bluuebunny/crossref_metadata_2025 huggingface dataset:
Thinning and retreat of Glaciar Upsala, and an estimate of annual ablation changes in southern Patagonia
Authors: Naruse Renji , Skvarca Pedro , Takeuchi Yukari
Published: September 2017

Glaciar Upsala, a freshwater calving glacier in southern Patagonia, has been retreating since 1978, and after a drastic recession of about 700 m a−1 in 1994 the retreat seems to have stopped in 1995. A large ice-thinning rate of 11 m a−1 was obtained between 1990 and 1993, by surveying surface elevations near the terminus of Glaciar Upsala. In 1993–94, the thinning was estimated at about 20 m a−1 near the lateral margin. Some possible causes of the thinning behavior are considered.

In the ablation area of Glaciar Perito Moreno, 50 km south of Glaciar Upsala, ablation rates were measured during 110 d in summer 1993–94, and air temperature was continuously recorded throughout 1994. Using a degree-day method with temperature data at the nearest meteorological station, Calafate, annual ablation during the last 30 years was estimated to fluctuate from about 12 ± 2 to 16 ± 2 m a−1 in ice thickness, with a mean of 14 ± 2 m a−1. Thus, the temperature anomaly alone cannot elucidate the thinning of 11 m a−1 at Glaciar Upsala. As a possible mechanism of the ice-thinning, it is suggested that the considerable retreat due to calving may have resulted in reduction of longitudinal compressive stress exerted from bedrock rises and islands near the glacier front, causing a considerable decrease in the emergence flow. Thus, the ice may have thinned at a rate close to the annual ablation rate.
Glacial variations on a meso-scale: examples from glaciers in the Aurland Mountains, southern Norway
Authors: Tvede Arve M. , Laumann Tron
Published: September 2017

Mass-balance measurements carried out since 1963 on six glaciers in southern Norway indicate that glaciers to the west of the main water divide have increased their mass, whereas glaciers further east have decreased. However, when small glaciers within the same mountain area are studied, the picture is more complicated. Different exposures, the terrain forms on the upwind side and area distribution of the glaciers are important factors with regard to the effect of the wind-driven snow.

The three glaciers studied in this paper are located within a 10 km radius in the Aurland Mountains, but differ in size, exposure and dynamic condition. Volume changes were calculated from two repeated mappings of two glaciers. Svartavassbreen increased in volume in all areas; Storskavlen decreased over most of its area and increased in some parts. Vargebreen has lost nearly all its ice mass in the eastern part, but has changed little in the western part. It is concluded that the volume changes found on these glaciers mainly reflect changes in local snow accumulation. Analysis of air-pressure data from the period 1952–94 is presented and supports the hypothesis that there has been a change to stronger westerly winds from the mid-1960s onwards.

It is recommended that glaciers chosen for long-term studies of volume changes should represent different exposures. The topography of the surrounding terrain is also important.
Areal melt and discharge modelling of Storglaciären, Sweden
Authors: Hock Regine , Noetzli Christian
Published: September 2017

A grid-based glacier melt-and-discharge model was applied to Storglaciären, a small valley glacier (3 km2) in northern Sweden, for the melt seasons of 1993 and 1994. The energy available for melt was estimated from a surface energy-balance model using meteorological data collected by automatic weather stations on the glacier. Net radiation and the turbulent heat fluxes were calculated hourly for every grid point of a 30 m resolution digital terrain model, using the measurements of temperature, humidity, wind speed and radiative fluxes on the glacier. Two different bulk approaches were used to calculate the turbulent fluxes and compared with respect to their impact on discharge simulations. Discharge of Storglaciären was simulated from calculated meltwater production and precipitation by three parallel linear reservoirs corresponding to the different storage properties of firn, snow and ice. The performance of the model was validated by comparing simulated discharge to measured discharge at the glacier snout. Depending on which parameterization of the turbulent fluxes was used, the timing and magnitude of simulated discharge was in good agreement with observed discharge, or simulated discharge was considerably underestimated in one year.
Comparison of radio-echo sounding (30–1000 MHz) and high-resolution borehole-temperature measurements at Finsterwalderbreen, southern Spitsbergen, Svalbard
Authors: Ødegård R. S. , Hagen J. O. , Hamranw S. -E.
Published: September 2017

Radio-echo soundings in four different frequency bands ranging from 30 to 1000 MHz were compared with temperature measurements in boreholes in the accumulation area and ablation area of Finsterwalderbreen (77°26′ N, 15°15′ E), southern Spitsbergen. Finsterwalderbreen is a polythermal surge-type glacier in the quiescent phase after its last surge around AD 1900. The objective of the study was to investigate the relation between internal echos and the glacier ice temperature to map the overall thermal structure of the glacier. The thermal structure is important for ice flow velocities and hydrology of glaciers, and it also affects their ability to surge. At the borehole site in the accumulation area (three boreholes within a range of 60 m), a change in the relative amplitude of the reflected signal is detected in the 320–370 and 600–650 MHz bands at 52–55 m depth. The high-resolution temperature measurements with 2 m intervals show that the transition zone between cold and temperate ice corresponds to the change in the relative amplitude on the 320–370 and 600–650 MHz bandwidth data. The overall thermal structure of the glacier was mapped based on the radar sounding. The radar results show (a) that the glacier is at the pressure-melting point over most of its bed except within 500–700 m of the terminus, and (b) that there is an upper cold ice layer of variable thickness (25–170 m) underlain by temperate ice. This thermal structure is confirmed by the thermistor-instrumented access holes to the bed in both the accumulation and ablation zones of the glacier. The variations in the thermal structure in lower parts of the accumulation area are explained by superimposed ice and ice layers that cause variations in the downward heat transfer by refreezing of meltwater.
Changes in Eurasian glaciation during the past century: glacier mass balance and ice-core evidence
Authors: Mikhalenko Vladimir N.
Published: September 2017

