Showing papers in "Annals of Glaciology in 2000"

The treatment of meltwater retention in mass-balance parameterisations of the Greenland ice sheet

Abstract: Retention of melt-water runoff by percolation and/ or refreezing in the snowpack cannot be neglected when studying the surface mass balance of the Greenlandice sheet. In this paper, we make a detailed comparison of several treatments proposed in the literature to account for this process in large-scale mass-balanceparameterizations. The melt on the Greenland ice sheet is calculated with a revised degree-day model using updated datasets of surface elevation and precipitationrate on a 5 km grid. Crucial model parameters are recalibrated by comparing mass-balance characteristics with available observations on a regional basis. Wediscuss the role of meltwater retention in the light of the overall mass-balance of the Greenland ice sheet and its sensitivity to climatic change, and displaypatterns of effective retention fractions for the various methods. As a main conclusion, it appears that overall results are quite similar for the various models, butthat meltwater retention has a large spatial variation not described by the simple treatments. Using the most comprehensive retention model, the sensitivity of therunoff is found to be +0.35 mm/ deg C of sea-level change per year. We also present a new map of the different zones (facies) that characterize the accumulationarea of the Greenland ice sheet, that is useful for interpreting field data and calibrating satellite observations.

Spatial distribution of net surface accumulation on the Antarctic ice sheet

Abstract: An isopleth map showing the spatial distribution of net mass accumulation at the surface on the Antarctic ice sheet, excluding Graham Land, the Larsen Ice Shelf and eastern Palmer Land, is produced based on field data from approximately 2000 sites. A database of accumulation values for 5365 gridpoint locations with 50 km spacing is interpolated from the isopleth map, giving a bulk accumulation of 2151 Gt a–1 and a mean of 159 kg m–2 a–1 for an area of 13.53 × 106 km2. Following the implementation of deflation and ablation adjustments applicable to sectors of the coastal zone, the accumulation values are reduced to 2020 Gt a–1 and 149 kg m–2 a–1. The new accumulation distribution is compared with another recent distribution, which was based on essentially the same field data using different analysis and interpolation criteria. Differences between the distributions are assessed using residuals for the 50 km gridpoint locations and by comparing average accumulation values for 24 drainage systems. The assessment based on residuals indicates that the two distributions show patterns of accumulation that are coherent at the continental scale, a shared attribute underscored by a small mean residual value of 6 kg m–2 a–1 (a difference of 22%) for six systems that collectively comprise approximately 4/10 of the ice-sheet area and 3/10 of the accumulation.

Consequences of climate change for runoff from Alpine regions

Abstract: The water balance of Alpine regions is strongly determined by the storage of water in the form of snow and ice On the basis of long time series of daily precipitation, air temperature and discharge, the conceptual runoff model HBV3–ETH9 was applied to various basins of the eastern Alps showing a glacierization of 0–80%. Using the results of regional climate modelling under the assumption of doubling of C02 , the meteorological input data files were altered taking into account more frequent hot days and additional connective precipitation events during the summer months, and the consequences of these changes for daily discharge were evaluated. The results show that in regions with insignificant glacierization, runoff reacts primarily to changes in precipitation, and less so to rising summer air temperature. In highly glacierized basins, however, the same scenarios suggest strongly enhanced water yields in an initial phase. Higher flood peaks will result when high melt rates and heavy summer rains coincide. If glacier mass losses continue in the more distant future, the glacierized area will diminish and summer discharge will be gradually reduced, resulting in drastic water shortages in hot, dry summers once the glaciers have disappeared.

Glacier monitoring within the Global Climate Observing System

Abstract: The fluctuation of mountain glaciers is recognized as a high-confidence indicator of air-temperature trends and as a valuable element of a strategy for early detection of possible Man-induced climate changes. The Terrestrial Observation Panel for Climate therefore recommended that glacier mass and area be monitored as part of the Global Climate Observing System (GCOS) established in 1992 by the World Meteorological Organization, the Intergovernmental Oceanographic Commission, the United Nations Environment Programme and the International Council of Scientific Unions. A tiered Global Hierarchical Observing Strategy was developed to be used for all GCOS terrestrial variables. According to this system of tiers, the regional to global representativeness in space and time of the records relating to glacier mass and area should be assessed by more numerous observations of glacier length changes as well as by compilation of regional glacier inventories repeated at time intervals of a few decades, the typical dynamic response time of smaller mountain glaciers. During the 1970s, Fritz Muller directed the Permanent Service on the Fluctuations of Glaciers and the Temporary Technical Secretariat for the World Glacier Inventory. These two bodies were combined in 1986 to form the World Glacier Monitoring Service, which is now responsible for internationally coordinated glacier monitoring, working in close collaboration with the World Data Center for Glaciology, Boulder.

