bluuebunny/crossref_metadata_2025 huggingface dataset:
Subaqueous melt rates at calving termini: a laboratory approach
Authors: Eijpen Kimjardine , R.Warren Charles , Benn Douglas I.
Published: November 2007

Numerous field and theoretical studies have pointed to an important role for subaqueous and waterline melting in the dynamics of calving glaciers. These processes remain unquantified because of the dangerous nature of the environment, a data gap which hampers both theoretical and numerical modelling studies. Here we report laboratory experiments designed to quantify waterline and subaqueous ice-melt rates in saline and fresh water. Experiments were conducted at temperatures of 1–10°C, a range typical of ice-contact environments, and at salinities of 0,17.5 and 35 ppt. Results indicate that melt rates are slightly faster under fresh-water conditions, and that different thermohaline combinations can produce contrasting ice-front geometries. Thermo-notches develop quickly at higher temperatures ( >4°C), and form most rapidly and to a greater extent in saline water. Contrasts in melt rates and ice-front geometry are controlled by temperature-driven density contrasts and circulation patterns. Rates of up to 0.8 md–1 suggest that mass loss by subaqueous melting is a significant process at calving termini.
Subglacial thermal balance permits ongoing grounding-line retreat along the Siple Coast of West Antarctica
Authors: Parizek Byron R. , Alley Richard B. , Hulbe Christina L.
Published: November 2007

Changes in the discharge of West Antarctic ice streams are of potential concern with respect to global sea level. The six relatively thin, fast-flowing Ross ice streams are of interest as low-slope end-members among Antarctic ice streams. Extensive research has demonstrated that these “rivers of ice” have a history of relatively high-frequency , asynchronous discharge variations with evolving lateral boundaries. Amidst this variability, a ∼1300 km grounding-line retreat has occurred since the Last Glacial Maximum. Numerical studies of Ice Stream D (Parizek and others, 2002) indicate that a proposed thermal-regulation mechanism (Clarke and Marshall, 1998; Hulbe and MacAyeal, 1999; Tulaczyk and others, 2000a, b), which could buffer the West Antarctic ice sheet against complete collapse, may be over-ridden by latent-heat transport within melt-water from beneath inland ice. Extending these studies to Ice Stream A, Whillans Ice Stream and Ice Stream C suggests that further grounding-line retreat contributing to sea-level rise is possible thermodynamically However, the efficiency of basal water distribution may be a constraint on the system. Because local thermal deficits promote basal freeze-on (especially on topographic highs), observed short-term variability is likely to persist.
Bed reflectivity beneath inactive ice streams in West Antarctica
Authors: Catania Ginny A. , Conway Howard B. , Gades Anthony M. , Raymond Charles F. , Engelhardt Hermann
Published: November 2007

Radio-echo sounding (RES) techniques are used to examine spatial changes in bed reflectivity across relict ice streams inWest Antarctica. Measurements from adjacent interstream ridges are used to correct the measured power returned from the bed for attenuation and losses due to geometric spreading, scattering and absorption. RES measurements near boreholes drilled on Ice Stream C (ISC) indicate high coefficients of bed reflectivity (R > 0.1) in locations where the bed was thawed and boreholes connected to the basal water system, and low reflectivity coefficients (R < 0.02) at locations that were frozen and not connected. Intermediate values of bed reflectivity were measured at locations where the connection to the basal water system was weak. Measurements across four relict margins show that bed reflectivity usually jumps from low to high values several kilometers inside the outermost buried crevasses. We interpret this to be a transition from frozen to thawed basal conditions and discuss implications of these observations.
The englacial stratigraphy of Wilkes Land, East Antarctica, as revealed by internal radio-echo sounding layering, and its relationship with balance velocities
Authors: Rippin David M. , Siegert Martin J. , Bamber Jonathan L.
Published: November 2007

The disruption of internal layering visible in radio-echo sounding (RES) data from East Antarctica has, to date, been attributed to ice flow around bedrock topography. However, observations of internal layer disruption in the Siple Coast ice streams of West Antarctica have led to the suggestion that increased strain at the margins of ice streams may also be responsible for the disruption of internal layers. Here we present a re-analysis of the extensive RES dataset collected between 1967 and 1979 over a large part of Wilkes Land, East Antarctica, and relate the location of continuous and disrupted internal layers to modelled balance velocities. We show that the mean balance velocity associated with all areas of disrupted layers is 2.5 times higher than that associated with areas of continuous layers. We also demonstrate that disrupted layers are associated not only with ice streams, but also with areas of enhanced ice flow, which penetrate inland from the grounding line up to several hundred kilometres into the interior of East Antarctica. Continuous layers always overlie disrupted layers, suggesting either a depth dependency in the process responsible for layer disruption, or subsequent deposition of continuous layers. In some cases, disrupted layers occur outside fast-flow features, and continuous layers occur within fast-flow features. Such regions are explained by short-term flow patterns, but might also be attributed to inaccuracies in the balance-velocity calculations.
From surge-type to non-surge-type glacier behaviour: midre Lovénbreen, Svalbard
Authors: Hansen Siri
Published: November 2007