Glaciers of both the Arctic and mid-latitude mountain systems within Eurasia have retreated intensively during the past century. Measured and reconstructed glacier mass balances show that glacier retreat began around the 1880s. The mean annual mass-balance value for 1880–1990 was −480 mm a−1 for glaciers with maritime climatic conditions, and −140 mm a−1 for continental glaciers. It can be concluded that warming in the Caucasus occurred during at least the last 60 years, according to the distribution of crystal sizes in an ice core from the Dzhantugan firn plateau. Temperatures measured in 1962 at 20 m on the Gregoriev ice cap, Tien Shan, were −4.2°C while in 1990 they were −2°C, a warming of 2.2°C over 28 years. Changes in the isotopic composition of glacier ice during the 20th century indicate recent and continuing warming in different regions of Eurasia. The δ18O records reveal an enrichment at the Gregoriev ice cap during the last 50 years, while surface temperatures at the Tien Shan meteorological station have increased 0.5°C since 1930.
Surface-velocity and strain-rate variations at the glacier Austre Okstindbreen, Okstindan, Norway, 1976–95
Authors: Jacobsen Frank M. , Theakstone Wilfred H. , Knudsen N. Tvis
Published: September 2017

Long-term observations of surface velocities and strain rates at the Norwegian glacier Austre Okstindbreen revealed both temporal and spatial variations. During a period of 6 years, the amount of ice passing through a cross-section slightly below the mean equilibrium-line altitude (1250 m) was some 30% less than the amount which accumulated above the equilibrium line. The mean horizontal component of surface velocity at the centre of the cross-section was of the order of 45–50 m a−1, whilst the thinner marginal ice moved less rapidly. At an altitude of about 1230–1200 m, surface velocities generally increased as the ice entered a steep icefall. In the lower part of the icefall, mean surface velocities again were of the order of 50 m a−1. From there, a general decrease down-glacier was evident, and longitudinal compression along the curving centre line of flow was accompanied by lateral extension. The contribution of internal deformation to surface flow at the lower part of the glacier, which was less than 150 m thick, is likely to have been relatively small, and between-year variations of the horizontal component of surface flow which affected a large area probably were a response to changes of basal sliding rates, reflecting variations of mass balance and water availability.
Kinematic global positioning system to monitor small Antarctic glaciers
Authors: Gandolfi S. , Meneghel M. , Salvatore M. C. , Vittuari L.
Published: September 2017

Results from analysis of kinematic global positioning system (GPS) surveying in Antarctica are examined to evaluate the feasibility of this technique for monitoring small glaciers. The experiment focuses on GPS field methods and a comparison of profiles surveyed in different years. Kinematic GPS has proved to be a useful method of monitoring glacier surfaces.
Signals of 20th-century warming from the glaciers in the Central Italian Alps
Authors: Pelfini Manuela , Smiraglia Claudio
Published: September 2017

The Lombard Alps in the central sector of the Italian Alps are one of the most intensively glacierized regions on the southern side of the chain (about 113 km2 of ice cover). All of the glaciers have been retreating since the beginning of the 20th century, but the trend is not uniform. Since the 1950s there has been a drop in the percentage of retreating snouts and an increase in stationary and advancing snouts. After 1985 the glaciers in the Lombard Alps began a new recession phase that is still going on. This pattern is consistent with temperature variations in Lombardy in the 20th century. According to data from Sondrio station, four alternating warming and cooling phases ran be distinguished in 20th-century warming. The Lombard glaciers well recorded the last three phases, showing evident signs of the 1955–88 cold phase, with a response time of about 20 years, and may be considered good indicators of 20th-century temperature trends.
Using ground-penetrating radar to image previous years’ summer surfaces for mass-balance measurements
Authors: Kohler Jack , Moore John , Kennett Mike , Engeset Rune , Elvehøy Hallgeir
Published: September 2017

In traditional mass-balance measurements one estimates winter snow accumulation by identifying the depth to the previous summer’s snow or ice surface using a snow probe. This is labor-intensive and unreliable for inhomogeneous summer surfaces. Another method is to image internal reflection horizons using a ground-penetrating radar (GPR), which has advantages in speed and areal coverage over traditional probing. However, to obtain quantitative mass-balance measurements from GPR images one needs to convert the time scale to a depth scale, not a straightforward problem. We compare a GPR section with dielectric profiles and visual stratigraphy of three snow cores, manual probings, and previous mass-balance measurements. We relate changes in snow-core dielectric properties to changes in density and to the travel times of reflecting horizons in the GPR section, and correlate some of these reflecting horizons with previous summer surfaces. We conclude that GPR can be used as a complementary tool in mass-balance measurements, giving a wide areal survey of winter accumulation and net balance for preceding years. However, proper calibration is essential for identifying specific surfaces in the radar data.
Effects of weather events on X-SAR returns from ice fields: case-study of Hielo Patagónico Sur, South America
Authors: Forster Richard R. , Smith Laurence C. , Isacks Bryan L.
Published: September 2017

