Previous studies of alpine glaciers have demonstrated that as water discharge increases through the summer, the predominant mode of subglacial drainage shifts from a distributed system to a more efficient conduit drainage system. We observed an early-melt-season speed-up and flood event lasting roughly 2 days in a small, uncomplicated Alaskan glacier that appears to have resulted from a sudden shift of the subglacial system in response to a significant accumulation of meltwater within the glacier. Calculated melt-water inputs exceeded discharge before the event; the implied change in storage over this 10 day period was equivalent to roughly 0.13 m averaged over the entire glacier bed. The pattern of discharge and suspended-sediment variations and the appearance of large ice chunks in the stream suggest that the speed-up occurred during a period of establishment of new subglacial conduits. A culminating flood and associated suspended-sediment pulse appear to have marked the final establishment of the new section of subglacial conduit. The flood ended the episode of high sliding velocity, but released water with high solute concentrations that reflect relatively long contact time with sediments. Discharge of stored water, inferred from high solute concentrations and lack of diurnal variation in discharge, continued for at least 3 days. While events such as this must recur through the melt season as the conduit system extends up-glacier and the locus of meltwater inputs shifts, their manifestations in the outlet stream will likely be more subdued later in the season.

For more than 30 years the quantitative method of evaluating stability (e.g. Roch, 1966; Föhn, 1987; Jamieson, 1995; Jamieson and Johnston, 1998a) has been focused on calculation of a strength-to-load ratio (or stability index): when the shear stress applied to the weak layer reaches the shear strength, failure is imminent. However, field observations combined with experience and measurements indicate that snow-slab temperatures and slab hardness can have a strong influence on dry-snow slab stability. In this paper, we present a simple static analysis of the stability index, and discuss the importance of slab temperatures and hardness and macroscopic size effects (factors not contained in the stability index) on snow-slab stability. Our conclusion is that the traditional method lacks some elements which are very important in snow-slab stability, particularly when skier triggering is involved.

The effect of subglacial lakes upon ice-sheet topography and the velocity patterns of subglacial water-sheet floods is investigated. A subglacial lake in the combined Michigan–Green Bay basin, Great Lakes, North America, leads to: (1) an ice-sheet lobe in the lee of Lake Michigan; (2) a change in orientations of flood velocities across the site of a supraglacial trough aligned closely with Green Bay, in agreement with drumlin orientations; (3) low water velocities in the lee of Lake Michigan where drumlins are absent; and (4) drumlinization occurring in regions of predicted high water velocities. The extraordinary divergence of drumlin orientations near Lake Ontario is explained by the presence of subglacial lakes in the Ontario and Erie basins, along with ice-sheet displacements of up to 30 km in eastern Lake Ontario. The megagrooves on the islands in western Lake Erie are likely to be the product of the late stage of a water-sheet flood when outflow from eastern Lake Ontario was dammed by displaced ice and instead flowed westward along the Erie basin. The Finger Lakes of northern New York state, northeastern U.S.A., occur in a region of likely ice-sheet grounding where water sheets became channelized. Green Bay and Grand Traverse Bay are probably the products of erosion along paths of strongly convergent water-sheet flow.

Two 10 cm lengths of firn core from the Dyer Plateau (70°31 ′S, 65°01 ′W), Antarctic Peninsula, were used to carry out a laboratory experiment to investigate the migration of methane sulphonate, the anion of methane sulphonic acid (MSA), in natural firn. Each length was cut vertically into four pieces, and a dopant solution containing Cl−, NO3−, SO42−, F− and MSA− pipetted onto the top of three sections, the fourth being kept as a blank. The doped sections were stored vertically, with the doped end uppermost, for 8 months at a range of temperatures (nominally –5°, –10° and –22°C) before subsampling at 1 cm resolution and analysis by ion chromatography. The two firn lengths were treated identically and the results were consistent. Profiles of the doped firn sections showed that C1−, NO3 and SO42− remained in the uppermost subsample, although the NO3− concentrations were variable compared to the blank. The F− profile shows slightly elevated concentrations in the second sample down compared to the blank, at temperatures of -10°C and above. The MSA− showed higher concentrations in the second, third and fourth samples down at -10°C and above, which indicates that some percolated downwards from its original position at the top of the core. This experiment shows that MSA is mobile in warm firn even over a short period of time. We propose that the mechanism for the mobility of MSA− in natural firn is via liquid MSA drainage, though we cannot yet discount vapour phase transport.

