Abstracts on presentations held at the

Skaftafell, Iceland, 20-24 June 1998
European Science Foundation
European Ice Sheet Modelling Initiative (EISMINT)

Abstracts on presentations


Basic characteristics of Vatnajökull: geometry, mass balance, flow, hydrology, subglacial geology.

Helgi Björnsson
Science Institute, University of Iceland
Dunhaga 3, 107 Reykjavik, Iceland

Vatnajökull, the largest ice cap in Europe (8100 km2 in area), is an important ice mass in glaciological, hydrological and climatological respect. The ice cap is situated close to the southeastern coast of Iceland, extending 150 km from west to east and 100 km from south to north; from 63 55' N to 64 50' N and 15 15' W to 18 10' W. The ice drains several domes where elevation is from 1400-2000 m to outlet glaciers whith margins at 600-800 m along the western to the northeastern margin and 50-100 m along the southern margin. The mean altitude of the ice cap is 1300 m. For a year of zero net balance, the equilibrium line is estimated to be about 1100 m for the southwestern outlets but 1200-1300 m for the northwestern and northern outlets; the accumulation area is typically about 60% of the total glacier area (Björnsson and others, 1998). The maximum thickness of the ice cap is about 900 m and the mean thickness about 400 m. The lowest point at the glacier base is at about 300 m beneath sea level and only about 10% of the base is above 1100 m elevation.
The annual precipitation falls from 4-7 m in the south where the mean temperature is 4 C to to 0.4 mm in the northern margin where the mean temperature is about 0 C. In the central areas of the ice cap, the winter balance has typically been about 2.5 m of water equivalent but the summer balance has varied from +0.5 to -0.5 m, and hence the net mass balance has varied from 2 to 3 m. At the glacier termini of 700-800 m elevation, the summer balance was typically about -5 m, and the winter balance 1.5 m on the western outlets and 0.5 m on the northern ones. At the elevation of 100 m on the southern outlets, the summer balance is about -10 m /a. The western outlets are situated in the active volcanic zone and the ice surface in ablation areas is covered with abundant ash cover which enhances melting. The activity index, defined as the gradient of the net balance with elevation at the equilibrium line, is about 60-90 mm/100 m.
The mass balance has been monitored since 1991. All mass balance components apply to a time interval between given measurement dates which are not fixed from one year to another. The dates in the autumn are separated by approximately one calender year which roughly coincides with the hydrolocical year defined as October 1 to September 30. The measurements in the spring were done in late April to mid-May. The spatial variability of the mass balance is described by observations along a limited number of flowlines which span the elevation range of the. Recently, modern over snow vehicles and helicopters have allowed fast traverses to ensure successful field work in spite of frequently poor weather conditions. During the period from 1991 to 1995 glacier mass balance has been monitored on the western and northern outlets of which altogether comprise about half of the total area of the ice cap (4000 km2) and extend from an elevation of 2000 m down to 600 m. After 1996, mass balance measurements have been carried out over an area covering 70-80 % of the ice cap and over the elevation range from 2000 m down to 100 m. Error limits for the area integrals of the mass balance components must be assigned no lower than 15%. The mean specific winter balance of the glaciers was fairly constant over the first four years but the summer balance, and hence the annual net balance, decreased year by year. The specific annual net balance was positive for all the northern outlets in the first three years due to cold spells and snowfall during the summers but slightly negative for the western ones in the third year. In the year 1994-95 the mass balance was in general negative but close to zero for one outlet. Over the last three years (1996-1998) the winter balance has been considerably lower than in the first four years (1991-95) and the net balance negative. These years, however, do not illustrate the full scale of climatic variability and extremes that can be expected (Veðráttan, 1924-1998).
Previous observations on the mass balance of Vatnajökull have been rather patchy. The first mass balance measurements on Vatnajökull were done in. 1919 (Wadell, 1920?). During the years 1936-39 detailed mass balance measurements were undertaken on Hoffellsjökull, a southeastern outlet of Vatnajökull (Ahlmann and Thorarinsson, 1937a, 1937b, 1938, 1939; Ahlmann, 1937, 1939, 1940; Thorarinsson, 1939). In 1951 and 1960 the winter balance was measured in a number of points all over the ice cap (Rist, 1952, 1961). Since 1954, systematic measurements of accumulation have been done in the Grímsvötn area in the interior of Vatnajökull (Björnsson, 1985) and sporadic measurements of accumulation and melting have been done on various parts of the glacier. Further, mass balance measurements were carried out in 1985-86 on Tungnaárjökull, a western outlet (Björnsson, 1988). Since 1991, mass balance has been regularily monitored along flowlines on several outlets of the ice cap (Björnsson and others, 1992, 1995a,b).

Glacio-meteorological measurements

Since 1994 automatic weather stations have been operated on the ice cap. The work has been carried out in collaboration between the National Power Company and the Science Institute, University of Iceland, and during the years 1996-98 also in collaboration with the Universities of Utrecht, Amsterdam and Innbruck, and supported by the fifth framework of EU (the prosjects TEMBA and ICEMASS).

Ice flow

Surges are common in Vatnajökull. All the outlets except the valley glaciers draining east of the ice caps have been observed to surge.
Surface velocity has been measured 30-50 points on Vatnajökull during the summer since 1992 using differential Global Position System. In general the flow has been considerably lower than required to transport downglacier the mass accumulated on the glacier; hence surges are expected in the future.