The space-shuttle-based SIR-C/X-SAR synthetic aperture radar (SAR) imaged part of Hielo Patagónico Sur (HPS; southern Patagonia ice field, South America) for five successive days during missions in April and October 1994. A significant meteorological event occurred during each mission, including a major storm in April and a sharp temperature decrease in October. Changes in backscatter are observed for both episodes in X-SAR returns from the mid-portions of one of the two large outlet glaciers in the study area. Ground-station and satellite meteorological, and hydrological data are combined with the daily X-SAR images to interpret changes in glacier surface conditions caused by meteorological events. Effects interpreted from the April storm are (1) wind- and precipitation-influenced surface roughening of a wet snowpack, and (2) the deposition of new wet snow at lower elevation and its subsequent retreat up-glacier. An abrupt decrease in regional temperature during October is thought to reduce the snow wetness and increase grain-size. The changes in the radar-defined glacier zones due to the April precipitation event are subtle, while the October temperature drop causes significant backscatter increases. Our results suggest that trends in HPS glacier surface and near-surface conditions observable from spaceborne SARs are not significantly masked by precipitation events.
Structural evolution of a surge-type polythermal glacier: Hessbreen, Svalbard
Authors: Hambrey Michael J. , Dowdeswell Julian A.
Published: September 2017

Hessbreen is a small valley glacier which last surged in 1974, and is typical of many polythermal glaciers in Svalbard. The present ice surface displays a wide range of structures that can be attributed to either quiescent-phase or surge-phase deformation. During quiescent-phase flow, primary stratification becomes slightly deformed into low-amplitude open folds, while a completely new structure, longitudinal foliation, develops in axial-planar relationship to these folds. The propagation of a surge front is associated with the formation of thrusts; however, not all of these break through to the surface. As the surge progresses, and the ice behind the surge front becomes extensional, the surface of the glacier breaks up into numerous crevasses, of which several hundred metre long transverse crevasses, convex up-glacier, are dominant. After a period of quiescence, these become degraded into crevasse traces, planar structures which are steeply dipping and have many different orientations. Overall, most ice which reaches the snout has not undergone significant cumulative strain, as indicated by the lack of deformation of both the primary structures and the later crevasse traces. The distribution of debris in Hessbreen is controlled strongly by the development of some of these structures. Angular supraglacial debris is intimately associated with stratification, while basal debris is lifted to en- and supraglacial positions in association with thrusting.
A flowline model for Nigardsbreen, Norway: projection of future glacier length based on dynamic calibration with the historic record
Authors: Oerlemans J.
Published: September 2017

Historic glacier-length variations are used to constrain a computer model for Nigardsbreen, Norway. A mass-balance history is derived that, when imposed on the flow model, gives an almost perfect match between observed and simulated glacier length since AD 1748. Being calibrated with past changes, the model predicts an 800 m advance of the glacier snout if mass-balance conditions remain as they were for the period 1962–93. Several greenhouse-warming scenarios are imposed to project the glacier’s behaviour into the next century. For a uniform heating rate of 0.02 K a–1, Nigardsbreen would advance slightly until AD 2020, but then a very rapid retreat would start: in AD 2100 glacier volume would be only 10% of the 1950 volume.
Studies of snow surface characteristics by Landsat TM in Dronning Maud Land, Antarctica
Authors: Winther Jan-Gunnar
Published: September 2017

Large scale melting phenomena like meltwater drainage channels and meltwater accumulation basins or frozen “lakes” have been surveyed on the land ice mass in Dronning Maud Land, Antarctica. These melting features were also detected in a Landsat Thematic Mapper (TM) image recorded on 12 February 1990. Image processing techniques such as principal component (PC) analysis, band ratioing and histogram-equalizing are carried out to emphasize the melting phenomena. Interestingly, a histogram-equalized single TM Band 5 image appears to be a good discriminator, leaving the melting phenomena as bright areas in the TM scene. The largest frozen “lake” is close to 1km wide while some of the drainage channels stretch more than 5 km. There is also a comparison of satellite-derived and in situ reflectance and temperature. The atmospheric corrected satellite data underestimate surface snow albedo by about 16% while TM measurements of surface temperature are 2° to 6°C lower than that measured on the ground.
AVHRR surface temperature and narrow-band albedo comparison with ground measurements for the Greenland ice sheet
Authors: Haefliger M. , Steffen K. , Fowler C.
Published: September 2017

