bluuebunny/crossref_metadata_2025 huggingface dataset:
The treatment of meltwater retention in mass-balance parameterizations of the Greenland ice sheet
Authors: Janssens Ives , Huybrechts Philippe
Published: November 2007

Retention of meltwater runoff by percolation and/or refreezing in the snowpack cannot be neglected when studying the surface mass balance of the Greenland ice sheet. In this paper, we make a detailed comparison of several treatments proposed in the literature to account for this process in large-scale mass-balance parameterizations. The melt on the Greenland ice sheet is calculated with a revised degree-day model using updated datasets of surface elevation and precipitation rate on a 5 km grid. Crucial model parameters are recalibrated by comparing mass-balance characteristics with available observations on a regional basis. We discuss the role of meltwater retention in the light of the overall mass balance of the Greenland ice sheet and its sensitivity to climatic change, and display patterns of effective-retention fractions for the various methods. As a main conclusion it appears that overall results are quite similar for the various models, but that meltwater retention has a large spatial variation not described by the simple treatments. Using the most comprehensive retention model, the sensitivity of the runoff is found to be +0.35 mm ˚C–1 of sea-level change per year. We also present a new map of the different zones (facies) that characterize the accumulation area of the Greenland ice sheet, which is useful for interpreting field data and calibrating satellite observations.
Climate on tundra and thoughts on causes of regional climate differences
Authors: Ohmura Atsumu
Published: November 2007

The Axel Heiberg Island Expedition of McGill University, Montreal, Canada, led by the late Fritz Müller was the cradle for a number of scientific investigations, not all of them glaciological. During the four years 1969–72, the tundra climate was investigated in the Expedition Area, encompassing about 100 km2 on the mid-west coast of Axel Heiberg Island, Nunavut, Canada. This series of studies identified the main features of the tundra climate, clarified its causal processes and contributed to an understanding of the differences between tundra climates and those of the neighboring regions with glaciers, ocean surfaces and boreal forests. A fundamental consideration of climate processes on a global scale was conceived during this time. Some significant results are recorded here to commemorate Fritz Müller’s contributions as a polar scientist and a university educator.
Derivation of ice thickness and bedrock topography in data-gap regions over Antarctica
Authors: Warner Roland C. , Budd W. F.
Published: November 2007

An approach to deriving the thickness of the Antarctic ice sheet and inferring the bedrock elevation to fill in gaps in the compilations of ice-thickness observations is presented. It combines assumptions about the general state of balance of the ice sheet and of the dynamics of ice flow, with information about ice accumulation and the topography of the ice-sheet surface elevation to infer the ice thickness and bedrock. A simplified version of the scheme already shows the potential of this approach, as is demonstrated by an application to the Lambert Glacier basin, East Antarctica.
Geometry and the deformation of pack ice: II. Simulation with a random isotropic model and implication in sea-ice rheology
Authors: Ukita Jinro , Moritz Richard E.
Published: November 2007

In this paper, we extend the analysis of geometry and deformation of pack ice initiated in part I by considering random isotropic geometry using the Poisson line process. The model is used to estimate opening, ridging and sliding coefficients for more realistic geometry than the idealized simple and regular geometry considered in part I. We then derive the shape of yield curves by applying minimization of the maximum shear stress to a linear combination of the estimated ridging and sliding coefficients. It is found that isotropic crack geometry results in a sine-lens shape for the yield curve if sliding makes no contribution to the energy dissipation. By contrast, when sliding contributes, the shape of the yield curve becomes teardropped. These results suggest the presence of a consistent relationship between large-scale characterization of inter-floe interactions and small-scale (crack and lead) ridging processes.
Sea-ice model validation using submarine measurements of ice draft
Authors: Shy Timothy L. , Walsh John E. , Chapman William L. , Lynch Amanda H. , Bailey David A.
Published: November 2007

Sea-ice thickness distributions from 12 submarine cruises under the North Pole are used to evaluate and enhance the results of sea-ice model simulations. The sea-ice models include versions with cavitating fluid and elastic-viscous-plastic rheologies, and versions with a single thickness and with multiple (5–27) thicknesses in each gridcell. A greater portion of the interannual variance of observed mean thickness at the Pole is captured by the multiple-thickness models than by the single-thickness models, although even the highest correlations are only about 0.6. After The observed thickness distributions are used to ˚tune" the model to capture the primary mode of the distribution, the largest model-data discrepancies are in the thin-ice tail of the distribution. In a 41 year simulation ending in 1998, the model results show a pronounced decrease of mean ice thickness at the Pole around 1990; the minimum simulated thickness occurs in summer 1998. The decrease coincides with a shift of the Arctic Oscillation to its positive phase. The smallest submarine-derived mean thickness occurs in 1990, but no submarine data were available after 1992. The submarine-derived thicknesses for 1991 and 1992 are only slightly smaller than the 12–case mean.
Spatial distribution of net surface accumulation on the Antarctic ice sheet
Authors: Glovinetto M. B. , Zwally H. J.
Published: November 2007