With the use of a numerical three-dimensional (3-D) model the flow dynamics of the confluence area of Unteraargletscher, Bernese Alps, Switzerland, are studied. Previous predictions, based on conceptual two-dimensional models, about flow characteristics at confluence areas are tested against results from the fully 3-D model. Measured winter velocities are used for model verification. Despite some consistent systematic differences, good overall agreement between measured and calculated surface velocities is obtained. The calculated vertical strain-rate variation with depth is in good agreement with available measurements from boreholes. The ice is found to be almost three times stiffer than standard estimates of rheological parameters for glacier ice would predict. The model predicts a complicated yet realistic pattern of vertical velocity variation along the surface. The most noticeable features of the vertical velocity distribution across the surface are listed, and their relation to topographic surface undulations and the overall dynamics of the confluence discussed. In accordance with previous results from analytical models, a strongly localized surface trough and a concomitant negative (downward orientation) vertical velocity anomaly develop at the junction point. Although depth-integrated strain rates are positive (extension), the basal layer is compressed vertically. The ice-cored medial moraine is formed by differential ablation. The flow mechanics of the confluence area play only an indirect role, by enabling transfer of debris-covered marginal ice towards the confluence center. In the absence of differential ablation, an elongated surface depression would be formed in the down-glacier direction from the junction point instead of an elevated ice-cored medial moraine.

Radar interferometry, ice-penetrating radar profiles and an elevation model are used to determine the velocity fields, rates of ice discharge, approximate states of balance and catchment area for three large outlet glaciers in northeast Greenland. Discharge through flux gates is calculated for Humboldt and Petermann Gletscher, which are found to be in balance (at the level that the accumulation is known). A large difference between the measured and estimated fluxes for Ryder Gletscher may be a reflection of unsteady flow behavior for this glacier. The patterns of ice flow for the three glaciers considered are each unique, showing that the nature of ice discharge varies substantially from basin to basin, controlled by bed conditions and the presence of subglacial troughs and obstructions.

A quasi-one-dimensional dense-snow avalanche model has been developed to predict avalanche runout and flow velocity in a general two-dimensional terrain. The model contains three different dense-snow-avalanche flow laws. These are: (1) a Voellmy-fluid flow law with longitudinal active/passive straining, (2) a Voellmy-fluid flow-law advanced by Russian researchers in which the Coulomb-like dry friction is limited by a yield stress, and (3) a modified Criminale—Ericksen—Filby fluid model proposed by Norwegian researchers. The application of the Voellmy-fluid law with active/passive straining to solve practical avalanche-dynamics problems is evaluated by applying the model to simulate laboratory experiments and field case-studies. The model is additionally evaluated by comparing simulation results using the Russian and Norwegian models. In a final analysis the influence of the initial conditions on avalanche runout is investigated. We conclude that the model resolves many of the shortcomings of the Voellmy–Salm model, which is traditionally used in Switzerland to predict avalanche runout. Furthermore, since the model contains the three well-calibrated parameters of the Swiss Guidelines on avalanche calculation it can be readily applied in practice. We discuss why we believe the Russian and Norwegian models are not ready for practical application. Finally, we show that many problems remain, such as the specification of the initial release conditions. We conclude that numerical models require a more detailed description of initial fracture conditions.