An ice-surface temperature retrieval algorithm for the Greenland ice sheet was developed using NOAA 11 thermal radiances from channels 4 and 5. Temperature, pressure and humidity profiles, cloud observations and skin temperatures from the Swiss Federal Institute of Technology (ETH) camp, located at the equilibrium line altitude at 49°17′W, 69°34′N, were used in the LOWTRAN 7 model. Through a statistical analysis of daily clear sky profiles, the coefficients that correct for the atmospheric effects were determined for the ETH-Camp field season (May to August). Surface temperatures retrieved by this method were then compared against the in situ observations with a maximum difference of 0.6 K. The NOAA 11 narrow-band planetary albedo values for channels 1 and 2 were calculated using pre-launch calibration coefficients. Scattering and absorption by the atmosphere were modelled with LOWTRAN 7. Then, narrow-band albedo values for the AVHRR visible and near infrared channels were compared with in situ high resolution spectral reflectance measurements. In the visible band (580–680 nm), AVHRR-derived narrow-band albedo and the in situ measurements corrected with radiative transfer model LOWTRAN 7 showed a difference of less than 2%. For the near infrared channel (725–1100 nm) the difference between the measured and modelled narrow-band albedo was 14%. These discrepancies could be either the result of inaccurate aerosol scattering modelling (lack of the in situ observation), or the result of sensor drift due to degradation.
Comparison of the passive microwave spectral signature of the Antarctic ice sheet with ground traverse data
Authors: Surdyk Sylviane , Fily Michel
Published: September 2017

Scanning Multichannel Microwave Radiometer (SMMR) data are compared with the snow characteristics measured during several traverses over Antarctica including average density over 0–2 m, 10 m temperature, grain-size distributions and number of strata. First a local study of specified areas and then a global study show some strong dependence between satellite data and ground data. The polarization ratios are affected by the stratification of snow (number and nature of layers); large polarization ratios correspond to strong stratification, mainly for the lower frequencies. Because low density induces strong stratification there is also a dependence between polarization ratios and density even if the density is not the critical factor. The gradient ratios are linked to the grain-sizes as determined from detailed stratigraphies; the larger the grain-size, the smaller is the gradient ratio between 6.6 and 18 GHz.
A synthesis of remote sensing data on Wilkins Ice Shelf, Antarctica
Authors: Vaughan D.G. , Mantripp D.R. , Sievers J. , Doake C.S.M.
Published: September 2017

Wilkins Ice Shelf has an area of 16000 km2 and lies off the west coast of the Antarctic Peninsula bounded by Alexander, Latady, Charcot and Rothschild islands. Several ice shelves, including Wilkins, exist close to a climatic limit of viability. The recent disintegration of the neighbouring Wordie Ice Shelf has been linked to atmopsheric warming observed on the Antarctic Peninsula. The limit of ice-shelf viability thus appears to have migrated south. Should this continue, the question arises; how long will Wilkins Ice Shelf survive?

Compared with the other ice shelves on the Antarctic Peninsula, few surface glaciological data have been collected on Wilkins Ice Shelf. We compare, contrast and combine a variety of remotely sensed data: the recently declassified GEOSAT Geodetic Mission altimetry, Landsat MSS and TM imagery, and radio-echo sounding data (RES), to study its structure and mass balance regime.

We find that this shelf has an unusual mass balance regime and relies heavily for sustenance on in situ accumulation. Its response to a continued atmospheric warming may be significantly different from that of Wordie Ice Shelf. Wordie Ice Shelf was fed by several dynamic outlet glaciers which accelerated the disintegration process when the ice shelf fractured. Wilkins Ice Shelf by contrast is almost stagnant and is expected to respond by normal calving at the ice front. Changes in the accumulation rate or basal melt-rate may, however, dominate any dynamic effect. Over the last two decades the ice front positions have remained stable.
The detectability of sea-ice leads in satellite data as a function of atmospheric conditions and measurement scale
Authors: Key J. , Stone R. , Maslanik J. , Ellefsen E.
Published: September 2017

The release of heat from sea-ice leads is an important component of the heat budget in the Arctic, but the impact of leads on regional scale climate is difficult to assess without information on their distribution in both space and time. Remote sensing of leads using satellite data, specifically AVHRR thermal and Landsat visible imagery, is examined with respect to one lead parameter: lead width. Atmospheric effects are illustrated through the concept of thermal contrast transmittance, where the brightness temperature contrast between leads of various ice thicknesses and the surrounding multi-year ice is simulated using a radiative transfer model. Calculations are made as a function of aerosol, ice crystal precipitation, and cirrus cloud optical depths. For example, at ice crystal optical depths of more than about 1.5 under mean January conditions in the central Arctic, the brightness temperature differences between 2 m and 5 cm thick ice are similar to the ice temperature variability so that there would be insufficient contrast to distinguish a lead from the surrounding ice. The geometrical aspects of the sensor are also simulated by degrading Landsat data so that the effect of sensor field-of-view on retrieved lead width statistics can be assessed. Large leads tend to “grow” with increased pixel size while small leads disappear. Changes in lead width and orientation distributions can readily be seen.
Toward the satellite monitoring of glacier changes on Mount Kenya
Authors: Hastenrath Stefan
Published: September 2017