Since the beginning of the 1900s, the polythermal valley glacier midre Lovénbreen (area 5 km2) in northwestern Svalbard has retreated approximately 1km. The glacier has not surged for at least 110 years, and the present smooth longitudinal profile and negative mass balance suggest that it is not currently building up to a surge. Based on limited evidence, it has often been assumed that midre Lovénbreen surged around 1880. The ongoing retreat is probably due to quiescent-phase stagnation amplified by 20th-century climate change, as is the case for several other glaciers in Svalbard. Recently, small eskers have been found in the glacier forefield. Based on their geomorphological appearance, these are interpreted as concertina eskers, and they add to the dynamic history of the glacier. Midre Lovénbreen is interpreted as an example of a change from surge-type to non-surge-type glacier behaviour.
Seasonal variations in ice deformation and basal motion across the tongue of Haut Glacier d’Arolla, Switzerland
Authors: Willis Ian , Mair Douglas , Hubbard Bryn , Nienow Peter , Fischer Urs H. , Hubbard Alun
Published: November 2007

Records of surface motion, englacial tilt and repeat inclinometry are used to determine patterns of surface, internal and basal motion across the tongue of Haut Glacier d’Arolla, Switzerland, over temporal scales ranging from days to months. Findings are interpreted with reference to contemporaneous measurements of subglacial water pressures, and prior knowledge of the glacier’s subglacial drainage-system structure. Long-term inclinometry results show pronounced extrusion flow over a subglacial drainage axis, with basal velocities up to twice those measured at the glacier surface. Deformation profiles are more conventional away from the drainage axis, with basal velocities ∼60–70% of surface velocities. Comparison of long-term tilt rates from repeat inclinometry and englacial tiltmeters shows close correspondence. Englacial tiltmeter data are used to reconstruct internal velocity profiles and to split surface velocities into internal deformation and basal motion contributions over spring, summer and autumn/winter periods. Although, spatial patterns of surface movement are similar between periods, patterns of internal and basal motion are not. Results are interpreted in terms of the location of sticky and slippery spots, with temporally changing patterns of basal drag reflecting changing distributions of water pressure.
Is Vostok lake in steady state?
Authors: Royston-Bishop George , Tranter Martyn , Siegert Martin J. , Lee Victoria , Bates Paul D.
Published: November 2007

Stable-isotope (δD and δ18O) data from the Vostok (East Antarctica) ice core are used to explore whether or not subglacial Vostok lake is in isotopic steady state. A simple box model shows that the lake is likely to be in steady state on time-scales of the order of 104–105 years (three to four residence times of the water in the lake), given our current knowledge of north–south and east–west gradients in the stable-isotopic composition of precipitation in the vicinity of Vostok station and Ridge B. However, the lake may not be in perfect steady state depending on the precise location of the melting area, which determines the source region of inflowing ice, and on the magnitude of the east–west gradient in isotopic compositions in the vicinity of Vostok station and Ridge B.
L-band ice-sheet brightness temperatures at Dome C, Antarctica: spectral emission modelling, temporal stability and impact of the ionosphere
Authors: Drinkwater Mark R. , Floury Nicolas , Tedesco Marco
Published: November 2007

The temporal thermal stability, size, structure and spatial uniformity of Antarctic Plateau regions such as Dome C make them ideal candidates for external calibration and validation, or radiometric performance monitoring of planned space-borne L-band radiometers such as the Soil Moisture and Ocean Salinity (SMOS) radiometer. As experimental observations do not exist at this frequency, a combination of historical Ka-, X- and C-band data are used together with electromagnetic and ionospheric models to extrapolate to this longer wavelength. Dome C is demonstrated to exhibit negligible seasonal variability (<0.1 K) and may prove to be a suitable candidate, provided the effects of atmospheric and ionospheric perturbations can be mitigated.
Topographic control of regional accumulation rate variability at South Pole and implications for ice-core interpretation
Authors: Hamilton Gordon S.
Published: November 2007

Snow-accumulation rates are known to be sensitive to local changes in ice-sheet surface slope because of the effect of katabatic winds. These topographic effects can be preserved in ice cores that are collected at non-ice-divide locations. The trajectory of an ice-core site at South Pole is reconstructed using measurements of ice-sheet motion to show that snow was probably deposited at places of different surface slope during the past 1000 years. Recent accumulation rates, derived from shallow firn cores, vary along this trajectory according to surface topography, so that on a relatively steep flank mean annual accumulation is ∼18% smaller than on a nearby topographic depression. These modern accumulation rates are used to reinterpret the cause of accumulation rate variability with time in the long ice-core record as an ice-dynamics effect and not a climate-change signal. The results highlight the importance of conducting ancillary ice-dynamics measurements as part of ice-coring programs so that topographic effects can be deconvolved from potential climate signals.
Monsoonal circulation of the McMurdo Dry Valleys, Ross Sea region, Antarctica: signal from the snow chemistry
Authors: Bertler Nancy A. N. , Mayewski Paul A. , Barrett Peter J. , Sneed Sharon B. , Handley Michael J. , Kreutz Karl J.
Published: November 2007

