The problem of measuring surface height and snowpack characteristics from satellite radar altimeter echoes is investigated. In this paper, we perform an analysis of the ERS1 altimeter dataset acquired during a 3 day repeat orbit. The analysis reveals that there are temporal variations in shapes of the radar altimeter echo and that these variations are linked to meteorological phenomena. The time- and space-scales over which these variations apply are a few to tens of days and a few hundred kilometres, respectively. This phenomenon, if not accounted for, can create error in the height measurement. A numerical echo model is used to recover snowpack characteristics by taking advantage of the temporal variations of the radar echoes. A map of penetration depth of the radar waves in the Ku band over the Antarctic continent is obtained and suggests that grain-size produces the dominant effect on radar extinction in the snowpack at this frequency. Finally, a procedure is proposed to correct the height measurement within the context of ice-sheet mass-balance survey.

Late Neogene Sirius Group strata from Tillite Spur and Quartz Hills in the Reedy Glacier area, Antarctica, demonstrate the variability in Sirius Group facies and contrasts Sirius Group strata deposited at high and low paleo-elevation, respectively. The Tillite Spur and Quartz Hills Formations (Pliocene) are formally defined here.The Tillite Spur Formation type section crops out on the edge of the Wisconsin Plateau overlooking Tillite Spur. It comprises 32m of alternating coarse gray conglomerate and muddy olive-brown diamictites. The Quartz Hills Formation type section crops out above the western margin of Reedy Glacier in a pre-existing cirque towards the southern end of the Quartz Hills. It comprises c.100m of alternating massive diamictites and rhythmically interbedded sandstone and laminated mudstones which were deposited close to sea level and subsequently rapidly uplifted (>500 m Myr−1) to their present elevation at c. 1500 m. Three orders of paleoclimatic variability are recorded in the Sirius Group strata from Reedy Valley: (1) recycled marine microfloras in glacial diamictites indicate intervals of marine incursion into the Antarctic cratonic interior co-occurring with reductions in the East Antarctic ice sheet; (2) an advancing and retreating paleo-Reedy Glacier deposited a glacial/interglacial sequence alternating on a 10-100 kyr scale; 3) Centimeter and millimeter stratification in strata of the Quartz Hills Formation record annual kyr scale variability.

Large subglacial lakes manifest themselves as flat regions on the ice surface. ERS-1 satellite radar altimetry of the Dome C region of East Antarctica was analyzed to correlate unusually flat areas on the ice surface with known locations of subglacial lakes identified from airborne radio-echo sounding (RES) data. The mean length of subglacial lakes which have an expression in the ice-sheet surface was ~8.3 km, whilst those that did not exhibit a surface morphological manifestation had a mean length of ~3.3 km. Thus, lakes up to about 4 km in length arc unlikely to be detected from satellite radar altimetry of the ice surface. Given that the spacing of radio-echo flight tracks within the SPRI-NSF-TUD Antarctic database is 50-100 km in many areas, a number of subglacial lakes probably lie undetected beneath the ice sheet. RES information from two large, flat surface regions within Dome C, and a further flat area located at 80° S, 127° E, indicates the absence of subglacial lakes beneath the ice-surface features. However, these areas are characterised by relatively strong radio-echo returns which may indicate the presence of water-saturated basal sediments. We suggest that (1) blankets of water-saturated basal sediments may cause similar surface morphological features to those produced by subglacial lakes; and (2) misidentification of subglacial lakes from satellite altimeter observations of the ice-sheet surface is possible without the support of RES information relating to the ice-sheet base. Furthermore, our study indicates a lack of subglacial lake signals from RES data over relatively thick regions of East Antarctica such as the Adventure Subglacial Trough. We conclude that subglacial water produced in such regions may be transported by a basal hydrological system, driven by overburden pressure, to less thick regions of the ice sheet where subglacial lakes have been identified.