A wealth of information is available over centuries about drainage of water from Vatnajökull (Björnsson, 1988). Runoff has been monitored for over 50 years, recently intensified as part of planning and managing hydro-power. The mass balance data provide the first estimates of the meltwater contribution to the various glacial rivers systems. For a year of zero net balance, the specific runoff corresponding to the summer balance was about 60 l s-1 km-2 averaged over the entire glacier and the whole year, dropping down to 30 l s-1 km-2 in the years of the most positive glacier mass balance. Rain on the glaciers during the five summer months may add 10-20 l s-1 km-2 to the specific discharge from the glacier
Glacier outburst floods (jökulhlaups in Icelandic) may profoundly affect landscapes, devastate vegatated areas, threaten lives, roads, bridges and hydroelectrical power plants on glacier-fed rivers. Their effects on the landscape are seen in the erosion of large canyons and in the transport and deposition of sediments over outwash plains. Jökulhlaups drain regularly from six subglacial geothermal areas in Iceland. From Grímsvötn in Vatnajökull, jökulhlaups have occurred at a 4-6 year interval since the 1940s with a peak discharge of 1,000 - 10,000 m3/s, 2 -3 weeks duration and total volumes of 0.5-3 km3. Prior to that, about one jökulhlaup occurred per decade with an estimated discharge of 5 km3 of water and a peak discharge of approximately 30,000 m3/s. Clarke's (1982) modification of Nye's (1976) general model of discharge of jökulhlaups gives in many respects satisfactory simulations for jökulhlaups from Grímsvötn. The best fit is obtained for the Manning roughness coefficients n = 0.08-0.09 m-1/3s and a constant lake temperature of 0.2 C (which is the present lake temperature). The rapid ascent of the exceptional jökulhlaup in 1938, which accompanied a volcanic eruption, can only be simulated for a lake temperature of the order of 4 C.
Jökulhlaups originating at geothermal areas beneath ice cauldrons located 10-15 km northwest of Grímsvötn have a peak discharge of 200-1,500 m3/s in 1-3 days, the total volume is 50-350x106 m3, and they recede slowly in 1-2 weeks. The form is in that respect a mirror image of the typical Grímsvötn hydrograph. The reservoir water temperature must be well above the melting point (10-20 C) and the flowing water seems not to be confined to a tunnel but spread out beneath the glacier and later gradually collected back to conduits.
At present, jökulhlaups originate from some fifteen marginal ice-dammed lakes in Iceland. Typical values for peak discharges are 1,000 - 3,000 m3/s, duration 2-5 days and total volumes of 2,000 x 106 m3. Hydrographs for jökulhlaups from marginal lakes have a shape similar to those of the typical Grímsvötn jökulhlaup. Simulations give reasonable ascent of the hydrographs for constant lake temperature of about 1 C but fail to show the recession. Some floods from marginal lakes, however, have reached their peaks exceptionally rapidly, in one day. That ascent could be simulated by drainage of lake water of 4-8 C.
An empirical power law relationship is obtained between peak discharge and total volume of the jökulhlaups from Grímsvötn; Qmax = K Vtb, where Qmax is measured in m3/s, Vt in 106 m3, K = 4.15 10-3 s-1 m-2.52 and b = 1.84.
In general, the jökulhlaups (excepting those caused by eruptions) occur when the lake has risen to a critical level, but before a lake level required for simple flotation of the ice dam is reached. The difference between the hydrostatic water pressure maintained by the lake and the ice overburden pressure of the ice dam is of the order 2-6 bar.

Since the time of the settlement of Iceland (870 A.D.), at least 80 subglacial volcanic eruptions have been reported in Vatnajökull, many of them causing tremendous jökulhlaups with dramatic impact on inhabitated areas and landforms. Meltwater created by subglacial volcanic eruptions may discharge instantly toward the glacier margins as determined by the rate of melting. Alternatively, benath large ice masses water may accumulate in subglacial lakes and finally drain in outburst floods; the best known example of such a flood was that of November 1996 in Vatnajökull in Iceland when meltwater was accumulated in the subglacial lake Grímsvötn before draining in a catastrophic flood. It is now apparent that the potentially largest and most catastrophic jökulhlaups may be caused by eruptions in the voluminous ice-filled calderas in northern Vatnajökull (of Bárdarbunga and Kverkfjöll). They may be the source of prehistoric jökulhlaups, with estimated peak discharge of 400,000 m3/s. The peak discharge of the largest floods from Katla in Mýrdalsjökull has been estimated at the order of 100-300,000 m3/s, duration was 3 -5 days and the total volume of the order of 1 km3.

Ahlmann, H. W. 1937. Vatnajökull in relation to other present-day Icelandic glaciers. Geografiska Annaler, 19, 212-229.

Ahlmann, H. W. 1939. The regime of Hoffellsjökull. Geografiska Annaler, 21, 171-188.

Ahlmann, H. W. 1940. The relative influence of precipitation and temperature on glacier regime. Geografiska Annaler, 22, 188-205.

Ahlmann, H. W. and S. Thorarinsson. 1937a. Object, resources and general progress of the Swedish-Icelandi investigations. Geografiska Annaler, 19, 140-160.

Ahlmann, H. W. and S. Thorarinsson. 1937b. Previous imvestigations of Vatnajökull, marginal oscillations of its outlet-glaciers and general description of its morphology. Geografiska Annaler, 19, 176-211.

Ahlmann, H. W. and S. Thorarinsson. 1938. Scientific results of the Swedish-Icelandic investigations 1936-37-38. The ablation. Geografiska Annaler, 20, 171-233.

Ahlmann, H. W. and S. Thorarinsson. 1939. Scientific results of the Swedish-Icelandic investigations 1936-37-38. The accumulation. Geografiska Annaler, 21, 39-66.

Björnsson, H. 1979. Glaciers in Iceland. Jökull, 29, 74-80.

Björnsson, H. 1985. The winter balance in Grimsvötn 1954-1985. Jökull, 35, 107-109.

Björnsson, H. 1988. Hydrology of Ice Caps in Volcanic Regions. Rit XLV, Societas Scientarium Islandica. Reykjavík. 139 pp (and maps).

Björnsson, H., F. Pálsson. 1991. Vatnajökull, northeastern part, Maps in scale 1 : 100 000. Ice divides, water divides. National Power Company and Science Institute.

Björnsson, H., F. Pálsson, and M. T. Gudmundsson. 1992. Vatnajökull, northwestern part, Maps in scale 1 : 100 000. Ice divides, water divides. National Power Company and Science Institute.

Björnsson, H., F. Pálsson, and M. T. Gudmundsson. 1995a. Afkoma, hreyfing og afrennsli á vestan- og norðanverðum Vatnajökli jökulárin 1992-1993 og 1993-1994. Science Institute Report RH-95-2.

Björnsson, H., F. Pálsson, and M. T. Gudmundsson. 1995b. Afkoma, hreyfing og afrennsli á vestan- og norðanverðum Vatnajökli jökulárið 1994-1995. Science Institute Report RH-95-25.

Björnsson, H., F. Pálsson, M. T. Gudmundsson and H. H. Haraldsson. 1998. Mass balance of western and northern Vatnajökull, Iceland, 1991-1995. Jökull, 45, 35-58.

National Energy Authority. Hydrological Service, 1994, 1998a,b. Data deliveries 1994; 04/1998; 06/1998.

Rist, S. 1952. Snow measurements on the Vatnajökull from March 27 to April 24, 1951. Jökull, 2, 6-7.

Rist, S. 1961. Rannsóknir á Vatnajökli 1960. Jökull, 11, 1-11.

Thorarinsson, S. 1939. Hoffellsjökull, its movement and drainage. Geografiska Annaler, 21, 189- 215.

Veðráttan, 1924-1995. Monthly climatic summary. Icelandic Meteorological Office. Reykjavík.