McMurdo Dry Valleys (MDV; Ross Sea region, Antarctica) precipitation exhibits extreme seasonality in ion concentration, 3–5 orders of magnitude between summer and winter precipitation. To identify aerosol sources and investigate causes for the observed amplitude in concentration variability, four snow pits were sampled along a coast–Polar Plateau transect across the MDV. The elevation of the sites ranges from 50 to 2400 m and the distance from the coast from 8 to 93 km. Average chemistry gradients along the transect indicate that most species have either a predominant marine or terrestrial source in the MDV. Empirical orthogonal function analysis on the snow-chemistry time series shows that at least 57% of aerosol deposition occurs concurrently. A conceptual climate model, based on meteorological observations, is used to explain the strong seasonality in the MDV. Our results suggest that radiative forcing of the ice-free valleys creates a surface low-pressure cell during summer which promotes air-mass flow from the Ross Sea. The associated precipitating air mass is relatively warm, humid and contains a high concentration of aerosols. During winter, the MDV are dominated by air masses draining off the East Antarctic ice sheet, that are characterized by cold, dry and low concentrations of aerosols. The strong differences between these two air-mass sources create in the MDV a polar version of the monsoonal flow, with humid, warm summers and dry, cold winters.
Variations of glacier frontal positions on the northern Antarctic Peninsula
Authors: Rau Frank , Mauz Fabian , De Angelis Hernán , Jaña Ricardo , Neto Jorge Arigony , Skvarca Pedro , Vogt Steffen , Saurer Helmut , Gossmann Hermann
Published: November 2007

Changes in the ice fronts on the Antarctic Peninsula north of 70˚ S are currently being investigated through a comprehensive analysis of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat Thematic Mapper (TM) data as part of the international research initiative ‘Global land Ice Measurements from Space’ (GLIMS). Regional case studies are presented that cover a variety of glacial systems distributed over the northern Antarctic Peninsula and provide data on glacier front variations during the period 1986–2002. The results confirm a general trend of regional glacier front recession, but a range of different glacier variations are observed throughout the study area. Areas of predominant retreat are located in the northeastern and southwestern sectors, while stationary ice fronts characterize glacial behaviour on the northwestern coast of the peninsula. In addition, a significant increase in glacier recession is identified on James Ross Island, where retreat rates doubled during the period 1988–2001 compared to the previous investigation period, 1975–88. These observations are interpreted as being direct consequences of the rapidly changing climate in the region, which differentially affects the local accumulation and ablation patterns of the glacial systems.
Deglacial and Holocene changes in accumulation at Law Dome, East Antarctica
Authors: Van Ommen Tas D. , Morgan Vin , Curran Mark A. J.
Published: November 2007

Dating constraints have been combined with an ice-flow model to estimate surface accumulation rates at Law Dome, East Antarctica, to approximately 80 kyr BP. Results indicate that the present high-accumulation regime (~0.7ma–1 ice equivalent) was established some time after ~7 kyr BP, following an increase of approximately 80% from early to mid-Holocene. The accumulation rate at the Last Glacial Maximum is estimated at less than ~10% of the modern value. The record reveals an approximately linear dependence between temperature (inferred from isotope ratio) and accumulation rate through the glacial period. This dependence breaks down in the early Holocene, and this is interpreted as a change to a mode in which moisture-transport changes have a stronger influence on accumulation than temperature (via absolute humidity). The changes in accumulation, including the large change in the early to mid-Holocene, are accompanied by changes in sea-salt concentrations which support the hypothesis that Law Dome climate has shifted from a glacial climate, more like that of the present-day Antarctic Plateau, to its current Antarctic maritime climate. The change between these two modes occurred progressively through the early Holocene, possibly reflecting insolation-driven changes in atmospheric moisture content and circulation.
Thermal structure of Dome Fuji and east Dronning Maud Land, Antarctica, simulated by a three-dimensional ice-sheet model
Authors: Saito Fuyuki , Abe-Ouchi Ayako
Published: November 2007

Three-dimensional structures of temperature focused on Dome Fuji and east Dronning Maud Land, Antarctica, simulated in a three-dimensional shallow ice model, are reported. With a geothermal heat flux of 54.6 mWm–2, as used in several modelling studies of the Antarctic ice sheet, and an enhancement factor of 1.3, which is smaller than in previous studies, the model result taking into account the glacial cycles is in good agreement with the borehole temperature and surface topography at Dome Fuji. The basal temperature at Dome Fuji must be at or very close to the pressure-melting point. The simulated amplitude of basal temperature through glacial/interglacial cycles is <1 K.
Coastal-change and glaciological map of the Saunders Coast area, Antarctica: 1972–97
Authors: Ferrigno Jane G. , Williams Richard S. , Foley Kevin M.
Published: November 2007

Satellite images from 1972 to 1997 have been used to prepare a map showing glaciological features of the Saunders Coast area, Antarctica. Analysis of the imagery shows a trend toward ice-front retreat that may be a result of changing environmental conditions.
Photochemical decomposition of hydrogen peroxide (H2O2) and formaldehyde (HCHO) in artificial snow
Authors: Jacobi Hans-Werner , Kwakye-Awuah Bright , Schrems Otto
Published: November 2007

