Showing papers on "Accumulation zone published in 2002"

Influence of subglacial drainage system evolution on glacier surface motion: Haut Glacier d'Arolla, Switzerland

Abstract: [1] The relationship between the evolution of subglacial drainage system morphology and spatial patterns of glacier surface velocity was investigated using dye tracing experiments and ground surveying throughout the 1995 melt season at Haut Glacier d'Arolla, Switzerland. With the onset of high and variable melt season discharges, subglacial drainage changed from a predominantly distributed system to a predominantly channelized system. The change occurred later farther up glacier. During the period of drainage evolution the glacier was subjected to three periods of rapidly rising meltwater discharge. The magnitude and spatial pattern of the glacier's velocity response differed between these periods and can be explained in terms of the impact of the evolving drainage system morphology on the amplitude and spatial distribution of basal hydrological forcing. Increasing discharge through a distributed drainage system caused widespread basal forcing and high glacier velocity. Increasing discharge through incipient channels below moulins, not yet connected to the main channel, caused more localized basal forcing and slightly increased glacier velocity. Increasing discharge through a fully channelized drainage system caused no significant basal forcing and glacier velocity was not significantly different from the annual deformation flow pattern. Empirical orthogonal function analysis of flow patterns defined two distinct spatial modes of surface velocity which corresponded closely with the drainage system morphologies inferred to be present during each event. The relative importance of these modes changed through the melt season, suggesting a temporal change in the spatial pattern of hydrologically induced basal forcing.

Spatial and temporal variability of snow accumulation using ground-penetrating radar and ice cores on a Svalbard glacier

Abstract: A 50 MHz ground-penetrating radar was used to detect horizontal layers in the snowpack along a longitudinal profile on Nordenskjoldbreen, a Svalbard glacier. The profile passed two shallow and one deep ice-core sites. Two internal radar reflection layers were dated using parameters measured in the deep core. Radar travel times were converted to water equivalent, yielding snow-accumulation rates along the profile for three time periods: 1986-99, 1963-99 and 1963-86. The results show 40-60% spatial variability in snow accumulation over short distances along the profile. The average annual accumulation rate for 1986-99 was found to be about 12% higher than for the period 1963-86, which indicates increased accumulation in the late 1980s and 1990s.

Sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions: a model study

Abstract: We use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5 13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5 10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.

Influence of Sub-Debris Thawing on Ablation and Runoff of the Djankuat Glacier in the Caucasus

Abstract: Superficial moraines grew in size during the entire 32-year-long period of direct monitoring of water and ice balance of the Djankuat Glacier in the Caucasus. The total area of debris cover on the glacier increased from 0.104 km 2 (3% of the entire glacier surface) in 1968 to 0.266 km 2 (8% of the glacier) in 1996. Such rapid dynamics of moraine formation greatly influences the ablation rate and distorts fields of mass-balance components. Sub-debris thawing can be calculated by means of a model, which describes the role of debris cover for the thermal properties of a glacier. Its meltwater equivalent depends mainly on debris thickness. In 1983 and 1994 the debris cover was repeatedly mapped over the whole glacier portion that was covered with morainic material. Sub-moraine ablation increases (vs. pure ice surface) under the thin, less than ca. 7-8 cm, debris layer, whereas the thicker debris cover reduces the liquid runoff due to its shielding effect. Zones differing due to their hydrological effect are depicted on the glacier map and the degree of debris influence on ablation is estimated quantitatively. As a whole runoff from debris-covered parts of the Djankuat Glacier has diminished due to the dominant shielding effect. Variation of the terminus is also shown to be dependent on the evolution of superficial moraine.

A rapidly moving small rock glacier at the lower limit of the mountain permafrost belt in the Swiss Alps

Abstract: Surface and internal movements, structure and thermal conditions were investigated on a small talus-derived rock glacier in the Upper Engadin. A borehole 5.4 m deep drilled on the upper part of the rock glacier displayed ice-saturated sediments, consisting of pebbles, cobbles and fine debris. The mean annual temperatures were close to 0°C within the uppermost 5 m of ground. Whereas the lower part moved downslope only slightly, the upper part showed rapid movement (50–150 cm a 1), accelerating over a 3-year period. Inclinometers also showed large deformation of permafrost at depth. A DC resistivity tomogram indicates that permafrost is thinner than 20 m. Permafrost close to the melting point is deformed rapidly and is sensitive to inter-annual variation in ground temperature. Such rapid movement is considered to be temporary and eventually followed by inactivation of the rock glacier because of permafrost melting and a lack of debris input. Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