Accumulation rates in Dronning Maud Land, Antarctica, as revealed by dielectric-profiling measurements of shallow firn cores

Abstract: The European Programme for Ice Coring in Antarctica includes a comprehensive pre-site survey on the inland ice plateau of Dronning Maud Land, Antarctica. The German glaciological programme during the 1997/98 field season was carried out along a 1200 km traverse on Amundsenisen and involved sampling the snow cover in pits and by shallow firn cores. This paper focuses on the accumulation studies. The cores were dated by dielectric-profiling and continuous-flow analysis. Distinct volcanogenic peaks and seasonal signals in the profiles served to establish a depth time-scale. The eruptions of Krakatoa, Tambora, an unknown volcano, Kuwae and El Chichon are well-documented in the ice. Variations of the accumulation rates over different times were inferred from the depth time-scales. A composite record of accumulation rates for the last 200 years was produced by stacking 12 annually resolved records. According to this, accumulation rates decreased in the 19th century and increased in the 20th century. The recent values are by no means extraordinary, as they do not exceed the values at the beginning of the 19th century. Variations in accumulation rates are most probably linked to temperature variations indicated in δ18O records from Amundsenisen.

Towards a palaeoclimatic model of rock-glacier formation in the Swiss Alps

Abstract: Climate and its long-term variability govern ground thermal conditions, and for this reason represent one of the most important impacts on creeping mountain permafrost. The decoding and better understanding of the present-day morphology and distribution of rock glaciers opens up a variety of insights into past and present environmental, especially climatic, conditions on a local to regional scale. The present study was carried out in the Swiss Alps using two different approaches: (1) kinematic analysis of specific active rock glaciers, and (2) description of the altitudinal distribution of relict rock glaciers. Two theoretical shape concepts of active rock-glacier morphology were derived’ a"monomorphic" type, representing presumably undisturbed, continuous development over several millennia and a ˚polymorphic" type, reflecting a system of (possibly climatically affected) individual creep streams several centuries old. The topoclimatic-based inventory analysis indicated an average temperature increase at relict rock-glacier fronts of approximately +2°C since the time of their decay, which is a sign of rock-glacier ages reaching back to the Alpine Late Glacial. The temperature difference of some tenths of a degree Celsius found for active/inactive rock glaciers is typical for the bandwidth of Holocene climate variations. These results confirm the importance of Alpine rock glaciers as highly sensitive indicators of past temperature evolution.

Modelling seasonal and spatial variations in the surface energy balance of Haut Glacier d’Arolla, Switzerland

Abstract: The impact of spatial and temporal variations in the surface albedo and aerodynamic roughness length on the surface energy balance of Haut Glacier d’Arolla, Switzerland, was examined using a semi-distributed surface energy-balance model (Arnold and others, 1996). The model was updated to incorporate the glacier-wide effects of albedo and aerodynamic roughness-length variations using parameterizations following Brock (1997). After the model’s performance was validated, the glacier-wide patterns of the net shortwave, turbulent and melt energy fluxes were examined on four days, representative of surface conditions in late May, June July and August. In the model, meteorological conditions were held constant on each day in order that the impact of albedo and aerodynamic roughness-length variations could be assessed independently. A late-summer snowfall event was also simulated. Albedo and aerodynamic roughness-length variations, particularly those associated with the migration of the transient snowline and the decay of the winter snowpack, were found to exert a strong influence on the magnitude of the surface energy fluxes The importance of meteorological conditions in suppressing the surface energy fluxes and melt rate following a fresh snowfall was highlighted

