bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental investigation of optical snow properties
Authors: Sergent Claude , Pougatch Evelyne , Sudul Marcel , Bourdelles Barbara
Published: September 2017

The authors have developed an experimental device in a cold laboratory with the purpose of measuring optical parameters of natural snow depending on grain-size, impurity content and density. Snow samples were prepared from homogeneous layers in order to measure the radiative properties of clearly identified snow types. The first part of this paper describes the working assumptions and the experimental device. In the second part, experiment results are described and discussed. We have compared albedo measurements of different natural snow types with theoretical values derived from physical optics, based on Mie scattering. The albedo evolution of three different snow types submitted to temperature-gradient metamorphism is analyzed.
Modelling the radiation budget of alpine snowfields with remotely sensed data: model formulation and validation
Authors: Duguay Claude R.
Published: September 2017

A model for estimating the radiation balance of alpine snowfields is presented. Shortwave and longwave downward flux densities are computed for sloping surfaces using a modified version of the two-stream radiative transfer scheme of Zdunkowski and others (1982). Surface albedo and thermal exitance values are estimated using Landsat-5 Thematic Mapper (TM) imagery and digital terrain data. The LOWTRAN7 radiative transfer code is utilized in order to remove atmospheric effects in satellite imagery as well as calculating solar irradiance within TM spectral bands, for the determination of the near-nadir reflectance of snow. Under a Lambertian assumption, near-nadir reflectance measurements obtained from a few TM bands are used to calculate the total hemispherical reflectance (albedo) of snow. The net all-wave radiation of snowfields is then simulated for the complete day on the same date as that of the Landsat overflight. The model is tested using Landsat TM data acquired in late June 1984, and results compared with field measurements acquired on Niwot Ridge, Colorado, U.S.A. Preliminary results are very encouraging but problems remain in the estimation of surface albedo from near-nadir satellite reflectance measurements of TM. These problems are discussed and recommendations for future model improvements are given.
Stereological determination of dry-snow parameters for discrete-scatterer microwave modeling
Authors: Shi Jiancheng , Davis Robert E. , Dozier Jeff
Published: September 2017

Modeling microwave back-scattering and emission from snow-packs requires the knowledge of snowpack characteristics and their dynamics to select an appropriate model. Both theory and field data show that microwave back-scattering coefficients and brightness temperatures are sensitive to parameters describing snow-microstructure. Stereological methods and other techniques can be applied to images of sections cut from undisturbed snow, and are used to obtain accurate and unbiased estimates of snow-microstructure parameters for discrete scatterer modeling. Assuming that the ice particle-size distribution can be characterized as a log-normal distribution function, we show that the parameters describing the distribution can be obtained from section images. The results show that, in addition to snow density and ice-particle size, the particle-size variation has great effect on dry-snow extinction properties. The optically equivalent ice-particle size for Rayleigh scattering in a snowpack with grain-size variations can be determined from the stereological measurements from snow sections.
Interpretation of basal ice conditions from radio-echo soundings in the eastern Heimefrontfjella and the southern Vestfjella mountain ranges, East Antarctica
Authors: Holmlund Per
Published: September 2017

Differences in radio-echoes received from the base of the ice sheet covering two different mountain areas in Dronning Maud Land, East Antarctica, are herein interpreted as differences in the physical properties in the ice and differences in basal conditions. An abnormally strong signal can be caused either by unusually little absorption of radio waves within the ice or by water at the base of the glacier.

One of the areas studied is totally covered by a 400–1400 m thick ice sheet. The altitude of the surface is about 2800 m a.s.l. The second area is a nunatak area situated relatively closer to the coast. The ice thickness in the valleys is about 500 m and the altitude of the surface is about 400 m.

The annual mean surface temperature for the areas is about −30° and −17°C respectively. In both cases ice depths are moderate and flow rates low, and it is thus probable that the ice is frozen to the bed.

However, in the colder locality I believe the strong echo is caused by low or negligible rate of mass flux. In the lower area strong bottom echoes are believed to be a cause of negative mass balance at the surface of the blue ice areas. The negative surface mass balance reverses the vertical component of ice flow and it also changes the temperature distribution within the ice. Basal melting conditions may thus occur at locations where the ablation rate is high. This process may also have important implications on the development of land forms under cold ice sheets.
Fast recession of the northern Larsen Ice Shelf monitored by space images
Authors: Skvarca Pedro
Published: September 2017

The rapid retreat and disintegration of the Larsen Ice Shelf sector extending north of Seal Nunataks (65° S), documented from the mid 1970s onwards by remote sensing, is presented and related to the Antarctic Peninsula climatic warming recorded over several past decades. A 1975 KOSMOS satellite photograph and a series of LANDSAT MSS and TM images taken in 1978, 1979, 1986, 1988 and 1989 were used to monitor the retreat of the ice shelf between Seal Nunataks and Prince Gustav Channel. The ice shelf has decreased by more than 30% during the period 1975–89 within the Christensen Island to Cape Longing region. Measurements of the ice front position carried out in the field during late 1991 indicate that the recession between Lindenberg Island and Sobral Peninsula is still continuing, in some places at a rate of up to 2.5 km a−1.
Interannual variations in snow melt over Arctic sea ice and relationships to atmospheric forcings
Authors: Serreze Mark G. , Maslanik James A. , Scharfen Gregory R. , Barry Roger G. , Robinson David A.
Published: September 2017