From push moraine to single-crested dump moraine during a sustained glacier advance

Abstract: In the 1980s, the NE part of the principal outlet glacier Kotlujokull on the east flank of the temperate ice cap Mýrdalsjokull was advancing; the maximum average rate of annual advance of 32 m was recorded during the period 1982-84. Along the advancing glacier margin a frontal push moraine was currently being produced by continuous mixing of proglacially thrust and folded outwash sediments with material avalanched down the steep front of the debris-covered glacier snout. During periods of fast glacier advance, the pushed ridge was mainly a combination of slabs of outwash deposits, but during periods of slow glacier advance, the pushed ridge chiefly consisted of front cliff-fall debris. By the end of the 1980s the glacier advance had stopped, and during the 1990s the most terminal part of the debris-mantled glacier developed into a 300-500 m wide zone of dead ice with a complex morphology of steep-sided mounds, ridges, collapsing edges and ice walls. At present the total lowering of the ice-cored terrain a...

Seasonal and spatial variations in the chemistry of a High Arctic supraglacial snow cover

Abstract: This paper describes the physical and chemical properties of the snowpack on John Evans Glacier, Ellesmere Island, Canadian Arctic Archipelago, and investigates the controls on snowpack solute concentrations and atmospheric deposition. The snowpack contains three layers that are traceable across the whole glacier. These represent fall accumulation that has been metamorphosed to depth hoar, winter accumulation mixed with snow reworked by wind from the underlying depth hoar, and spring accumulation mixed with wind-reworked snow. The seasonal cycle in snow chemistry closely reflects changes in the composition of the atmospheric aerosol at Alert, with some modification of NO 3 - concentrations by post-depositional processes. Mean water-weighted solute concentrations in the snowpack are largely independent of accumulation, while atmospheric deposition tends to increase with accumulation. This suggests that, for most species, wet deposition is the dominant depositional process throughout the year. However, concentrations of Ca 2+ and K + increase with both accumulation and elevation, implying an enhanced input from dry deposition of soil dust above 800 m elevation. Concentrations of SO 4 2- are inversely related to accumulation, especially in the winter layer, suggesting a significant input from non-precipitating events, such as dry deposition or riming, during this period of very limited snowfall.

Increase of 10 m ice temperature: climate warming or glacier thinning?

Abstract: In 1972 and 1995, shallow ice temperatures were measured at identical locations and depths on polythermal McCall Glacier, Brooks Range, Alaska, U.S.A. Mean annual ice temperatures at 10 m depth (T 10 ) have systematically increased by > 1 K for the ablation area (1400-1900 m), while closer to the firn area, where meltwater percolation and refreezing play a role, they remained approximately unchanged. Interpreting these findings in terms of climate change requires careful consideration of the observed thinning of the glacier, which causes lowering of the surface through an existing vertical temperature gradient. Such temperature gradients can be particularly large in the ablation areas of polythermal glaciers; on McCall Glacier they are on the order of 0.2 K m -1 . We also study the evolution of a 75 m deep temperature profile measured at one location in 1972 using a one-dimensional heat-diffusion model. We find that this profile was in approximate equilibrium with the mean surface temperature extrapolated from the 1972 data. Using the observed rate of surface lowering and measured rates of vertical advection, we find that both the measured temperature change and the vertical temperature gradient at 10 m depth can be reproduced only if the mean annual surface temperature on McCall Glacier has increased by 1.1 ± 0.3 K between 1972 and 1995. This result is consistent with the observed trend toward more negative mass balances on the glacier in the 1990s.

Unsteady flow inferred for Thwaites Glacier, and comparison with Pine Island Glacier, West Antarctica

Abstract: We present a comparison of surface velocities in 1996, derived from interferometric synthetic aperture radar, with an estimate of the long-term, depth-averaged velocity based on the assumption of steady-state flow for both Pine Island Glacier and its neighbour, Thwaites Glacier, West Antarctica. The results show that the former was close to balance conditions in 1996 (within 9%). The ice flux and velocity of the latter is significantly different in magnitude and distribution from that required to maintain the basin in a state of mass balance over the long term. The balance flux was found to be 32 ± 19% less than the measured outgoing flux. We conclude that the mass imbalance and dramatic difference in the pattern of flow is most likely due to a substantial change in the flow regime of Thwaites Glacier in the recent past.