Canals under sediment-based ice sheets

Abstract: Classical theories of channelized subglacial drainage identify a melting vs creep balance at the channel-ice boundary that is crucial in determining its steady flow properties, but this consideration neglects the role of erodible-deformable basal sediments, especially in controlling channel morphology. Here we present a mathematical model for channel/underneath ice sheets, which incorporates the detailed mechanics of sediment transport over a mobile till-bed interface, and which allows a variable, wide channel cross section also. The resulting drainage conditions depend on the sediment flux (q) as well as the water flux (Q) through the channel - these quantities being controlled by the rates at which sediment and meltwater are captured by the flow. An approximate analytical solution indicates a "canal"-type drainage law (Walder and Fowler, 1994) of the form where N2 is the effective channel pτessUτe, n is the exponent in Glen’s flow law for ice, and Ψ is the imposed hydraulic gradient (due to topography). By solving the downstream problem numerically, the canal characteristic has been confirmed and found to be insensitive to upstream conditions. Thus, low effective pressures can result from high basal water flux or melt rates. This supports the contention that canals may be a central component in fast ice flow over soft beds The proposed model also establishes a fundamental link between the plumbing system of a glacier and its sediment budget.

Evidence for Recent Climate Change from Ice Cores in the Central Himalaya

Abstract: Comparison of the terminus locations of Rongbuk Glacier, Mount Everest, measured in 1966 and 1997 shows that in the past 30 years the glacier has retreated 170–270 m equivalent to a retreat speed of 5.5–8.7 m a–1 . During summer 1997, a 15 m firn core was recovered from Dasuopu glacier (28°23’ N, 85°44’ E; 7000 m a.s.D on the northwest margin of Xixabangma Feng, Xizang (Tibet). The seasonal variations of δ 18O values in the core indicate that monsoon signals are clearly recorded in the glacier. δ18O values are controlled by the amount effect in the monsoon season; more negative δ18O is representative of the monsoon season in snow layers. Analysis of the relationship between ice-core δ18O, sampled from 6500 m a.s.l. on the north side of Mount Everest, and instrumental series representing regional-scale precipitation, atmospheric circulation and temperature suggests a change in the relative influence of these parameters on δ18O since the 1940s. The results of the comparison add to and lengthen the sparse array of instrument data available for the Tibetan (Qinghai-Xizang) Plateau and demonstrate a recent decline in moisture flux for at least the southern part of the plateau. Glacier retreat, associated with a recent increase in temperature in the region, is coincident with this period of decreased moisture flux.

Alpine Younger Dryas glaciers as palaeo-precipitation gauges

Abstract: Moraines of the Younger Dryas "Egesen-Stadial", which are widespread features in the Alps, are a valuable terrestrial data source for quantitative palaeoclimatic studies. The depression of the early Younger Dryas (Egesen-I) ELA shows a distinct spatial pattern. It was highest (ca. -450 - -500 m against "present-day") in those areas, which are exposed towards the West and Northwest. In the central, more shielded valleys, it was in the order of -300 m and less. Summer temperature depression, which can be derived from the Younger Dryas timberline depression is in the order of -3.5 K. The stochastic glacier-climate model by Ohmura and others (1992), which relates summer temperature and precipitation at the ELA is used to infer precipitation change. Results are compared with those obtained from the glacial-meteorological approach by Kuhn (1981a). Both models lead to highly similar results. During the early Younger Dryas, climate in the central valleys of the Alps seems to have been considerably drier than today. In those areas, which are open to the West and Northwest, precipitation seems to have been the same as today or even slightly higher. These results which are based on a rather dense network of data points agree well with the results from permafrost-climate studies and the more qualitative information from palaeobotanical research. They also support the results from Atmospheric General Circulation models for the Younger Dryas in Europe, which point towards a more zonal type of circulation.