An isopleth map showing the spatial distribution of net mass accumulation at the surface on the Antarctic ice sheet, excluding Graham Land, the Larsen Ice Shelf and eastern Palmer Land, is produced based on field data from approximately 2000 sites. A database of accumulation values for 5365 gridpoint locations with 50 km spacing is interpolated from the isopleth map, giving a bulk accumulation of 2151 Gt a–1 and a mean of 159 kg m–2 a–1 for an area of 13.53 × 106 km2. Following the implementation of deflation and ablation adjustments applicable to sectors of the coastal zone, the accumulation values are reduced to 2020 Gt a–1 and 149 kg m–2 a–1. The new accumulation distribution is compared with another recent distribution, which was based on essentially the same field data using different analysis and interpolation criteria. Differences between the distributions are assessed using residuals for the 50 km gridpoint locations and by comparing average accumulation values for 24 drainage systems. The assessment based on residuals indicates that the two distributions show patterns of accumulation that are coherent at the continental scale, a shared attribute underscored by a small mean residual value of 6 kg m–2 a–1 (a difference of <4%). However, the regional assessment based on average accumulation values for the drainage systems shows differences that are larger than the assessment error (>22%) for six systems that collectively comprise approximately 4/10 of the ice-sheet area and 3/10 of the accumulation.
Verification of a hydrometeorological model of glacierized basins
Authors: Kuhn Michael
Published: November 2007

A hydrometeorological model designed for climatological applications with monthly time-steps and 100 m altitude intervals was calibrated with runoff and climate data on five small, glacierized basins in the Silvretta mountains, Austria (46°55’ N, 10° 10’ E). These basins are exposed to similar synoptic weather conditions but differ in their altitude ranges and glacier covers, enabling the model to be tested for a variety of physical conditions. The structure of the model, the data input and the calibration and tuning procedures are described. Sensitivities of the model to temperature and precipitation changes are derived and applied to annual and decadal values of T and P, respectively. Judged by the difference between observed and calculated sums of runoff, simulation of decadal values is satisfactory, while annual simulations and measurements have rms differences of the order of 200 mm.
Alpine Younger Dryas glaciers as palaeo-precipitation gauges
Authors: Kerschner Hanns , Kaser Georg , Sailer Rudolf
Published: November 2007

Moraines of the Younger Dryas ˚Egesen Stadial", which are widespread features in the Alps, are a valuable terrestrial data source for quantitative palaeoclimatic studies. The depression of the early Younger Dryas (Egesen-I) equilibrium-line altitude (ELA) shows a distinct spatial pattern. It was greatest (about –450 to –500 m vs present day) m areas exposed towards the west and northwest. In the central, more sheltered valleys it was on the order of –300 m or less. Summer temperature depression, which can be derived from the Younger Dryas timberline depression, was on the order of –3.5 K. The stochastic glacier-climate model of Ohmura and others (1992), which relates summer temperature and precipitation at the ELA, is used to infer precipitation change. Results are compared with those obtained from the glacial-meteorological approach of Kuhn (1981a). The two models produce highly similar results. During the early Younger Dryas, climate in the central valleys of the Alps seems to have been considerably drier than today In areas open to the west and northwest, precipitation seems to have been the same as today or even slightly higher. These results, which are based on a rather dense network of data points, agree well with results from permafrost-climate studies and the more qualitative information from palaeobotanical research. They also support the results from atmospheric general circulation models for the Younger Dryas in Europe, which point towards a more zonal type of circulation.
Characteristics of summer energy balance on the west coast of the Antarctic Peninsula
Authors: Braun Matthias , Schneider Christoph
Published: November 2007

The characteristics of summer energy budgets in the ablation zones during the summer at the surface of two glaciers in the Antarctic Peninsula are investigated and compared to the findings of previous studies. The study areas are located on King George Island (62° S) and in Marguerite Bay (68° S). The summer energy balance was computed from automatic weather station data. The results reveal that turbulent heat fluxes dominate over radiation balance in Marguerite Bay whereas on King George Island ablation is driven by net radiation. The summer energy balance on King George Island reflects the more maritime subpolar climate along the northwest tip of the peninsula in contrast to a more continentally toned polar climate in Marguerite Bay and areas further south. The terms of the energy balances are partitioned very differently but ad-vection from northerly directions causes the highest summer snowmelt rates at both study sites. It is concluded that sensitivity studies should consider not only the mean air-temperature increase, but also possible changes in other climate parameters.
In situ stress tensor measured in an Alaskan glacier
Authors: Pfeffer W. T. , Humphrey N. F. , Amadei B. , Harper J. , Wegmann J.
Published: November 2007