Isotopic study on Dokriani Bamak glacier, central Himalaya: implications for climatic changes and ice dynamics

Abstract: Measurements of natural and artificial radioisotopes ( 32 Si, 210 Pb and 137 Cs) and oxygen isotopes (δ 18 O) have been carried out on surface snow and ice, shallow snow pits and an ice core collected from Dokriani Bamak glacier, central Himalaya, to study the dynamics of glacier ice and short-term climatic changes. Based on the 32 Si and 210 Pb activities in the meltwaters, the age of the snout ice is 400 years and the flow rate of ice along the glacier length is ∼14 m a -1 . The specific activity of 137 Cs, corresponding to 1963 fallout, in the surface ice at the equilibrium line yields a flow rate of 32 m a, a factor of two higher than that derived for the snout ice. The depth variation of 137 Cs concentration in a shallow ice core yields a mean accumulation rate of 0.43 m a -1 for the glacier ice over the past decade. The δ 18 O of snout ice (-13.4‰) is significantly depleted compared to the average value of -9.2‰ in the shallow ice core, indicating that cooler climatic conditions prevailed around AD 1600. Based on the oxygen isotopic ratios in the shallow pits, an altitude effect of 0.9‰ per 100 m in δ 18 O variation is documented for this glacier.

Simulating the Greenland atmospheric boundary layer

Abstract: A three-dimensional dynamic downscaling model of the Greenland atmospheric boundary layer, with a horizontal resolution of 20 km, is descibed and applied to the Greenland ice sheet for the 1998 ablation season. The model uses ECMWF analysis data fields of synoptic pressure, free atmospheric temperature, cloud cover, humidity and sea surface temperature to force the model. The model calculates the perturbation component of the temperature and pressure field to describe the atmospheric boundary layer dynamics. The aim of this study, the first of two papers, is to investigate the role of the turbulent heat fluxes in the surface energy balance of the ice sheet and their response to changes in atmospheric temperature. In this first paper, results from the simulation are compared with observations from six automatic weather stations situated on the ice sheet, three in the ablation zone and three in the accumulation zone. The comparison shows that the boundary layer model can reproduce the nearsurface meteorological variables of wind, temperature and specific humidity quite well and improve significantly on 2 m values taken directly from the ECMWF analysis. The increased spatial resolution of the model is essential in order to model accurately katabatically forced winds near the margin of the ice sheet. The calculated and observed melts at two sites in the ablation zone are also compared. At one site close to the margin, which is situated in a well drained ice region, the comparison with observations is very good, within 1%. At a higher site, where subsurface processes not included in the model are important for the total ablation, the calculated melt is 35% larger than the observed ablation.

Surface lowering over the ablation area of Lirung Glacier, Nepal Himalayas

Abstract: Repeated surveys for six transverse surface profiles on the debris-covered ablation area of Lirung Glacier, Nepal Himalayas, reveal that the glacier surface has lowered from 1996 to 1999 The average annual rate of the surface lowering is evaluated to be in a range from 1 to 2 m a -1 The emergence velocity is also estimated to be about 02 m a -1 on average for the ablation area, using surface flow and ice thickness data A relation with mass balance in the continuity equation roughly supports these results The surface lowering is suggested to have accelerated possibly since late 1980s In addition, the lowering rate is discussed in comparison with those on other glaciers in the Nepal Himalayas

Modeling the Variation of Glacier System——Taking the Southern Tibet Region as an Example

