of 1986-89 and the Kosmos KATE-200 photographic products of late 1975. By comparing them, it was possible to estimate the change along the 600 km north-south ice-shelfs seaward margin where substantial calving has occurred in the recent past. In overall extent, the ice shelf has decreased by ~9300km2 since 1975. The interpretation of the available satellite data also allowed estimates of the ice-shelf’s inland boundary and detection of new surface ice features, especially in the very disturbed region south of Jason Peninsula, which may help to explain the significant calving events occurring in this region. Estimates of surface velocities were derived by photographically co-registering sequential imagery in the chaotic rifted-crevassed area east of Kenyon Peninsula, where conspicuous ice features could be clearly detected even after a 10.4 year time interval. The Landsat TM mosaic, though uncontrolled, provides a basic data set for future studies of global change of the major ice shelf on the Antarctic Peninsula which is probably very sensitive to any climatic trend.
Studies of the grounding-line location on Ice Streams D and E, Antarctica
Authors: Jacobel Robert W. , Robinson Audrey E. , Bindschadler Robert A.
Published: February 2017

Landsat Thematic Mapper (TM) images were used prior to the 1991–92 field season to infer the position of the grounding line at the mouths of Ice Streams D and E, West Antarctica. Our field plan for mass-balance studies was based on this determination, and thus the imagery played a central role in both the scientific and logistics planning. A radar profile along the flow direction was made across the inferred grounding line at one location, and ice-thickness measurements together with surface surveying enable us to compare the hydrostatic surface and the actual topography to determine the point at which the ice becomes grounded. The profile transits from floating to grounded ice at the same location as the grounding line inferred from the imagery. Changes in the radar-echo strength also occur at this location, giving further support to this interpretation. Tilt studies of the ice flexure caused by tidal variations at locations on either side of this grounding point give additional evidence that grounding is occurring close by. The combination of these three measurements therefore confirms the grounding-line location derived from the satellite imagery and supports the use of this technique to determine grounding.
An empirical relation between overburden pressure and firn density
Authors: Hitoshi Shoji Takao Kameda , Kawada Kunio , Watanabe Okitsugu , Clausen Henrik B.
Published: February 2017

Two empirical equations for firn densification have been obtained,considering firn porosity as a function of overburden pressure. In the first equation, thereduction ratio of porosity in firn is assumed to be proportional to the increasing ratioof overburden pressure and the mth power of the porosity. The porosity exponentmshould be close to -2, so as to have a best-fit with 14 depth-density profiles fromGreenland and Antarctica. In the second equation, the reduction ratio of porosity wasassumed to increase proportionally to the increment of overburden pressure and thenth power of the porosity. The most satisfactory values of the exponent range from -1 to 1. It has been suggested that firn density, determined primarily by overburdenpressure and firn temperature, contribute to a lesser degree.
Velocities of the Smith Glacier ice tongue and Dotson Ice Shelf, Walgreen Coast, Marie Byrd Land, West Antarctica
Authors: Lucchitta B.K. , Mullins K.F. , Smith C.E. , Ferrigno J.G.
Published: February 2017

Velocity measurements were made for two time intervals on the Smith Glacier ice tongue (1973–88 and 1988–90) and three on the Dotson Ice Shelf (1972–88, 1973–88 and 1988–90). The Smith Glacier ice tongue velocities for the two intervals are similar near the grounding line but show a progressive increase toward the terminus in the late 1980s. The Dotson Ice Shelf velocities remained virtually constant during all three time intervals. The increased velocities of the Smith Glacier ice tongue may be attributed to a general loss of densely packed icebergs that buttressed the terminus during the 1970s but drifted out to sea during the late 1980s. The Smith Glacier ice tongue receded as much as 10 km between 1973 and 1988, the Dotson lee Shelf 5–7 km in the same time period. Similar observations of drifting and ca1ving were made for the adjacent Thwaites Glacier ice tongue. The cause of the loss of ice in the region is unknown but it may have been a change in winds or a warming of the air or water during the late 1980s.
Electrical resistivity measurements on Ice Stream B, Antarctica
Authors: Shabtaie S. , Bentley C.R.
Published: February 2017

Electrical resistivity sounding using the four-electrode Schlumberger array was carried out at station UpB on Ice Stream B to an electrode spacing of 3 km. Measured apparent resistivities were compared with theoretical models based on known relations between resistivity, density and temperature. Densities were measured in a pit and two coreholes; temperatures were measured in the upper 200 m of the ice stream and have been calculated for greater depth from an ice-stream temperature model. The resistivity, after correction for density and temperature, increases with depth down to 650-700 m. Below that is a marked decrease over the next 100m or so that we correlate with the Holocene-Wisconsin transition zone. Still deeper there is an orders-of-magnitude increase to a value, in the basal ice, of 30 MΩ m or more. This extremely high resistivity is similar to that reported for temperate glaciers and deep in the Antarctic ice sheet elsewhere. We attribute it to the destruction, by extensive metamorphism, of impurity-conduction paths at two-grain boundaries
Ice thickness, bed topography and basal-reflection strengths from radar sounding, Upstream B, West Antarctica
Authors: Novick A.N. , Bentley C.R. , Lord N.
Published: February 2017