For a pilot study of change detection of small tropical mountain glaciers from SPOT satellite, Mount Kenya was chosen because a map at scale 1 : 5000 in a local coordinate system, based on aerial photography in September 1987, provided a control for mapping of horizontal ice extent, and because the precipitous relief offered a particular challenge for technique development. A terrain model in compatible map projection was constructed from the topography of the 1987 map. A SPOT subscene of February 1988 was geocoded and terrain corrected to conform with the terrain model and identifiable terrain features. Another 26 control points established on the ground were entered in the digital array. The pixels were re-sampled to a nominal resolution of 10 m. This digital map was output at scale 1 : 5000. The glacier boundaries and the location of the terrain control points agreed with the September 1987 map within the mapping accuracy commensurate with the 1 : 5000 scale. This satellite-based mapping provides an internally consistent reference for determining the changes of Mount Kenya’s glaciers from February 1988 to a SPOT sampling intended for the mid-1990s. This pilot study further indicates the feasibility of glacier change detection on various other high mountains of the tropics.
Relief and decay of flow stripes on Byrd Glacier, Antarctica
Authors: Casassa G. , Brecher H. H.
Published: September 2017

Curvilinear flow stripes appear on aerial photographs and satellite imagery of polar ice. On Byrd Glacier, Antarctica, flow stripes are especially prominent and can be detected on AVHRR imagery down to the ice shelf margin.

Aerial photographs of Byrd Glacier (Brecher, 1986) are used to determine photogrammetrically the relief associated with flow stripes on two transverse profiles separated by a distance of 65 km. Two kinds of stripes are found, topographic and textural flow stripes. Topographic flow stripes are associated with a ridge-trough topography with double amplitudes of 7 m to 45 m and slopes of 1–7 %. The valleys of the topographic flow stripes appear bright in the photographs and correspond to snow-covered areas, while ridges appear in general as dark stripes that correspond to bare ice areas with abundant crevasses. Textural flow stripes correspond to bands of distinct crevasse pattern which are not associated with topography.

The photogrammetric information is correlated with the brightness pattern across flow stripes that appears on Advanced Very High Resolution Radiometer (AVHRR) data. Flow stripes decay rapidly on the AVHRR imagery over the first 40 km downstream from Byrd Glacier and some persist for nearly 400 km to the front of the Ross Ice Shelf.

Velocity was measured across a transect on the upstream photogrammetric profile. There are no velocity discontinuities across the boundaries of flow stripes, which shows that lateral shear between flow stripes is not a valid mechanism for explaining their persistence.
Snow conditions and hydrology of the upper Colorado River basin from satellite passive microwave observations
Authors: Josberger Edward G. , Campbell William J. , Gloersen Per , Chang Alfred T.C. , Rango Al
Published: September 2017

Satellite passive microwave observations can provide unique mesoscale (25 km) information on snowpack properties; however, the mountainous terrain of the upper Colorado River basin compounds the difficulty of the problem. Nevertheless, observations of this region from the Scanning Multichannel Microwave Radiometer (SMMR) have provided unique, synoptic, mesoscale snowpack information from 1979 to 1987 on the snowpack extent. For this nine-year period, the SMMR 18 and 37 GHz brightness temperature observations, combined to form a parameter called NGR, show the average maximum snowpack extent covers 70% of the basin and occurs on water year day 130 (mid-February). The minimum snowpack extent took place in 1981 and covered 35% of the basin. The maximum snowpack extent took place in 1979 and covered 99% of the basin. Summation of the NGR values from each SMMR mesoscale pixel within the basin provides an index of the regional snowpack properties on both an intra- and inter-annual basis and exhibits behavior similar to the snowpack extent. When compared to the nine-year average, 1981 is the minimum year and 1979 is the maximum year. Furthermore, the sum over the basin of the annual maximum NGR from each pixel correlates with the annual discharge, r = 0.6. This correlation increases to 0.8 when digital elevation data are used to characterize each SMMR pixel and only the April through July discharge is used in the regression. Hence, this study combines the small scale elevation data with the mesoscale SMMR observations to investigate the basin-wide or regional snowpack characteristics and its hydrology.
Geodetic airborne laser altimetry of Breidamerkurjökull and Skeidarárjökull, Iceland, and Jakobshavns Isbræ, West Greenland
Authors: Garvin James B. , Williams Richard S.
Published: September 2017

Two geodetic airborne laser altimeter (ALA) systems coupled to Global Positioning System receivers acquired submeter-resolution topographic profiles of the lower parts of Breidamerkurjökull and Skeidarárjökull, Iceland, in May 1989 and September 1991 (Skeidarárjökull) and of Jakobshavns Isbræ, Greenland, in April 1992. Maximum measured crevasse depths on Breidamerkurjökull, Skeidarárjökull and Jakobshavns Isbræ were 20.7, 36.1 and 50.2 m, respectively. Crevasse spacings were 43 m (45 crevasses km−1) for Breidamerkur jökull, 46–51 m (25 crevasses km−1) for Skeidarárjökull and 20–40 m (lower part) or 50–80 m (upper part) of Jakobshavns Isbræ (27 crevasses km−1). Surface slopes were ~2.4° for the lower 11 km of Breidamerkurjökull, ~0.8° for the lower 10 km of Skeidarárjökull and 1.55° for the lower 28 km of Jakobshavns Isbræ (with a range of 0.55° for the final 17 km to ~6.3° for a steep central part several km in length). Average longitudinal strain-rate values, estimated by assuming a bulk ice temperature of 0°C and a density of 880 kg m−3, ranged from 0.12 a−1 for Breidamerkurjökull, to 0.63 a−1 for Skeidarárjökull; values for Jakobshavns Isbræ fell between 1.3 and 1.7 a−1. Remote sensing of glacier microtopography by ALA offers a potential new tool for determining crevasse morphology, spatial density and spacing, meter-scale local slopes, long-wavelength gradients and derived strain rates.
Cloud screening in AVHRR digital data over Arctic regions
Authors: Gallegos Sonia C. , Hawkins Jeffrey D. , Cheng Chiu Fu
Published: September 2017

