Arctic precipitation as depicted in the National Center for Environmental Prediction (NGEP) and the National Center for Atmospheric Research (NCAR) reanalysis effort is evaluated using 6 hourly model output for the period 1986–93 in conjunction with gauge-corrected climatologies. Climatological fields from the model agree favorably with observations in terms of general spatio-temporal patterns, but with some notable differences. In particular, the precipitation maximum over the central Arctic (the region north of 70°N) is depicted in July, one month too early. Values are too low from August through December, resulting in underestimates of annual precipitation of about 40 mm. Despite these shortcomings, the modeled precipitation fields appear to be sufficiently realistic to represent a base for blending with other data to provide gridded fields suitable for use in climate studies and sea-ice models.

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

Sediments from the Antarctic continental margin may provide detailed palaeoenvironmental records for Antarctic shelf waters during the late Quaternary. Here we present results from a palaeoenvironmental study of two sediment cores recovered from the continental shelf off Mac. Robertson Land, East Antarctica. These gravity cores were collected approximately 90 km apart from locations on the inner and outer shelf. Both cores are apparently undisturbed sequences of diatom ooze mixed with fine, quartz-rich sand. Core stratigraphies have been established from radiocarbon analyses of bulk organic carbon. Down-core geochemical determinations include the lithogenic components AÍ and Fe, biogenic components opal and organic carbon, and palaco-redox proxies Mn, Mo and U. We use the geochemical data to infer past variations in the deposition of biogenic and lithogenic materials, and the radiocarbon dates to estimate average sediment accumulation rates. The Holocene record of the outer-shelf core suggests three episodes of enhanced diatom export production at about 1.8, 3.8 and 5.5 ka BP, as well as less pronounced bloom episodes which occurred over a shorter period. Average sediment accumulation rates at this location range from 13.7 cm ka−1 in the late Pleistocene early Holocene to 82 cm ka−1 in the late Holocene, and suggest that the inferred episodes of enhanced biogenic production lasted 100-1000 years. in contrast, data for the inner-shelf core suggest that there has been a roughly constant proportion of biogenic and lithogenic material accumulating during the middle to late Holocene, with a greater proportion of biogenic material relative to the outer shelf. Notably, there is an approximately 7-fold increase in average sediment accumulation rate (from 24.5 to 179 cm ka−1) at this inner-shelf location between the middle and late Holocene, with roughly comparable increases in the mass accumulation rates of both biogenic and lithogenic material. This may represent changes in sediment transport processes, or reflect real increases in pelagic sedimentation in this region during the Holocene. Our results suggest quite different sedimentation regimes in these two shelf locations during the middle to late Holocene.

This paper describes the calibration of sediment traps in melt-water intake structures of the Grande Dixence hydro-electricity scheme for the purpose of generating medium-term estimates of suspended sediment and bedload transport by pro-glacial streams. The frequency of emptying/purging of sediment from traps in 17 glacier basins in the Val d’Hérens is combined with their design sediment volume to provide first approximations of the mean annual volume of sediment purged. This is followed by more detailed consideration of 11 years of records from three of the basins, to investigate mean seasonal and inter-annual patterns of purging. The difficulties of relating sediment purging to sediment transport are considered. A sediment calibration strategy based on 2 years of turbidity records, hand sampling of suspended sediment concentration, and monitoring of the accumulation of sediment in sediment traps is proposed for one of the glacier basins, and is checked using hourly and 4 h suspended sediment observations from two further ablation seasons. The calibration is applied to 10 years of discharge and purge records to establish estimates of mean seasonal and inter-annual patterns of suspended sediment and bed-load transport.

The catchments of Pine Island Glacier and Thwaites Glacier in the Amundsen Sea Embayment are two of the largest, most rapidly changing, and potentially unstable sectors of the West Antarctic Ice Sheet. They are also neighboring outlets, separated by the topographically unconfined eastern shear margin of Thwaites Glacier and the southwest tributary of Pine Island Glacier. This tributary begins just downstream of the eastern shear margin and flows into the Pine Island ice shelf. As a result, it is a potential locus of interaction between the two glaciers and could result in cross-catchment feedback during the retreat of either. Here, we analyze relative basal reflectivity profiles from three radar sounding survey lines collected using the UTIG HiCARS radar system in 2004 and CReSIS MCoRDS radar system in 2012 and 2014 to investigate the extent and character of ocean access beneath the southwest tributary. These profiles provide evidence of ocean access ~12 km inland of the 1992–2011 InSAR-derived grounding line by 2014, suggesting either retreat since 2011 or the intrusion of ocean water kilometers inland of the grounding line.