An experimental program at Worthington Glacier, Alaska, U.S.A., has yielded the first in situ measurement of the full stress tensor in glacier ice. Measurements were made with an array of stiff (low-compliance) normal-force sensors frozen into a borehole at 120 m depth. Freezing in temperate ice was accomplished by a down-hole heat exchanger which extracted heat at a rate of 15 W. Under slowly varying stress conditions, relaxation of stress anomalies by viscous creep following drilling of the hole and installation of the sensors allows for equilibration of measured stresses with far-field stresses. Equilibration of local and far-field stresses was confirmed and pressure sensors calibrated in laboratory experiments prior to the field program. Results of the stress measurements show principal axes of the stress tensor oriented in directions consistent with the geometry of the glacier and broadly consistent with measured englacial strain rate. The magnitudes of stress-tensor components are more error-prone and more sensitive to uncertainty in sensor magnitude than uncertainty in sensor orientation. Mean stress determined by pressure measurements agrees with estimated lithostatic overburden to within approximately 15%. Unexpected results include a stress perturbation lasting about 5 days that caused a rotation of the orientations of the principal stress axes of approximately 5° about an axis pointing in the down-flow direction.
Simulation of microwave emission from physically modeled snowpacks
Authors: Wiesmann Andreas , Fierz Charles , Mätzler Christian
Published: November 2007

Detailed knowledge of snowpack properties is crucial for the interpretation and modeling of thermal microwave radiation. Here we use two well-known snow models, Crocus and SNTHERM, to obtain snow profiles from meteorological data. These profiles are compared with pit profiles and used as input to the Microwave Emission Model of Layered Snowpacks (MEMLS) for the simulation of microwave radiation. The snow-profile data can be applied almost directly. Adaptation is needed only in the conversion of the grain-size used in the snow models to the correlation length used in the emission model; it is based on empirical fits. The resulting emissivities are compared with in situ microwave measurements. The computed snow depths are in good agreement with observations. Comparison of selected profiles shows that Crocus is in good agreement with the pit profile, but the density of simulated melt-freeze crusts is underestimated. The SNTHERM profiles show no such crusts, and the density deviates from the pit profiles. The computed temporal behavior of the snowpack emissivity is reasonable. Comparison of selected situations with in situ measurements indicates good agreement. However, the polarization difference tends to be underestimated because of inaccuracies in the simulation of density profiles. The results show the potential of combined snow-physical and microwave-emission models for understanding snow signatures and for developing snow algorithms for microwave remote sensing. Based on the frequency-selective penetration and on the high sensitivity to snow texture, density and wetness, microwave radiometry can offer a new dimension to snow physics. Potential applications are described.
Evidence for deep icequakes in an Alpine glacier
Authors: Deichmann N. , Ansorge J. , Scherbaum F. , Aschwanden A. , Bernard F. , Gudmundsson G. H.
Published: November 2007

To obtain more reliable information about the focal-depth distribution of icequakes, in April 1997 we operated an array of seven portable digital seismographs on Unteraargletscher, central Swiss Alps. Over 5000 events were detected by at least two instruments during the 9 day recording period. P-wave velocities (3770 m f) were determined from several calibration shots detonated at the glacier surface as well as in a 49 m deep borehole, whereas S-wave velocities (1860 ms–1) were derived from a simultaneous inversion for Vp/Vs6 applied to 169 icequakes. So far, hypocentral locations have been calculated for over 300 icequakes. Besides confirming the occurrence of shallow events associated with the opening of crevasses, our results show that a small but significant fraction of the hypocenters are located at or near the glacier bed. One event was found at an intermediate depth of about 120 m. Three-dimensional particle-motion diagrams of both explosions and icequakes clearly demonstrate that all vertical component seismograms from shallow sources are dominated by the Rayleigh wave. On the other hand, for events occurring at depths greater than about 40 m, the Rayleigh wave disappears almost entirely. Therefore, a qualitative analysis of the signal character provides direct information on the focal depth of an event and was used as an independent check of the locations obtained from traditional arrival-time inversions. Thus, our results demonstrate that deep icequakes do occur and that simple rheological models, according to which brittle deformation is restricted to the uppermost part of a glacier, may need revision.
Structural glaciology of the fast-moving Jakobshavn Isbræ, Greenland, compared to the surging Bering Glacier, Alaska, U.S.A.
Authors: Mayer Helmut , Herzfeld Ute Christina
Published: November 2007

Crevasse patterns revealing mostly brittle deformation on glacier surfaces are analyzed based on video images collected during systematic overflights of Jakobshavn Isbræ, West Greenland, the Earths continuously fastest moving ice stream, in 1996 and 1997. Crevasse patterns on the surface of the central ice stream are distinct. All crevasses are closed, the surface appears rather smooth. Towards the margins, typical shear patterns with conjugate shears and still-closed crevasses prevail, curved patterns indicate the bending of crevasse lines into the flow direction. Outward from this zone different patterns of open crevasses occur. This suite of patterns is compared to similar data collected over Bering Glacier, Alaska, U.S.A., during its recent surge from 1993–95. There a number of patterns of mostly open crevasses is characteristic: parallel crevasses, two-directional orthogonal open crevasses, arrays of wavy crevasses, en-echelon crevasses. These patterns of the surging glacier are completely different from those of the fast-moving ice stream indicating different underlying kinematics and dynamics.
Modelling self-organization in ice streams
Authors: Hulton N. R. J. , Mineter M. J.
Published: November 2007