Spring snow melt over the Beaufort and East Siberian seas is examined using visible-band DMSP imagery, SMMR brightness temperatures, and surface air temperatures for the years 1979, 1980 and 1984–86. Regional melt onset, as identified from the SMMR data, typically begins in early June, but can vary by up to two weeks between years. The subsequent pace of melt, estimated from the difference in days between the SMMR signal and visual identification of melt features in the DMSP data, exhibits a similar range. Interannual differences in snow melt are examined with respect to variations in cloud cover and geostrophic meridional winds. While over the Beaufort Sea, early melt onset and rapid melt progression may be favored by a combination of limited cloud cover and, as inferred from the wind data, strong northward air advection, no firm conclusions can be drawn from the available data. Presumably, potential relationships between melt and atmospheric forcings tend to be masked by additional factors, such as variations in initial snow depth and uncertainties in the data sets.
Active and passive microwave signatures of Antarctic firn by means of field measurements and satellite data
Authors: Rott Helmut , Sturm Klaus , Miller Heinz
Published: September 2017

The angular dependence and polarization behaviour of back-scattering and emission of polar firn at 5.2 GHz and 10.3 GHz were measured during an oversnow traverse in Dronning Maud Land, Antarctica. The signatures emphasize the importance of snow stratification in the interpretation of microwave remote sensing measurements. Highest backscattering coefficients and little angular variations were observed for refrozen firn near the coast. In permanently dry snow, areas with high accumulation rates and homogeneous snow morphology showed low backscattering coefficients and high emissivities. Pronounced layering and related density variations in low accumulation zones resulted in increased polarization differences of brightness temperatures and increased like-polarized backscattering coefficients. This behaviour is confirmed by the analysis of C-band scatterometer measurements of the Active Microwave Instrument aboard the European Space Agency's ERS-1.
Comparison of SSM/I ice-concentration algorithms for the Weddell Sea
Authors: Burns Barbara A.
Published: September 2017

Four different algorithms for retrieving ice concentration from passive microwave imagery are applied to SSM/I data collected over the Weddell Sea in September 1989. Comparison of the results along a typical transect from the ice edge to the coast shows point-wise differences of up to 45% in ice concentration. The observed differences can largely be explained by the different combinations of the 19 and 37 GHz, horizontal and vertical polarization, data channels utilized by each algorithm. Through their frequency/polarization signatures, and with the aid of coincident surface observations, the differences are further related to ice type, specifically the presence of grease ice or thin ice, and surface conditions.
A thermomechanical model of ice-shelf flow
Authors: Rommelaere Vincent , Ritz Catherine
Published: September 2017

An ice-shelf model which features efficient numerical techniques is developed to determine the back-force exerted by sides and pinning points, such as islands of an embayed ice shelf. The model is applied to three ideal geometries and shows that the restraint exerted by a small island, even far downstream from the grounding line, can represent about one-half of the total restraint due to the embayment. Our results are further interpreted to suggest several criteria useful for testing any ice-shelf model.
A computer scheme for rapid calculations of balance-flux distributions
Authors: Budd W. F. , Warner R. C.
Published: September 2017

A simple computer scheme developed by Budd and Smith (1985) and modified by D. Jenssen has been further developed to provide a rapid computation of steady-state balance fluxes over arbitrary ice masses, given the surface elevations and net accumulation distribution. The scheme provides a powerful diagnostic tool to examine the flux and state of balance over whole ice masses or limited regions to interpret field observations for dynamics or the state of balance.

In many cases the uncertainty in the state of balance may be much less than the uncertainty in the deformation and sliding properties of the ice and so the flux and velocities derived from balance could provide a useful guide for the dynamics where direct observations are sparse.

The scheme assumes that, on a horizontal scale of many ice thicknesses, the ice-flow direction is approximately down the steepest surface slope. The continuity equation is used to compute steady-state implied downslope fluxes at each grid point from integrations of the net accumulation over the area from the summits to the edges. The algorithm ensures the exact integral balance of the surface net flux over the area with flow through boundaries.

Applications are demonstrated for the whole of Antarctica and for regional areas. Comparisons are made between fluxes computed from observed ice thicknesses and velocities and those computed from balance. The observed ice thicknesses can also be used to compute surface velocities from assumed column-to-surface velocity ratios. The combined fluxes from observations and balance can be used to compute rates of change of elevation with time.
Mass-balance modelling of the Greenland ice sheet: a comparison of an energy-balance and a degree-day model
Authors: van de Wal R. S. W.
Published: September 2017

A degree-day model and an energy-balance model for the Greenland ice sheet are compared. The two models are compared at a grid with 20 km spacing. Input for both models is elevation, latitude and accumulation. The models calculate the annual ablation over the entire ice sheet. Although on the whole the two models yield similar results, depending on the tuning of the models, regional discrepancies of up to 45% occur, especially for northern Greenland. The performance of the two types of model is evaluated by comparing the model results with the sparsely available (long-term) mass-balance measurements. Results show that the energy-balance model tends to predict a more accurate mass-balance gradient with elevation than does the degree-day model.