Balance velocities and measured properties of the Antarctic ice sheet from a new compilation of gridded data for modelling

Abstract: This paper presents a new compilation of gridded data sets for three-dimensional modeling of the Antarctic ice sheet. These are for surface elevation, icethickness, bedrock elevation and accumulation rate as interpolated on a 281 x 281 mesh with 20 km spacing, and encompass all of the ice sheet and thesurrounding continental shelf. Data sources include the Bamber digital elevation model from ERS-1 radar altimeter data, a redigitization of available ice thicknessdata, the Giovinetto accumulation data, recent ice-thickness data from British and German expeditions as well as accumulation data from German and Norwegianexpeditions. In particular, new data were incorporated for the Filchner-Ronne-Schelfeis and for Dronning Maud Land arising from the EPICA pre-site survey.Special attention was devoted to carefully match the various data sources, both among themselves as across the grounding line and below the ice shelves toenable ice-sheet expansion and retreat in dynamic situations. As an application, the balance flow is calculated over the entire ice sheet using a two-dimensionalfinite difference scheme and compared with a previous assessment. This brought to light the existence of ice-streaming features extending well inland of the icesheet. A detailed zoom over Dronning Maud Land exhibits the general flow characteristics of interest for locating a future deep-drilling site. As a by-product, anupdated value of 26.4 x 106 km3 was obtained for the total volume of the ice sheet and ice shelves, or equivalent to 61.1 m of global sea-level rise after removalof the ice sheet and subsequent oceanic invasion and isostatic rebound. The total accumulation over the grounded ice sheet, including the Antarctic Peninsula, is1924 Gta-1, or between 5 and 20% higher than earlier estimates. Including all of the ice shelves, the value is 2344 Gta-1.

Snow and firn permeability at Siple Dome, Antarctica

Abstract: The nature of air-snow exchange processes depends upon both the physical characteristics of the snow and forcing from the atmosphere. An understanding of snow-air transport processes and models of these processes are important for interpreting ice cores and for predicting remotely sensed snow-surface characteristics. Accurate modeling requires knowledge of the physical characteristics of the snow and firn. In this paper, measurements of snow and firn permeability from the surface down to 12 m depth at Siple Dome, Antarctica, are presented. The permeability varies greatly as a function of layer and depth, generally increasing to approximately 3 m, and generally decreasing below that. The maximum permeability,280 × 10–10 1 0 m2, occurs at approximately 3 m depth. The lowest permeability in the top 12 m of snow and firn, 10 × 10–10w m2, was measured in the surface wind pack/The measurements show that layering and microstructure have large effects on permeability A mathematical expression for the overall variation of permeability with depth is presented It is shown that snow density is a poor indicator of permeability Quantitative microscopy may be helpful in relating observed snow characteristics to permeability, and useful in modeling firnification and chemical-species transport.

Evidence for deep icequakes in an Alpine glacier

Abstract: To obtain more reliable information about the focal-depth distribution of icequakes, in April 1997 we operated an array of seven portable digital seismographs on Unteraargletscher, central Swiss Alps. Over 5000 events were detected by at least two instruments during the 9 day recording period. P-wave velocities (3770 m f) were determined from several calibration shots detonated at the glacier surface as well as in a 49 m deep borehole, whereas S-wave velocities (1860 ms–1) were derived from a simultaneous inversion for Vp/Vs6 applied to 169 icequakes. So far, hypocentral locations have been calculated for over 300 icequakes. Besides confirming the occurrence of shallow events associated with the opening of crevasses, our results show that a small but significant fraction of the hypocenters are located at or near the glacier bed. One event was found at an intermediate depth of about 120 m. Three-dimensional particle-motion diagrams of both explosions and icequakes clearly demonstrate that all vertical component seismograms from shallow sources are dominated by the Rayleigh wave. On the other hand, for events occurring at depths greater than about 40 m, the Rayleigh wave disappears almost entirely. Therefore, a qualitative analysis of the signal character provides direct information on the focal depth of an event and was used as an independent check of the locations obtained from traditional arrival-time inversions. Thus, our results demonstrate that deep icequakes do occur and that simple rheological models, according to which brittle deformation is restricted to the uppermost part of a glacier, may need revision.

A snowdrift index based on SNOWPACK model calculations

Abstract: The SNOWPACK model developed at the Swiss Federal Institute for Snow and Avalanche Research is a one-dimensional momentum, mass- and energy-balance model. Its current version includes important features such as a numerical solution of the instationary heat-transfer and creep/settlement equations, a complete surface-energy balance, phase changes, water transport and snow microstructure development (metamorphism). The microstructural parameters are linked to the thermal conductivity and the snow viscosity. The new snowdrift routine uses the modelled snow to determine a threshold friction velocity The drift model describes the local mass flux of snow, distinguishing between a saltation and suspension contribution. The snow-cover model adapts to erosion (or deposition) due to drifting and blowing snow. The drift formulation improves the seasonal snow-cover simulation considerably for stations that show a significant influence of snow transport. From the mass-flux calculation, a snowdrift index results that provides valuable information on local snowdrift for avalanche warning.