The total air content (V) of ice has been measured along the Dome Summit South (DSS) core from Law Dome, East Antarctica. The main features of this record are the very well-preserved sub-annual fluctuations of V (down to at least 900 m depth) and the significant increase of the V values during the last deglaciation. The sub-annual variations reflect changes in close-off porosity, and we interpret the V seasonal peaks as tracers of depth-hoar layers. For the longer time-scale, the large V fluctuations observed are interpreted in terms of elevation and/or close-off porosity changes under different assumptions. We analyze the possible influence of a different global pressure field and/or a change in seasonal temperature and precipitation cycles during the last glacial period. Our estimates of surface elevation changes derived from the V data are then compared with independent reconstructions of past elevations.

Observations on the spatial distribution and on the strain dependence of the crack density are given for cracks formed during compressive, unidirectional, constant-strain-rate deformation of columnar-grain ice. Specimens, in the grain-size range of about 2 9 mm, were strained at the nominal rates of 10−3, 10−4 and 10−5 s−1 at 10°C. The axis of hexagonal crystallographic symmetry of each specimen grain tended to be in the plane perpendicular to the long direction of the grains and to have a random orientation in that plane. For stress applied perpendicular to the long direction of the grains, the deformation was practically two-dimensional. It was found that the log-normal distribution function provided a good approximation to the strain dependence of the crack density. Statistical characteristics of the distribution had a maximum in the same range of strain rate as found for the strength of columnar-grain ice. Analysis of the spatial distribution of the cracks indicated some deviation from randomness for specimens of grain-size less than 5 mm and total strain less than 50 × 10−4. The observations provide further evidence that crack formation under the experimental conditions is a random process.

In the Jutulgryta area of Dronning Maud Land, Antarctica, subsurface melting of the ice sheet has been observed. The melting takes place during the summer months in blue-ice areas under conditions of below-freezing air and surface temperatures. Adjacent snow-covered regions, having the same meteorological and climatic conditions, experience little or no subsurface melting. To help explain and understand the observed melt-rate differences in the blue-ice and snow-covered areas, a physically based numerical model of the coupled atmosphere, radiation, snow and blue-ice system has been developed. The model comprises a heat-transfer equation which includes a spectrally dependent solar-radiation source term. The penetration of radiation into the snow and blue ice depends on the solar-radiation spectrum, the surface albedo and the snow and blue-ice grain-sizes and densities. In addition, the model uses a complete surface energy balance to define the surface boundary conditions. It is run over the full annual cycle, simulating temperature profiles and melting and freezing quantities throughout the summer and winter seasons. The model is driven and validated using field observations collected during the Norwegian Antarctic Research Expedition (NARE) 1996–97. The simulations suggest that the observed differences between subsurface snow and blue-ice melting can be explained largely by radiative and heat-transfer interactions resulting from differences in albedo, grain-size and density between the two mediums.

The hydraulic properties of the firn on Storglaciären, Sweden, were investigated in firn cores by water-table measurements and pumping tests. The mean density of the firn was 800 850 kg m3, giving an effective porosity of 0.073. The lower part of the firn layer was saturated with water, producing a maximum saturated layer of 5 m in late July. Hydraulic conductivity of the firn aquifer was determined from pumping tests to be 4.9 × 105 m s1. Percolation velocity, calculated from the time lag of maximal water input at the glacier surface and the water-level peaks, was 0.25 m h1. Percolation velocity increased over the ablation season, indicating a widening of the percolation pathways. A decrease in percolation velocity with percolation depth was found, reflecting decreasing permeability. The firn–water table responded to water input at the glacier surface with a delay of about 3 days. No diurnal variations were found in an area which was not influenced by fast drainage, indicating a diffusion of diurnal variations in meltwater production. One borehole intersected a water-filled cavity. Water level in this cavity showed diurnal variations, which probably were caused by diurnally produced meltwater waves moving fast through englacial conduits.