Laboratory-made snow doped with either hydrogen peroxide (H2O2) or formaldehyde (HCHO) was exposed to radiation in the ultraviolet and visible range, resulting in a decomposition of both compounds. These experiments demonstrate that, besides the photolysis of nitrate, further photochemical reactions of atmospheric relevant compounds can take place in snow. Under similar conditions the decomposition of H2O2 is more efficient than that of HCHO. Since the decompositions in the experiments follow first-order reaction kinetics, we suggest that the same products as in photolysis reactions in the liquid phase are produced. If similar reactions also take place in natural snow covers, these reactions would have several important consequences. The reactions could represent pathways for the generation of highly reactive radicals in the condensed phase, enhancing the photochemical activity of surface snow and modifying the oxidation capacity of the atmospheric boundary layer. The photolysis could also constitute an additional sink for H2O2 and HCHO in surface snow, which should be taken into account for the reconstruction of atmospheric concentrations of both compounds from concentration profiles in surface snow and ice cores.
The EPICA deep ice cores: first results and perspectives
Authors: Stauffer Bernhard , Flückiger Jacqueline , Wolff Eric , Barnes Piers
Published: November 2007

Two deep ice cores are being drilled in Antarctica in the frame of the European Project for Ice Coring in Antarctica (EPICA). The Dome C ice core will provide more information about mechanisms of global climatic changes over several climatic cycles. The DML core, drilled at Kohnen station, will provide a detailed record over the last climatic cycle, which can be compared with Greenland records. The drilling at Dome C reached 3200 m depth during field season 2002/03, and the age of the ice at the bottom of the hole could be 900 000 years according to preliminary estimates. The depth at Kohnen station is 1564.6 m at present, corresponding to an age of about 55 000 years. Analyses along the top parts of both ice cores have provided interesting first results. A few selected results from these parts, mostly published already, are summarized. Only a few measurements are available from the deeper parts of both cores. Dielectric profiling and electrical conductivity measurements, performed in the field, provide continuous and high-resolution records concerning the acidity and the salt concentration of the ice. Continuous flow analyses and Fast Ion Chromatography also provide high-resolution records of several chemical compounds. These records give some clues as to the age scale of the EPICA Dome C ice core, but they also leave us with many open questions.
Snow surface height variations on the Antarctic ice sheet in Princess Elizabeth Land, Antarctica: 1 year of data from an automatic weather station
Authors: Dahe Qin , Cunde Xiao , Allison Ian , Lingen Bian , Stephenson Rod , Jiawen Ren , Ming Yan
Published: November 2007

The net surface snow accumulation on the Antarctic ice sheet is determined by a combination of precipitation, sublimation and wind redistribution. We present a 1 year record of hourly snow-height measurements that shows its seasonal variability. The measurements were made with an ultrasonic sensor mounted on an automatic weather station (AWS) installed at LGB69, Princess Elizabeth Land, Antarctica (70.835˚S, 77.075˚E; 1850 ma.s.l.). The average accumulation at this site is approximately 0.70 m snow a–1. Throughout the winter, between April and September, there was little change in surface snow height. The strongest accumulation occurred during the period October–March, with four episodic increases occurring during 2002. These episodic events coincided with obvious humidity ‘pulses’ and decreases of incoming solar radiation as recorded by the AWS. Observations of the total cloud amount at Davis station, 160 km north-northeast of LGB69, showed good correlation with major accumulation events recorded at LGB69. There was an obvious anticorrelation between the lowest cloud height at Davis and the daily accumulation rate at LGB69. Although there was no correlation over the total year between wind speed and accumulation at LGB69, large individual accumulation events are associated with episodes of strong wind. Strong accumulation events at LGB69 are associated with major storms in the region and inland transport of moist air masses from the coast.
Palaeoproductivity in the Ross Sea, Antarctica, during the last 15 kyr BP and its link with ice-core temperature proxies
Authors: Salvi Cristinamaria , Salvi Gianguido , Stenni Barbara , Brambati Antonio
Published: November 2007

A detailed study of organic carbon content obtained from two sediment cores collected in the Joides basin, western Ross Sea, Antarctica, was carried out. The variations observed during the last deglaciation and the Holocene were compared to the high-resolution climatic records (EPICA DC and Taylor Dome) preserved in the ice. The importance of the carbon content as a proxy for palaeoclimatic and palaeoenvironmental changes was investigated. A dramatic decrease in the Ross Sea palaeoproductivity was observed during the Antarctic Cold Reversal (12.5–14 kyr BP). Another decrease in total organic carbon in the second half of the Holocene (after 5–6 kyr BP) confirms the climate worsening observed in previous studies.
Ultra-high-resolution seasonality of trace-ion species and oxygen isotope ratios in Antarctic firn over four annual cycles
Authors: Mcmorrow Alison , Van Ommen Tas D. , Morgan Vin , Curran Mark A. J.
Published: November 2007