In calculations of the variation in the 2 m temperature along glaciers, the lapse rate is generally assumed to be constant. This implies that the ratio of changes in the 2 m temperature above a glacier to changes in the temperature outside the thermal regime of that glacier (“climate sensitivity”) is equal to 1. However, data collected during the ablation season on several mid-latitude glaciers show that this sensitivity is smaller than 1. The lowest measured value (0.3) was obtained on the tongue of the Pasterze, a glacier in Austria. The measured temperature distribution along the Pasterze cannot be described by a constant lapse rate either. However, there is almost a linear relationship between potential temperature and the distance along die glacier. This paper introduces a simple, analytical, thermodynamic glacier-wind model which can be applied to melting glaciers and which explains the observed “climate sensitivities” and temperature distributions much better than calculations based on a constant lapse rate.

This way of modelling the 2 m temperatures has implications for the sensitivity of the surface mass balance to atmospheric warming outside the thermal regime of the glacier. The magnitude of this sensitivity is computed with a surface energy-balance model applied to the Pasterze. When a constant lapse rate is used instead of the proposed glacier-wind model to compute changes in the 2 m temperature along the glacier, the negative change in mass balance due to 1°C warming is overestimated by 22%.

The set of force equations and stress strain-rate relations for ice masses can be solved with the method of lines and shooting the stress-free conditions at the free surface. Single- and multiple-shooting schemes with fixed point or Newton iterations for solving the stress equations including the deviatoric stress gradients are described and their performances arc discussed. The single-shooting Newton iteration proved to be the fastest seheme for typical valley glaciers, although its horizontal grid limitation is restrictive. Grid resolution can be improved substantially with a multiple-shooting scheme but computation time and storage requirements increase substantially. The Newton iteration allows the handling of mixed basal boundary conditions, partly basal velocity and partly basal shear traction being prescribed. A stick slip free gravity flow illustrates the performance of the scheme.

Changes in the Earth’s gravity field with time have important applications to a broad range of disciplines. Any process that involves a large enough horizontal redistribution of mass, either within the Earth or on or above its surface, is potentially detectable. In particular, when ice sheets grow or shrink, gravity changes as mass is redistributed in the solid earth and between the oceans and the ice sheets. The sources of global sea-level rise (about 2 mm a−1 over the last century) and in particular the contribution of the Antarctic ice sheet thereto are not well understood. Gravity measurements can help to diminish this uncertainty.

The technology currently exists to measure gravity with high accuracy by a dual-satellite mission in which the distance between the satellites is precisely monitored. We estimate from recent studies that temporal changes in the gravity field as determined by a satellite gravity mission lasting 5 years at an orbital height of 400 km would be sensitive to changes in the overall mass of the Antarctic ice sheet to a precision corresponding to better than 0.01 mm a−1 of sea-level change. However, the effects of three other phenomena that could each produce a temporally varying gravity signal with characteristics comparable to that caused by a change in Antarctic ice—postglacial rebound, inter-annual variability in snowfall, and atmospheric pressure trends — also need to be evaluated. Postglacial rebound could be partly separated from ice-mass changes with the aid of global positioning system campaigns and numerical models of rebound that use improved determinations of mantle viscosity also provided by the gravity mission. Determination of inter-annual ice-mass changes will be aided by measurements of moisture-flux divergence around the perimeters of the ice sheets and direct observations of inter-annual changes by the gravity satellite itself. The removal of pressure effects over Antarctica will become more effective as the number of automatic weather stations in the interior of the continent increases.

Even after corrections are made for these factors, the uncertainties they cause limit the accuracy in the détermination of the contribution of the Antarctic ice sheet to sea-level change to about 0.5 mm a−1. However, there is a strong complementarity between gravity measurements and the surface-height measurements that will be produced by NASA’s laser altimeter mission early next century. Together, they should be able to determine that contribution to an accuracy of about 0.1 mm a−1.

Numerical methods are used to examine the interaction between the spatial distribution of the basal shear traction and the corresponding basal velocity for an inclined slab geometry. In our improved treatment, we reject the common assumption that basal velocity is a simple function of local variables in favour of a non-local treatment that includes normal deviatoric stress and takes basal velocity to be an integrated response to spatially varying influences. Computationally, one must either iterate the basal velocity with a friction parameterization that relates basal shear traction to basal velocity or, alternatively, prescribe the basal shear traction that results from bed decoupling and substrate déformation.