The EISMINT II experiments revealed the tendency for idealized model ice sheets to produce spatially variable flow under certain uniform thermal, mass-balance and topographic boundary conditions. Warm, fast-flowing streams with enhanced creep were separated by zones of colder, slower flow. Similar but different spatial patterns of differentiated flow were produced by all authors. We present further experiments that explore the formation and function of such ice streams at higher modelled resolutions. These are explored by the use of flat, but stochastically rough (10 m amplitude) beds, idealized, parallel-sided model ice sheets and models of finer (12.5 and 5 km) resolutions. Ice streams self-organize irregularly, but with consistent typical spacings which vary with thermal and miss-balance boundary conditions. More radial features are produced at finer scales indicating a dependency on the grid resolution used although this is not linear; at finer resolutions streams occupy increasingly more gridcells. This variation in scale may be related to the finer resolution of the warm/cold streaming/non-streaming boundary. The numerical solution of the thermodynamic ice equation is also highly sensitive to the orthogonality of the model grid. A major deficiency is that the numerical solution appears to fail where the flow is parallel to the grid axes, suggesting that artificial diffusion in the numerical scheme helps to smooth streams lying across the axes directions. The inclusion of sliding produces fewer, more concentrated, flow features, but these also display a level of scale-dependent organization. The spatial arrangement of such streams adjusts in response to the global mass flux of the ice sheet between "warm" and "cold" flow end-member. The results point to a mechanism in which ice sheets respond to climate by altering the large-scale arrangement of their flow patterns.
Comparison of finite-element and homogenization methods for modelling the viscoplastic behaviour of a S2–columnar-ice polycrystal
Authors: Meyssonnier Jacques , Philip Armelle
Published: November 2007

The main homogenization schemes used to model the behaviour of polycrystalline ice are assessed by studying the particular case of a two-dimensional polycrystal which represents natural S2–columnar ice. The results of the uniform-stress, uniform-strain-rate and one-site self-consistent models are compared to finite-element computations. The comparisons were made using the same model of grain, described as a continuous transversely isotropic medium, in the linear and non-linear cases. The uniform-stress and uniform-strain-rate models provide upper and lower bounds for the macroscopic fluidity which are too far from each other to be useful when a degree of anisotropy relevant to ice is considered. Although the self-consistent model gives a weak representation of the interaction between a grain and its surroundings, due to the strong anisotropy of the ice crystal, the resulting macroscopic behaviour is found to be acceptable when compared to the results from finite-element computations.
Intercomparison of subglacial sediment-deformation models: application to the Late Weichselian western Barents margin
Authors: Howell Daniel , Siegert Martin J.
Published: November 2007

Numerical experiments, where a simple ice-sheet model was coupled with sediment-deformation models, were performed to investigate the transport of glacigenic material to the western Barents Shelf during the Late Weichselian. The ice-sheet model, and its environmental inputs, has been matched previously with a series of geological datasets relating to the maximum extent of the ice sheet (Howell and others, http://www.ggg.qub.ac.uk [rp05/1999]). Additional geological data on the volumes of sediment delivered to the Bear Island fan (Barents continental margin) are available for comparison. The experiments indicate the sensitivity of sediment transport and deposition to variations in (a) the ice-stream model and (b) a variety of model parameters. Two ice-stream models were used: (1) a height-above-buoyancy model, in which basal velocity is controlled by basal driving stress and a buoyancy-induced reduction in the normal load beneath a marine-based ice sheet; and (2) a modified version of the method presented by Alley (1990) in which basal velocity is related to pore-water pressure, sediment thickness, and driving basal stress. The results of the two different models were then compared. An extensive set of sensitivity tests was carried out to determine sediment-transport response to changes in the model’s parameters. Results indicate that, using physically realistic parameters for deforming subglacial sediment, both models reproduce the volume of Late Weichselian sediment measured on the Bear Island fan. Results from both models are sensitive to (1) cohesion of the sediment and (2) the thickness of deforming sediment beneath the ice sheet. The two models exhibited different degrees of sensitivity to the sediment parameters, with the height-above-buoyancy model proving to be less sensitive to variations in the thickness of the deforming sediment layer than the model proposed by Alley (1990). The differences between the two models examined here highlight the need for a comprehensive comparison of all the methodologies for calculating basal-ice motion currently in use.
Simulation of anisotropic ice flow and fabric evolution along the GRIP–GISP2 flowline, central Greenland
Authors: Gagliardini Olivier , Meyssonnier Jacques
Published: November 2007

A model for the anisotropic behaviour of polar ice and the evolution of its strain-induced anisotropy is applied to the flowline between GRIP and GISP2. The linear constitutive law for polycrystalline ice is derived from the uniform-stress model under the assumptions of a linear transversely isotropic behaviour of the ice single crystal and of an orthotropic fabric described by a continuous parameterized orientation-distribution function (ODF). The velocities and fabrics obtained for the observed flowline and bedrock geometry assuming stationary flow, are compared to those measured at the ice-sheet surface and down the two boreholes.
Surface elevation, ice thickness, and subglacial-bedrock topography of Ekström Ice Shelf (Antarctica) and its catchment area
Authors: Sandhäger H. , Blindow N.
Published: November 2007