Ultra-high-resolution firn-core records covering four annual cycles of oxygen isotope ratios (Δ18O) and trace-ion species were generated from a high-accumulation site on Law Dome, East Antarctica. Event-scale dating of the records was established using hourly snow accumulation measurements from a co-located automatic weather station (AWS). These net accumulation events were used to examine the seasonal timing of Δ18O and a suite of trace-ion species including marine biogenic sulphur compounds (methanesulphonic acid (MSA), non-sea salt sulphate), nitrate and major sea-salt species (sodium, chloride, magnesium). The ultra-high-resolution nature of this study and independent dating scale provide an opportunity to examine exact timings in the seasonality of each species. The traditional summer-maximum species of Δ18O and MSA show consistent relative phasing during midsummer over the four annual cycles. Nitrate shows an erratic seasonal cycle with a general trend characterized by narrow peaks during spring and early summer, preceding the mid-summer peaks in Δ18O and MSA. Non-sea-salt sulphate cycles indicate similar characteristics to MSA signals during summer, but are more comparable to nitrate signals during spring, autumn and winter. This suggests the summer non-sea-salt sulphate signal is driven by biological activity, although this species appears to be linked with nitrate signals outside the summer season. Finally, the sea-salt species indicate a seasonal cycle characterized by maximum concentrations during spring, winter and autumn. Event-scale dating of the firn-core records allows direct comparisons between the seasonal cycles and meteorological conditions. Contemporaneous local air-temperature measurements are compared with the high-resolution Δ18O record. This allows a detailed investigation of the relationship between site temperature and Δ18O signals in the ice core.
Space geodesy as a tool for measuring ice surface velocity in the Dome C region and along the ITASE traverse
Authors: Vittuari Luca , Vincent Christian , Frezzotti Massimo , Mancini Francesco , Gandolfi Stefano , Bitelli Gabriele , Capra Alessandro
Published: November 2007

Dome C was chosen by the European Project for Ice Coring in Antarctica (EPICA) as the site for the drilling of a deep ice core. This paper presents results from geodetic surveys of ice velocities (absolute and relative) at Dome C and along a transect to Terra Nova Bay. The purpose of the surveys was to provide accurate data for the study of ice dynamics, particularly a strain network comprising 37 poles surveyed in 1995 and again in 1999. Data indicate that the ice surface at the poles closest to the topographic summit moves horizontally by up to a few mm a–1 in a direction consistent with downslope motion of the ice sheet, while 25 km from the summit it moves up to 211 mma–1. The EPICA drilling site yields an interpolated velocity of about 15 ±10mma–1 in a north-northwesterly direction. Analysis of the velocity field and surface topography reveals that the surface flow centre is nearly co-located with the dome summit, and that both are in a steady-state condition. The measured horizontal velocities are consistent with the remote-sensing result and provide accurate ground-truth control for flow mapping. Seven snow–firn cores, up to 53m deep, were drilled during the Terra Nova Bay–Dome C traverse. Submerged velocity systems were installed at the borehole and measured using the global positioning system (GPS). First results show a steady-state condition. Measured (horizontal) ice velocities increase from the summit of the ice sheet to the coast, reaching about 28 ma–1 at site GPS2A.
Methanesulphonic acid movement in solid ice cores
Authors: Smith Barbara T. , Van Ommen Tas D. , Curran Mark A. J.
Published: November 2007

Methanesulphonic acid (MSA) is an important trace-ion constituent in ice cores, with connections to biological activity and sea-ice distribution. Post-depositional movement of MSA has been documented in firn, and this study investigates movement in solid ice by measuring variations in MSA distribution across several horizontal sections from an ice core after 14.5 years storage. The core used is from below the bubble close-off depth at Dome Summit South, Law Dome, East Antarctica. MSA concentration was studied at 3 and 0.5 cm resolution across the core widths. Its distribution was uniform through the core centres, but the outer 3 cm showed gradients in concentrations down to less than half of the central value at the core edge. This effect is consistent with diffusion to the surrounding air during its 14.5 year storage. The diffusion coefficient is calculated to be 2 ×10–13 m2 s–1, and the implications for the diffusion mechanism are discussed.
Changes in Antarctic temperature, wind and precipitation in response to the Antarctic Oscillation
Authors: van den Broeke Michiel R. , van Lipzig Nicole P. M.
Published: November 2007

Output of a 14 year integration with a high-resolution (55 km ×55 km) regional atmospheric climate model is used to study the response of Antarctic near-surface climate to the Antarctic Oscillation (AAO), the periodical strengthening and weakening of the circumpolar vortex in the Southern Hemisphere. In spite of the relatively short record, wind, temperature and precipitation show widespread and significant AAO-related signals. When the vortex is strong (high AAO index), northwesterly flow anomalies cause warming over the Antarctic Peninsula (AP) and adjacent regions in West Antarctica and the Weddell Sea. In contrast, cooling occurs in East Antarctica, the eastern Ross Ice Shelf and parts of Marie Byrd Land. Most of the annual temperature signal stems from the months March–August. The spatial distribution of the precipitation response to changes in the AAO does not mirror temperature changes but is in first order determined by the direction of flow anomalies with respect to the Antarctic topography. When the vortex is strong (high AAO index), the western AP becomes wetter, while the Ross Ice Shelf, parts of West Antarctica and the Lambert Glacier basin, East Antarctica, become drier.
Heat flow from wet to dry snowpack layers and associated faceting
Authors: Jamieson Bruce , Fierz Charles
Published: November 2007