Wadell, H. 1920. Vatnajökull. Some studies and observations from the greatest glacial area in Iceland. Geografiska Annaler, 4, 300-323.
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Interferometry of Vatnajökull

Helmut Rott
Institute of Meteorology and Geophysics
University of Innsbruck, A-6020 Innsbruck, Austria

The phase differences of repeat pass SAR images are sensitive to changes in surface elevation and to displacement in direction of the radar beam. Phase coherence between two SAR images is a precondition for generating an interferogram which is the basis for mapping surface motion and elevation. Studies of coherence of ERS-1 and ERS-2 SAR data of Iceland from one-day, three-day and 35-day repeat intervals showed good coherence over glacier surfaces only for one-day repeat intervals. Therefore the investigations focused on the use of one-day repeat data which were obtained by means of the tandem operation of ERS-1 and ERS-2. Even for these data the signals over the glaciers decorrelate at certain dates partly or completely. The main reason for loss of coherence is surface melt, but also snowfall, wind drift, and small scale surface deformation may cause disturbing phase shifts. The best coherence conditions on the glaciers are found during winter. ERS-1 and ERS-2 operated simultaneously between July 1995 and June 1996, the so-called Tandem Mission, providing 24-hours repeat pass SAR images. During the Tandem Mission a significant number of interferometric pairs was acquired over Vatnajökull. After June 1996 only ERS-2 SAR was in operation continuously. However, to enable the study of ice deformation after the eruption in October 1996, ESA switched on ERS-1 SAR over western Vatnajökull several times on special request to operate in tandem with ERS-2. This data base includes interferometric pairs from five different dates between 21/22 October 1996 and 8/9 October 1997.

The one-day repeat data over Vatnjökull represent an excellent basis for studying the motion field. However, certain restrictions should be kept in mind: (1) It is necessary to separate the topography-dependent and motion-dependent phase shifts. This is possible by using two interferometric pairs, if the motion and topography did not change between the dates when these pairs were acquired. This is not the case for surging glaciers and for the deformation related to volcanic activity. A practical solution to overcome this problem is the use of an interferometric pair with a short baseline, which is not sensitive to topography, and a synthetic interferogram calculated from an available digital elevation model to subtract the topographic phase. (2) SAR interferometry measures only the motion component in direction of the radar beam. In order to obtain the motion vector, SAR images from different view directions (ascending and descending orbit) can be combined. Another possibility is to assume ice-flow parallel to the surface if accurate topographic data are available or can be derived differentially.

Analysis of interferometric pairs, acquired between 1/2 January 1997 and 8/9 October 1997, revealed detailed features of ice motion in the region of the Gjálp eruption site and deformation patterns of the ice shelf of Grímsvötn. As another example, the motion field of Sylgjujökull was derived from a tandem pair of 27/28 March 1996, when the glacier was in the final stage of a surge, showing the motion maximum not far from the glacier front. Ongoing work is focusing on the analysis of ice motion around the eruption site at different dates in 1997. As next step of the interferometric analysis it is planned to map the surface motion of glaciers of western Vatnajökull.
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Dating of Vatnajökull by ash layers

Gudrun Larsen, Magnús T. Gudmundsson and Helgi Björnsson
Science Institute, Dunhaga 3, 107 Reykjavík

A Science Institute project involving the identification and dating of tephra layers on the ablation areas of Vatnajökull ice cap was initiated in 1994. The project has been supported by the Icelandic Science Fund (Rannís). The aims of the project are threefold (Larsen et al. 1996, 1998):

(1) To date the ice in the outlet glaciers using tephra layers that crop out in their ablation areas; (2) To connect dated tephra layers to internal reflections seen on ice radars, thereby providing a tool to study glacier mass balance for certain periods of the past; (3) To add to the knowledge of the eruption history of volcanoes below the Vatnajökull ice cap.

Tephra that falls on accumulation areas of glaciers is subsequently buried by the accumulation of snow and carried downglacier by ice flow. The extent of each tephra layer within Vatnajökull depends on the extent of the tephra fall onto the accumulation areas: A tephra layer that was deposited all over Vatnajökull should appear in the ablation areas of all the outlet glaciers while layers that covered only limited area may appear in only one glacier. The tephrostratigraphy of two glaciers, Tungnaárjökull in the western part and Brúarjökull in the northern part of Vatnajökull, has been studied in detail and work on the third, Skeidarárjökull in the southern part, is in progress. Work on the fourth outlet glacier, Dyngjujökull, will commence this summer and preliminary surveying of two others, Köldukvíslarjökull and Breidamerkurjökull, has been carried out.
On the two first mentioned glaciers, both of which are surging glaciers, 35-40 dark bands crop out in the ablation areas. Most of the bands consist of ash- to lapilli-sized tephra particles. Debris bands are only found close to the snout. The tephrostratigraphy is undisturbed by repeated surging and individual tephra layers can be followed for tens of km on the ice.
The volcanic systems on the Eastern Volcanic Zone, including the ones below Vatnajökull, have chemical characteristics that allow the tephra layers to be connected to their sources. Comparison to written records of eruptions and to the regional tephrochronology allow certain tephra layers to be dated to a precise calendar year. They serve as key layers to date the remaining tephra layers (by extrapolation) and the ice, as well as to connect the tephrostratigraphy on the ablation areas of the various glaciers.
Key layers include Öræfajökull 1362 AD, Veidivötn 1477 AD, Grímsvötn 1619 AD, Katla 1625 AD and 1755 AD, and Laki/Grímsvötn 1783/84 AD. The oldest tephra layer to which a precise calendar date can be attributed is a Katla layer thought to be from 1262 AD in Brúarjökull while extrapolation indicates that the oldest discernable tephra layers in both glaciers are from the middle/early 12th century AD. The age of the oldest ice in both glaciers, which is sheared and contains debris, cannot be determined by tephrochronology, but dates at least back to the 11th century AD.


Larsen, G., M.T. Gudmundsson and H. Björnsson 1996. Tephrastratigraphy of Ablation Areas of Vatnajökull Ice Cap, Iceland.
In S.C. Colbeck (ed): Glaciers Ice Sheets and Volcanoes: A tribute to Mark Meier. CRREL Special Report 96-27: 75-80.

Larsen, G., M.T. Gudmundsson and H. Björnsson 1998. Eight centuries of periodic volcanism at the center of the Iceland. Back to top

Radio echo soundings of volcanic ash layers and mass balance rates of Vatnajökull, Iceland

Helgi Björnsson, Finnur Pálsson1, Yngvar Gjessing2, Sven Erik Hamran3 , Björn Erlingsson4
1: Science Institute, University of Iceland, 2: University of Bergen,
3: Environmental Surveillance Technology Program, PFM, 4: Norwegian Polar Institute.

