The transition from a dry, sub-freezing snow cover to isothermal snow freely conducting water was observed near timberline in the Sierra Nevada, California, U.S.A. Although there were few major stratigraphic boundaries in the snowpacks observed, minor textural discontinuities were sufficient to both route water laterally downslope and later create ice layers at these strata. During the first few days of snowmelt, downslope movement of water parallel to the strata allowed water to bypass lower layers of the snow cover on slopes and converge in microtopographic depressions on level terrain, creating isolated drains through the snowpack. In addition to these drains, flow fingers associated with holes in the ice sheets delivered water to the base of the snow several days prior to the thorough wetting of the snowpack. Formation of a series of ice lenses just below the snow surface every night released much more latent heat than did ice-layer development within the bulk of the snowpack or at its base.

Extensive damage to ice occurs during ice structure interaction by microcracking, recrystallization and melting. The objective of this work was to investigate this damage process under confined-stress conditions believed to be associated with impact zones that occur during ice–structure interaction. “Damage” refers to microstructural modification that causes deterioration of the mechanical properties. Prior experimental work has shown that a small amount of deformation causes permanent damage in ice, leading to enhanced creep rates during subsequent loading. To investigate this softening, freshwater granular ice was deformed under moderate confinement (20 MPa) at –10°C, at two rates which bracket ductile and brittle behavior (10−2 s−1 and 10−4 s−1). Samples were deformed to different levels of axial strain up to 28.8%. Thin sections were examined to assess the progressive changes in microstructure.

Both grain-boundary and intra-granular cracking began at strains corresponding to the peak stress (1–2%) for tests at both strain rates. The peak stresses were 23.4 MPa for the tests at 10−2 s−1 and 9.8 MPa for the tests at 10−4 s−1. At strains of > 1–2%, dense clusters of intra-granular cracks began to develop in the samples tested at the higher rate. At the lower rate, dynamic recrystallization was apparently the dominant deformation mechanism beyond the peak stress. The average grain-size decreased strongly during the first few per cent strain and then maintained a relatively stable value.

This work investigates the deformation of ice under deviatoric stresses and confining pressures expected during ice–structure interaction. Granular ice was tested under a range of confining pressures (5–60 MPa) and deviatoric stresses (up to 25 MPa), with sample temperatures between –8° and –10°C. Samples were deformed to increasing end-levels of axial strain, and were thin-sectioned and photographed immediately following testing.

At all confinement levels, the original texture of the sample is completely transformed within the first 10–15% strain, to a fine-grained matrix with a few larger, isolated grains. At low confinements, grain-size reduction is associated with extensive microcracking. At high confinements, few cracks are observed. Observations suggest that microcracking, melting and recrystallization are active at all levels of confinement, though the relative importance of each depends on the confinement, stress and accumulated strain.

Deviatoric stress is a strong factor in controlling the creep, reflected in both the time required for the sample to reach accelerated creep and the tertiary creep rate itself. Two exceptions to this pattern were noted. First, some samples experienced strain localization and eventual rupture. These specimens tended to have higher creep rates even in the initial stages of strain. Second, prior damage resulted in rapid softening compared with the behavior of undamaged specimens. However, when strain rates are compared among all samples at a given level of cumulative axial strain, the creep behavior obeys a power law over the whole range of strain levels tested. Effective viscosity decreased from 107.8 to l06.4 MPa−n s within the first 10% strain, during which the most substantial microstructural changes occurred, and then stayed relatively stable. The stress exponent, n, remained within the range 4.0–4.6.

The dominant deformation mechanism appears to depend strongly on confining pressure (cracking at low pressure and dynamic recrystallization at high pressure). Creep rates at high confinement appear to increase relative to those at intermediate confinement, but the influence of temperature must be addressed further.

During a full-scale iceberg-impact study conducted in July 1995 on the Labrador coast, Canada, a sample of ice was retrieved from the impacted surface of an iceberg. The sample was thin-sectioned and the observations of the contact-zone microstructure are presented in this paper.

Thin sections were prepared from two slabs cut parallel to the impacted surface. In each of the thin sections taken from the impacted-surface slab, fine-grained material was found to surround parent-size grains (as observed in the second slab). A boundary between the parent grains and the grains of modified microstructure was found running approximately parallel to the impacted surface in each of the thin sections taken from the impacted-surface slab. This boundary was pronounced towards the edges of the contact zone. Lateral movement of grains outward along this boundary was observed in thin sections near the edges but not near the centre of the contact zone.

The thin sections were compared to the results of medium-scale indentation tests in 1989 and 1990 from the Arctic Ocean. The same type of fine-grained material and layer formation of modified microstructure was found in the contact zones.