Layers of faceted crystals adjacent to crusts form the failure layers for some unexpected dry-slab avalanches. This paper focuses on the case of facets that form when dry snow overlies wet snow. From a basic equation for heat flow in solids, the approximate freezing time of the wet layer is derived. Seven experiments are described in which a wet layer was placed between two dry-snowlayers in a cold laboratory. Measured freezing times are comparable to the freezing times from the approximate solution assuming that latent heat from the irreducible water content flowed up. In four of the experiments, evidence of faceting was observed at the base of the upper dry snow layer within 5 hours and before the wet layer froze. In all seven experiments faceting was observed in the upper dry layer after the wet layer froze. Simulations performed with the snow-cover model SNOWPACK yield freezing times that agree reasonably with the approximate solution and allow the influence of various parameters on the results to be explored. In addition, simulated temperatures and grain evolution are compared with observations, showing good agreement.
Hazard scenarios for avalanche actions on bridges
Authors: Margreth Stefan , Ammann Walter J.
Published: November 2007

Engineers require impact scenarios when developing hazard mitigation strategies to protect structures against snow avalanches. Since direct measurements of avalanche impacts on large obstacles are rare, the documentation and post-event analysis of avalanche damages is essential to understand the interaction of avalanches with obstacles. The objective of this paper is to develop hazard scenarios for avalanche actions on bridges, based on a case study of a well-documented avalanche event. The 40 m long pre-stressed road bridge Ri di Rialp in the Swiss Alps collapsed after being struck by a dense-flow avalanche in 1998. The post-event analysis shows that the controlling hazard scenario was an inclined avalanche impact, not a horizontal impact as one would assume given the topographical situation. Using a failure analysis, an impact angle of 40° and a minimal impact pressure of 172 kN m−2 were found. Finally the insights are summarized so that engineers can apply the results when designing a bridge at risk from avalanches.
Modeling the density variation in the shallow firn layer
Authors: Li Jun , Zwally H. Jay
Published: November 2007

Vapor-transfer theory is incorporated into a previous firn-densification model to investigate the effect of vapor-transfer processes on densification in firn within 10 m of the surface. The densification rate in the model is governed by the change of overburden pressure (determined by the accumulation rate), the firn temperature, and the temperature gradient. The time of exposure to temperature gradients at shallow depths is a critical factor determining the importance of vapor-transfer processes. In high-accumulation and high-temperature conditions such as for the Greenland ice sheet, the temperature gradient and vapor transfer are less important due to the shorter exposure times. The high summer temperatures dominate the rate of densification and annual variations in density. In low-accumulation and low-temperature conditions, such as for inland Antarctica, the vapor transfer driven by the temperature gradient has a stronger effect on the densification rate, and temperature-driven processes are less important. These factors determine both the rate of density increase with depth and the amplitudes of annual variations in density with depth.
Regular packing of grains as a model of snow structure
Authors: Golubev Vladimir N. , Sokratov Sergey A.
Published: November 2007

This paper presents a model of snow structure (model of regular grain packing) that is based on experimental determination of various geometrical characteristics of fine-, medium- and large-grained granular snow. Data analysis supports the possibility of approximating the ice-matrix configuration as a regular lattice of nearly spherical ice grains connected by rigid ice bonds. The model was successfully used for relating microstructural parameters of snow to snow density, compaction behavior and evolution of mechanical properties.
Mechanical vibration responses of snow samples near the melting temperature
Authors: Takei Iwao , Maeno Norikazu
Published: November 2007

Mechanical properties of snow were investigated by means of a vibration response technique in a frequency range from 10Hz to 1MHz and a temperature range from –15° to –0.1°C with heating and cooling processes. The response signals were divided into two kinds of propagation, transverse and longitudinal waves through the snow sample. The temperature dependence of elastic-wave velocities showed a large decrease above –0.6°C. Poisson’s ratio and Young’s modulus of snow samples were derived from the longitudinal and transverse wave velocities. Poisson’s ratio of snow samples showed a value of 0.35 ± 0.01 below –0.6°C, and dropped to 0.29 or less at –0.1°C. Young’s modulus of snow samples at –0.1°C showed values seven-tenths as large as (25–34%less than) those below –0.6°C. These phenomena suggest weakening and slipping of boundaries between ice particles in snow samples near the melting temperature. The elastic-wave velocities and Young’s modulus change with the density of samples and with time and temperature cycling. These changes are related to the number and state of bonds between ice particles in snow samples.
Shear strength and angle of repose of snow layers including graupel
Authors: Abe Osamu
Published: November 2007

Snow avalanches with failure layers including graupel are sometimes observed in Japan. A snow layer including graupel may be formed close to the melting point (0°C) after passage of a cold front with cumulus clouds. Graupel particles have a large diameter and a spherical shape, so they have a relatively small contact area per unit volume and may form weak layers within the snowpack. In this paper, characteristics of snow layers that include graupel are described by angle of repose, shear frame index (SFI), grain-size distribution, density and temperature. The mean value of the angle of repose was found to be 45°, independent of the grain-size. This suggests that a graupel layer near 0°C will not be formed on a slope exceeding 45°. SFI data for three different graupel layers were measured over 1–3 days for each layer, and were correlated with density. An exponential relationship between shear strength and density was found. The results indicate that the SFI of graupel layers is approximately one-third that of dry new snow with the same density.
Dense and powder avalanches: momentumreduction generated by a dam
Authors: Naaim-Bouvet Florence , Naaim Mohamed , Faug Thierry
Published: November 2007