A cloud screening method initially generated to mask cloud contaminated pixels over the ocean in visible/infrared imagery, has been revised and adapted to detect clouds over Arctic regions with encouraging results. Although the method is quite successful in eliminating very cold clouds, it underestimates low level clouds. However, this does not appear to interfere with monitoring of ice related features such as leads or the ice edge in Advanced Very High Resolution Radiometer (AVHRR) scenes. The method uses: a multiple-band approach to produce signatures not readily available in single channel data, an edge detection/dilation technique to locate features in the clouds and to join isolated edges, and a polygon identification technique to remove noise in the form of isolated pixels and separate clear regions from cloud contaminated areas. The method has been tested over a limited set of data with consistent results. Initial evaluation of the usefulness of this cloud-detection algorithm in data-fusion experiments indicate a potential in locating areas in AVHRR data which are cloud contaminated and which could yield a far superior representation of the ice features if replaced with data from a different sensor such as the Special Sensor Microwave/lmager (SSM/I).
A simplified multi-scale model for predicting climatic variations of the ice-sheet surface elevation in central Antarctica
Authors: Salamatin Andrey N. , Ritz Catherine
Published: September 2017

The equation describing the surface evolution of a large ice sheet is examined on the basis of a scale analysis applied to Antarctic conditions. Changes in the surface elevation are mainly driven by mass-balance fluctuations which approximately follow global atmospheric temperature variations. The essential spatial non-uniformity of the accumulation rate and the resultant difference between central and coastal regions in reaction time-scales are taken into account. The dynamic interaction of the time-lagging interior with the quasi-stationary margin is described. As a result, a simplified model is deduced to simulate the surface-elevation variations in the central parts of the Antarctic ice sheet caused by mass-balance perturbations corresponding to the main Milankovich cycles with the periods of 19–100 kyears. Special computational tests are performed to validate the model through intercomparison with the predictions obtained with a two-dimensional thermomechanical model. The sensitivity of the model to physical factors (represented by dimensionless tuning parameters) is discussed. Climatically controlled variations of the ice-sheet thickness in the vicinity of Vostok Station during the past 200 kyears are estimated.
Modelled ice-sheet margins of three Greenland ice-sheet models compared with a geological record from ice-marginal deposits in central West Greenland
Authors: Van Tatenhove Frank G. M. , Fabre Adeline , Greve Ralf , Huybrechts Philippe
Published: September 2017

Ice-sheet modelling is an essential tool for estimating the effect of climate change on the Greenland ice sheet. The large spatial and long-term temporal scales of the ice-sheet model limits the amount of data which can be used to test model results. The geological record is useful because it provides test material on the time-scales typical for the memory of ice sheets (millennia). This paper compares modelled ice-margin positions with a geological scenario of ice-margin positions since the Last Glacial Maximum to the present in West Greenland. Morphological evidence of ice-margin positions is provided by moraines. Moraine systems are dated by 14C-dated marine shells and terrestrial peat. Three Greenland ice-sheet models are compared. There are distinct differences in modelled ice-margin positions between the models and between model results and the geological record. Disagreement between models and the geological record in the near-coastal area is explained by the inadequate treatment of marginal processes in a tide-water environment. A smaller than present ice sheet around the warm period in the Holocene (Holocene climatic optimum) only occurs if such a period appears in the forcing (ice-core record) or used temporal resolution. Smoothing of the GRIP record with a 2000 year average eliminates the climatic signal related to the Holocene climatic optimum. This underlines the importance of short-term and medium-term variations (decades, centuries) in climatic variables in determining ice-margin positions in the past but also in the future.
Time-step limits for stable solutions of the ice-sheet equation
Authors: Hindmarsh Richard C. A. , Payne Antony J.
Published: September 2017

Various spatial discretizations for the ice sheet are compared for accuracy against analytical solutions in one and two dimensions. The computational efficiency of various iterated and non-iterated marching schemes is compared.