Continuous internal reflections have been mapped by radio echo soundings over large areas on Vatnajökull, Iceland. Four such reflecting planes are found in the Grímsvötn area, in the interior of the ice cap, at depths about 130 m, 150 m, 180 m and 200 m. These four reflecting planes are interpreted as tephra layers from volcanic eruptions in the Grímsvötn volcano. The eruptions, presumably, date from A.D. 1934, 1922, 1903 and 1883, respectively. If so, the layers were burried at an average rate of 2.3 -1.9 m/a.

Extensive radio echo soundings have been carried out on the ice caps Vatnajökull, Hofsjökull and Mýrdalsjökull, using a low frequency pulse radar (Björnsson, 1986, 1988; Sverrisson and others, 1980). The purpose of the work was to map the bedrock topography, but in several places continuous reflectors were recorded inside the glacier, at depths ranging from tens to hundreds of metres (Fig. 1). These reflectors were conspicuous in large areas of W-Vatnajökull, where volcanic eruptions are frequent, and it has been considered likely that the reflectors are tephra layers. As a further step in the studies of these reflecting layers radio echo soundings were done on Vatnajökull in 1991 by a multiband synthetic pulse radar developed by the Norwegian Environmental Surveillance Technology Program (Hamran, 1989). This radar has higher spatial resolution than the Icelandic pulse radar. The present paper reports some results of the studies of these internal reflecting layers by both radars.

Both low and high frequency radars can be used to detect internal reflecting planes in Icelandic glaciers. These planes are presumably tephra layers and radio echo sounding of these layer is a powerful tool for tephrochronological studies in Iceland. Through such a work valuable data will be collected for estimating average mass balance of the glaciers. Back to top

Basal processes and conditions beneath ice masses with deformable beds.

Tavi Murray
School of Geography, University of Leeds, LS2 9JT, UK

This presentation reviewed the techniques and measurements which have been used to assess bed properties beneath glaciers with temperate ice overlying soft beds, the basal conditions thought to be prevalent beneath much of the Vatnajökull icecap. Sufficient measurements have now been made with identical instruments beneath a variety of glaciers which allow the first real inter-comparison of the properties of basal conditions and assessment of the variability of the properties of basal till (Tables 1-3). The problem of coupling between the glacier sole and its bed, and the variation of sliding and deformation with basal water pressure is not yet resolved: insufficient simultaneous measurements of sliding, deformation and water pressure have led to equivocal results. These difficulties result in part from the inhomogeneity of the basal mechanical and hydrological systems, and the complexity of interactions between them. Future advances are likely to come most rapidly from the combination of geophysical techniques which illuminate basal conditions at scales 10s metres (such as high resolution seismics), with bore-hole instrumentation, which provides temporal coverage and access to the basal processes involved.

The presentation also outlined new results from bore-hole instruments installed beneath Falljökull, an outlet glacier from Öræfajökull in the south western region of Vatnajökull (Figure 1 and 2). Penetration testing of the bed beneath the glacier suggests that it is comprised of patchy till and bedrock. The instruments installed include ploughmeters and pressure sensors. Such instruments allow the calculation of till properties if certain assumptions are made (Fischer and Clarke 1994; Porter and others, 1997). If the entirety of the ice surface motion (measured at 35.5 m yr-1) is ascribed to basal motion then the yield strength of the basal material can be calculated to be 7.3 - 29.8 kPa and the viscosity to be 6.0 x 108 - 2.4 x 109 Pa s. The basal shear stress beneath this steep valley glacier (~ 125 kPa) is greater than this yield strength which suggests that the sediments should be deforming, but the bedrock hummocks may be acting as "sticky-spots" supporting a greater than average proportion of the basal shear stress. The force experienced on the ploughmeters is weakly negatively correlated to basal water pressure with no apparent lag (Figure 2). An increase in basal water pressure would be expected to either decouple the glacier from its bed and increase sliding or to increase deformation in the basal sediments through elevated pore water pressures. In the first scenario, force could increase, if the sediments are considered to be viscous in their behaviour, however, if the response is plastic no change in force would be expected. The second scenario, that of increased deformation would decrease the force experienced on a ploughmeter, but a delay between the change in water pressure and the corresponding change in force, corresponding to diffusion of water through the sediments would be expected. In this case we believe that the change in forcing could result from uplift of the glacier at high water pressure reducing the insertion depth of the ploughmeter. This would require the response of the sediments to be plastic or only very weakly strain-related. Measurement of surface uplift during 1998 has subsequently shown uplift of several centimetres associated with increased surface velocity can result from high basal water pressure supporting this suggestion.

Yield strength (kPa)Viscosity (Pa.s)Basalshear stress (kPa)
Trapridge Glacier48 - 573.0 x 109- 3.1 x 101077
Bakaninbreen16.6 - 87.51.1 x 1010 - 4.3 x 10105.8 - 28.8
Falljökull7.3 - 29.86.0 x 108 - 2.4 x 109125
Table 1. Glaciers beneath which till properties have been measured using ploughmeters.

Effective viscosity
Pa s-1
s(kPa)Basal shear stress (kPa)f()Reference
Trapridge1.7 x 1010 - 8.0 x 10103048 - 5777Blake,1992,Clarke, 1987
Bakaninbreen1.1 x 1010 - 4.3 x 101017 - 8726Porter&Murray,in prep
Table 2. Glaciers beneath which till properties have been measured using tiltcells. The geotechnical properties σ (the friction angle) and ( (the sediment yield strength) are also given.

References: Blake, E.W. 1992. The deforming bed beneath a surge-type glacier: measurement of mechanical and electrical properties. PhD thesis, University of British Columbia Vancouver.

Boulton, G. S., Dent, D. L. and Morris, E. M. 1974. Subglacial shearing and crushing, and role of water pressures in tills from south-east Iceland. Geograftska Annaler, 564: 135-145.

Clarke, G. K. C. 1987b. Subglacial till: a physical framework for its properties and proce Journal of Geophysical Research, 92(B9): 9023-9036.

Engelhardt, H., Humphrey, N. and Kamb, B. 1990b. Borehole geophysical observations of ice stream B, Antarctica. Antarctic Journal of the U.S., 25(5): 80-82.

Fischer, U. H., and Clarke, G. K C. 1994. Ploughing of subglacial sediment, Journal of Glaciology, 40(134), 97-106.

Fountain, A. G. 1994. Borehole water-level variations and implications for the subglacio hydraulics of South Cascade Glacier, Washington, U.S.A. Journal of Glaciology 40(135): 293-304.

Hubbard, B., Sharp, M.J., Willis, I., Nielsen, M. K. and Smart, C.C. 1995. Borehole water level variations and the structure of the subglacial hydrological system of Haut d'Arolla, Valais, Switzerland. Journal of Glaciology, 41(139): 572-583.