The average of basal shear traction over the entire bed of the ice mass is invariant under changes in sliding distribution and thus constitutes a useful reference; any local relative reduction of traction leads to basal movement, either sliding over the bed or moving with a deforming subglacial layer. The local stress réduction is accompanied by a concentration of traction up-and down-glacier of the moving base. Growth, decay and possible migration of basal stress concentrations may be closely related to short-lived sliding events and to surges.

For glaciological and meteorological reasons Mount Elbrus, Caucasus, was chosen as a site for physical and chemical pilot studies of ice cores. This study was the first step towards systematic studies of impurities in glacier ice on Mount Elbrus. In 1900 two ice cores, each 17 m deep and spaced 10 m apart, were taken at an elevation of 4100 m on the Bolshoy Azau glacier on the western slopes of Mount Elbrus. The cores were used for different physical and chemical analyses. Structure, texture and ice microstructure were studied. Chemical analyses of major ions (SO42−, NO3−, K+ and Na+) and measurements of insoluble light-absorbing material and radioactivity (137Cs and total β activity) were made. With the results of the physical and chemical analyses of these two ice cores, the possibilities of utilising the ice for the study of trace substances deposited after long-range transport from Europe were explored. Ice-stratigraphic methods made it possible to establish the annual accumulation rate. A reference horizon was established from the depth variation of 137Cs and total β activities that showed a well-defined peak of their activities coinciding in time with the 1986 Chernobyl accident. The results of this study show that Mount Elbrus is a useful archive for atmospheric composition downwind of Europe.

Previous research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated that δD and δ18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site.

This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. The sub-annual variation of the δD record at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of H2O2 data. The slope of the multi-year T vs δ correlation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution. This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores.

The annual layering in the Greenland and Antarctic ice sheets revealed by the b 0 record becomes less distinct with depth because of self-diffusion. However, the calculated diffusion rates are loo slow to explain die observations. It is suggested that the presence of veins of liquid water increases the effective diffusion constant by a factor of about 20.

A “tethered stake” apparatus is used to measure basal sliding in a borehole on Ice Stream B, West Antaretica, about 300 km upstream (east) from its grounding line near the head of the Ross Ice Shelf. A metal stake, emplaced at the top of a laver of unfrozen till underlying the ice, is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding. The measured sliding motion includes any actual slip across the ice–till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface. This 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment, based on features of the record of apparent sliding. Alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm. In any case it is small compared to the seismically inferred till thickness of 9 m. Measured apparent sliding averages 69% of the total motion of 1.2 m d−1 over 26 days of observation if a 3.5 day period of slow apparent sliding (8% of the total motion) is included in the average. The occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c.80% and c. 8% of the total motion, on a time-scale of a few days. However, it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole. If the slow period is therefore omitted, the indicated average basal sliding rate is 83% of the total motion. In either case, basal sliding predominates as the cause of the rapid ice-stream motion. In the last 2 days of observation the average apparent sliding rate reached 1.17 m d−1, essentially 100% of the motion of the ice stream. If till deformation contributes significantly to the ice-stream motion, the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer. These observations, if applicable to the West Antaretic ice sheet in general, pose complications in modeling the rapid ice-streaming motion.

Push-morainal banks at the grounding lines of tidewater termini of temperate glaciers are the source of two types of restraining forces operating at the glacier terminus. Horizontal normal forces derive from the lateral support and transport of the bank of sediment at the terminus, whereas a horizontal shear force operates along the base of a bank pushed in front of an advancing glacier. The simple model we present suggests that bank-related restraining forces are significantly larger than the restraining force derived from the hydrostatic pressure of water adjacent to the submerged terminus of a glacier. During glacier advance, restraining forces continually increase, resulting in decreasing flow rates, glacier thickening and the eventual cessation of advance. During retreat, restraining forces continually decrease, resulting in increasing flow rates, glacier thinning and the potential for unstable, rapid, sustained retreat. The normal, seasonal, oscillatory advance retreat cycle of a glacier is moderated by restraining forces associated with push moraines. Unstable retreat is likely initiated when bank-related restraining forces fall below some threshold value during the seasonal retreat cycle. Calving is not a primary cause of glacier retreat, but is more likely a short-term response to increased flow rates. Increased flow rates result in glacier thinning and an approach toward buoyancy, both of which fluctuate seasonally in accordance with bank-related restraining forces.