Since so little is known about the present-day climate of the ice sheet, it is more useful to consider the sensitivity of the ablation to various climate elements than to consider the actual present-day ablation. Results show that for a 1 K temperature perturbation, sea-level rise is 0.31 mm year−1 for the energy-balance model and 0.34 mm year−1 for the degree-day model. After tuning the degree-day model to a value of the ablation, equivalent to the ablation calculated by the energy-balance model, sensitivity of the degree-day model increases to 0.37 mm sea-level change per year. This means that the sensitivity of the degree-day model for a 1 K temperature perturbation is about 20% higher than the sensitivity of the energy-balance model. Another set of experiments shows that the sensitivity of the ablation is dependent on the magnitude of the temperature perturbation for the two models. Both models show an increasing sensitivity per degree for larger perturbations. The increase in the sensitivity is larger for the degree-day model than for the energy-balance model. The differences in the sensitivity are mainly concentrated in the southern parts of the ice sheet.

Experiments for the Bellagio temperature scenario. 0.3°C increase in temperature per decade, leads to sea-level rise of 4.4 cm over a period of 100 years for the energy-balance model. The degree-day model predicts for the same forcing a 5.8 cm rise which is about 32% higher than the result of the energy-balance model.
Models of ice-atmosphere interactions for the Greenland ice sheet
Authors: Braithwaite Roger J.
Published: September 2017

Simple models to calculate melt and refreezing are reviewed. Both degree-day and energy-balance models can give distributed melt inputs to ice-dynamics models but have only been tested extensively in West Greenland, and more data are needed from the remoter parts of Greenland. The energy-balance model is more realistic but needs input data that are not generally available over the whole ice sheet. On the other hand, degree-day factors vary from situation to situation although a value of about 8 kg m−2 d−1 deg−1 for ice ablation is a reasonable first approximation as assumed in recent ice-dynamics models. Meltwater refreezing in the accumulation area can be modelled very simply but more detailed physical models are needed to describe the shifts in accumulation zones under different climates.
Climate sensitivity of the ice cap of King George Island, South Shetland Islands, Antarctica
Authors: Knap Wouter H. , Oerlemans Johannes , Cabée Martin
Published: September 2017

A two-dimensional vertically integrated ice-flow model has been used to simulate the current state of the ice cap of King George Island, South Shetland Islands, Antarctica, as well as the sensitivity of this state to climate change. The model was forced by an energy-balance model that generates the specific mass balance from climatological input data of two research stations. It proved difficult to simulate-satisfactorily the entire geometry of the present-day ice cap. Nevertheless, it was possible to simulate a steady-state ice cap whose volume and areal extent approximate the (estimated) current situation. Several experiments have indicated that this state is highly sensitive to climate change. The model predicts that cooling by 1 K will increase the ice volume by 10% and warming by 1 K will decrease it by 36%. A 10% change in precipitation will alter the ice volume by less than 8%. Application of the IPCC-90 Business-as-Usual scenario leads to a 55% reduction in the ice volume by the year AD 2100, compared to the present-day situation. The response of the ice cap to warming is therefore totally different from the response of the main Antarctic ice sheet which is believed to gain mass by increasing temperatures.
On elevation models as input for mass-balance calculations of the Greenland ice sheet
Authors: van de Wal R. S. W. , Ekholm S.
Published: September 2017

In this paper the elevation model for the Greenland ice sheet based upon radio-echo-sounding flights of the Technical University of Denmark (TUD) (Letréguilly and others, 1991) are compared with the satellite-altimetry model (Tscherning and others, 1993) improved with airborne-laser and radar altimetry (IA model). Although the general hypsometry of both data sets is rather similar, differences seem to be large at individual points along the ice margin. Over the entire ice sheet, the difference between the IA model and the TUD model is 33 m with a root-mean-square error of 112 m. Differential GPS measurements collected in the ice-marginal zone near Søndre Strømfjord show that the IA model is more accurate than the TUD model. The latter data set underestimates the elevation by approximately 150 m in the ice-marginal zone near Søndre Strømfjord.