Passive protection structures reduce avalanche run-out distance and hence the potential damages brought about by an avalanche, by reducing its velocity and mass. This paper starts with a summary of the main existing results on interactions between snow avalanches and dams. In the case of dense avalanches, the effects of dams are re-examined and previous results are theoretically justified. For a powder-snow avalanche a dam has two primary effects. The momentum is reduced by the retarding force upstream of the dam and when the jet collides with the ground after the dam. Entrainment of air during the jet reduces its velocity since the mass increases. For a dense avalanche, there is a retarding force upstream of the dam and when the jet collides with the ground, but there is no significant effect of air entrainment. There is, however, deposition of snow which reduces the momentum of the avalanche without affecting the velocity. The reduction of the volume due to deposition reduces the spreading length and therefore the run-out distance of the avalanche. Downstream of the obstacle, the avalanche flow is regarded as an airborne jet. A comparative study shows that density and ambient-fluid entrainment effects are significant for gravity currents, powder and dense snow avalanches whereas they are negligible in granular flows.
A system for prediction of avalanche hazard in the windy climate of Iceland
Authors: Haraldsdóttir Svanbjörg H. , Ólafsson Haraldur , Durand Yves , Giraud Gerald , Mérindol Laurent
Published: November 2007

A system for predicting avalanche hazard in the windy climate of Vestfirðir, northwest Iceland, is presented. The system is based on (a) numerical simulations of the snowpack and evaluation of avalanche hazard due to overloading by the snow models SAFRAN–Crocus and MÉPRA, and (b) observations of wind speed in the mountains used as a proxy for snowdrift. The system was tested during two winter seasons and correctly predicts 30 out of 35 avalanche days. The 5 missing days feature either large spatial gradients in precipitation or very strong winds a few days before the avalanches, indicating much snowdrift. A comparison of the simulated avalanche hazard and observations of avalanches confirms that snowdrift is of primary importance not only for large avalanches, as already established, but also for small and medium-size avalanches in Vestfirðir. The system is a first step towards an objective evaluation of avalanche hazard in Iceland.
Linear trends of the length of snow-cover season in the Northern Hemisphere as observed by the satellites in the period 1972–2000
Authors: Rikiishi Kunio , Hashiya Eisuke , Imai Masafumi
Published: November 2007

The dataset of Northern Hemisphere EASE-Grid weekly snow cover and sea-ice extent (U.S. National Snow and Ice Data Center) for the period September 1972–August 2000 is analyzed to examine the possible influence of recent global warming on the seasonal change of snow cover in the Northern Hemisphere. It is found that the total snow-cover area in the 1980s and 1990s is diminished by 36106 km2, and the length of snow-cover season is reduced by 2 –3 weeks, as compared with the 1970s. In general, the contribution from earlier snowmelt is greater than that from delayed snow accumulation. In addition, the maximum snow-cover area during January–February has gradually decreased by about 36106 km2 within the two decades. Geographically, the rate of decrease of snow-cover duration is 50.1 week per year (wpy) in the high-latitude regions such as the Siberian Plains and Northwest Territories of Canada and 40.2 wpy in the high-elevation regions such as the Scandinavian Peninsula, Tibetan Plateau and Rocky Mountains. The earlier snowmelt in the high-elevation regions suggests that the snowfall amounts there are decreasing owing to global warming.
Dynamics of avalanches along general mountain slopes
Authors: Pudasaini Shiva Prasad , Wang Yongqi , Hutter Kolumban
Published: November 2007

A continuum dynamical hydraulic model for the flow of a finite mass of cohesionless shallow granular material down an inclined plane was proposed by Savage and Hutter (SH) in 1989. This model consists of depth-averaged equations for the time and space evolution of the velocity profile and the avalanche depth. Their model was generalized to incorporate more complicated topography in which the coordinate lines in the downhill direction are curved but not twisted. We present recently developed model equations by Pudasaini and Hutter for free-surface geophysical gravity-driven flows (e.g. avalanches, debris and pyroclastic flows) down complicated realistic mountain terrain generated by arbitrary space curves with slowly varying curvature and torsion. These are very important extensions to the successful SH theory, which incorporate curvature and torsion effects of the sliding surface. Shock-capturing numerical schemes are used to integrate the hyperbolic conservation system of equations. The physical significance of the numerical solutions is discussed.
Surface mass-balance observations and automatic weather station data along a transect near Kangerlussuaq, West Greenland
Authors: van de Wal R.S.W. , Greuell W. , van den Broeke M.R. , Reijmer C.H. , Oerlemans J.
Published: November 2007