Ground surface-temperature reconstruction based on data from a deep borehole in permafrost at Janssonhaugen, Svalbard

Abstract: Analyses of the geothermal gradient in permafrost areas constitute a key signal of the ground-surface temperature history. Permafrost temperatures in selected areas are particularly well suited to reconstructing past surface-temperature changes, mainly because there is no thermal disturbance due to circulating groundwater. One year of temperature data from an instrumented 102 m deep borehole in permafrost on Janssonhaugen, Svalbard, is presented. Ground thermal properties are calculated. The average value for the thermal conductivity is 1.85 ±0.05 W m–1 K–1 , and the average value for the thermal diffusivity is 1.1m2 s–1, which gives a phase speed for the annual wave of 5.65 × KT2 m d–1. The depth of zero annual amplitude is 18 m The permafrost thickness is estimated as approximately 220 m. Analysis of the temperatures reveals an increasing temperature gradient with depth. Using a heat-conduction inversion model, a palaeoclimatic reconstruction is presented, showing a warming of the surface temperature over the last 60–80 years. The temperature profile represents a regional signal on Svalbard, which shows an inflection associated with near-surface warming of 1.5 ± 0.5°C in the 20th century.

First results and interpretation of energy-flux measurements over Alpine permafrost.

Abstract: The interaction of energy-exchange processes between the atmosphere and the Earth surface determines the surface temperature regime. It is of fundamental importance to the question whether frozen ground exists at a given site and how rapidly it may decay in response to a climatic perturbation. To further our understanding of these processes, measurements concerning near-surface energy-exchange processes were initiated in January 1997 on creeping permafrost at a high mountain site, Murtel-Corvatsch, upper Engadin, Swiss Alps. Data on all important energy-balance fluxes were collected. In this paper, we present ground-temperature and energy-balance measurements from Murtel-Corvatsch for a 2 year period, 1997–99. We will examine the relative importance of the energy-balance components and discuss special problems relating to the coarse surface layer. The results indicate a non-zero energy budget, with a positive deviation of up to 78 W m 4 in winter and a negative deviation of up to –130 W nT2 in summer. We propose that this overall imbalance of the energy-exchange fluxes, as well as the significant difference between mean annual surface and ground temperatures/can be explained by unmeasured advective energy fluxes that occur within the layer of large boulder blocks at the top of the permafrost.

Remote-sensing science and technology for studying glacier processes in high Asia

Abstract: A large number of multispectral and stereo-image data are expected to become available as part of the Global Land Ice Measurements from Space project. We investigate digital elevation model extraction, anisotropic reflectance correction and selected glacier analysis tasks that must be developed to achieve full utility of these new data. Results indicate that glaciers in the Karakoram and Nanga Parbat Himalaya, northern Pakistan, exhibit unique spectral, spatial and geomorphometric patterns that can be exploited by various models and algorithms to produce accurate information regarding glacier extent, supraglacial features and glacier geomorphology The integration of spectral, spatial and geomorphometric features, coupled with approaches for advanced pattern recognition, can help geoscientists study glacier mass balance, glacier erosion, sediment-transfer efficiency and landscape evolution.

Holocene electrical and chemical measurements from the EPICA–Dome C ice core

Abstract: The comparison between electric (electric-conductivity measurement (ECM) and dielectric profiling (DEP)) and chemical sulphate and chloride) depth profiles along the first 400 m of the EPICA-Dome C ice core revealed a very good fit, especially for peaks related to volcanic emissions. From the comparison between these profiles, a dominant contribution of sulphuric acid to the ionic balance of Antarctic ice for the Holocene was confirmed. A progressive increase with depth was observed for chloride concentrations, showing a change of relative contribution between sulphate and chloride. A higher increase of chloride was evident between 270 and 360 m depth, probably due to a change in source or transport processes or to an increase of the annual snow-accumulation rate. The DEP, ECM and sulphate ice signatures of Tambora (AD 1816) and El Chichon (?) (AD 1259) eruptions are described in detail. A characteristic peak series, due to HCl deposition, was identified at 103–109 m depth from the ECM, DEP and chloride profiles.