Calculation of the ablation with an energy-balance model and with a degree-day model points to a 20% decrease in the ablation if the IA model is used. Not only does this show the sensitivity of ablation calculations to the orographic input but it also indicates that the ablation calculated by the models used nowadays is relatively overestimated.
Present and future mass balance of the ice sheets simulated with GCM
Authors: Ohmura Atsumu , Wild Martin , Bengtsson Lennart
Published: September 2017

A high-resolution GCM ECHAM3 T106 was used to simulate the climates of the present and of the future under doubled CO2 The ECHAM3 T106 was integrated for an equivalent time of 5 years (1) with the observed SST of the 1980s and (2) with the SST for the 2 × CO2 climate generated from the ECHAM1 T21 coupled transient experiment. The main motivation for using the GCM to simulate the mass balance is the level of skill in simulating precipitation and accumulation recently achieved in the high-resolution GCM experiment. The ablation is computed, based on the GCM internal surface fluxes and the temperature/ablation relationship formulated on the Greenland field data. The two ice sheets show very different reactions towards doubling the CO2. As the decrease in accumulation and the increase in ablation in Greenland cause an annual mean specific mass balance of −225 mm (eq. −390 km3), the increase in accumulation and virtually non-melt conditions in Antarctica result in a mean annual specific mass balance of + 23 mm (eq. + 325 km3). The sum of the mass balance on both ice sheets is equivalent to the annual sea-level rise of 0.2 mm. This experiment shows that other mechanisms for sea-level change, such as the thermal expansion of the sea water and the melt of small mountain glaciers, will remain important in the coming century.
Modelling fabric development along the GRIP ice core, central Greenland
Authors: Castelnau O. , Thorsteinsson Th. , Kipfstuhl J. , Duval P. , Canova G. R.
Published: September 2017

The preferred c-axes orientation (fabric) observed in cold polar ice is induced by intracrystalline slip only when the grain-boundary migration rate is low enough (i.e. corresponding to grain growth or rotation recrystallization regimes). Fabrics reflect the entire thermomechanical history of the ice and strongly influence its mechanical behaviour. Large viscoplastic anisotropy is always associated with pronounced fabrics. We use a viscoplastic self-consistent (VPSC) polycrystal deformation model to calculate fabric development. In this model, stress- and strain-rate fields are not uniform within the polycrystal and both equilibrium and compatibility conditions are fulfilled. We compare fabrics measured on thin sections along the GRIP ice core (central Greenland) with those calculated down to a depth of 2800 m. Behaviours predicted by uniform stress and uniform strain bounds are presented for comparison. Predictions of the VPSC model are in close agreement with measurements within the upper 650 m, which corresponds to the entire grain-growth zone. Deeper clown, the simulated fabric strength appears to be too high. A simple calculation shows that this discrepancy may be fully attributed to the effects of rotation recrystallization.
A model for the tangent viscous behaviour of anisotropic polar ice
Authors: Meyssonnier Jacques , Philip Armelle
Published: September 2017

During the deformation of polar ice, a fabric develops which results in a macroscopically anisotropic behaviour. Since the plastic anisotropy of the ice single crystal is very high, the effect of a strong (single maximum) fabric on the macroscopic flow law cannot be neglected when simulating the flow of an ice sheet. As this is already a difficult task when using the familiar isotropic power law for ice, the fabric evolution and related macroscopic mechanical behaviour model, to be implemented in such a simulation, must be realistic yet simple enough to achieve results within a reasonable level of complexity, at least as a first step.

To this aim, we propose to model polar ice as a transversely isotropic medium; while simplifying the problem, this captures the essential features of the in-situ observed fabrics. The macroscopic mechanical behaviour of the ice polycrystal is obtained by using an orientation distribution function (ODF) for the c axes of the grains, which characterizes the fabric, and a self-consistent scheme, considering each single crystal as transversely isotropic. The evolution of the ODF is described by a conservation equation. In the first stage, this model was run in the simple cases of uniaxial compression and tension along the orthotropy symmetry axis.
A continuum approach for modelling induced anisotropy in glaciers and ice sheets
Authors: Svendsen Bob , Hutter Kolumban
Published: September 2017

This paper presents a formulation of a continuum model for so-called (stress or deformation) induced anisotropy in natural ice which, unlike computer-based Taylor-type models, can be incorporated in numerical simulations of large ice masses to account for the effects of this process on the flow of these bodies in a physical fashion. To do this, we treat natural ice as a rigid-elastic, non-linear inelastic material which can develop transverse isotropic behaviour (accounting for the simplest kind of induced anisotropy in natural ice masses), where the degree of such anisotropy at each point is controlled by the distribution of crystal glide-plane orientations there. This distribution is described by a so-called orientation-distribution function, for which an evolution relation can be derived. The central constitutive assumption of this formulation relates this distribution to the “structure” tensor representing the transverse isotropy of the material. On the basis of this relation, the model predicts in particular isotropic (e.g. classical Glen’s flow-law type) behaviour at a given point when the distribution of crystal glide-plane orientations is uniform there.
The relative strengths of debris-laden basal ice and clean glacier ice: some evidence from Taylor Glacier, Antarctica
Authors: Lawson Wendy
Published: September 2017

An understanding of the mechanical behaviour of the basal zone of an ice mass is fundamental to understanding the overall dynamics of that ice mass. Despite the fact that debris-laden ice is found in the basal zones of many glaciers and ice sheets, its mechanical behaviour is only poorly understood. This paper attempts to expand our knowledge of the mechanical behaviour of debris-laden ice by examining the uniaxial compressive strength of debris-laden basal ice sampled from the snout of the Taylor Glacier, Antarctica.