The interannual variability of glacier mass balance is expressed by the standard deviation of net balance, which varies from about ±0.1 to ±1.4 m a−1 for a sample of 115 glaciers with at least 5 years of record. The standard deviation of net balance is strongly correlated with the mass-balance amplitude (half the difference between winter and summer balances) for 60 glaciers, so the amplitude can be estimated from net balance standard deviation for the other 55 glaciers where winter and summer balances are unavailable. The observed and calculated mass-balance amplitudes for the 115 glaciers show contrasts between the Arctic and lower latitudes, and between maritime and continental regions. The interannual variability of mass balance means that balances must be measured for at least a few years to determine a statistically reliable mean balance for any glacier. The net balance of the Greenland ice sheet is still not accurately known, but its standard deviation is here estimated to be about ±0.24 m a−1, in agreement with other Arctic glaciers. Mass-balance variability of this magnitude implies that the ice sheet can thicken or thin by several metres over 20–30 years without giving statistically significant evidence of non-zero balance under present climate.

The annual mean oxygen-isotope content of Antarctic snow is strongly influenced by the seasonal variability of accumulation. Since the annual mean δ18O value is frequently used to derive mean annual temperatures from ice cores, changes in atmospheric circulation pattern can lead to large errors in the deduced temperature record. At the German Antarctic wintering base, Neumayer, accumulation measurements have been carried out continuously over the last 16 years. Weekly readings of accumulation stakes combined with snow pits and shallow firn cores are used to investigate the influence of the seasonal variability of accumulation on the annual mean δ18O values and to estimate the possible error in the determination of annual mean temperatures from ice cores by using the oxygen-isotope record.

This paper describes the current state of a complete automatic system of three numerical models that simulate snow-cover stratigraphy and avalauche risks for operational avalanche forecasting. The first model, SAFRAN, estimates relevant meteorological parameters affecting snowpack evolution. The second Crocus, is a snow numerical model which simulates the physical processes inside the snowpack and its stratigraphy. The last model, MÉPRA, is an expert system; based on an assessment of snowpack stability, it deduces natural and accidental avalanche risks. To describe the great Variability of the snowpack and the associated avalanche risks, this automatic system simulates snow-cover evolution and its stability for many typical slopes, elevations and aspects representative of the different French massifs. To achieve this result, different kinds of validations have been carried out since winter 1981; they are mainly based on comparisons with different sets of measurements and on the opinion of users.

Although the routinely obtained results do not yet take into account all small-scale effects such as wind transport, they have been considered as valuable information by avalanche forecasters since 1992 93 and used operationally since then.

As polar ice descends from the free surface to depth in a large ice sheet, it undergoes deformations which give rise to the formation and subsequent evolution of a fabric and associated anisotropy. In this paper two orthotropic models of such strain-induced anisotropy are considered. Model A is based on analysis of the microscopic behaviour of an individual ice crystal with transversely isotropic response and assumed uniform stress in a polycrystal. The macroscopic response of the ice aggregate is then derived by applying the concept of an orientation distribution function, and the resulting viscous law relates the strain rate to the stress and three structure tensors. In model B it is assumed that the macroscopic response of ice is determined by the fabric induced entirely by macroscopic deformations, and all microprocesses taking place at the grain level are ignored. A constitutive relation is derived from a general frame-indifferent law for orthotropic materials, and expresses the stress in terms of the strain rate, strain and throe structure tensors. The two models are applied to determine the viscous response of ice to continued uniaxial compression and simple shearing in order to compare the predictions of both theories.

We employed a commercial wireline drill rig to investigate the subglacial conditions of Black Rapids Glacier, a well-studied surge-type glacier in the central Alaska Range. The four main goals were: to assess the capabilities of the commercial drilling industry for sampling subglacial material, to investigate the basal morphology, to determine the subglacial geology and to emplace borehole instruments. The drilling was done in an area where seasonal and secular variations in speed are large, and where seismic studies suggested the presence of a till layer. Four holes were drilled at three locations to a maximum depth of 620 m. Three holes yielded samples of basal ice and till, although recovery of the latter was generally poor. Bedrock was sampled in one or possibly two of the holes. In the area sampled, t he glacier is underlain by a till layer some 4–7 m thick, confirming the seismic interpretation. It consists of a sandy matrix at least 20–30% of which comprises larger clasts. Limited samples of the matrix indicate that near the top of the till the porosity is 40%, and t hat some of the pore water is frozen. Geologic studies suggest that the drilling area lies to the north of the Denali Fault, a major tectonic boundary followed by the glacier, and that most of the till is locally derived with transport distances of <2 km.