Glacier facies from the Greenland ice sheet and the Wrangell-St Elias Mountains, Alaska, are analyzed using multi-temporal synthetic aperture radar (SAR) data from the European Space Agency ERS-1 satellite. Distinct zones and facies are visible in multi-temporal SAR data, including the dry-snow facies, the combined percolation and wet-snow facies, the ice facies, transient melt areas and moraine. In Greenland and south-central Alaska, very similar multi-temporal signatures are evident for the same facies, although these facies are found at lower altitude in West Greenland where the equilibrium line appears to be found at sea level at 71°30?N during the year analyzed (1992-93), probably because of the cooling effect of the eruption of Mount Pinatubo. In Greenland, both the percolation and dry-snow facies are excellent distributed targets for sensor calibration, with backscatter coefficients stable to within 0.2 dB. However, the percolation facies near the top of Mount Wrangell are more complex and less easily delineated than in Greenland, and at high altitude the glacier facies have a multi-temporal signature which depends sensitively on slope orientation.

The synthetic-aperture radar-interferometry technique is used to detect the migration of the limit of tidal flexing, or hinge line, of the floating ice longue of Peter-mann Gletscher, a major outlet glacier of north Greenland. The hinge line is detected automatically from differential interferograms using a model-fitting technique based on an elastic-beam theory. The statistical noise of the model fit is less than 3 mm, and the hinge line is mapped with a precision of 30 m. Following automatic registration of multidate image data to a precision of 5m, hinge-line migration is subsequently detected with a precision of 40 m in the horizontal plane across the glacier width. The results show that the hinge line of Petermann Gletscher migrates back and forth with tide by ±70m, in excellent agreement with the migration calculated from ocean tides predicted by a tidal model combined with the glacier surface and basal slope measured by an ice-sounding radar. Superimposed on the short-term hinge-line migration due to tide, we detect a hinge-line retreat of 270 m in 3.87 years which varies across the glacier width by ± 120 m. The retreat suggests glacier thinning at a rate of 78 + 35 cm ice a−1. Coincidentally, an analysis of ice-volume fluxes indicates that the hinge-line ice flux of Petermann Gletscher exceeds its balance flux by 0.88±1km3 ice a−1, which in turn implies glacier thinning at 83+95cm ice a−1 in the glacier lower reaches. Both methods therefore suggest that Petermann Gletscher is currently losing mass to the ocean.

We analyse data on solar radiation measured with an automatic weather station on Morteratschgletscher, Switzerland, for the period 1 October 1995–30 September 1996. The station is in the lower ablation zone. Due to shading by surrounding mountains and atmospheric attenuation, only 49% of the annual extraterrestrial irradiance (mean: 292 W m−2) reaches the glacier surface. About 48% of this is absorbed at the surface (mean: 79 W m−2; annual albedo of 0.53).

We present a simple albedo scheme for use in glacier mass-balance models. We fit the model to the 1 year dataset by optimizing five control parameters (optimal values in brackets): albedo of snow (0.75), albedo of firn (0.53), albedo of ice (0.34), e-folding constant for effect of ageing on snow albedo (21.9 days) and e-folding constant for effect of snow depth on albedo (3.2 cm). The input consists of daily albedo, snow depth and dates of snowfall events. The correlation coefficient between observed and simulated albedo is 0.931, the corresponding rms difference being 0.067.

Observations of slab-avalanche releases in alpine terrain have led to the hypothesis that rocky outcrops can influence the spatial distributions of temperature and heal flow in dry alpine snow covers and Unis control the local distribution of depth hoar. We investigate the effects of terrain on crystal growth by using a two-dimensional finite-clemcnl model of heal How coupled with a model of crystal growth from vapor. We used the model to examine the influence of snow properties, terrain geometry and snow depth on this phenomenon. The efleel is stronger in the early winter than in the late winter, because the rock has then had time to cool. In all cases, we found that depth-hoar growth occurs preferentially over the rock. This suggests that snow-pit investigations made over soil can he misleading if rocky outcrops are present.