The mechanical behaviour of debris-laden ice (debris content 5–20% by volume) under uniaxial compression, and the relationship between the behaviours of debris-laden basal ice and ‘clean’ glacier ice, is complex and variable. At the relatively warm temperatures at which uniaxial compressive strength tests were conducted in the field, debris-laden ice was generally weaker than clean glacier ice. At these temperatures, between 0° and −5°C, pressure melting was the dominant deformation mechanism in the debris-laden ice and cracking the dominant deformation mechanism in clean ice. At −25°C, however, debris-laden ice reached higher strengths than lite clean glacier ice and cracking was the dominant deformation mechanism in both ice types. The change in relationship between the strengths of debris-laden and clean ice with temperature is inferred to be attributable to the temperature dependence of the rate of pressure melting.

These results suggest that the dynamic effects and significance of the presence of a debris-laden ice layer in the basal zone of an ice mass are likely to be highly variable in space and time.
Glaciation, erosion and the evolution of the Transantarctic Mountains, Antarctica
Authors: Kerr Andrew , Gilchrist Alan
Published: September 2017

Modelling studies of the tectonic evolution of the Transantarctic Mountains in Antarctica have drawn differing conclusions as to the primary mechanisms involved. None has considered the role of the East Antarctic ice sheet in detail. We use a denudation—flexural model to examine the isostatic response of the continental margin to glacial erosion to determine whether glacial processes have played a role in forcing mountain uplift.

The conclusion is that, although there are insufficient data formally to delimit the role of glacial erosion, available geophysical and geomorphological data are not inconsistent with the results of the differential denudation model, providing certain conditions are met. These results indicate that the current topography of the Transantarctic Mountains can be simulated, in part, from the isostatic response of the lithosphere to glacial erosion. The short wavelength and high amplitude of the Transantarctic Mountains do not require a low flexural rigidity in the unrated lithosphere, provided there is a fast escarpment retreat from the rift hinge, high escarpment denudation rates and a large differential in denudation between the coastal zone and the interior.
Dynamic/thermodynamic simulations of Laurentide ice-sheet instability
Authors: Greve R. , MacAyeal D. R.
Published: September 2017

A crucial element of several leading theories of Laurentide ice-sheet instability (i.e. Heinrich events and advance/retreat cycles of the southern margin) is the evolution of melting conditions at the subglacial bed. Despite the great importance basal-temperature conditions play in these theories, relatively little has been done to test their physical plausibility. We therefore undertake a numerical model study of the ice-sheet temperature field along an important transect which extends from the lobate southern margin of the Laurentide ice sheet to the iceberg-calving from at the terminus of Hudson Strait. Our experiments illustrate the influence of important aspects of ice-sheet thermodynamics on ice-sheet instability, including horizontal advection and the development of an internal temperate-ice reservoir. Free oscillations of the basal temperature and ice thickness in Hudson Strait are possible under a restricted range of parameters elucidated by the model. These free oscillations may provide a basis for understanding ice-sheet instability, e.g. Heinrich events, with time-scales in the range of 103–104 a.
Antarctic topography and kilometre-scale roughness derived from ERS-1 altimetry
Authors: Brisset Laurence , Rémy Frédérique
Published: September 2017

The ERS-1 satellite has delivered altimetric data since 1992, enabling us to map most of the Antarctic ice-sheet topography south to 82° S with better precision than all previous techniques. An algorithm has been developed such that the accuracy of the height data reaches the sub-metre level. As a first step, an inverse method has been designed to map the large-scale global topography, which is of interest to the study of the ice-sheet flow dynamics. As a second step, an adapted inverse algorithm displays precisely the short-scale undulations which are controlled by the bedrock below the ice. Finally, variations in the back-scattered altimetric signal allow us to map directly the kilometre-scale roughness that is related to the basal-flow conditions. Together, these maps constitute an important data base for modelling the ice sheet.
Movement and structural features observed in ice masses, Framnes Mountains, Mac.Robertson Land, East Antarctica
Authors: Marmo B. A. , Dawson J.
Published: September 2017

Ice-movement surveys in the Framnes Mountains from austral summers 1993–94 and 1994–95 are presented with complementary surface elevations and structural interpretations. Surface velocities of 21 m a−1 have been recorded in Ice Stream B and 31 ma−1 in Ice Stream C. The flow rate varied between 11.6 and 21.0 m a−1 over a lateral range of 2.1 km west of Rumdoodle Peak and between 17.1 and 31.5 m a−1 over 2 km west of Mount Parsons. Two ice streams of relative high flow between the central Masson and David Ranges and several zones of incremental increase in flow rate have been identified within Ice Stream B, using shear-sense criteria based as ice-fracture geometry and subsequent deformation of crevasse traces. These zones of localized strain correlate with ice-flow survey measurements.
Surface energy exchange at the equilibrium line on the Greenland ice sheet during onset of melt
Authors: Steffen Konrad
Published: September 2017