Radar profiling in the area around Upstream B camp (82.5° S, 128.7° W) was performed during the austral 1991−92 summer. About 150 km of lines covering 80 km2along The Ohio State University strain grid were profiled. The high density of data along the profiling lines allowed us to use median filtering schemes to remove or decrease interference from near-surface diffractors. Bed and surface elevations are poorly correlated. Hydraulic-head gradients suggest ponding of water in some places and drainage from others. Reflection amplitudes vary over a range of about 24 dB, partly because of scattering by crevasses, but partly also because of real differences in bed reflection.
Indication of a dilatant bed near Downstream B Camp, Ice Stream B, Antarctica
Authors: Atre S. R. , Bentley C. R.
Published: February 2017

Phases of seismic P-wave reflections from the bed of Ice Stream B ata site on its ice plain have been examined. The survey comprised a 36 km line at ashallow angle (18°) to ice movement and four 3.6 km cross lines. Reversed-phase and unreversed-phase reflections each characterize about half the bed. The corresponding zones can be correlated in stripes quasi-parallel to ice movement. We take this as support for a model previously developed that relates the zones to different types of subglacial sediment dragged along by the ice. There is also evidence for patches of pooled water.
A water-piracy hypothesis for the stagnation of Ice Stream C, Antarctica
Authors: Alley R. B. , Anandakrishnan S. , Bentley C. R. , Lord N.
Published: February 2017

Water piracy by Ice Stream B, West Antarctica, may have caused neighboring Ice Stream C to stop. The modern hydrologic potentials near the upstream end of the main trunk of Ice Stream C are directing water from the C catchment into Ice Stream B. Interruption of water supply from the catchment would have reduced water lubrication on bedrock regions projecting through lubricating basal till and stopped the ice stream in a few years or decades, short enough to appear almost instantaneous. This hypothesis explains several new data sets from Ice Stream C and makes predictions that might be testable.
Recent survey of brine infiltration in McMurdo Ice Shelf, Antarctica
Authors: Morse D. L. , Waddington E. D.
Published: February 2017

The brine infiltration zone of Mc Murdo Ice Shelf, Antarctica, has been extensively studied by previous authors. Brine percolates inland laterally from the ice front, opposite to the direction of ice-shelf motion. Inland propagation of brine pulses following ice-shelf break-outs appears to be the primary brine-infiltration mechanism. During the 1992-93 field season, we used radio-echo sounding to resurvey the inland limit of the brine-infiltration zone. The boundary had been similarly mapped in 1977. We observed that since the earlier survey, one 7km section his retreated seaward by approximately 800 m while another 5 km section is unchanged. These displacements are consistent with the earlier hypothesis that brine infiltration occurs by the influx of brine pulses
Detailed elevation map of Ice Stream C, Antarctica, using satellite imagery and airborne radar
Authors: Bidschadler R.A. , Vornberger P.L.
Published: February 2017

Surface elevations collected by airborne radar arc interpolated between flight lines using Landsat I'M image data to produce a detailed digital elevation model of a part of Ice Stream C, West Antarctica. The photodinometric method developed is general and allows the derivation of surface elevation along any line parallel to the solar illumination from a single known elevation on that line. Accuracies of the derived elevations are improved with additional elevation control that permits an empirical determination of parameters in the photoclinometric equation accounting for albedo and atmospheric scattering. Elevation errors increase approximately linearly with the integration distance. An explicit relationship is derived that shows the image brightness is insensitive to the cross-Sun component of small surface slopes typical of ice sheets. The greatest impediment to the accuracy of this technique is the radiance resolution of the sensor which limits the signal-to-noise ratio of the image data and can mask variations in albedo that are falsely converted to variations in surface slope.
A comparison of satellite-altimetry and ice-thickness measurements of the Ross Ice Shelf, Antarctica
Authors: Jonathan Bamber , Charles r Bentley
Published: February 2017

The launch of ERS-l provides coverage, by satellite altimetry, of a large part of the Ross Ice Shelf ineluding areas of input from Byrd Glacier and Ice Streams D and E. Five 35 d repeats of fast-delivery data, comprising approximately 100000 height estimates, have been used to produce a Digital Elevation Model (DEM) of the Ross Icc Shelf north of 81.5° S. Careful filtering of the altimeter data, which removed about 30% of the measurements, ensured that only valid values were used. The data were grldded to produce a DEM with a cell size of 10km. Repeatability of the data was assessed from an analysis of crossing points of ascending and descending tracks. The rms cross-over difference for the ice shelf was 0.94 m. Using the five repeat tracks gave a random error of 0.30 m for an averaged height measurement. Regionally correlated errors in the orbit and geoid add a systematic long wavelength bias of approximately 2m to the final elevation estimate. Two of the latest geoid models, OSU91-A and JGMI, were compared with the available in situ data and hYdrostatic models based on ice and water densities.