The stability properties of different marching schemes, with and without iterations on the non-linear equations, are compared. Newton–Raphson techniques permit the largest time steps. A new technique, which is based on the fact that the dynamics of unstable iterated maps contain information about where the unstable root lies, is shown to improve substantially the performance of Picard iteration at a negligible computational cost.
An adaptive-grid finite-volume model of glacier-terminus fluctuations
Authors: Lam Joseph K.-W. , Dowdeswell Julian A.
Published: September 2017

An adaptive-grid finite-volume glacier model is described. The model is an implicit one-dimensional flowline model. The discretized implicit finite-volume equations are solved by an iterative predictor–corrector method. The grid adapts as the terminus moves in response to changes in surface mass balance. Only the terminus grid point and the penultimate grid point are adapted as the glacier-terminus position changes in order to minimize computation. Several modelling experiments are carried out to demonstrate the performance of the model. Comparisons are made with a fixed-grid finite-volume model and a fixed-grid finite-difference model. Comparisons are made on two levels. The differences in methods, finite-volume method versus finite-difference method, arise from differences in accuracy and programming efficiency. The differences in grids, adaptive-grid versus fixed-grid, arise from differences in the numerical smoothness of the motion of the moving terminus. This affects questions of stability and accuracy.
Steady-state thermomechanical modelling of ice flow near the centre of large ice sheets with the finite-element technique
Authors: Hvidberg Christine Schøtt
Published: September 2017

A finite-element model is developed in order to calculate the coupled ice and heat flow and the surface topography in cold, steady-state ice sheets. The model decouples the heat-flow equation and the surface mass-balance condition from the rest of the equations and solves the problem by an iterative method. The model is used to examine the thermomechanics of ice divides. Initial studies of a symmetric, plane ice divide and an axisymmetric ice divide have led to the following conclusions, which are consistent with previous results. The ice-divide zone is a narrow region, only a few ice thicknesses wide, where the surface slope drops to zero and the flow solution changes. The longitudinal strain rate is high, especially in the upper layers, and the vertical velocity is smaller than away from the divide. This causes the basal temperatures to increase and the isochrones to rise. Divergent-flow conditions widen the ice-divide zone, whereas they do not influence the solution at the ice divide.
On the gravity-driven shear flow of an ice–till mixture
Authors: Wu T. , Jöhnk K. , Svendsen B. , Hutter K.
Published: September 2017

In this work, we formulate a model for the isothermal flow of a (basal) ice–till mixture that is overlain by a layer of pure ice. Such a model is relevant to the case of a glacier or ice sheet possessing a till at its base. To this end, ice is treated as usual as a constant true-density, very viscous fluid, while till, which is assumed to consist of sediment and bound (i.e. moving with the sediment) interstitial water, is also assumed in a first approximation to behave as such a fluid. Since the mixture is assumed isothermal, only the mass- and momentum-balance relations for till and ice need be considered. To complete the model, no-slip and stress-free boundary conditions are assumed at the base and free surface, respectively. By working with the former conditions, we neglect the process of entrainment of sediment into the basal layer, concentrating rather on its flow behaviour and thickness. The transition from the till–ice mixture layer to the overlaying pure ice layer is idealized in the model as a moving interface representing in the simplest case the till material boundary, at which jump-balance relations for till and ice apply. As in the basal layer, till and ice are assumed to interact mechanically at this interface. In the context of the thin-layer approximation, numerical solutions of the lowest-order form of the model show that it is predominantly the thickness of the basal (mixture) layer that is influenced by the ice–till momentum interaction.
Increased ablation of Wisconsin ice in eastern north Greenland: observations and modelling
Authors: Bøggild Carl Egede , Oerter Hans , Tukiainen Tapani
Published: September 2017

In Kronprins Christian Land at 80° N in eastern north Greenland, it has been observed that the surface of the Wisconsin ice is significantly darker than the Holocene ice found immediately upstream from a transition located 710 m from the ice margin. δ18O analysis has shown the dark surface is of Wisconsin origin. Deep ice cores from the Greenland ice sheet all indicate that the Wisconsin ice contains orders of magnitude more microparticles which could he the reason for the dark appearance of the Wisconsin surface.

Photographic documentation, spectral surface-albedo measurements and satellite-image analysis all indicate a reduced albedo of Wisconsin ice. The effect of this reduced albedo is not confirmed by the ablation measurements. However, measurements of ablation variability within small test sites has documented that large errors will arise if only one stake per measuring point is used. Energy-balance calculations show ablation rates should be 10–70%, i.e. 1.8–8.4 mm d−1, respectively, less than experienced. Additionally, a satellite-image analysis shows even higher albedo contrasts 20–30 km to the south of our transect locality.

Immediately after the termination of the ice age, most of the surface in the ablation zone consisted of this low-reflectance ice. So, in the early Holocene, the dark ice of Wisconsin origin is likely to have resulted in higher ablation rates than previously considered. This may probably partly explain the fast rate of retreat/disintegration of the ice sheets in the Northern Hemisphere, after the termination of the Wisconsin ice age.
A seasonal energy-balance climate model for coupling to ice-sheet models
Authors: Paul André
Published: September 2017

An energy-balance climate model designed for coupling to ice-sheet models is presented. Its independent variables are longitude, latitude and time of the year. The model is based on the vertically integrated equations of conservation of energy and humidity. It can predict the vertically averaged temperature. Since it includes a hydrological cycle, it can also diagnose the net fresh-water flux and hence the annual snow budget at the atmosphere–ice-sheet interface. To this end, the model does not require observed precipitation rates. The computational cost is reduced by using an analytically computed Fourier–Legendre representation of daily insolation. For a highly idealized test-case configuration, two simple sensitivity experiments are carried out.
An anisotropic flow law for ice-sheet ice and its implications
Authors: Azuma Nobuhiko , Goto-Azuma Kumiko
Published: September 2017