The energy balance was measured at the ETH/CU research camp, located at the equilibrium-line altitude of the Greenland ice sheet (69°34′N, 49°17′W), in spring 1993. Prior to the onset of melt the latent energy flux is the only energy sink, with an average value of 1.18 MJ m−2d−1. This turbulent transfer of moisture is responsible for an average daily snow sublimation of 5 × 10−4 m. The sensible heat flux (0.91 MJ m−2d−1) is the major energy source, and the net radiation only a minor contributor with a positive balance of 0.12 MJ m−2d−1. The residual of the turbulent fluxes and the net radiation is about 0.14 MJ m−2d−1, which agrees surprisingly well with the in situ ground-heat flux measurements of 0.15 MJ m−2d−1 made at the same location. Assuming a 3°C temperature increase in a global-change scenario, and using the information gained from the energy-balance measurements, a simple calculation showed that 5.4 × 10−2 m w.e. of snow would be lost by sublimation for an area of approximately 400 000 km2. The increased sublimation is due to an increase in the net radiation balance caused by an albedo reduction. This would lead to a total annual mass reduction of 22 km3 w.e. of snow for the entire Greenland ice sheet, which is 4.2% of today’s annual accumulation.
The effect of ice-sheet thickness change on the accumulation history inferred from GISP2 layer thicknesses
Authors: Cutler Nadine N. , Raymond C.F. , Waddington E. D. , Meese D.A. , Alley R.B.
Published: September 2017

Net accumulation rates at the Greenland summit have been inferred using layer-thickness data from the GISP2 ice core with corrections for strain using a non-linear, one-dimensional flow model of an ice sheet. The flow model accounts for thickness changes in ice-sheet in response to mass-balance variations. The model is used to investigate how net accumulation-rate changes affect the time evolution of: (1) the ice-sheet thickness. (2) the vertical strain rate, and (3) the corresponding internal annual-layer structure. The model, parameterized to fit the present net accumulation rate and thickness of the Greenland ice-sheet summit, has a characteristic time constant for adjustment to accumulation-rate changes of about 6000a and yields an ice sheet 200-400 m thinner than its present thickness during the last glacial period.

Accumulation-rate histories inferred from GISP2 layer-thickness data using both a constant- and a variable-thickness model are compared. The variable-thickness model predicts accumulation rates about 25% lower than the constant-thickness model. Our results also indicate that high-frequency changes in accumulation rates (i.e. alter the Younger Dryas event) are consistent with earlier analyses. However, sensitivity tests indicate that the accumulation-rate history cannot be precisely determined. Our analysis defines an envelope of likely accumulation histories bounded above by the accumulation history inferred by the constant-thickness model. Predictions become increasingly uncertain for old ice because of (1) intrinsic difficulties associated with this inverse problem, and (2) decreased accuracy of the data.
Ground-based radar observations of snow stratigraphy and melt processes in the percolation facies of the Greenland ice sheet
Authors: Zabel I.H.H. , Jezek K.C. , Baggeroer P.A. , Gogineni S. P.
Published: September 2017

Summer melt is a primary source of mass loss on the Greenland ice sheet. An understanding of melt processes on the ice sheet, their connection with atmospheric processes, and the redistribution of meltwater is important for ascertaining the mass balance of the ice sheet. High-resolution radar measurements made in the percolation zone of the Greenland ice sheet reveal the evolving radar signature of summer surface melting and subsequent refreezing of meltwater. A traverse over the snow surface has resulted in the first radar map of snow stratigraphy over an extended distance. The dominant sources of back-scatter in the study area are the snow surface and effectively continuous annual ice layers. We suggest applications of our results to help define the extent of the percolation zone and to discriminate between regions where surface melt is lost to the sea and those where melt refreezes nearly in place.
Characterizing the long-term variability of snow-cover extent over the interior of North America
Authors: Brown Ross D. , Hughes Marilyn G. , Robinson David A.
Published: September 2017

Historical and reconstructed snow-cover data show evidence of a gradual increase in snow cover over the continental interior of North America (NA) during much of the 20th century, primarily in response to increasing snowfall. A rapid decrease in Canadian-prairies snow cover after 1970 is not observed over the Great Plains. Analysis of snow-cover-climate relationships revealed systematic increases in the sensitivity of snow cover to Northern Hemisphere (NH) temperatures over the 1940-65 period. This change is mainly due to an increase in snowfall-temperature sensitivity during this period. Seasonal analysis revealed that the observed increase in snow-cover and snowfall temperature sensitivity is primarily a spring phenomenon. A marked increase in the importance of the spring period is observed around 1960, which coincides with a well-documented change in atmospheric circulation over NA. The post-1960 period is characterized by a significant inverse relationship between snow cover and hemispheric air temperature over the Canadian prairies and northern Great Plains regions.
Melt features in ice cores from Site J, southern Greenland: some implications for summer climate since AD 1550
Authors: Kameda T. , Narita H. , Shoji H. , Nishio F. , Fujii Y. , Watanabe O.
Published: September 2017