Of several problems associated with theoretical explanations of the jökulhlaups which emerge from the outlet glacier Skeiðarárjökull of the ice cap Vatnajökull in southeast Iceland, the mechanism of flood initiation is one that has hitherto defied explanation. We provide such an explanation based on a careful analysis of the classical Nye-Röthlisberger model; near the subglacial lake Grímsvötn, the hydraulic potential gradient is towards the lake, and there is therefore a catchment boundary under the ice, whose location depends on the subglacial meltwater drainage characteristics. As the conditions for a flood approach, we show that the water divide migrates towards the lake, while at the same time the lake pressure increases. When the hydraulic potential gradient towards the lake is low and the refilling rate is slow, the seal will "break" when the catchment boundary reaches the lake, while the lake level is still below flotation pressure, whereas if refilling is rapid, flotation can be achieved before a flood is initiated. This theory can thus explain why the seal is normally broken when the lake level at Grímsvötn is still some 60 m below flotation level. In addition, we are able to explain why the jökulhlaup following the 1996 eruption did not occur until flotation level was achieved, and we show how the cyclicity and magnitude of jökulhlaups can be explained within this theory.

Stable-oxygen-isotope ratios (δ18O) of precipitation and ice-core samples collected from the headwaters of the Urümqi river, Tien Shan, China, were used to test the relationship between δ18O and contemporaneous surface air temperature (Ta). A strong temporal relationship is found between δ18O in precipitation and Ta, particularly for the monthly averages which remove synoptic-scale influences such as changes in condensation level, condensation temperature and moisture sources ( Yao and others, 1996). Linear fits as high as 0.95‰° C-1 for precipitation events and 1.23‰° C-1 for monthly averages are found. Although the δ18O amplitude in ice cores drilled at the nearby Ürümqi glacier No.1(~2 km from the precipitation sampling site) decreased dramatically compared to the precipitation samples, the ice-core records of annually averaged δ18 are still positively correlated with contemporaneous air temperature, especially summer air temperature, at the nearby Daxigou meteorological station. Nevertheless, the relationship between the ice-core δ18O records and contemporaneous air temperature is less significant than that for the precipitation samples due to depositional and post-depositional modification processes, which are highlighted by the successive snow-pit δ18O profiles from the Ürümqi glacier No.1. Our results might extend the application of high-altitude and subtropical ice-core δ18O —Ta records for paleoclimate reconstruction.

Detailed ground-penetrating radar (GPR) surveys at 50 and 200 MHz on Hansbreen, a polythermal glacier in southern Svalbard, are presented and interpreted. Comparison of the variations in character of the radar reflections with borehole thermometry and water levels in moulins suggests that GPR can be used to study the hydrothermal properties of the glacier. The high resolution of the GPR data shows that the hydrothermal structure of the glacier is highly variable both along the centre line and on transverse profiles. Water contents for many places and depths within the glacier were calculated by estimating radar-wave velocities to point reflectors. We find typical water contents of 1-2% for the temperate ice, but wetter ice associated with surface crevassing and moulins (typically 4% water content). There is evidence that wet ice sometimes overlays drier ice. The hydrothermal structure is thus shown to be very complex. Temperature gradients in the cold ice indicate freezing rates of temperate ice below cold ice of 0.1-0.5 ma-1, while isolated point reflectors within the cold ice indicate large water-filled bodies that are probably related to the regular drainage structure of the glacier.

Until now, an assumption of surface-parallel glacier flow has been used to express the vertical velocity component in terms of the horizontal velocity vector, permitting all three velocity components to be determined from synthetic aperture radar interferometry. We discuss this assumption, which neglects the influence of the local mass balance and a possible contribution to the vertical velocity arising if the glacier is not in steady state. We find that the mass-balance contribution to the vertical surface velocity is not always negligible as compared to the surface-slope contribution. Moreover, the vertical velocity contribution arising if the ice sheet is not in steady state can be significant. We apply the principle of mass conservation to derive an equation relating the vertical surface velocity to the horizontal velocity vector. This equation, valid for both steady-state and non-steady-state conditions, depends on the ice-thickness distribution. Replacing the surface-parallel-flow assumption with a correct relationship between the surface velocity components requires knowledge of additional quantities such as surface mass balance or ice thickness.

Snow glide is the translational slip of the entire snowpack over a sloping ground surface, and it is thought that rapid rates of snow glide precede full-depth avalanches. The nature of avalanches that release at the ground makes them difficult to predict and difficult to control using explosives.

On-slope instrumentation comprised of stainless-steel "glide shoes" was used to measure rates of snow glide for two winters on a bedrock slope adjacent to the Coquihalla Highway, Cascade Mountains, British Columbia, Canada. Climate data and avalanche occurrences were recorded by the British Columbia Ministry of Transportation and Highways.