Surface mass-balance data from the Kangerlussuaq transect (K-transect) located on the western part of the Greenland ice sheet near 67° N are presented. The series covers the period 1990-2003 and is the longest series of surface mass-balance measurements in Greenland. The surface mass-balance measurements cover an altitude range of 390-1850 m and show a linear increase of the specific mass balance, with a mass-balance gradient of 3.7 × 10–3 m m–1 and a mean equilibrium-line altitude of 1535 ma.s.l. Interannual variability shows a weak 4 yearly periodicity. In addition to the surface mass-balance data, automatic weather station data at an elevation of approximately 1010m are available for the period 1997-2002. These data are used to explain observed surface mass-balance anomalies over the same 5 years. It is shown that variations in shortwave radiation dominate interannual variability. The mean annual cycle of temperature is characterized by a maximum in summer around the melting point, leading to a mean summer outgoing longwave radiation of approximately 314 W–2. The mean annual cycle in wind speed shows a maximum in winter (on average around 8 m s–1) and a minimum in summer (on average around around 5 m s–1), which is characteristic for a katabatic forcing. During summer the net radiation is on average about 61 Wm–2, which is used for ice melting at a rate of typically 2 cm w.e.d-1. Net radiation contributes 84% of the total energy used for summer melting averaged over the 5 years.
The methanesulfonic acid (MSA) record in a Svalbard ice core
Authors: Isaksson Elisabeth , Kekonen Teija , Moore John , Mulvaney Robert
Published: November 2007

Svalbard ice cores have not yet been fully exploited for studies of climate and environmental conditions. In one recently drilled ice core from Lomonosovfonna, we have studied the methanesulfonic acid (MSA) records in relation to temperature and sea ice. Under the present climatic conditions, MSA appears to be negatively correlated with the sea-ice conditions in the Barents Sea, and positively correlated with the instrumental temperature record from Svalbard. However, prior to about 1920 the MSA concentrations were about twice as high, despite the more extensive sea-ice coverage. After exploring different possibilities, we suggest that MSA concentrations were higher in the 19th century than in the 20th century due to increased primary production, in response to increased vertical stability of the sea surface layers, caused by increased meltwater production from the more extensive sea-ice cover. Thus, the MSA record from Lomonosovfonna probably both is a measure of the regional sea-ice variability on the multi-decadal scale and reflects locally favorable conditions for marine biogenic dimethyl sulfide (DMS) production on the sub-decadal scale.
Velocity measurements on Engabreen, Norway
Authors: Jackson Miriam , Brown Ian A. , Elvehøy Hallgeir
Published: November 2007

Horizontal velocity measurements on the lower part of Engabreen, Norway, were made from repeat aerial photography. IMCORR software, which has been widely used to measure velocities from satellite images, was used to make the measurements. This is the first known successful use of IMCORR on aerial photographs. Supplementary horizontal velocity measurements were made from repeat measurements of stakes, giving velocities over different periods and also in areas that are too slow-moving to register a measurable velocity after only a few days.
Problems with the retrieval of glacier net surface balance from SAR imagery
Authors: Brown Ian A. , Klingbjer Per , Dean Andy
Published: November 2007

There are relatively few comparisons between synthetic aperture radar (SAR) observations and glacier mass-balance measurements. More typically, SAR has been deployed to identify changes in the end-of-summer snowline and other facies boundaries. In this paper, we analyze the geophysical processes affecting SAR amplitude data over two Arctic glacier systems in northern Scandinavia to assess the potential of SAR observations for the retrieval of surface balance parameters. Using a backscatter model and in situ data, we identify the controls on SAR imagery in terms of mass-balance measurement and discuss the glaciological parameters which can reasonably be derived from multi-temporal SAR data. Our results show that amplitude SAR imagery, in the absence of in situ measurements, is not capable of providing meaningful mass-balance data. We show that backscatter from temperate glaciers is affected primarily by snow grain-size and density, and therefore processes such as firnification or depth-hoar formation can complicate the analysis of imagery. We conclude that SAR imagery over temperate glaciers can provide valuable proxy information but not direct mass-balance terms.
Temporal changes in the radiophysical properties of a polythermal glacier in Spitsbergen
Authors: Jania J. , Macheret Yu.Ya. , Navarro F.J. , Glazovsky A.F. , Vasilenko E.V. , Lapazaran J. , Glowacki P. , Migala K. , Balut A. , Piwowar B.A.
Published: November 2007

In order to study the seasonal and inter-seasonal variations in radio-wave velocity (RWV), radiophysical investigations were made at Hansbreen, a polythermal glacier in Spitsbergen, in July– August 2003 and April 2004. These investigations included repeated radar profiling (20 and 25 MHz) along a transverse profile, repeated common-midpoint measurements, continuous radar measurements during 8 days at a fixed site, meteorological observations, and continuous ice surface velocity monitoring by differential GPS. Seasonal and inter-seasonal RWV changes in the temperate ice layer are attributed, respectively, to rapid water redistribution within it during the summer, and to variations in water content from 2.1% in summer to 0.4% in spring. The reflection properties of the temperate ice layer correlate well with the air temperature, with a nearly semi-diurnal time lag. The temporal variability of the reflection properties of the internal horizon suggests enlargement of water inclusions or water drainage from the horizon. Repeated profiling shows a stable spatial pattern in bed reflection power interpreted as changes in water content controlled by bedrock topography. The spatial variations of internal reflection energy along the repeated profile correlate with the thickness of the cold ice layer and the occurrence of drainage and crevasse systems.