Tests have been performed on fine-grained, columnar, freshwater ice sheets 40 to 70 mm thick grown in a refrigerated model basin. Cantilever beams of various geometries were tested for lengths ranging from 200 to 2 000 mm and widths of 50 to 250 mm. Analysis of the results in terms of simple elastic beam theory indicated that modulus increased with increasing beam length and decreasing bean width. An analytical model for beam deflection was developed, taking into account the effects of buoyancy, shear, and rotation and deflection at the root. This model satisfactorily explained the observed deflection behaviour and the apparent geometry dependence of the modulus. Flexura! strength was independent of beam length, but decreased with increasing beam width. Flexural strength was independent of loading rate, whereas modulus decreased with increased loading time.

The composition of air in bubbles of ice reveals important information on the composition of the atmosphere at the time at which the ice was formed. The enclosed air has essentially an atmospheric composition if the ice formation occurred by dry sintering of snow and firn at low temperatures. The history of atmospheric CO2 concentration is of special interest due to its present increase and its climatic implications. Currently, CO2 analysis of air extracted from ice cores seems the best and most direct method of determining the natural CO2 concentration before 1950 and possible pre-industrial variations. Analyses of ice samples representing the past 40 ka show that there were significant changes in concentration of atmospheric CO2 at the end, and probably during part, of the last glaciation. δ13C measurements on CO2 extracted from ice cores can indicate possible mechanisms causing these changes in the concentration of atmospheric CO2.

We contrast the frontal dynamics of dilute powder snow avalanches with the behavior of their tail. While the former can be regarded as a fast-moving eruption current dominated by synergistic material injection into a short head, the latter behaves as a nearly arrested dilute cloud of particles expanding by progressive incorporation of ambient air, or by entrainment of snow-cover material by late avalanches trailing well behind the front.

We measured the velocities of six glacier tongues and a few tongues within ice shelves distributed around the Antarctic coastline by determining the displacement of crevasse patterns seen on sequential Landsat images. The velocities range from less than 0.2 km a−1 for East Antarctic ice-shelf tongues to more than 2.5 km a−1 for the Thwaites Glacier Tongue. All glacier tongues show increases in velocity toward their distal margins. In general, the tongues of glaciers draining the West Antarctic ice sheet have moved significantly faster than those in East Antarctica. This observation may be significant in light of the hypothesized possible disintegration of the West Antarctic ice sheet.

Sea ice is a highly mobile component of the Antarctic environment. Its velocity and deformation are critical processes, important in global climate models. These variables are determined by the balance of atmospheric and oceanic forces on each ice Hoe and variations in these forcings, and can produce regions ofdivcrgence or convergence. Surface drag coefficients relate the forces due to wind or water to the stress applied to the ice floe. This study adds to the limited drag coefficients reported previously for Antarctic data.

Surface elevation profiles were collected during two ship-based voyages to the Weddell Sea in 1992 and 1994, and were also recorded on Ice Station Weddell in 1992. These data are used to derive surface drag coefficients using an empirical formulation following Banke and others (1980). The eastern and western regions of the Weddell Sea contain primarily first- and second-year ice, respectively. Despite these different ice types, the drag coefficients calculated are similar. The difieren! ice-drifl/wind-speed ratio in the two regions suggests a difference in ocean currents, internal ice stress or water drag. The drag coefficients calculated ranged between 1.2 X 10 −3 and 2.2 X 10 −3 The results compare well with other published Antarctic coefficients, and are generally smaller than those reported for the Arctic.

Our understanding of ice algal responses to the recent changes in Arctic sea ice is impeded by limited field observations. In the present study, environmental characteristics of the landfast sea-ice zone as well as primary production and macromolecular composition of ice algae and phytoplankton were studied in the Kitikmeot Sea near Cambridge Bay in spring 2017. Averaged total chlorophyll-a (Chl-a) concentration was within the lower range reported previously for the same region, while daily carbon uptake rates of bottom-ice algae were significantly lower in this study than previously reported for the Arctic. Based on various indicators, the region's low nutrient concentrations appear to limit carbon uptake rates and associated accumulation of bottom-ice algal biomass. Furthermore, the lipids-dominant biochemical composition of bottom-ice algae suggests strong nutrient limitation relative to the distinctly different carbohydrates-dominant composition of phytoplankton. Together, the results confirm strong nitrate limitation of the local marine system.

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.

Based on sedimenlological and micropaleontological work on three sediment cores collected at about 167° Ε in the western Ross Sea, Antarctica, and accelerator mass spectrometer l4C ages of organic carbon, we have reconstructed environmental changes in the area during the late Quaternary. Since 38 ka BP at latest, this area was a marine environment with low productivity. A grounded ice sheet advanced and loaded the sediments before about 30-25 ka BP. After 25 ka BP, the southernmost site (76°46'S) was covered by floating ice (shelf ice), preventing deposition of coarse terrigenous materials and maintaining a supply of diatom tests and organic carbon until 20 ka BP. The northernmost site (74°33'S) was in a marine environment with a moderate productivity influenced by shelf ice/ice sheet after about 20 ka BP. Since about 10 ka BP, a sedimentary environment similar to the present-day one has prevailed over this area.