Abstract: In the basins of the Ganges, Yarlung Zangbo and Indus there are 19 575 km 2 of glaciers, accounting for one-third of the total glacier area in China. On the basis of the structure of the glacier system and the nature of the equilibrium line altitudes at the steady state, functional model of the variation of glacier system are established, using Kotlyakov-Krenke's equation relating annual ablation of glacier and mean summer temperature. The models are applied to the study of the response of glacial runoff to climatic change. The effect of decreasing air temperature due to rising of %ELA% and reduction of glacier area is considered in these model simultaneously. The modeling results under the climatic scenarios with temperature increasing rates of 0^01, 0^03 and 0^05 K·a -1 indicate that the glacial runoff fed by the marine-type glaciers with high levels of mass balance are very sensitive to climate change, it will take 10～30 years to reach a climax, and go back to their initial state in less than 100 years. However, the increasing rate of glacial runoff is small. During a peak period, the discharge-increasing rate will range from 1^01 to 1^14. In contrast, the glacial runoff of the continental-type glaciers, characterized by lower level of mass balance smaller decreasing rate of glacier size, and longer life-span responds climate variation slowly; it will take over 100 years to a climax and hundreds of years to return to their initial state. During a peak period, the discharge-increasing rate will ranged between 1^6～4^0 for extrexly continental-type glaciers. The decreasing rate of size for the marine type glaciers has larger than the continental-type on in case of 0^05 K·a+{-1} and in 100 a, the former one is {-0.82} and the later one is {-1.8}. At the similar levels of mass balance, smaller glaciers respond quickly to the climate change and also retreat relatively fast. The glacier system with very large elevation difference have the longest life span. The life span of the Rongbuk Glacier on the northern slope of the Himalayas will be over 1 800 years at a temperature-increasing rate of 0.01 K·a -1.

Some shrinking features in the uppermost ablation area of the Khumbu Glacier, east Nepal, 1995-1999

Abstract: An accelerated surface lowering of the Khumbu Glacier, one of debris-covered glaciers in the Nepal Himalayas, was detected by repeated mappings in the uppermost ablation area of the glacier. The surface lowered at a rate of about 2 m a -1 in 1995-1999 whereas about 1 m a -1 in 1978-1995. Mean ice flow velocity decreased to about 18 m a -1 in 1995-1999 from about 26 m a -1 in 1987-1995. Using ice thickness data and flow velocity in 1999, annual ablation over the respected area was roughly evaluated to be about 3.5 m a -1 . This value was almost compatible to the value obtained by in-situ observation in the late 1970s. Decrease of influx of ice from the upglacier was suggested as a possible cause of recent accelerated surface lowering.

ロシア連邦サハ共和国スンタル・ハイアタ山脈のNo. 31氷河に関する予察研究

Abstract: We carried out a preliminary glaciological research on the No.31 Glacier in the Suntar Khayata Range, Sakha Republic, Russian Federation, in the summer of 2001. This glacier was intensively studied, including mass balance, ice temperature measurements and surveying, by Russian researchers in 1957-58 (the 3 rd International Geophysical Year) (Koreisha, 1963). We aimed to obtain the change of the glacier volume since 1958 to study climate change during the last 40 years, and to know the possibility of ice core drilling and analyses for paleoclimatic study in the eastern Siberia. The glacier is a valley-type cold glacier of approximately 3.85 km long and covers the altitude from 2728 m to 2050 m a.s.l. The accumulation area of the glacier is mostly underlain by superimposed ice, which is slightly capped with water-saturated firn. The ablation area is characterized by well-developed longitudinal foliations, and basal ice layers at the very end of the terminus. The air temperature was as high as 12.7 °C in average at the Base Camp (ca. 2000 m a.s.l.) during the observation period (July 21-27, 2001), and we observed intensive melting at the whole area of the glacier. From this observation, we conclude that the glacier is not suitable for the ice core study because it is probably difficult to reconstruct a continuous ice core climate record. Beside glaciological studies, we conducted a topographical survey of the glacier, which showed that the glacier terminus had retreated approximately 200 m in distance and lowered by approximately 20 m from 1957-59 to 2001.

Chemical characteristic study of snow and sno w pit in Mount Qomolangma region

Abstract: Based on the analysis results of δ 18 O and major ion concentrations of snow and snow pit samples collected from the accumulation zone of the East Ron buk Glacier, which lies on the northern slope of Mt Qomolangma, the chemical ch aracteristics and climatic significance of snow and snow pit are discussed. The results indicate that δ 18 O and major ion concentrations of the precipitation in this region has distinct seasonal variations. In summer monsoon season, the δ 18 O and major ion concentrations in precipitation a re much lower than that in non summer monsoon season, suggesting different moist ure origins and climate conditions.