Iverson, N. R., Jansson, P. and Hooke, R. LeB. 1994. In-situ measurement of the strength deforming subglacial till. Journal of Glaciology, 40(136): 497-503.

Murray, T. and Clarke, G.K.C. 1995. Black-box modeling of the subglacial water system Journal of Geophysical Research, IOO(B7), 10231-10245.

Porter, P. R., Murray, T. and Dowdeswell, J. A. 1997. Sediment deformation and basal dynamics beneath a glacier surge front: Bakaninbreen, Svalbard. Annals of Glaciology, 24: 21-26.

Porter, P. R., T. Murray and M. D. Crabtree. In prep. Characteristics of the bed of Falljökull, Iceland. To be submitted to: Earth Surface Processes and Landforms.

Waddington, B. S. 1993. Hydraulic properties of subglacial sediments determined from mechanical response of water-filled boreholes. MSc thesis, University of British Columbia, Vancouver.
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Controls on glacier surging: multivariate analysis of surge-type glaciers using glacier inventories.

Hester Jiskoot
School of Geography, University of Leeds, LS2 9JT, UK

Surge-type glaciers manifest long periods of slow flow (I5 - 100+ yr.) intermitted by short periods of fast flow (1-10 yr.). The switch from the slow mode to the fast mode is through an intemally triggered flow instability, related to water pressure fluctuations, but the exact surge mechanisms are still largely unknown. Surge-type glaciers have restricted outflow during the quiescent phase, which is reflected by flow velocities lower than the balance velocity. During quiescence, the upper part of the glacier thickens, while the lower part is thinning and retreating. During a surge the surplus volume of ice in the upper reservoir zone discharges rapidly into the lower receiving zone, usually as a wave/surge bulge travelling down glacier. This redistribution of ice results in marked changes in the glacier's geometry and morphology, and surges frequently result in an advance of the glacier front.

Surge-type glaciers occur in clusters. Besides the frequent occurrence of surges in Iceland, surges have been observed in parts of Alaska, Canada, Greenland, Svalbard, the Caucasus, the Karakoram, the Pamir, the Tien Shan, the Andes, Kamchatka, and surging is suggested for the Russian High Arctic. As this spatial distribution of surge-type glaciers is markedly nonuniform it may be possible to distinguish these from the population of non-surge-type glaciers on the basis of glacial and environmental variables. By identifying those factors related to surging we are able to test proposed surge theories and indicate physical properties controlling the surge process. With this aim we developed LOGIT models of glacier surging in Svalbard and the Yukon Territory, while models for East Greenland and Iceland are in preparation. LOGIT models are a type of linear multivariate statistics, where both continuous and categorical data can be used to calculate surge probabilities for individual glaciers. The Svalbard LOGIT model was based on 132 surge-type and 372 non-surge-type glaciers, which were selected from the Glacier Atlas of Svalbard and Jan Mayen (Hagen et al., 1993). The surge classification was based on the Glacier Atlas, publications, and airphoto interpretation. All glaciers were tested for 19 glacial, geological, and mass-balance related variables. The optimal model includes the variables length, slope, aspect, lithology and geological age. Results indicate that long glaciers (>10 km) with relatively steep surface slopes overlying shale or mudstone have the highest surge probabilities. Our findings for geology and slope do not support Kamb's linked-cavity theory but lend some support to soft-bed surge theories.

Residual analysis was used to test the model performance for individual glaciers. Large residuals can be found in two classes: glaciers that are classified as non-surge-type but have high surge probabilities, and glaciers classified as surge-type but have low surge probabilities. The first class contains 8 glaciers with surge probabilities over 0.7. Of these, morphological evidence showed that 7 must be re-classified as surge-type. The discovery of these previously undetected surge-type glaciers shows that our model is an effective tool in predicting glacier surging. The second class consists of 35 glaciers. For some of these, the LIA advance may have been misinterpreted as a surge advance. Detailed studies of the remaining glaciers should give further clues for controls on surging.

Additionally, I compare surge behaviour (onset, velocity development, impact, periodicity, duration of the surge, etc.) of surge-type glaciers in different regions and relate these surge characteristics to substrate, mass balance properties and possible climate variations.
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Response to the eruption in 1996.

Magnús T. Guðmundsson
Science Institute, University of Iceland, Dunhaga 3
107 Reykjavik, Iceland

The eruption in October 1996 occurred under 500-750 m thick ice in NW-Vatnajökull, between the two central volcanoes of Bárðarbunga and Grímsvötn. It provided a unique opportunity to study an eruption under a thick ice cap. Such eruptions under the Pleistocene ice sheets formed the hyaloclastite tuyas and ridges found in the volcanic zones in Iceland. For the first time the formation of such a hyaloclastite ridge in a subglacial eruption could be studied. Ice melting rates could be measured and the response of the ice cap to large changes in ice surface geometry was observed. An eruption occurred at the same place in 1938 and the site has been given the name Gjálp, after a giantess in Nordic mythology.

The eruption took place on a six kilometre long fissure trending roughly NNE. On the second day the eruption broke through the ice cover opening a subaerial vent that was active for the reminder of the eruption. The eruption lasted from September 30 until October 13 and only a minor part of the activity was subaerial. Heat transfer from magma to ice was extremely fast. Melting rates were 0.5-0.8 km3/day for the first four days, as large depressions were formed in the ice surface above the fissure. By October 13 the eruption had melted 2.4 km3 of water, all of which was drained into the Grímsvötn subglacial lake. A further 0.4 km3 had melted by November 5 when all the meltwater was released from Grímsvötn in a very swift jökulhlaup lasting less than 2 days. No water accumulation occurred at the eruption site during the eruption; the meltwater was apparently drained away continuously. Ice melting over the path of the meltwater suggests that it had a temperature of 15-20C as it was drained from the vents. Calorimetric calculations of ice melting indicate that the total volume of magma erupted was 0.4 km3, which corresponds to a hyaloclastite ridge of 0.7 km3. The ridge is 6 km long and rises 150-350 m over the pre-eruption bedrock. It may in future modify ice flow in the area to some extent.

The draining of the meltwater caused the creation of the ice cauldrons over the fissure, since the volume removal from the glacier was only partly compensated by the formation of the hyaloclastite ridge. Ice flow during the cauldron formation apparently occurred by rapid internal deformation with little or no basal sliding. By the end of the eruption the maximum width of the depression was 4 km perpendicular to the trend of the fissure, increasing gradually to 7 km in August 1997. The depressions caused considerable migration of ice- and water divides in the area between Grímsvötn and Bárðarbunga. The water drainage area of Grímsvötn increased in size from 154 km2 before the eruption, to 207 km2 in January 1997. A similar increase occurred in the size of the ice drainage area. The eruption removed about 5 km3 of ice from the ice cap and lowered the ice surface elevation by up to 150 m in an area of 60 km2. If it assumed that the affected area has a positive mass balance of 1.5-2 m/yr and it will eventually reach a size of 100 km2, it will take 25-35 years for the ice cap to regain its former elevation. However, further volcanic activity may increase this "healing" time.