Tidewater glaciers: frontal flow acceleration and basal sliding

Abstract: A numerical glacier-flow model (finite-element method) is used to suggest the processes that control the flow behind the calving front of a tidewater glacier. The model is developed for grounded calving glaciers and includes an effective-pressure-dependent sliding law. The sliding law is implemented by adding a soft basal layer with a variable viscosity. The model is applied on Hansbreen, a tidewater calving glacier in Svalbard. Comparison between modeled surface velocities and observed velocity data shows good agreement. We conclude that the flow of a grounded calving glacier can be modeled with an effective-pressure-dependent sliding law.

Spatial distribution of net surface mass balance on Greenland

Abstract: Abstract The spatial distribution of surface mass balance on the Greenland ice sheet is mapped on a 50 km grid using a combination of methods depending on a zonal characterization of the diagenetic snow fades. In the zones of dry snow and upper percolation fades, the accumulation rate is calculated from microwave emissivities derived from satellite measurements using a model that is calibrated with field accumulation data. In the lower percolation zone, accumulation rates are obtained from visual interpolation of previously compiled field data, with some modification so the balance is zero at the equilibrium line In the ablation zone, ablation rates are calculated as a function of ice-surface elevation and latitude. Average values of the surface balance are 263 kg m–2a–1 in the accumulation zone, (-) 1259 kg m–2 a–1 in the ablation zone and 1286 kg m–2 a–1 overall. Compared to the findings of a previous study using practically the same approach but different models, our bulk estimate of balance (216 Gt a–1) is 57% smaller, but the differences in the estimates of net accumulation and net ablation are, respectively, 30% and 172% larger. In this and other comparisons, there is evidence that the differences in estimates are primarily due to differences in the delineation of the equilibrium line and the estimate of ablation, and secondarily to the estimate of accumulation and interpolation of field data. The differences noted with six other estimates reported in the last two decades are all of a size close to the composite variation of the difference (±50Gtσ–1) . Our surface balance is smaller than three estimates, larger than one and in agreement with two. If substituted in the latest mass-budget estimate that indicates equilibrium, our surface balance estimate would suggest a negative budget of 55 Gt a–1 and thus a positive contribution to sea-level change of 0.15 mm a–1.

Simulation of microwave emission from physically modeled snowpacks

Abstract: Abstract Detailed knowledge of snowpack properties is crucial for the interpretation and modeling of thermal microwave radiation. Here we use two well-known snow models, Crocus and SNTHERM, to obtain snow profiles from meteorological data. These profiles are compared with pit profiles and used as input to the Microwave Emission Model of Layered Snowpacks (MEMLS) for the simulation of microwave radiation. The snow-profile data can be applied almost directly. Adaptation is needed only in the conversion of the grain-size used in the snow models to the correlation length used in the emission model; it is based on empirical fits. The resulting emissivities are compared with in situ microwave measurements. The computed snow depths are in good agreement with observations. Comparison of selected profiles shows that Crocus is in good agreement with the pit profile, but the density of simulated melt-freeze crusts is underestimated. The SNTHERM profiles show no such crusts, and the density deviates from the pit profiles. The computed temporal behavior of the snowpack emissivity is reasonable. Comparison of selected situations with in situ measurements indicates good agreement. However, the polarization difference tends to be underestimated because of inaccuracies in the simulation of density profiles. The results show the potential of combined snow-physical and microwave-emission models for understanding snow signatures and for developing snow algorithms for microwave remote sensing. Based on the frequency-selective penetration and on the high sensitivity to snow texture, density and wetness, microwave radiometry can offer a new dimension to snow physics. Potential applications are described.