A new method for preparing thin sections of wet snow is described. Samples are flash-frozen to immobilise the liquid water content (LWC). Next, the pore space is filled with ester, and the entire sample is frozen to a solid, sliceable block, Micrographs of slices are obtained using transmitted and specular illumination. The proportion of LWC is measured on the micrographs and compared with LWC measured independently using calorimetry. Section analysis is both facilitated and complicated by small bubbles which form during sample preparation. Flash-freezing and bubble formation are discussed theoretically.

Reconnaissance energy-balance studies were made for the first time at two sites in North Greenland to compare with conditions in West Greenland. The field experiments were planned to save weight because it is expensive to operate in North Greenland. The larger energy components (incoming radiation and ablation) were measured for 55 days altogether, and the smaller components were evaluated by indirect methods, e.g. turbulent fluxes are calculated from air temperature, humidity and wind speed, to save the weight of instruments. The energy-balance model is “tuned" by choosing surface roughness and albedo to reduce the mean error between measured ablation and modelled daily melting. The error standard deviation for ablation is only ± 5 kg m −2 d−1’, which is much lower than found in West Greenland, due to better instruments and modelling in the present study. Net radiation is the main energy source for melting in North Greenland but ablation is relatively low because sublimation and conductive-heat fluxes use energy that would otherwise be available for melting. There is a strong diurnal variation in ablation, mainly forced by variations in shortwave radiation and reinforced by nocturnal cooling of the ice surface by outgoing longwave radiation and sublimation. The model frequently predicts a frozen glacier surface at night even when air temperatures are positive.

The time evolution of a subglacial tunnel cross-section is examined usine a two-dimensional finite-element ice-flow model coupled to an idealized drainage system. Simulations are driven by physically based calculations of surface water-input variations at Slorgiaciaren, Sweden. Highlights of the model are its ability to handle unsteady conditions and irregular tunnel shapes. Agreement between modelled water pressure and borehole water levels is good. The following conclusions are reached: (i) Tunnels adapt to fluctuating inflow on time-scales of days. Storms, during which effective pressure ranges from 0 to 0.9 MPa, cause significant adjustments but daily fluctuations due solely to melt-water inflow are minor, (ii) Open-channel flow may become commonplace late in the ablation season, (iii) Initial tunnel shape influences subsequent tunnel evolution and seasonal water-pressure variation. Over the course of a summer, tunnels retain some of their initial shape, though in all experiments the width-to-height ratio increased with time, (iv) Tunnel contraction forms broad low tunnels. However, (v) given two tunnels of equal initial area, the higher narrower one expands more rapidly. Thus, more semi-circular tunnels may capture How from broader neighbours early in the summer.

Numerical models for sea-ice thickness distribution and velocity are used for ice-dynamics research and ice forecasting. In the modeling work, ERS-1 SAR is an excellent tool, in particular by providing spatial ice-velocity fields as described in the present Baltic Sea study. Ice velocities were extracted from SAR data with 3 and 6 day time intervals using the optical-flow method. A considerable stiffening of the ice pack was observed due to the change in the character of ice déformation under compression from rafting to ridging as the minimum ice thickness increased from 10 to 3O cm. The coastal alignment was strong in the ice motion and the coastal boundary layer width was 20-30 km. An analysis of the SAR data with an ice-dynamics model showed that the observed overall ice-velocity field could be produred using the Hibler viscous-plastic ice rheology. The compressive strength of the ice (over 10 km sc ales) was 2.5x 104 N m−2 ±50% for ridging and negligible for rafting of very thin ice. The shear strength was significant and the normal yield ellipse aspect ratio of 2 was valid. The 3 day time interval is val id for updating an ice model but for detailed ice-dynamics investigations a data frequency of 1 d−1 or higher would be preferable.