A new flow law for anisotropic polycrystalline ice is presented. The strain-rate tensor is related by a geometrical factor tensor (G) to the stress tensor. The G factor tensor can be obtained front the c-axis fabric data and stress condition. This new flow law describes well the direction-dependent mechanical properties of anisotropic ice which cannot be demonstrated by Glen’s flow law. For example, the new flow law can explain the fact that a strong single-maximum fabric ice, such as Dye 3 Wisconsin ice, can deform several times faster than isotropic ice under horizontal shear but can hardly deform under vertical or horizontal normal stress. We also show that at a deeper part of an ice sheet, where a single-maximum fabric develops, a positive vertical strain rate can be produced with only a horizontal shear stress.
Deformation rates in combined compression and shear for ice which is initially isotropic and after the development of strong anisotropy
Authors: Jun Li , Jacka T.H , Budd W.F.
Published: September 2017

Laboratory-prepared fine-grained, initially isotropic polycrystalline ice samples were deformed under conditions of simple shear with simultaneous uniaxial compression at a constant temperature of −2.0°C. The aim was to investigate the effects of stress configuration on the flow rate of initially isotropic ice and on ice with subsequent stress and strain-induced anisotropy. Experiments were carried out for various combinations of shear and compression with shear stress ranging from 0 to 0.49 MPa and compressive stress ranging from 0 to 0.98 MPa, but such that for every experiment the octahedral shear stress was 0.4 MPa.

The strain curves resulting from the experiments clearly exhibit minimum strain rates while the ice is still isotropic, and steady-state tertiary strain rates along with the development of steady-state anisotropic fabric patterns. With constant octahedral stress (root-mean-square of the principal stress deviators), the minimum octahedral shear-strain rate has no dependence on stress configuration. This result supports the hypothesis that the flow of isotropic ice is dependent only on the second invariant of the stress tensor. This fundamental assumption has been used to provide a general description of ice-flow behaviour independent of the stress configuration (e.g. Nye, 1953; Glen, 1958; Budd, 1969).

For the tertiary flow of anisotropic ice, the octahedral strain rate is stress-state dependent as a consequence of the developed crystal-orientation fabric, which is also stress-state dependent, and which develops with strain and rotation. The present tests indicate that the enhancement factor for steady-state tertiary octahedral shear-strain rate depends on the shear or compression fraction and varies from about 10 for simple shear (with zero compression) to about 3 for uniaxial compression (with zero shear).
Development of microstructure in the high-temperature deformation of ice
Authors: Wilson Christopher J. L. , Zhang Yanhua
Published: September 2017

Microstructural changes in three sets of experiments involving crystallographic slip in anisotropic polycrystalline ice are described and interpreted with the aid of computer models. The development of microstructure was followed using time-lapse photography and transmitted light observations with deformation undertaken in plane strain and at a temperature of approximately –1°C. The deformation within a grain aggregate that accompanies axial shortening is always heterogeneous on a grainscale. The extent of inhomogeneity varies depending on the pre-existing grain structure and the way it can accommodate intragranular slip. Grain interactions are extremely important in determining the bulk deformation and the degree of grain-boundary migration. A consequence of shortening of the aggregate is the formation of high stresses between neighbouring grains and under the appropriate conditions there may be either grain-boundary migration or melting at these sites. Where a sample undergoes translation and shear during deformation, anisotropic grains in the appropriate orientation undergo bending. A buckle instability may then develop and much of the strain is accommodated by grains in easy-glide orientations. In such situations, the ice undergoes extensive recrystallization and grain growth that is concentrated in the areas of greatest buckling.
A comparison of different ways of dealing with isostasy: examples from modelling the Antarctic ice sheet during the last glacial cycle
Authors: Le Meur Emmanuel , Huybrechts Philippe
Published: September 2017

The bedrock isostatic response exerts a strong control on ice-sheet dynamics and is therefore always taken into account in ice-sheet models. This paper reviews the various methods normally used in the ice-sheet modelling community to deal with the bedrock response and compares these with a more sophisticated full-Earth model. Each of these bedrock treatments, five in total, is coupled with a three-dimensional thermomechanical ice-sheet model under the same forcing conditions to simulate the Antarctic ice sheet during the last glacial cycle. The outputs of the simulations are compared on the basis of the time-dependent behaviour for the total ice volume and the mean bedrock elevation during the cycle and of the present rate of uplift over Antarctica. This comparison confirms the necessity of accounting for the elastic bending of the lithosphere in order to yield realistic bedrock patterns. It furthermore demonstrates the deficiencies inherent to the diffusion equation in modelling the complex deformation within the mantle. Nevertheless, when characteristic parameters are varied within their range of uncertainty, differences within one single method are often of the same order as those between the various methods. This overview finally attempts to point out the main advantages and drawbacks of each of these methods and to determine which one is most appropriate depending on the specific modelling requirements.