Horizontal and vertical distributions of melt features (ice layers) were examined using two ice cores (206.6 and 101.5 m deep, 1 m apart) from Site J (66°51.9′ N, 46°15.9′W, 2030 m a.s.l.). The temperature at 10 m was −16.3°C. We observed 2804 melt features, with a total thickness of 30.32 m, in the 206.6 m core, corresponding to 16.4% by volume of the ice-equivalent core length. Horizontal distribution of melt features was examined by correlating melt-feature thicknesses in the two cores. The correlation coefficient was 0.71 (n = 514) for each melt feature in the two cores. It was maximum for data passed through 5 and 40 year low-pass filters. A significant relationship (P = 0.005, n = 36) was obtained for the vertical distribution of melt features and the June temperature on the west coast of Greenland (Jakobshavn). Using this, June temperatures at Jakobshavn since 1550 were estimated. There are three periods (1685-1705, 1835-70 and 1933-45) during which mean June temperatures clearly decreased, when they were estimated to he 0.1°, 0.4° and 0.2°C lower than the average for the whole period (1550-1989). The first two “cold” periods have been identified in melt features of the Dye 3 and Devon Island ice cores and in a tree-ring profile from Yukon Territory, Canada. Melt-feature percentages in the Site J ice core have increased since about 1945, probably reflecting summer-temperature warming on the ice sheet.
Variations in melt-layer frequency in the GISP2 ice core: implications for Holocene summer temperatures in central Greenland
Authors: Alley Richard B. , Anandakrishnan Sridhar
Published: September 2017

The rare melt features in the GISP2, central Greenland deep ice core have decreased in frequency over the most recent 7000 years. Calibration of this change in melt frequency against modern spatial variation of melt frequency and temperature in central Greenland, and against modern temporal variability of temperatures in central Greenland, indicates that mean mid-summer temperatures have cooled over the most recent 7000 years, probably by slightly more than l°C if variability of summer temperatures has not changed. Comparison to GRIP isotopic records from central Greenland and to the melt record from the Agassiz Ice Cap, Arctic Canada, suggests some seasonal and regional coherence for this cooling signal, as well as for a cold event about 8000-8500 BP.
Climate change and the Arctic hydrologic cycle as calculated by a global coupled atmosphere–ocean model
Authors: Miller James R. , Russell Gary L.
Published: September 2017

A global coupled atmosphere–ocean model is used to examine the hydrologic cycle of the Arctic Ocean. The model has a horizontal resolution of 4° × 5°, nine vertical layers in the atmosphere and 13 in the ocean. River discharge into the Arctic Ocean is included by allowing runoff from each continental grid box to flow downstream according to a specified direction file and a speed that depends on topography. A 74 year control simulation of the present climate is used to examine variability of the hydrologic cycle, including precipitation, sea ice, glacial ice and river discharge. A 74 year transient simulation in which atmospheric CO2 increases each year at a compound rate оf 1% is then used to examine potential changes in the hydrologic cycle. Among these changes are a 4°C increase in mean annual surface air temperature in the Arctic Ocean, a decrease in ice cover which begins after 35 years, and increases in river discharge and cloud cover. There is little change in the net difference between precipitation and evaporation. Also in the transient simulation, glacial ice on Greenland decreases relative to the control.
Thermomechanical modelling of Northern Hemisphere ice sheets with a two-level mass-balance parameterization
Authors: Huybrechts Philippe , Siobbel Stephen T’
Published: September 2017

A three-dimensional time-dependent thermomechanical ice-sheet model was used together with a two-level (snow-accumulation/runoff) mass-balance model to investigate the Quaternary ice sheets of the Northern Hemisphere. The model freely generates the ice-sheet geometry in response to specified changes in surface temperature and mass balance, and includes bedrock adjustment, basal sliding and a full temperature calculation within the ice. The mass-balance parameterization makes a distinction between snowfall and melting. Yearly snowfall rates depend on the present precipitation distribution, and are varied proportionally to changes in surface temperature and the moisture content of the air. The ablation model is based on the positive-degree-day method, and distinguishes between ice and snow melting. This paper discusses steady-slate characteristics, conditions for growth and retreat, and response time-scales of ice sheets as a function of a prescribed lowering of summer temperature. Most notably, the modelled extents of the Eurasian ice sheet for a summer temperature lowering of 6–7 K and of the Laurentide ice sheet for a cooling of 9–10 K are in reasonable agreement with most reconstructions based on geological evidence, except for the presence of a large ice sheet stretching from Alaska across the Bering Strait to most of eastern Siberia. In addition, wet basal conditions turned out to be always confined to the margin, whereas central areas in these reconstructions remained always cold-based. This is of relevance for processes involving reduced basal traction.
A 485 year record of atmospheric chloride, nitrate and sulfate: results of chemical analysis of ice cores from Dyer Plateau, Antarctic Peninsula
Authors: Dai Jihong Cole , Thompson Lonnie G. , Mosley-Thompson Ellen
Published: September 2017

Detailed ionic analyses of Dyer Plateau snow show that major soluble impurities in snow consist of sodium (Na+), chloride (Cl−), nitrate (NO3−), sulfate (SO42−), and acidity (H+). The ratios of Na+ to Cl− concentrations are close to that of sea water, indicating little or no fractionation of sea-salt aerosols. The analyses of core sections from three sites along a 10 km transect show that local spatial variation of snow chemistry in this area is minimal and that temporal (decadal, inter-annual and sub-annual) variations in snow chemistry are very well preserved.

