Showing papers on "Accumulation zone published in 1997"

Stick-slip sliding behaviour at the base of a glacier

Abstract: Measurement of basal sliding is an important component in studying the mechanical and hydrological coupling between a glacier and its bed. During the 1992 summer field season we used a "drag spool" to measure sliding at the ice/bed interface ofTrapridge Glacier, a small surge-type glacier in the St Elias Mount ains, YukonTerritory, Canada. Measured diurnal variations in sliding appear to be correlated to subglacial water pressure fluctuations. In contrast to other observations where peak subglacial water pressure and glacier motion appear to coincide, our data imply that maximum sliding rates coincide with rises in water pressure. If the growth of water-filled cavities at the glacier bed is associated with these pressure increases, then our observations may corre­ spond to numerical results by Iken (1981 ) which indicate that the largest sliding velocity occurs during cavity growth and not when the steady-state size of cavitation is attained. Howe ver, our data suggest the idea that a loca lized stick -slip relaxation process is at work. As the water pressure rises, a local strain build-up in the ice is released, resulting in a momentary increase in sliding rate; once the finite relaxation has occurred, further rises in water pressure do not produce additional enhancement of basal sliding, and the stick­ slip cycle begins again by accumulation of elastic strain. We have developed a theoretical model for the sliding motion of ice over a surface having a basal drag that varies tem­ porally in response to changes in subglacial water pressure. Our model results support the proposed stick-slip sliding process at the glacier base, whereby accumulated elastic strain in the ice is released as the rising water pressure decouples the ice from the bed.

Recent Glacier Variations in the Southern Patagonia Icefield, South America

Abstract: The area variations of 48 outlet glaciers of the Southern Patagonia Icefield, South America, were elucidated for a period between 1944/45 and 1985/86, utilizing various remotely-sensed data. All but two of these glaciers are currently calving into a proglacial lake or a fiord. During the 41-yr period, most glaciers retreated, while a few glaciers remained almost stagnant and two glaciers had a net advance. The largest rate of area loss was 1.21 km2 yr-1 (or a distance of 484 m) at O'Higgins Glacier, which is by far the largest loss in Patagonia. On the other hand, Pfo XI Glacier, the largest glacier in South America, advanced at a rate of 1.45 km2 yr-1 (or a distance of 288 m) during the same period. Moreno Glacier oscillated frequently, with a net gain of 4.1 km2 in area in 39 yr; however, it is regarded to be currently in equilibrium. Some locational contrasts in the retreating rates can be recognized. For example, those on the northern half and on the east retreated at greater rates than those on the southern half and on the west, respectively. Possible causes for such variation patterns include temperature warming, although weak, in this century in the area south of latitude 46?S. On top of such climatic forcing, glacier dynamics affected by the size of the glacier, surface gradient around the equilibrium line (EL), and calving status and channel geometry exerted influence, causing a wide variety of the changing rates. The volume of ice loss due to snout retreats during the 41-yr period is estimated to be on the order of 40 to 80 km3. The volume loss in the ablation area due to thinning is roughly estimated at about 100 to 300 km3 for the same period.

Glaciological and oceanographic evidence of high melt rates beneath Pine Island Glacier, West Antarctica

Abstract: Satellite imagery indicates that the floating terminus of Pine Island Glacier has changed little in extent over the past two decades. Data on the velocity and thickness of the glacier reveal that calving of 28 ± 4 Gt a-1 accounts for only half of the ice input near the grounding line. The apparently steady configuration implies that the remainder of the input is lost by basal melting at a mean rate of 12 ± 3 m a-1. Ocean circulation in Pine Island Bay transports +1°C waters beneath the glacier and temperatures recorded in melt-laden outflows show that heat loss from the ocean is consistent with the requirements of the calculated melt rate. The combination of iceberg calving and basal melting lies at the lower end of estimates for the total accumulation over the catchment basin, drawing into question previous estimates of a significantly positive mass budget for this part of the ice sheet.