A thin tephra layer was deposited over Vatnajökull to the north, east and the south of the eruption site. Summer ablation in 1997 was clearly enhanced by decreased albedo as tephra became exposed. The 1996-1997 annual layer was melted completely below 1600 m elevation on the western part of Dyngjujökull and in places up to 1900 m elevation on the eastern slopes of Bárðarbunga. In 1998 the tephra layer was also exposed in places, but not as widely as in 1997.
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Data on the flow of Vatnajökull

Guðfinna Aðalgeirsdóttir
ETH Zentrum, Gloriastrasse 37/39
CH 8092, Zurich, Switzerland

Mass balance and surface velocity have been measured on Vatnajökll from 1992 until present and additionally in 1986. The measurements are done by drilling poles in the accumulation area but long wires in the ablation area and the height or length of those is measured along with the location using differential Global Position System. This is usually done in the spring and in the fall but sometimes more frequently. The glaciers that have been measured regularly are Síðujökull, Tungnaárjökull, Köldukvíslarjökull, Dyngjujökull, Brúdarjökull and Grímsvötn area; all on the north and west sides of Vatnajökull. Eyjabakkajökull and Breiðamerkurjökull have been measured since 1995 and 1996, respectively. Those measurements include the surges on Tungnaárjökull and Síðujökull, but after the surges it was impossible to measure for several years due to highly crevassed surface.

The glaciers on the northern side have had a slight increase in velocity but more notable is that the mass balance has been gradually decreasing during this time period. This results in a decrease in the velocity that is required to move the mass that is accumulated in order to keep the glacier in balance. The calculated balance flow at the equilibrium line is thus converging to the measured flow. For example has the flow measured on Dyngjujökull changed from being 7 per cent of the calculated balance flow to be equal to it at the same time as the net balance has changed from being positive 1.3 m to negative 0.6 m. The summer velocity on Brúarjökull is about 1.5 times the year velocity but for the other glaciers the summer velocity is about the same size as the year velocity.
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Modelling ice flow

Guðmundur Hilmar Guðmundsson
ETH Zentrum, Gloriastrasse 37/39, CH 8092
Zurich, Switzerland

Almost all flow models of ice caps determine the flow and the flux of ice by solving numerically the continuity equation which relates the rate of elevation changes and the spatial flux gradients with the mass balance distribution. A large class of models makes the assumption that the flow field at each spatial point depends on the surface elevation and the surface slope at that location only. This may be justified if spatial gradients in the stress field do not have a significant effect on the flow field. Weather spatial gradients may be ignored or not in a glacier flow model depends not only on the spatial scale which needs to be resolved in the model, but also, and this may be less well appreciated, on the temporal resolution to be achieved. If temporal resolution comparable to or larger than the volume time scale is being aimed at, a numerical model, which is incorrect in the sense that it gives wrong estimates of the propagation and the diffusion time scales, can nevertheless be used for accurate predictions of general changes in the geometry of an ice cap as a result of, for example, a shift in climate. This fact has been know for some time and it constitutes the main justification for the use of flow models ignoring stress gradients for estimating the effects of climatic shifts on the geometries of ice caps and ice sheets.

Recent theoretical work has made it possible to calculate, at least in some limiting cases, exactly the effects of basal perturbations on glacier flow in three dimensions. These theoretical developments are discussed and examples are given for the transient response of glaciers to both basal undulations and spatial variations in resistance to basal were the effects of stress gradients on the flow field are taken rigorously into account. The properties of the resulting anomalous flow fields are complicated and often, at first sight, counterintuitive. Under certain circumstances, for example on ice streams, localized bedrock perturbations are found to generate surface disturbances which stretch over long distances as compared to the mean ice thickness.
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Ice Dynamical Modelling of the Vatnajökull Ice Cap

Shawn Marshall, Fern Webb, Garry Clarke and Gwenn Flowers 1) Helgi Björnsson 2)
1) Earth and Ocean Sciences, University of British Columbia, Vancouver, B.C., Canada
2) Science Institute, University of Iceland, Reykjavik, Iceland

Surface and bed topography of the Vatnajökull Ice Cap have been mapped in exceptional detail, with digital elevations maps at 200 m resolution covering most of the ice cap. The historical record of margin fluctuations at Vatnajökull is also exceptional, making it the best-observed major ice body in the world. Very few modelling studies of Vatnajökull's ice dynamics have been undertaken, however.

We present initial efforts to simulate dynamics of the ice cap with a three-dimensional ice dynamics model developed by Marshall and Clarke (1996, 1997). Ice flow is pure vertical shear deformation in preliminary tests, via Glen's flow law, with no basal sliding or subglacial sediment deformation. Grid resolution is 2 km, which should be sufficient to capture major outlets of the ice cap but is expected to be inadequate for some of the detailed structure of the southwestern margin. Resolutions better than 2 km are not viable with ice dynamics models of this class, however, as assumptions in the shallow-ice approximation require horizontal resolution to be greater than 2 to 3 ice thicknesses. Higher resolution studies will certainly require consideration of longitudinal stresses and strains. This may also prove necessary at 2 km, as a large proportion of Vatnajökull consists of outlet glaciers and margins which undergo basal motion and significant longitudinal strain.

Preliminary model explorations have simply sought out present-day equilibrium states of the ice cap, using observational climate records and a degree day mass balance model. These tests illuminate several difficulties associated with simulation of steady-state ice thicknesses and surface topography. Southern, eastern, and western margins of the ice cap are retracted and over-steepened in the model, while there is a pervasive tendency for ice expansion to the north. Stiff (high-viscosity) ice reaches a steady state equilibrium with reasonable (but over-steepened) northern margins, within a few km of what is observed. Softer ice rheologies (Glen flow law parameters which give good reconstructions of Greenland and East Antarctica) result in major northward advance of the ice cap, however. Vatnajökull is thought to be temperate, hence ice is very deformable; Greenland rheologic tuning may not be appropriate, however, so the exact flow law parameters for Vatnajökull should be studied. Constraint should be possible through surface velocity measurements in areas which are believed to exhibit no basal flow.