Sea ice, which forms in polar and nonpolar areas, transmits light to ice-associated (sympagic) algal communities. To noninvasively study the distribution of sea-ice algae, empirical relations to estimate its biomass from under-ice hyperspectral irradiance have been developed in the Arctic and Antarctica but lack for nonpolar regions. This study examines relationships between normalised difference indices (NDI) calculated from hyperspectral transmittance and sympagic algal biomass in the nonpolar Saroma-ko Lagoon. We analysed physico-biogeochemical properties of snow and land-fast sea ice supporting 27 paired bio-optical measurements along three transects covering an area of over 250 m × 250 m in February 2019. Snow depth (0.08 ± 0.01 m) and ice-bottom brine volume fraction (0.21 ± 0.02) showed low (0.06) and high (0.58) correlations with sea-ice core bottom section chlorophyll a (Chl. a), respectively. Spatial analyses unveiled the patch size of sea-ice Chl. a to be ~65 m, which is in the same range reported from previous studies. A selected NDI (669, 596 nm) explained 63% of algal biomass variability. This reflects the bio-optical properties and environmental conditions of the lagoon that favour the wavelength pair in the orange/red part of the spectrum and suggests the necessity of a specific bio-optical relationship for Saroma-ko Lagoon.

During two winter seasons, we observed avalanche winds and the air movement in the snow dust region. The wind velocity near the avalanche front was comparable to the averaged internal velocity of the front region. The avalanche wind blew down the valley. However, there was a rising air current just before the avalanche front. The wind velocity and direction showed a periodic change in the snow cloud region, from which the size of the ordered structure (vortex) was estimated to be 35–70 m. The density of the frontal part of one avalanche was estimated to be 1.9–3.8 kg m−3.

We report new data on the position of ice edges in the eastern and southern Weddell Sea for the years 1983 and 1984. The data are derived from ship-borne radar measurements of individual points along the ice edge together with ship’s positions obtained by a satellite navigation system. They are accurate within 0.23 to 0.4 nm (426 - 741 m). Comparisons of ice shelf margins for the years 1980, 1983 and 1984 allow estimates of apparent ice advance rates during this period. Together with quantitative ice edge velocity estimates first conclusions about net changes along the ice front and the ablation along the margin of ice shelves in the eastern and southern Weddell Sea are derived.

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

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

Immediately after the termination of the ice age, most of the surface in the ablation zone consisted of this low-reflectance ice. So, in the early Holocene, the dark ice of Wisconsin origin is likely to have resulted in higher ablation rates than previously considered. This may probably partly explain the fast rate of retreat/disintegration of the ice sheets in the Northern Hemisphere, after the termination of the Wisconsin ice age.

We measured the depth profiles of soluble ions in the Dome Fuji (Antarctica) ice core to search for possible paleoclimate indications of seasonal climate variations in the last glacial period. A 523 mm long core section between 587.65 and 588.18 m depth was selected for this pilot study, and the high-resolution chemical analysis was done on 2 mm thick samples. Our results indicate that anion– cation trapping in ice affects the profiles of the soluble ions and [Na+] in the core may preserve its seasonal signal. Correlation coefficients and the equivalent balances of the soluble ions suggest the following selective coexistences: (1) [Cl–] = [Na+], (2) [NO3–] = 1/2[Ca2+] + [K+], and (3) [SO42–] = 1/2[Ca2+] + [H+] + [Mg2+]. These coexistences are probably due to (1) a sea-salt source of Na+ and Cl–, (2) reaction of NO3– with dust for Ca2+ and NO3–, and (3) anion–cation trapping in ice for SO42– and Ca2+ (Mg2+), respectively.

The nature of the fall-off at lower stresses from power-law behavior to a lower-order stress dependency is of particular interest in glacier and ice-sheet modeling. Preliminary experiments show that the stress level at which the fall-off occurs is a function of the specimen’s dislocation density. The analysis employs a dislocation-based model of anelasticity that provides a quantitative relationship between the effective dislocation density and the area of hysteresis loops observed in cyclic loading experiments. Combining this technique with a staged creep experiment makes it possible to calculate the dislocation density as a function of strain, thereby supporting a quantitative dislocation analysis of the deformation process.Work on saline ice established that the threshold stress associated with power-law behavior increased as a result of prior straining, power-law behavior emerged when the effective dislocation density increased measurably during deformation, and approximately linear behavior was evident when the dislocation density remained relatively constant. Those findings motivated the experiments on fresh-water ice presented here. The preliminary experiments show that pre-straining increases the stress associated with the fall-off from power-law behavior, and the results are interpreted in the context of a dislocation-based constitutive model developed for sea ice.