Ekström Ice Shelf and its catchment area form a comparatively small (∼29 000 km2) drainage system in northern Dronning Maud Land, Antarctica. Aerial-altimetry and radio-echo-sounding data of this region have been used to derive detailed maps of ice-surface and bedrock topographies and ice thickness. With the new database the volumes of the floating and grounded ice in the drainage system are calculated to be ∼3200 km3 and ∼16 000 km3, respectively. This corresponds to a total ice mass of ∼17 000 Gt. Four significant graben-like depressions in the bedrock topography have been identified, which incline from inland towards the grounding line and are up to ∼16 km wide there. These structures coincide with the particular zones of concentrated ice flux into the ice shelf. The total mean annual mass discharge over the grounding line of the larger western part and the smaller eastern part of Ekström Ice Shelf is estimated to be about 3.7 Gt and 0.4 Gt, respectively. Both parts represent individual ice-shelf systems with different catchment areas, geometric characteristics and flow regimes.
Ice-age ice-sheet rheology: constraints from the Last Glacial Maximum form of the Laurentide ice sheet
Authors: Peltier W. Richard , Goldsby David L. , Kohlstedt David L. , Tarasov Lev
Published: November 2007

State-of-the-art thermomechanical models of the modern Greenland ice sheet and the ancient Laurentide ice sheet that covered Canada at the Last Glacial Maximum (LGM) are not able to explain simultaneously the observed forms of these cryospheric structures when the same, anisotropy-enhanced, version of the conventional Glen flow law is employed to describe their rheology. The LGM Laurentide ice sheet, predicted to develop in response to orbital climate forcing, is such that the ratio of its thickness to its horizontal extent is extremely large compared to the aspect ratio inferred on the basis of surface-geomorphological and solid-earth-geophysical constraints. We show that if the Glen flow law representation of the rheology is replaced with a new rheology based upon very high quality laboratory measurements of the stress-strain-rate relation then the aspect ratios of both the modern Greenland ice sheet and the Laurentide ice sheet, that existed at the LGM, are simultaneously explained with little or no retuning of the flow law.
Ice-stream-ice-shelf transition: theoretical analysis of two-dimensional flow
Authors: Wilchinsky Alexander V. , Chugunov Vladimir A.
Published: November 2007

Two-dimensional steady isothermal flow of a marine ice stream is studied. Gases of different relations between shear stress and longitudinal deviatoric stress in the ice stream are considered. Analysis of the ice-stream-ice-shelf transition zone shows that even if the longitudinal stress deviator in the ice stream is much larger than the shear stress (as it is in the ice shelf), the ice-stream-ice-shelf transition zone if singular and the full system of Stokes equations must be solved in it. Scales of fields in the transition zone and the relation between the ice thickness and the horizontal mass flux at the grounding line are found.
The simulation of Antarctic sea ice in the Hadley Centre Climate Model (HadCM3)
Authors: Turner John , Connolley William , Cresswell Doug , Harangozo Steven
Published: November 2007

An assessment is presented of the extent and variability of Antarctic sea ice in the non-flux-corrected version of the Hadley Centre’s coupled atmosphere-ocean general circulation model (HadCM3). The results are based on a 100 year segment of a long control run of the model with the sea ice being compared to ice extents and concentrations derived from passive microwave satellite data. Over the year as a whole, the model ice extent (the area with >15% ice concentration) is 91% of that determined from satellite imagery, but, not surprisingly, the regional-scale distribution differs from the observed. Throughout the year there is too much ice near 90° E, which is believed to be present as a result of incorrect ocean currents near Kerguelen. In contrast to the satellite data, there is too little ice to the west of the Antarctic Peninsula as a result of anomalously northerly atmospheric flow, compared to observations. During the winter the sea-ice concentrations in the model are too high, possibly as a result of the simple representation of the sea ice, which does not simulate complex dynamical interactions within the pack. The annual cycle of sea-ice advance/retreat in the model has a phase error, with the winter sea-ice maximum extent being too late by about 1 month.
The role of sea ice in the fresh-water budget of the Weddell Sea, Antarctica
Authors: Timmermann R. , Beckmann A. , Hellmer H. H.
Published: November 2007

A coupled sea-ice-ocean model of the Weddell Sea, Antarctica, has been developed as part of the Bremerhaven Regional Ice-Ocean Simulations (BRIOS) project. It is based on the s-Coordinate Primitive Equation ocean Model (SPEM) and a dynamic-thermodynamic sea-ice model with viscous-plastic rheology which also provides the thermohaline forcing at the base of the Antarctic ice shelves. Model runs are forced with wind, cloudiness, temperature and precipitation fields of the European Centre for Medium-range Weather Forecasts and U.S. National Centers for Environmental Prediction re-analyses. Model results show good agreement with observations of ice extent, thickness and drift. Water-mass properties and the large-scale circulation are in good agreement with observations. Fresh-water fluxes from sea-ice formation as well as from ice-shelf basal melting and from precipitation are computed and compiled to the fresh-water budget of the Weddell Sea. Supporting estimates based on hydrographic observations, model results indicate that fresh-water loss due to sea-ice formation and export (34mSv) is roughly balanced by ice-shelf basal melting (9 mSv) and net precipitation (19 mSv). Furthermore, sea-ice formation appears to be a necessary condition for bottom-water production in the Weddell Sea.
Spatial variability of the sea-ice radiation budget and its effect on aggregate-area fluxes
Authors: Wang Xuanji , Key Jeffrey R.
Published: November 2007