Retreat of mountain glaciers of northern Eurasia since the Little Ice Age maximum

Abstract: Analysis of aerial photographs of about 1000 glaciers located in the mountain ranges of the former Soviet Union –Caucasus, Polar Urals, Pamir-Alay, Tien Shan, Altay, Kodar, Cherskiy range, Suntar-Khayata, Koryakskoye Nagorye, Kamchatka–shows that variations in the magnitude of glacier retreat since the Little Ice Age maximum are significant and probably connected to climatic continentality. On average, the scale of glacier shrinkage is much smaller in continental Siberia than in central Asia and along the Pacific margins.

Verification of geophysical models in Alpine permafrost using borehole information

Abstract: At two permafrost sites in the Swiss Alps a range of geophysical methods were applied to model the structure of the subsurface. At both sites, borehole information was used to verify the quality of the model results. On the Murtel-Corvatsch rock glacier (2700 m a.s.L; upper Engadine) a 58 m deep core drilling was performed in 1987. D. c resistivity measurements, refraction seismics, ground-penetrating radar (GPR) and gravimetric surveys allowed the shape of the permafrost table beneath the marked surface microtopography to be determined and the lateral extent of a deeper shear horizon to be established The validity of each method was verified by the borehole information (cores, density log and temperature). A coherent model of the rock-glacier structure was developed. At the Schilthorn (2970 m a.s.L; Bernese Oberland), it was not clear whether permafrost is in fact present. Various geophysical surveys (d.c. resistivity tomography, refraction seismics, GPR and EM-31) gave results that were not typical of permafrost environments. A 14 m percussion drilling revealed warm permafrost and a very low ice content. These geotechnical and geothermal data allowed reinterpretation of the geophysical results, improving modelling of ground conditions. The paper demonstrates that in the difficult terrain of Alpine permafrost, boreholes may be critical in calibration and verification of the results of geophysical methods. The most useful combinations of geophysical techniques proved to be (a) seismics with d.c. resistivity, and (b) gravimetry with GPR.

Dating ice cores from a high Alpine glacier with a flow model for cold firn

Abstract: A flow model for cold firn proves to be very successful in calculating the age-depth relation of several deep ice cores drilled on Colle Gnifetti, Monte Rosa, Swiss Alps. The compressibility of firn is taken into account by an appropriate constitutive equation, first employed in glaciology by Gagliardini and Meyssonnier (1997), which is implemented in a finite-element code. Flow models of the Colle Gnifetti saddle glaciation in two and three dimensions are based on digital elevation maps of the surface σnd the bedrock, based on radio-echo soundings of the ice thickness. Firn density and the englacial temperature fields are either prescribed or calculated in coupled models. Measured surface velocities, density profiles, the ages of chemically dated layers in ice cores and the closure of a 100 m deep borehole provide benchmarks for the models. The good agreement of modeled and measured quantities confirms that the model includes the relevant physical processes and particularly that the firn flow law is well suited for this type of glacier. The study provides new constraints on the age of the ice near the base as well as the source regions of the ice in the cores. An exceptional flow behavior of the basal ice layer was detected in measurements of borehole closure and inclination. Measurtxl6ed deformation rates exceed upper bounds derived from the flow models, and are thus attributed to altered rheological properties.

Mass balance of Xiao Dongkemadi glacier on the central Tibetan Plateau from 1989 to 1995

Abstract: Data on the mass balance of Xiao Dongkemadi glacier in the Tanggula mountains, central Tibetan Plateau, were obtained over 5 5 years from 1989 to 1995. These are the first continuous mass-balance data for a continental-type glacier on the Tibetan Plateau, where the glacier accumulates during the summer monsoon (summer-accumulation-type glacier). Mass-balance vs altitude profiles were steeper in the negative than in the positive mass-balance years. This is considered to have resulted from the effect of summer accumulation. The annual mass balance is compared with air temperature, precipitation, and black-body temperature in the area including the glacier, which is calculated from infrared radiation observations by theJapanese Geostationary Meteorological Satellite. It was found that the interannual variation in the glacier mass balance was not closely related to maximum monthly mean air temperature, while it did have a relatively good correlation with maximum monthly mean black-body temperature.

Holocene glacier fluctuations: is the current rate of retreat exceptional?