Temperature, density and accumulation data were obtained from shallow firn cores, drilled during an overland traverse through a previously unknown part of Dronning Maud Land, East Antarctica. The traverse area is characterised by high mountains that obstruct the ice flow, resulting in a sudden transition from the polar plateau to the coastal region. The spatial variations of potential temperature, near-surface firn density and accumulation suggest that katabatic winds are active in this region. Proxy wind data derived from firn-density profiles confirm that annual mean wind speed is strongly related to the magnitude of the surface slope. The high elevation of the ice sheet south of the mountains makes for a dry, cold climate, in which mass loss owing to sublimation is small and erosion of snow by the wind has a potentially large impact on the surface mass balance. A simple katabatic-wind model is used to explain the variations of accumulation along the traverse line in terms of divergence/convergence of the local transport of drifting snow. The resulting wind- and snowdrift patterns are closely connected to the topography of the ice sheet: ridges are especially sensitive to erosion, while ice streams and other depressions act as collectors of drifting snow.

In this paper we present a simulation approach to mapping avalanche risk with application to settlements in Iceland. Two simulation models are developed to calculate the probability of avalanches travelling a certain distance, and of the flow being a specific width. These two simulation models, in combination with knowledge of the average frequency of avalanche occurrence, the variability in avalanche direction and the degree of loss caused by an avalanche, permit risk values to be determined for the areas of concern.

The conductive heat flux through the snow cover (Fa) is used as a proxy to examine the hypothesis that there is a significant heat flow from the Alaskan North Slope to the atmosphere because of the large number of lakes in the region. Fa is estimated from measurements of snow depth, temperature and density on tundra, grounded ice and floating ice in mid-April 1997 at six lakes near Barrow, northwestern Alaska. The mean Fa values from tundra, grounded ice and floating ice are 1.5, 5.4 and 18.6 W m2, respectively. A numerical model of the coupled snow/ice/water/soil system is used to simulate Fa and there is good agreement between the simulated and measured fluxes. The flux from the tundra is low because the soils have a relatively low thermal conductivity and the active layer cools significantly after freezing completely the previous autumn. The flux from the floating ice is high because the ice has a relatively high thermal conductivity, and a body of relatively warm water remains below the growing ice at the end of winter. The flux from the grounded ice is intermediate between that from the tundra and that from the floating ice, and depends on the timing of the contact between the growing ice and the lake sediments, and whether or not those sediments freeze completely. Using the estimated Fa values combined with the areal fractions of tundra, grounded ice and floating ice derived from synthetic aperture radar images, area-weighted Fa values are calculated for six areas. Fa values for the ice vary between 9.8 and 13.8 W m−2, and those from the ice plus tundra vary between 3.9 and 5.3 W m−2. The Fa values are similar to those observed in the sea-ice-covered regions of the south and north polar oceans in winter. The North Slope of Alaska may thus make a significant contribution to the regional energy budget in winter.

Repeat-pass L-band interferometric synthetic aperture radar (InSAR) data for part of Hielo Patagónico Sur, Chile, were collected by the space-shuttle-based Spaceborne Imaging Radar C (SIR-C) over a 4 day span in October 1994. Three co-registered complex SAR images are used to generate phase-coherence maps, a digital elevation model (DEM) and an ice-velocity map. The phase-coherence maps indicate low coherence in the 5–15 km approaching the termini due to large displacements, ice deformation and melting. However, the coherence is high over nearly all of the remaining imaged icefield. Ice-velocity precision is greater than 2 cm d−1, while the DEM is good to about 25 m. A flow divide between two of the glaciers is mapped by locating a narrow band of near-zero ice velocity. Horizontal ice-surface velocity profiles calculated along flowlines show there is a high degree of spatial variability reaching a peak value of 5.5 m d−1 located 3.5 km from the terminus of Glaciar Europa. Longitudinal strain rates along the center lines calculated from these velocities at the locations of the initiation of crevassing are used to compute the tensile strength of ice (169–224 kPa).