The spatial and temporal variability of surface, cloud and radiative properties of sea ice are examined using new satellite-derived products. Downwelling short- and longwave fluxes exhibit temporal correlation over about 180 days, but cloud optical depth and cloud fraction show almost no correlation over time. The spatial variance of surface properties is shown to increase much less rapidly than that of cloud properties. The effect of small-scale inhomogeneity in surface and cloud properties on the calculation of radiative fluxes at ice- and climate-model gridscales is also investigated. Annual mean differences between gridcell fluxes computed from average surface and cloud properties and averages of pixel-by-pixel fluxes are 9.46% for the downwelling shortwave flux and −7.04% for the longwave flux. Therefore, using mean surface and cloud properties to compute surface radiative fluxes in a gridcell results in an overestimate of the shortwave flux and an underestimate of the longwave flux. Model sensitivity studies show that such biases may result in substantial errors in modeled ice thickness. Clearly, the sub-gridscale inhomogeneity of surface and atmospheric properties must be considered when estimating aggregate-area fluxes in sea-ice and climate models.
Summer ice dynamics during SHEBA and its effect on the ocean heat content
Authors: Richter-Menge Jackie A. , Perovich Donald K. , Pegau W. Scott
Published: November 2007

Winter ice dynamics plays an important role in the energy budget of the air-ice-ocean system, through the formation of leads and ridges. In summer, thermodynamic processes cause a transition in the ice pack from a mechanical continuum to an ensemble of floes that move in a state of free drift, with little floe-floe interaction. Results from the recent Surface Heat Budget of the Arctic Ocean (SHEBA) experiment have demonstrated that even under summer conditions, ice dynamics can still cause dramatic changes in the characteristics of the ice-ocean matrix that affect the energy budget. To illustrate this, we present observations taken before and after a period of sustained, moderate winds in late July 1998, which was preceded by an extended period of low winds. These conditions resulted in significant differential motion of ice floes in the vicinity of SHEBA. The measurements include the mass balance of the ice cover, the distribution of ice and open water, and salinity and temperature profiles in leads. The data show that after the storm there was a significant change in the amount and distribution of open-water areas, that there was an increase in the rate of bottom ablation, and that a stratified layer of warm fresh water that had formed at the top of leads during melt had become mixed.
The seasonal cycle of ERS scatterometer signatures over perennial Antarctic sea ice and associated surface ice properties and processes
Authors: Haas Christian
Published: November 2007

Time series of satellite radar backscatter coefficients of the European Remote-sensing Satellite (ERS-1/-2) scatterometer are presented for perennial sea-ice regions in the Antarctic from June 1991 to June 1999. There is a pronounced seasonal cycle, with higher backscatter in summer than in winter. On average, backscatter increases from spring values of −16.3 dB to summer values of −10.7 dB within 96 days. This rapid rise and the summer maximum are occasionally interrupted by sudden strong signal drops. After late summer/early autumn, backscatter decreases again and slowly approaches winter values. The seasonal cycle is interpreted in terms of processes at the ice surface. The spring backscatter rise is associated with internal snowmelt and the formation of superimposed ice. This process commences once the surface energy balance becomes positive. Sudden backscatter drops are caused by temporary meltwater saturation of the snow during episodic events of warm-air advection. In the autumn, when superimposed-ice formation ceases due to surface cooling, gradual surface flooding with sea water becomes the dominant process, causing the observed decreasing backscatter. The reoccurrence of the seasonal backscatter cycle in most regions points to the widespread formation of superimposed ice on perennial Antarctic sea ice.
A comparison of East Antarctic sea-ice motion derived using drifting buoys and remote sensing
Authors: Heil P. , Fowler C.W. , Maslanik J. A. , Emery W. J. , Allison I.
Published: November 2007

Results of East Antarctic (20−160° E) sea-ice motion derived from drifting buoys (collected in eight selected years between 1985 and 1997) are compared with sea-ice motion derived from sequential images from the 37 and 85.5 GHz channels of the Scanning Multichannel Microwave Radiometer (SMMR, 1979−87) and the Special Sensor Microwave/Imager (SSM/I, 1987−97). For the latitudes of interest, buoys yield velocity fields at 2 hourly resolution/while SMMR and SSM/I provide daily data. The spatial coverage of the drifting buoys is relatively sparse compared to the complete coverage of the southern sea-ice zone by SMMR and SSM/I, but the accuracy of the buoy position is better than ±280 m, while the resolution for the 85.5 GHz channel is only about 12.5 km, and about 25 km for the 37 GHz channel. The buoy measurements represent point motion, equal to the velocity of an individual ice floe, while the satellite data derived via the maximum cross-correlation method represent the motion of surface patterns within a gridcell. Comparison of the mean distributions of daily velocity derived from the datasets shows that while the broad-scale velocity patterns agree, the magnitude of the satellite-derived sea-ice velocities is significantly lower (typically by 40% or less) than the velocities derived from buoy measurements. This is in contrast with previous studies for the Arctic Ocean and the Weddell Sea, Antarctica. To elucidate differences in the velocity magnitude, individual trajectories from a number of drift regimes off East Antarctica are directly compared. Discrepancies between the two datasets are discussed with regard to the difference in spatial resolution of the data, the data-collection and −processing methods and the observed region.
Modelling the variability of the sea-ice conditions in the Baltic Sea under different climate conditions
Authors: Haapala Jari , Juottonen Arja , Marnela Marika , Leppäranta Matti , Tuomenvirta Heikki
Published: November 2007