Abstract: Most glaciers in the Northern Hemisphere reached their postglacial maximum in recent times, that is, after the medieval period. During the last 100 or 150 years a significant retreat has taken place, and there is little sign that this is coming to an end. The current worldwide shrinkage of glaciers is considered to be a strong indication of global warming. However, glacier retreat should be judged against the natural variability of glacier systems. Numerical glacier models can be used to quantify this variability. I have studied the natural variability of three glaciers for which long historic records of glacier length exist: Nigardsbreen, Norway; Rhonegletscher, Switzerland; and Franz Josef Glacier, New Zealand. Integrations for a 10 000 year period, driven by random forcing of a realistic strength, show that the current retreat cannot be explained from natural variability in glacier length and must be due to external forcing.

Tributaries to West Antarctic ice Streams: Characteristics Deduced from Numerical Modelling of Ice Flow

Abstract: A network of relatively fast-flowing tributaries in the catchment basins of the West Antarctic ice streams transport ice from the inland reservoir to the heads of the ice streams. Branches of the network follow valleys in basal topography but not all valleys contain tributaries. We investigate the circumstances favoring tributary flow upstream of Ice Streams D and E, using a combination of observation and numerical modelling. No consistent pattern emerges. The transition from tributary to ice-stream flow occurs smoothly along the main tributary feeding into the onset of Ice Stream D, with ice thickness being relatively more important upstream, and sliding being relatively more important downstream. Elsewhere, the downstream pattern of flow is more complicated, with local increases and decreases in the contribution of sliding to ice speed. Those changes may be due to variations in basal water storage, subglacial geologic properties or a combination of the two.

Laboratory measurement of the shear strength of ice-filled rock joints

Abstract: To assess the safety against failure of rock slopes in cold regions, such as high mountain areas, where stability is potentially maintained by ice in rock discontinuities, the shear strength of ice-filled rock joints was investigated in a series of direct shear-box tests. To permit control and repeatability, the experiments were conducted using simulated rock specimens. These were cast in the laboratory using high-strength concrete. Laboratory measurements showed that at a constant rate of shearing, the interface shear strength between ice and a joint surface of repeatable roughness is a function of both temperature and normal stress.

Deformation and recrystallization processes of ice from polar ice sheets

Abstract: Information on deformation modes, fabric development and recrystallization processes was obtained by study of deep ice cores from polar ice sheets. It is shown that intracrystalline slip is the main deformation mechanism in polar ice sheets. Grain-boundary sliding does not appear to be a significant deformation mode. Special emphasis was laid on the occurrence of "laboratory" tertiary creep in ice sheets. The creep behavior is directly related to recrystallization processes. Grain-boundary migration associated with grain growth and rotation recrystallization accommodates dislocation slip and counteracts strain hardening. The fabric pattern is similar to that induced only by slip, even if rotation recrystallization slows down fabric development. Fabrics which develop during tertiary creep, and are associated with migration recrystallization, are typical recrystallization fabrics. They are associated with the fast boundary migration regime as observed in temperate glaciers. A decrease of the stress exponent is expected from 3, when migration recrystallization occurs, to a value ≤ 2 when normal grain growth occurs.

Modeling interdecadal variations of lake-ice thickness and sensitivity to climatic change in northernmost Alaska

Abstract: A physically based finite-element heat-transfer model with phase change is used to simulate ice growth and thickness variability on shallow, thaw lakes on the North Slope of Alaska during the period 1947–97 The basic inputs to the model are air temperature and snow depth as recorded at the US National Weather Service station, Barrow, Alaska The simulated long-term mean maximum ice thickness was 191 ±021 m with a range from 133 m (1962) to 247 m (1976) Variations in the seasonal snow cover played a much greater role than air temperatures in controlling ice-thickness variability during the 50 year simulation period The sensitivity of lake-ice growth to extremes of snow depth, air temperature and snow bulk thermal conductivity is investigated This study shows that lake-ice thickness has varied significantly from year to year in northern Alaska Continued variability combined with potential climate change could affect the area of ice that freezes completely to the bottom of lakes each winter, resulting in changes in water storage and availability, permafrost thermal regime and talik dynamics beneath lakes, and methane efflux and energy fluxes to the atmosphere It is concluded that quantification and a full understanding of these potential effects will require systematic and continuous field measurements that will provide better forcing and validation fields for improved models