Our results show that the supply of free water to the snow/ground interface by rain or snowmelt is the most important influence on full-depth avalanche release. Full-depth avalanche release responds to rainfall and snowmelt events within 12-24 hours. Occasionally, full-depth avalanches occur unexpectedly during clear, cold periods. Snowmelt by radiation is thought to contribute enough meltwater during these cold periods to induce higher rates of snow glide and full-depth avalanche release. The results also indicate that snow glide alone is not a reliable indicator for full-depth avalanche release.

A digital image-processing approach is proposed which allows the extraction of two-dimensional polycrystalline ice microstructure (grain boundaries) from thin sections observed between cross-polarisers. It is based on image segmentation of colour images. The method is applied to the preliminary analysis of the shallow ( Holocene) ice of the European Project for Ice Coring in Antarctica (EPICA) ice core at Dome Concordia. Structural parameters, such as the mean cross-sectional area, shape anisotropy and grain morphology, are obtained. The interest and limitations of this automatic procedure are discussed.

Slushflows were first recorded in the Iberian Peninsula on 18 December 1997. Three slushflows were released at the ski resort of El Port del Comte, in the Catalan Pyrenees, northeast Spain, during intense rainfall. Two of the slushflows originated on the pistes, and the third affected another piste. Three ski lifts were damaged. This paper analyzes the hydrogeological characteristics of the massif, the geomorphic features of the terrain and the meteorological and snowpack conditions that caused the release of the slushflows. Man's role in triggering the slushflows by compacting snow on the pistes is also considered. Drainage control for reducing the hazard is outlined, taking into consideration the low frequency of the phenomenon.

A mass-balance model based on the energy balance at the snow or ice surface is formulated, with particular attention paid to processes affecting absorption of radiation. The model is applied to a small glacier, Glacier AX010 in the Nepalese Himalaya, and tests of its mass-balance sensitivity to input and climatic parameters are carried out. Calculated and observed area-averaged mass balances of the glacier during summer 1978 (June-September) show good agreement, namely -0.44 and -0.46 m w.e., respectively.

Results show the mass balance is strongly sensitive to snow or ice albedo, to the effects of screening by surrounding mountain walls, to areal variations in multiple reflection between clouds and the glacier surface, and to thin snow covers which alter the surface albedo. In tests of the sensitivity of the mass balance to seasonal values of climatic parameters, the mass balance is found to be strongly sensitive to summer air temperature and precipitation but only weakly sensitive to relative humidity.

Quadrature solutions for temperature fields in the central regions of ice sheets where flow is by internal deformation are presented and compared with numerical computations and the European Ice Sheet Modelling Initiative (EISMINT) Benchmark standards. The solution appears to be more accurate than the EISMINT Benchmark standards for thermomechanically uncoupled temperature distributions. The ability of finite-difference and pseudo-spectral methods to reproduce this solution is considered.

Errors in the EISMINT Benchmark are larger than expected. The possibility that they could arise from inaccurate evaluation of the vertical velocity is considered, and found to be unlikely. Formulae for computing the vertical velocity in Jenssen's σ coordinate are complicated and may lead to programming errors. A simple form of the heat-transport equation in the σ coordinate for flow by sliding and internal deformation is derived arid presented, and it is shown how this form is particularly suitable for finite-element computations. It is also shown that only two quadratures are necessary to compute the vertical and horizontal heat transport and the ice flux.

The kinematic boundary condition al the glacier surface can be used to give glacier mass balance at a point as a function of changes in the surface elevation, and of the horizontal and vertical velocities. Vertical velocity can in turn be estimated from basal slope, basal ice velocity and surface strain. In a pilot study on the tongue of Griesgletscher, Swiss Alps, the applicability of the relation for modelling area-wide ice flow and mass-balance distribution is tested. The key input of the calculations, i.e. the area-wide surface velocity field, is obtained using a newly developed photogrammetric technique. Ice thickness is derived from radar-echo soundings. Error estimates and comparisons with stake measurements show an average accuracy of approximately ±0.3 ma-1 for the calculated vertical ice velocity at the surface and ±0.7 ma-1 for the calculated mass balance. Due to photogrammetric restrictions and model-inherent sensitivities the applied model appeared to be most suitable for determining area-wide mass balance and ice flow on flat-lying ablation areas, but is so far not very well suited for steep ablation areas and most parts of accumulation areas. Nevertheless, the study on Griesgletscher opens a new and promising perspective for the monitoring of spatial and temporal glacier mass-balance variations.