Momentum, Heat, and Moisture Budgets of the Katabatic Wind Layer over a Midlatitude Glacier in Summer

Abstract: In the summer of 1994, meteorological measurements were performed on Pasterze Glacier in the eastern Alps. One of the most remarkable observations concerning the observed climate was the persistent glacier wind. On the relatively large glacier, which has a length of 9.6 km, the gravity wind at the lower parts of the glacier is well developed and has a thickness of about 100 m. To determine the mechanisms that cause the steady-state glacier wind, the author calculated its vertically integrated budgets of momentum, heat, and moisture at a spot on the lower glacier tongue. It was found that the sum of interfacial and surface friction account for only half of the momentum dissipation of the glacier wind; the rest of the katabatic force is most probably balanced by the mesoscale pressure gradient that drives the valley wind above the katabatic layer. The heat and moisture budgets of the boundary layer show simple two-term balances: heat is lost through the turbulent flux of sensible heat at the surfac...

Structure and diurnal variation of the atmospheric boundary layer over a mid-latitude glacier in summer

Abstract: During the summer of 1994, a meteorological experiment (PASTEX) was performed over the Pasterze Glacier, Austria. In this paper we describe the average horizontal and vertical structure of the atmospheric boundary layer (ABL) above the melting glacier, as well as its diurnal variation during a period of fair weather. It was found that very persistent glacier winds with a vertical extent of 100 m dominate the summertime structure of the ABL, because the gravity force acting on the near surface air parcels is many times larger than the synoptic-scale pressure gradient. During fair weather, we find a well developed mountain-valley wind circulation above the katabatic layer. During daytime, the valley wind advects warm and humid air from the ice-free valley towards the glacier, limiting the development of the glacier wind. During the night, the downslope flows that develop above the ice-free valley walls (mountain wind) merge with the glacier wind and enhance the downslope transport of air. The associated subsidence is the most probable cause for the drying of the lower part of the atmosphere during the night. During periods of weak synoptic winds, the glacier wind effectively generates turbulence in the strongly stratified surface layer. On average, the turbulent fluxes of sensible and latent heat provide 25% of the total melting energy at the surface of the glacier tongue, and the influence of the glacier winds on the surface energy budget can therefore not be neglected.

The flow of a polythermal glacier: McCall Glacier, Alaska, U.S.A

Abstract: We have analyzed the flow of polythermal McCall Glacier in Arctic Alaska. Using measurements of surface velocity from the 1970s and 1990s, together with measurements of ice thickness and surface slope, we have investigated both the present flow and seasonal and long-term flow variations. Our analysis of the present flow reveals that (i) longitudinal stress coupling is important along the entire length of the glacier, and (ii) there is significant basal sliding beneath a 2 km long section of the lower glacier. This sliding exists year-round and it accounts for more than 70% of the total motion there. We have developed a numerical model which shows that such a sliding anomaly causes an asymmetric decrease in ice thickness. Accompanying this decrease in thickness is a decrease in surface slope at the center of the anomaly and an increase in slope up-glacier from it. Both effects are reflected in the observed surface profile of McCall Glacier.The longitudinal stress-coupling length of McCall Glacier is three times the ice thickness, almost twice that typical of temperate glaciers. This is a direct effect of lower strain rates, which themselves are associated with the smaller mass-balance gradients of Arctic and continental glaciers. Long-term variations in surface velocity between the 1970s and 1990s are explained solely by the effects of changes in glacier geometry on the deformational flow contribution. This means that long-term variations in the spatial patterns of longitudinal stresses and basal sliding must have been small. Seasonally, Velocities reach their annual minimum in spring and increase during the short summer nick season by up to 75% above mean winter values. However, the extra motion associated with the period of elevated velocities is only about 5% of the total annual motion. The speed-up is due to an increase in basal sliding. This implies that most of the glacier bed is at the melting point. The zone a affected by the melt-season speed-up extends well up-glacier of any moulins or other obvious sources for melt water at the bed.