The problem of northward margin migration is likely to persist, however, and it is anticipated from previous modelling studies. With no basal motion or surging in the ice sheet model applied in these tests, margins are excessively thick and steep. This gives too much ice flux and a very narrow ablation zone; flux exceeds ablation and ice moves northward. Bed topography on the other margins is low enough to prevent this, with modelled ablation rates high enough to suppress ice advance. It may turn out that a treatment of surging will be essential to model reconstruction of Vatnajökull, as surge cycles move thin, low-sloping ice into ablation areas and are a very effective means of ice removal/margin containment. In this light, a "steady-state" reconstruction is not especially meaningful for Vatnajökull, which has a very large number of surge-type outlets. This is an intriguiing question, and we see Vatnajökull as a tremendous place to learn about the processes of surging and basal flow, and to test quantitative models of these phenomena. We view subglacial hydrology as essential to these processes, and will concentrate our efforts on the development of basal hydrology models and basal flow parameterizations in further, more sophisticated modelling of Vatnajökull. It may prove very constructive to model Vatnajökull outlets using ice stream physics (longitudinal and horizontal shear strains), as outlined in Marshall and Clarke (1997).

Marshall, S. J. and G. K. C. Clarke, 1996. Sensitivity tests of coupled ice-sheet/ice-stream dynamics in the EISMINT experimental ice block. Ann. Glaciol. 23, 336-347.

Marshall, S. J. and G. K. C. Clarke, 1997. A continuum mixture model of ice stream thermomechanics in the Laurentide Ice Sheet 1: Theory. J. Geophys. Res. 102(B9), 20,599-20,614.
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A glacio-meteorological experiment on Vatnajökull, Iceland

J. Oerlemans#, H. Björnsson, M. Kuhn$, F. Obleitner$, F. PaissonT,
P. Smeets&, H.F. Vugts& and J. de Wolde#

# Institute for Marine and Atmospheric Research, Utrecht University
T Science Institute, University of Iceland
$ Institute for Meteorology and Geophysics, University of Innsbruck
& Department of Earth Sciences, Free University, Amsterdam

In the summer of 1996 a glacio-meteorological experiment was carried out on Vatnajökull, Iceland (area 8000 km2; altitude range: from sea level to about 2000 m). The main goal was to understand how the energy used in the melting of snow and ice is delivered to the surface and what is the role of the ice cap's microclimate. In addition, the experiment should deliver a data set that is useful for validation of satellite-derived surface properties like albedo. Many meteorological stations were operated simultaneously on the ice cap for a 100-day period (see map below). Cable balloons and radiosondes were used to probe the vertical structure of the boundary layer.

Data analysis is in full swing. It appears that katabatic flow shapes the microclimate of the glacier to a large extent. The height of the wind maximum varies between a few and a few tens of meters. It is only during the passage of intense storms that the katabatic wind in the melt zone disappears. Because of the low wind maximum present most of the time, the evaluation of turbulent fluxes requires great care. Also, föhn-type flows occur every now and then. Such events are significant, because turbulent exchange is quite high.

Global radiation increases significantly with altitude. Surface albedo varies enormously in space and time. Very low values ((0,1) are found at many places because of the melt out of volcanic ash layers. When considering the total melt in the period 22 May - 31 August 1996, radiation provides typically two-thirds of the melt energy, turbulent exchange one-third. At the stations high on the ice cap, turbulent exchange becomes less significant.
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On the energy budget at site I6, Breiðamerkurjökull, Iceland

Friedrich Obleitner
Leopold-Franzens-University. Institute of Meteorology and Geophysics
Faculty of Natural Sciences. Innsbruck, Austria

Within the framework of the Vatnajökull experiment, measurements of the energy budget have been performed at a site near the estimated equilibrium line of Breiðamerkurjökull (64o11.5N, 16o25.5W, 715m a.s.l. and 19km off the Atlantic coast).

Heading an optimum evaluation of the energy budget components, aspects of instrumentation, micrometeorology and effects of surface development had been investigated thoroughly. Amongst the most interesting micrometeorological features appeared a kink in the wind profiles, which developed after transition from snow to ice. Thorough profile analysis indicated combined stability and roughness effects and initiated extended methodological studies on corresponding effects with respect to the evaluation of the turbulent fluxes mainly. A thus optimised evaluation of the energy budget at site I6 confirmed net radiation to contribute most effectively to the energy being available for the melt of ice. During the period of snow the ablation rates were low (1cm/d), mainly because of high albedo, but because of evaporation was effectively withdrawing energy too. During the period of ice at the other hand, high temperatures, condensation and precipitation were continuously contributing energy, thus enhancing melt rates towards 5cmd-1 typically. Investigations of the directional dependencies of the energy budget components and of their driving meteorological parameters were preliminary steps towards relating the energy budget and distinct regimes of atmospheric flow. The vertical and regional extent of katabatic flow were documented by analyses of tethered balloon soundings and mesoscale analysis of the wind fields across the ice sheet. Typically, corresponding flow reached depths in the order of decameters and appeared to be more sensitive to synoptic disturbances as compared to the lower tongue of Breiðamerkurjökull. The calm weather conditions and relatively high temperatures induced effective melt during such conditions.

Amongst the key impressions in the field were frequent occurrences of a low cloudiness phenomenon, i.e. a wall cloud approaching from lower Breiðamerkurjökull, but rarely sweeping beyond the elevation of I6. Corresponding cross sectional analyses revealed an advective nature of such events. Low level southerly winds advecting moist air and topographic lifting were identified as key factors, where complex interactions with katabatic and upper air winds aloft were documented to play their role also. As usually not sweeping beyond the crest of the ice sheet, such events had little effect on the energy budget at site I6 itself, but were argued to be of more importance at the lower tongue of Breiðamerkurjökull. Topographically induced luv/lee effects were strikingly obvious in the field already and were judged to be of relevance for the energy- and mass budget of the ice sheet at all. At site I6 northerly approach flow induced most pronounced effects, which manifested themselves in overriding the katabatic regime by violent, warm and dry winds. Such events were indicated to enhance evaporation, which effectively reduced the available melt energy. Regional variations of the energy/mass budget were apparent from a network of stakes within the wider measurement area. Differential ablation developed mainly because of local variability in surface roughness, albedo, slope and distance from the local ice margin.
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Observed wind profiles and turbulence fluxes over an ice surface with changing surface roughness

C.J.P.P. Smeets (1), P.G. Duynkerke (2), and, H.F. Vugts (1)
(1) Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan
1085, 1081 HV, Amsterdam, The Netherlands
(2) Institute for Marine and Atmospheric Research,
Princetonplein 5, 3584 CC, Utrecht, The Netherlands.