The altimetry was compared with ice-thickness data from Ross Icc Shelf Geophysical and Glaciological Survey (RIGGS) stations and Scott Polar Research Institute radio-echo-sounding surveys undertaken in the 1970s. Differences between the DEM and heights calculated from ice thicknesses and a standard density -depth equation lie, in general, within the combined measurement errors. There are, however, several areas where this is not the case. Prominent north-south stripes of different ice thickness shown on a RIGGS map apparently do not exist. Low elevations are associated with high-density ice draining from East Antarctic outlet glaciers. The grounding line of Icc Streams D and E and an ice plain behind it are clearly demarcated by the break in surface slope. Grounded ice north of Steershead is also observed
20th century behaviour of Drygalski Ice Tongue, Ross Sea, Antarctica
Authors: Frezzotti M. , Mabin M.C.G
Published: February 2017

Drygalski Ice Tongue is the floating seaward extension of David Glacier, a large outlet glacier draining from Talos and Circe Domes of the East Antarctic ice sheet. Several explorers mapped and described Drygalski Ice Tongue in the early years of the 20th century and, although this information does not allow detailed interpretation of ice-tongue behaviour, it is clear that from 1900–12 it was a significant feature extending 65—75 km from the coast. More detailed information has been compiled from aerial photographs and satellite images. In December 1956, the ice tongue was about 110 km long. By December 1957, a major calving event had occurred and the outer 40 km of the ice tongue had broken away. This is the only major 20th century calving event identified, and it may have occurred during a violent storm that affected the Ross Sea area in mid-June 1957. By 1960, further minor ice loss had occurred but, since that time, Drygalski Ice Tongue has maintained the same shape. In January 1993, the ice tongue was 95.8 km long and at its terminus was flowing at 880 900 m a-1. Drygalski Ice Tongue is an important regulator of the size of the Terra Nova Bay polynya. The average size of the Polynya has varied from nearly 2000 km2, in 1956, to 650 km2in 1957. This has a. significant impact on sea-ice production in the Ross Sea. In 1956, about 115 km3,) of sea ice would have been produced, sufficient to cover 30%of the Ross Sea area with a 1 m thickness of sea ice.
Flow regime of the Lambert Glacier-Amery Ice Shelf system, Antarctica: structural evidence from Landsat imagery
Authors: Hambrey Michael J. , Dowdeswell Julian A.
Published: February 2017

High-resolution visible and near-infrared satellite imagery provides a means of investigating the structural glaciology, and in turn the dynamics, of large ice masses. The Lambert Glacier-Amery Ice Shelf system is one of the largest ice drainage basins in Antarctica and has previously yielded conflicting evidence concerning its dynamic behaviour: either that the system has a propensity for surging or that it has a constant flow regime. Digital manipulation of Landsat imagery allows analysis of the structure of the glacier system, showing longitudinal foliation, medial moraines and crevasse patterns that provide no evidence of surging behavior during the residence time of ice in the glacier system.
Melting and freezing beneath ice shelves: implications from a three-dimensional ocean-circulation model
Authors: Determann J. , Gerdes R.
Published: February 2017

The thermohaline circulation below a regularly shaped ice shelf comparable in dimensions to the Ronne Ice Shelf, Antarctica, is investigated by means of a three-dimensional primitive-equation model. To handle the strongly inclined ocean surface as well as the bottom topography, we use scaled coordinates in the vertical direction. Preliminary results of two model versions containing open and closed open-ocean boundaries, respectively, prove the internal circulation is almost independent of the hydrography outside the ice shelf.

Merely driven by the pressure-dependence of the sea-water freezing point (-0.000753 K dbar-1), a stationary circulation pattern evolves, forced by the buoyancy flux due to melting and freezing. The redistribution of ice from deep to shallow ice-shelf drafts resembles the ice-pump mechanism. Its strength depends on the ice-thickness gradients, namely, the differences of the freezing point at the ice-shelf base. As simulated melting and accumulation rates convincingly reproduce the interactions observed at the base of the Ronne Ice Shelf, the present model can act as a tool coupling ice-ocean dynamics.