Anion analyses of the upper 181 m section of two 235 m ice cores yield a data set of 485 years (1505-1989) of annual snow accumulation and fluxes of Cl−, NO3−, and non-sea-salt (nss) SO42−. No significant long-term trends are observed in any of the anion fluxes. This is consistent with other Antarctic ice-core records showing no significant anthropogenic atmospheric pollution in the high southern latitudes. Linear regression analysis shows that Cl− flux is independent of snow-accumulation rate. Significant positive correlations are found between accumulation rate and both NO3− flux and background nss-SO42− flux. These results suggest that dry deposition is primarily responsible for air-to-ground Cl− flux while wet deposition dominates the NO3− and nss-SO42− flux (≥90% and ≥75%, respectively). The nss-S042− fluxes provide a chronology of explosive volcanic emissions reaching the Antarctic region for the past 485 years.
1000 years of climatic change in China: ice-core δ18O evidence
Authors: Lin P. N. , Thompson L.G. , Davis M.E. , Mosley-Thompson E.
Published: September 2017

Since 1987, ice cores have been drilled from the Dunde and Guliya ice caps on the Tibetan Plateau, western China. Here, the oxygen isotopic (δ18O) records for the last 1000 years from both these cores are compiled and compared. Using surface temperature observations since the mid-1960s from meteorological stations on the plateau and δ18O measured on precipitation collected contemporaneously, the empirical relationship: δ18O = 0.6 Ts – 12 is established. δ18O appears to serve as a reasonable proxy for regional surface temperatures and a reasonable basis for reconstructing 1000a proxy temperature records from Dunde and Guliya. The reconstructed temperature histories for Dunde (on the eastern Tibetan Plateau) and Guliya (on the western Tibetan Plateau) show some centennial-scale similarities, but reveal quite different histories for higher-frequency variability over the last millennium. The ice-core δ18O histories from Dunde and Guliya are compared with a tree-ring index from western China and the dust-fall record from eastern China, but show no consistent relationship. The most prominent similarity between the reconstructed temperature histories for Dunde and Guliya is the marked warming of the last few decades. From the 1000a perspective provided be these ice-core records, the recent warming on Dunde is unique in its strength and persistence; however, the warming on Guliya (inferred from 18O enrichment) is more recent (since 1985) and not unprecedented. This recent warming over the Tibetan Plateau is evident in the limited meteorological records.
Mass-balance variability as a base for interpreting ice-core data
Authors: Mikhalenko V. N.
Published: September 2017

The spatial extrapolation of data from ice cores depends on the complexity of the glacier system where the drilling site is located. The correlation between net mass balance,bn, of a specific point and of the whole glacier is different for each point. Analysis of net mass balance of Tuyuksu glacier in the Tien Shan, central Asia, confirms that the distribution of mass balance with height is more-or-less constant from year to year except in years with extreme valuesbn. Two types of “similarity” are described, additive and multiplicative. The “similarity” changes gradually from additive at the peripheral parts of the Tien Shan to multiplicative in the most continental central and eastern parts. Glacier mass-balance fluctuations of the frontal ridges are connected to the oscillations of accumulation and consequently to precipitation. Where the climate is more continental the mass-balance variability depends much more on the melting conditions than on accumulation. For the spatial interpretation of ice-core drilling results, a special analysis of “similarity type” is necessary. It allows the fixing of the spatial borders of the glacier system for which the dhilling site is representative.
An ice-core chemistry record from Snøfjellafonna, northwestern Spitsbergen
Authors: Goto-Azuma Kumiko , Kohshima Shiro , Kameda Takao , Takahashi Shuhei , Watanabe Okitsugu , Fujii Yoshiyoshiyui , Ove Hagen Jon
Published: September 2017

A 2.2 m deep pit and the top 42.5 m of an ice core recovered at Snøfjellafonna, northwestern Spitsbergen, were continuously analyzed for Na+, Cl−, NO3−, SO42− and pH. Seasonal variations in ionic concentrations seem to have remained in the pit and the core, in spite of the relatively severe summer melting. We dated the core by counting annual peaks of Na+ and made an adjustment with the use of a tritium peak in 1963 as a reference horizon. It turned out that the depth of 42.5 m went back to the early 1930s or late 1920s. The 60–70 year record of snow chemistry showed that the concentrations of both NO3− and SO42− had increased in the 1950s and had decreased in the late 1970s and the 1980s. The increase would be explained in terms of anthropogenic inputs from the industrial areas. The later decrease of the same ions may have been caused by a combination of the reduction of the atmospheric precursors due to pollution controls and the meltwater-associated processes.