The present and future ice conditions around 100 years from present in the Baltic Sea are examined by two 10 year integrations of an ice-ocean model. Results from atmospheric climate-model simulations constitute the atmospheric forcing, one representing present climate conditions (control simulation), and the other global warming due to CO2 doubling (scenario simulation). The present-day climatological ice conditions and the interannual variability were realistically reproduced by the ice-ocean model. The modelled range of the maximum annual ice extent in the Baltic was 190−420 × 103km2 in the control simulation and 50−270 × 103km2 in the scenario simulation. The range of the annual maximum level-ice thickness was 45−85 and 20−58 cm in the control and scenario simulations, respectively.
Lifetime estimation for a land-fast ice sheet subjected to ocean swell
Authors: Langhorne P. J. , Squire V. A. , Fox C. , Haskell T. G.
Published: November 2007

It is well known that an incoming ocean swell produces a strain field in a land-fast ice sheet. The attenuation and spectral content of this strain field can be calculated and has been measured. The response of the sea ice to this type of cyclic forcing has also been measured, and in particular we are able to estimate the number of cycles to failure for sea ice loaded at constant amplitude. In this paper we consider the response of the land-fast ice sheet or vast floe to a measured ice-coupled wave field of variable amplitude. We use the Palmgren-Miner cumulative damage law and stress-lifetime curves taken from field experiments to predict the lifetime of the sea-ice sheet as a function of significant wave height and sea-ice brine fraction. Calculations are performed to account for the swell entering a land-fast sea-ice sheet at arbitrary angle, and the influence of c-axis alignment and the presence of pre-existing cracks are discussed.
Spatial variations in the winter heat flux at SHEBA: estimates from snow-ice interface temperatures
Authors: Sturm Matthew , Holmgren Jon , Perovich Donald K.
Published: November 2007

The temperature of the snow-ice interface was measured every 2.4 h throughout winter 1997/98 at 30 locations near the Surface Heat Budget of the Arctic Ocean (SHEBA) camp in the Beaufort Sea. Measurements were obtained from young ice, ridges, refrozen melt ponds and ice hummocks. Average snow depths at these locations were 567 cm, while mean interface temperatures ranged from −8° to −25°C, with minimums varying from −12° to −39°C. Interface temperatures were linearly related to snow depth, with increasing scatter at greater depths. The conductive heat flux during the winter, Fc, was estimated for each location using air and interface temperatures, snow depths and measured snow thermal conductivities. Fc was integrated to determine total heat loss for the winter at each site. Losses varied by a factor of four, with variations over short distances (10 m) as large as the variations between ice floes. Spot measurements along traverse lines confirm that large variations in interface temperature are common, and imply that small-scale spatial variability in the conductive flux is widespread. A comparison of the dependence of Fc on snow depth and ice thickness based on our observations with the dependence predicted by a one-dimensional theoretical model suggests that spatial heterogeneity may be an important issue to consider when estimating the heat flux over large aggregate areas. We suggest that the small-scale variability in the conductive flux arises because the combined snow and ice geometry can produce significant horizontal conduction of heat.
The Laptev Sea flaw polynya, Russian Arctic: effects on the mesoscale hydrography
Authors: Dmitrenko I. , Hölemann J.A. , Tyshko K. , Churun V. , Kirillov S. , Kassens H.
Published: November 2007

A detailed analysis of hydrographic data from a period of 20 years (1980−99) has shown that the persistent presence of a flaw polynya influences mesoscale hydrography of the Laptev Sea, Russian Arctic. Based on these data, the interannual variability of surface water salinity within the polynya has been estimated. As the salinity increase in the surface water layer is mainly caused by the formation of new ice within the polynya, the average ice-production rate of the polynya was calculated. The results indicate an ice production of 3−4 m per season. A further aim of this study was to calculate the probability that the convective mixing in the polynya penetrates to the sea-floor. It is demonstrated that the probability is maximal in the flaw-polynya area, but does not exceed 20% in the eastern and 70% in the western part of the polynya as a result of strong vertical density stratification from river runoff, especially in the eastern Laptev Sea. Additional studies of water circulation in the marginal zone of the flaw polynya were carried out during field observations in April-May 1999. On the basis of conductivity-temperature-depth and current measurements we deduce that high current velocities (62 cm s-1) recorded in surface waters near the fast-ice edge are caused by a convectively driven circulation system under the polynya. Our measurements indicate that these high-velocity currents are part of a cellular circulation, which results from the rejection of brine during intensive ice formation in the polynya. The observed azimuthal alignment of the crystalline structure of sea ice is also, most probably, the consequence of this quasi-stationary cellular circulation.
Determining the floe-size distribution of East Antarctic sea ice from digital aerial photographs
Authors: Paget M.J. , Worby A. P. , Michael K. J.
Published: November 2007