Ice at depth in ice sheets can be softer in bed-parallel shear than Glen’s flow law predicts. For example, at Dye 3, Greenland, enhancement factors of 3 4 are needed in order to explain the rate of borehole tilting Previous authors have identified crystal fabric as the dominant contributor, but the role of impurities and crystal size is still incompletely resolved. Here we use two formulations of anisotropic flow laws for ice (Azuma’s and Sachs’ models) to account for the effects of anisotropy, and show that the measured anisotropy of the ice at Dye 3 cannot explain all the detailed variations in the measured strain rates, the jump in enhancement across the Holocene–Wisconsin boundary is larger than expected from the measured fabrics alone. Dust and soluble-ion concentration divided by crystal size correlates well with the residual enhancement, indicating that most of the “excess deformation” may be due to impurities or crystal size. While the major features of the deformation at Dye 3 are explained by anisotropy and temperature, results also suggest that further research into the role of impurities and crystal size is warranted.

Thermal conductivity of snow has been investigated experimentally using the thermal-probe method, which is a transient method of measurement. The measurements have been made over a wide range of snow density (for fresh and dense snow), for varying temperatures and for different conditions of water content, snow-grain type, etc., both in the field and in the laboratory. The results are presented along with detailed sample descriptions. Thermal conductivity of snow increases with density and water content. It also increases with temperature, and the effect is more pronounced for temperatures between –15° and 0°C.

During the 1992 summer field season we installed arrays of “plough-meters” and water-pressure transducers beneath Trapridge Glacier. Yukon Territory, Canada, to study hydromechanical coupling at the ice–bed interface. Diurnal signals recorded with two of these ploughmeters appear to correlate with fluctuations in sub-glacial water pressure. These diurnal variations can be explained by changes in basal resistance to sliding as mechanical conditions at the bed vary temporally in response to changes in the subglacial hydrological system. We propose that a lubricating water film, associated with high water pressures, promotes glacier sliding, whereas low pressures cause increased basal drag resulting in “sticky” areas. Using a theoretical model, we analyze the sliding motion of glacier ice over a flat surface having variable basal drag and show that a consistent explanation can be developed. Results from our model calculations provide strong support for the existence of time-varying sticky spots which are associated with fluctuations in subglacial water pressure.

The rate of margin migration of an ice stream can be determined using repeat measurements of the surface velocity profile within the marginal shear zone. The method relies on the assumption that the velocity profile is a characteristic feature of the margin, and that this profile is shifted laterally as the margin migrates. Application of the method to Ice Stream B, Antarctica, indicates that the southern margin is moving outward into the inland ice at a rate of at least 9.7 ± 1.1 m a−1.

Synthetic aperture radar- (SAR-)derived ice-motion vectors and SAR interferometry were used to study the sea-ice conditions in the region between the coast and 75° N (~ 560 km) in the East Siberian Sea in the vicinity of the Kolyma River. ERS-1 SAR data were acquired between 24 December 1993 and 30 March 1994 during the 3 day repeat Ice Phase of the satellite. The time series of the ice-motion vector fields revealed rapid (3 day) changes in the direction and displacement of the pack ice. Longer-term (≥ 1 month) trends also emerged which were related to changes in large-scale atmospheric circulation. On the basis of this time series, three sea-ice zones were identified: the near-shore, stationary-ice zone; a transitional-ice zone;and the pack-ice zone. Three 3 day interval and one 9 day interval interferometric sets (amplitude, correlation and phase diagrams) were generated for the end of December, the begining of February and mid-March. They revealed that the stationary-ice zone adjacent to the coast is in constant motion, primarily by lateral displacement, bending, tilting and rotation induced by atmospheric/oceanic forcing. The interferogram patterns change through time as the sea ice becomes thicker and a network of cracks becomes established in the ice cover. It was found that the major features in the interferograms were spatially correlated with sea-ice deformation features (cracks and ridges) and major discontinuities in ice thickness.