Glacial regime of the highest Tien Shan mountain, Pobeda-Khan Tengry massif

Abstract: Major processes controlling the existence of a large sub-continental glacier system were identified on the basis of glaciological, meteorological and isotopic analyses using expeditionary and long-term data. Observations were made on the southern Inylchek glacier located in the Pobeda-Khan Tengry massif, the largest sub-continental glacier system on the northern periphery of central Asia. More than 1200 glaciers with a total area of about 4320 km comprise the massif. Melt is for the most part caused by radiation and is most intensive during periods of anticyclonic weather with fohn development. The proportion of solar radiation input in relation to heat balance is more than 90%. Evaporation and condensation are negligible during most times and comprise 7% of heat expenditure. Accumulation was associated with cold cyclonic weather. Four ice-formation zones were identified, the upper boundary of liquid runoffis at 5200 m and the recrystallization zone is above 5900 m. The calculated net glacier mass is negative, -318 kg m -2 a -1 , and indicates the degradation of modern Pobeda-Khan Tengry glaciers.

Snow-accumulation distribution in the interior of the Lambert Glacier basin, Antarctica

Abstract: A prime input variable to uncoupled ice-sheet models, or for estimating the mass budget of present-day ice sheets, is the distribution of net surface mass balance. In most eases this is extrapolated from relatively few direct measurements over a limited time period, and parameterised in terms of continentality, surface elevation and other broad-scale indicators. Between 1989 and 1995 a series of oversnow traverses around the interior of the Lambert Glacier basin gathered a comprehensive set of data on snow accumulation and surface properties, surface climatology, ice-sheet velocities, elevations and thicknesses. Above the 2000 m level accumulation averages were found to be 76 kg ma−2a−1 (σ = 74), much lower than at similar elevations in Wilkes Land. The traverse route contains three distinct accumulation regimes: a relatively high accumulation zone along the western side despite higher average elevations, a very low accumulation zone in the south due to the effect of inereased continentality and an eastern sector that exhibits a rain-shadow effect in predominantly easterly wind fields. Inter-annual variability is high- with 1993 a colder year, recording only half the longer term average accumulation over the portion of the route that was measured.

Climatic effects and bedrock control on rapid fluctuations of Chhota Shigri glacier, northwest Himalaya, India

Abstract: Active tectonism in the Himalaya, and the long-fault lineament controlling the east-west river valleys, play an important role in melt at the snouts of glaciers, or their fluctuations during the year. Lower glacier limits are controlled by high winds from the Indian plains flowing along the valleys. Fluctuations of the Chhota Shigri glacier depend on temperature variations at the junction with the Chandra River valley and in the unconsolidated morainic debris below the glacier ice. High flow rates in the ablation zone indicate that basal sliding is the main movement mechanism. Melt takes place mainly from advection, and this process of melting during the summer season occurs both day and night.

Hydrodynamic effects on the basin expansion of Tsho Rolpa Glacier Lake in the Nepal Himalaya

Abstract: In order to clarify the physics of basin expansion of a supraglacial lake, we investigated the meteorology, hydrodynamics and basin topography in Tsho Rolpa Glacier Lake (27°51'N, 86°29'E : lake level, 4580 m above sea level ; length, about 3 km ; surface area, 1.39 km 2 ; maximum depth, 131 m) in the premonsoon season of 1995. Tsho Rolpa Lake has been expanded by a large recession of the glacier (the Trakarding Glacier) since 1950's, when it was about 1 km long accompanied by six closed ponds 100 to 200 m long. The rapid lake basin expansion accompanying the glacier recession is probably due to an increase of meltwater discharge from the global warming. Vertical and continuous measurements of water temperature and suspended sediment concentration (SSC) in the lake revealed that under the density stratification made up by large SSC, thermal conditions at the bottom are controlled by dynamic behaviors of sediment-laden underflows and wind-driven currents. The underflows were generated off a subaqueous mouth of englacial tunnels, which is located at the base of the cliff-shaped glacier terminus. The leeward transport of radiative heat by wind-driven currents is responsible for enhancing the ice carving at the glacier terminus ; a receding speed of the terminus was 18 cm/d on average in 1993-1994, being 280 times as high as the rate of lake-bottom subsidence, 0.064 cm/d at uplake of the terminal moraine.