Floe size within the Antarctic pack ice is an important parameter that affects both the ocean-ice-atmosphere energy exchange and the mechanical properties of pack ice. In this paper we present a computer-based algorithm to extract floe-size distribution information from digital aerial photographs of Antarctic sea ice. The algorithm segments digital images of sea ice into distinct floes from which the size of each floe is calculated and floe-size distributions are derived. Through a recursive application of the morphological erosion operator, each floe is eroded towards its centre while each pixel’s erosion number is recorded. This ensures no information is lost while individual floes are identified. The algorithm combines image-processing techniques with some manual input to accurately identify the boundaries of individual floes within an image. To demonstrate the functionality of the algorithm, six images, representing regions along a 22 km south-to-north transect through a transitional zone of pack ice near 65° S, 140° E, have been processed. Regional variations in the floe-size distributions show an increase in the number of smaller floes relative to larger floes and a decrease in the dominant floe size (in terms of areal coverage) from south to north. These results are consistent with ship-based observations.
Simulation of pancake-ice dynamics in a wave field
Authors: Hopkins Mark A. , Shen Hayley H.
Published: November 2007

From many field observations, it has become well known that pancake ice is ubiquitous in a wave-dominated sea. These strikingly uniform circular floes are consistently found in the Antarctic seas during the ice-formation season. Their presence has also been reported in the Bering Sea, the Greenland Sea and in polynyas and leads within pack ice. In one estimate made by P. Wadhams (personal communication, 1991) the areal coverage of pancake ice in the Southern Ocean marginal ice zone is around 6 × 106 km2. Pancake ice forms through a combination of thermodynamic growth and mechanical thickening, caused by rafting of floes that is driven by wave motion This complex growth process is much faster than pure thermodynamic growth and hence may be the main factor responsible for ice-edge advance in marginal ice zones. In this paper we simulate wave/ice-floe dynamics by combining a new three-dimensional discrete element model with wave hydrodynamics. In one set of simulations we calculate ice-drift speed as a function of wave amplitude and ice collisional dynamics. In a second set of simulations we place a non-reflecting vertical barrier meant to simulate the fast-ice edge. The simulation results show the rate of increase of the ice impact force on the barrier, and the accompanying thickening of the rafted region in front of the barrier, as a function of wave amplitude. Results from this study will help quantify the formation process for pancake-ice covers in a wave field.
The Arctic Ocean Circulation as simulated in a very high-resolution global ocean model (OCCAM)
Authors: Aksenov Ye. , Coward A.C.
Published: November 2007

To investigate the Arctic Ocean Circulation, results from a high-resolution fully global ocean model have been analyzed. The results come from two runs of the Ocean Circulation and Climate Advanced Modelling project (OCCAM) model, developed and run by the Southampton Oceanography Centre, at 1/4° × 1/4° and 1/8° × 1/8° resolution. The model is based on the Bryan-Semtner-Cox model and has 36 vertical levels. Enhancements include a free surface, an improved advection scheme and an improved treatment of the surface fresh-water flux. The model is forced with a monthly European Centre for Medium-range Weather Forecasts wind-stress climatology. It reproduces many of the fine-scale features found in the Arctic Ocean. The analysis concentrates on several of the key features, including the highly energetic eddy system in the western part of the Beaufort Sea, East Greenland West and Spitsbergen Currents and the detailed structure of the marginal currents along the Siberian and Canadian coasts. Much of the paper is focused on the water transport through the Bering and Fram Straits and through the Canadian Archipelago. Comparisons of the model net fluxes through the straits against observations are presented. The analyses of the results demonstrate the ability of the fine-resolution model to simulate features such as small-scale eddies and jets, which have some agreement with the limited observations available.
Biomass, production and microhabitat characteristics near the freeboard of ice floes in the Ross Sea, Antarctica, during the austral summer
Authors: Fritsen Christian H. , Coale Susan L. , Neenan Diann R. , Gibson Angela H. , Garrison David L.
Published: November 2007

The physical, chemical and biological characteristics of surface and freeboard habitats in the summer pack ice in the eastern Ross Sea, Antarctica, were documented in a continuing effort to determine the factors controlling the distribution, production and succession of sea-ice biota. Three longitudinal transects from approximately 65° to 74° S in the western Ross Sea along 135°, 150° and 165° W were visited where samples of slush and slush interstitial water from surface and freeboard habitats as well as sea water were collected at every degree of latitude. Freeboard and surface habitats, found at all stations in the pack ice, contained a large range (five orders of magnitude) of microalgal biomass (measured as chlorophyll a concentrations) and nutrients ranging from below levels of detection to those of the surrounding sea water. The geophysical attributes of the freeboard habitat (i.e. a layer of semi-consolidated ice overlying a layer containing unconsolidated ice crystals and sea water) are consistent with previous descriptions of this environment. However, additional information is presented on the range of biomass concentrations as well as the small-scale distributions of the habitat and biota.