Glaciological features of Koryto Glacier in the Kronotsky Peninsula, Kamchatka, Russia

Abstract: Glaciological as well as meteorological features of Koryto Glacier at Kronotsky Peninsula, eastern Kamchatka, were studied in the summer of 1996. The glacier lays in the altitudes between 320 m and 1180 m, and is classified as a temperate glacier. Shallow corings at several altitudes on the glacier in July indicate that the balance increases linearly with increasing altitude in the accumulation area of the glacier. More than 600 cm of the balance from the end of summer in 1995 was found at the highest part of the glacier in July, 1996. The ablation rate less depends on altitude in the accumulation area, although there was significant difference in ablation rates between bare ice at the terminus and clean snow above the transient snow line. Valley-wall effect on ablation rate was found only near the terminus of the glacier. Degree day factors were calculated at two different altitudes : 0.72 (cm °C -1 day -1 ) at the altitude of 545 m and 0.55 (cm °C -1 day -1 ) at the altitude of 1005 m. Simple estimation of mass balance during the balance year of 1995-1996 suggests that the extremely high positive specific net balance of the year can be ascribed to the heavy snowfall in the winter of 1995-1996.

Formation processes of ice body revealed by the internal structure of perennial snow patches in Japan

Abstract: To clarify the transformational processes from firn to ice of perennial snow patches in Japan on the basis of their characteristics of internal structure, core drillings were carried out during the ablation periods of 1995 and 1996 on eight perennial snow patches, located in different mountain areas. The core analyses including stratigraphy, density and liquid water content revealed that the existence of an ice body was a common feature of perennial snow patches nourished mainly by drifting snow in Japan. In addition, a water-saturated firn layer (firn aquifer) is formed just above the firn-ice transition during the ablation period. In this type of snow patch, the transformation from firn to ice may be completed in a single year by early-winter freezing of water-saturated firn which was highly densified in the firn aquifer. In perennial snow patches nourished mainly by snow avalanches, the existence of ice bodies could not be ascertained, because debris embedded in the snow interrupted the drilling. However, it is unlikely that an ice body can be formed by the same processes as in snow patches nourished by drifting snow. Avalanche-fed snow patches exist at relatively low altitudes and air temperatures prior to new snow accumulation are not low enought to freeze the water-saturated firn layer.

Experimental studies on the transformation from firn to ice in the wet-snow zone of temperate glaciers

Abstract: The densification of water-saturated firn, which had formed just above the firn-ice transition in the wet-snow zone of temperate glaciers, was investigated by compression tests under pressures ranging from 0.036 to 0.173 MPa, with special reference to the relationship between densification rate, time and pressure. At each test, the logarithm of the densification rate was proportional to the logarithm of the time, and its proportionality constant increased exponentially with increasing pressure. The time necessary for ice formation in the firn aquifer was calculated using the empirical formula obtained from the tests. Consequently, the necessary time decreased exponentially as the pressure increased, which shows that the transformation from firn in ice can be completed within the period when the firn aquifer exists, if the overburden pressure acting on the water-saturated firn is above 0.12–0.14 MPa. This critical value of pressure was in good agreement with the overburden pressure obtained from depth–density curves of temperate glaciers. It was concluded that the depth of firn–ice transition was self-balanced by the overburden pressure to result in the concentration between 20 and 30 m.

INSAR at ASF: Analysis of the 1993 Bering Glacier Surge

Abstract: This talk will begin with a brief discussion of the development of SAR interferometry at the Alaska SAR Facility (ASF) and will proceed to an analysis of interferometrically derived data on the Bering Glacier. The Bering Glacier and its associated accumulation area, the Bagley Ice Field, are located in South Central Alaska in the Chugach-St.Elias Mountains. Together they comprise the largest temperate valley glacier in North America. Differential SAR interferometry is applied to the Bagley Ice Field prior to and after the onset of a major event in the spring of 1993 using data from the two ERS-1 Ice Phases (early in 1992 and 1994). Variations in surface elevation and surface ice velocity in the quiescent accumulation area upstream of the catastrophically disrupted surging region are discussed in the context of surge mass balance estimation. The flow directions in the region of interest tend to run roughly in the cross-track direction but conversion of line-of- sight displacement over three day repeat-pass intervals to surface velocity vector fields is neverthe less problematicaL Attention is also given to the difficulties in analyzing low-coherence signals from temperate-climate glacier surface ice near the Gulf Coast of Alaska. Despite these obstacles, interferometry is providing high-resolution data input to the general problem of understanding the dynamic nature of this glacier system.