Wind profile and eddy-correlation data are presented which were obtained at site A4 and A5 on the Breiðamerkurjökull glacier at an altitude of approximately 279 and 381 m, and 4 and 2.5 degrees, respectively. The experiments were run by the Faculty of Earth Sciences of the Vrije Universiteit of Amsterdam (VUA) and carried out from 20 May (Day Of Year, DOY 140) to 31 August 1996 (DOY 245). Both sites were equipped with a 9 m high profile mast, and an eddy-correlation system at about 3 m height.

Throughout the experiment the surface roughness increased rapidly from smooth to very rough due to differential ice melt. (an increasing roughness element height from 0.3 towards 1,7 m at the roughest site). From DOY 140 to 190 the "growing" obstacles approximately resemble a symmetric hemispheric shape, and from DOY 190 to 240 their shape becomes asymmetric and elongated in line with the predominant wind and slope direction whereas their height remains constant. In a layer up to approximately 2 times the roughness element height (=h) we observed decreased wind speed gradients, and horizontal inhomogeneities in the wind profiles in accordance with results from others close to the rough surface in a so called roughness sublayer. Above this layer the wind speed profiles behave conform similarity theory as expected in an inertial sublayer.

Since the measurements were performed on top of a growing roughness element the sensor heights, as measured relative to the top of the roughness element, became increasingly too small due to a displaced zero reference plane. We calculated zero-plane corrections from our profile and turbulence measurements, and compared these with results from a drag partitioning model. In general the agreement was reasonable considering the ranges of uncertainty. The measurements appear to show a trend that contrasts with the model predictions, and suggests that due to the increasing horizontal anisotropy of the surface the use of the model is limited. These limitations should be considered when using the model an alternative for calculating a displaced zero plane correction over a rough ice surface. The magnitudes and evolution of the roughness lengths throughout the experiment agree with a simple model presuming linearity with the frontal area index (the total silhouette area of the elements normal to the wind occupying a unit surface area). The latter results encourage us to propose this model as a good alternative if appropriate profile measurements are not available. To obtain good results from the model it is stressed that an accurate determination of the input surface parameters is necessary, and a wind directional dependence should be considered with care.

Eddy-correlation data reveal that normalised standard deviations of vertical velocity, and the turbulence kinetic energy budget (studied in the stability range 0 Back to top

Simulated and Observed Turbulent Fluxes under Conditions of Katabatic Flowon the Breiðamerkurjökull.

Bruce Denby
Institute for Marine and Atmospheric Research (IMAU), Utrecht University,
Princetonplein 5, 3584 CC Utrecht, The Netherlands. Tel: 030-2533155

During the glacio-meteorological experiment on Vatnajökull the atmospheric boundary layer was found to be dominated by katabatic flows especially inthe lower steeper regions of the icecap. The katabatic boundary layer (KBL) is characterised by a low level wind maximum just a few meters above the surface and a strong temperature inversion at this height. This katabatic wind is the result of near surface cooling, predominantly through the exchange of heat with the surface by turbulent fluxes, which leads to the sinking of denser colder air along the sloping glacier surface.

In this paper use has been made of the three profile masts (U3, A4 and A5) which were positioned along the flow line of the Breiðamerkurjökull outlet glacier. At the lowest of these (U3) wind maxima below 9m (maximum mast height) were detected for 65% of the entire observational period indicating the strong influence of the KBL on the local climate. A 2-D mesoscale second order turbulence closure model, which has been especially designed to cope with conditions of katabatic flow, was applied to the melting ice topography of the Breiðamerkurjökull. The results of these simulations were compared with observations made during periods when synoptic scale pressure gradients were small allowing a pure katabatic layer to develop. Using observed free atmospheric temperature profiles to force the model and appropriate surface roughness lengths for the ice surface the model was found to be quite capable of simulating observed wind and temperature profiles as well as observed eddy correlation derived turbulent fluxes.

Given the qualitatively good results of the simulations a series of climate sensitivity experiments were carried out by varying the free atmospheric temperature. As a result of these simulations the following conclusions could be drawn.

1. Due to the dynamics of katabatic flows the surface turbulent sensible heat flux is not a linear function of external forcing temperature but scales with the square of the temperature. This is due to the fact that wind speeds under katabatic conditions are directly related to the temperature forcing. This result is born out by both simulations and observations.

2. During conditions of melt, when the surface ice temperature is constant, the 2m temperature is not representative of the air temperature outside of the KBL. From observations and simulations it was found that 2m temperatures were between 1/4 and 1/2 of the external forcing temperature, dependent on position along the glacier. This has important repercussions for climate experiments with mass balance models that assume changes in 2m temperatures to be equal to changes in free atmospheric values.

3. The model was used to simulate 3 components of the surface energy balance, i.e. long wave radiation fluxes and sensible and latent turbulent heat fluxes, assuming average cloud cover for the observational period. The resultant climate sensitivity for these components of the surface energy balance to an external temperature forcing was found to be approximately 25 Wm-2Kl for the average summer temperature of 7C.

4. Turbulence in the KBL is in general not similar due to the importance of advection and turbulent transport and so the use of Monim-Obukhov theory in determining surface fluxes is not necessarily appropriate. However it has be shown that the bulk method appears to give quite good estimates of the surface sensible heat flux if surface roughness lengths are assumed to be known.
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Towards a calibrated mass balance model

Martijn de Ruyter de Wildt
Institute for marine and Atmospheric Research
University of Utrecht, Princetonplein 5, 3584 CC Utrecht, Netherlands

Goals of this project.

To determine the mass balance of glaciers or icecaps several meteorological variables are needed, but the size and inaccessibility make wide-spreaded and frequent in situ measurements difficult. Modelling can replace or extrapolate the scarce in situ measurements to obtain the mass balance over large areas and during long periods, but good model parametrisations often are hard to construct. Remote sensing can provide us with better measurements and may help to improve the model:

1) The albedo is an important factor for the energy balance but is difficult to parameterise because of its strong variability in space (especially in volcanic regions) and time. Satellite images constructed from the visible/infrared part of the e.m. spectrum may be used to accurately measure the albedo over vast areas.

2) The mass balance model can be calibrated by comparing the output with measured locations of the equilibrium line. The equilibrium line / meltline may be detected with satellite microwave/radar images.

The Vatnaj6kull icecap is an obvious subject for this research project because the dataset acquired in the 1996 experiment can be used for parameterisation and validation.

Mass balance model

A grid-based mass balance model will be evaluated for a limited but still considerable number of grid points. These points have been distributed manually according to the steepness of the surface; this means that the density of evaluation points is largest near the margins of the icecap where the surface is steep. The resulting gridpoint mass balances will be interpolated to the entire icecap. The most important contributing factors in the mass balance equation are surface melt and precipitation, respectively corresponding to ablation and accumulation. Refreezing of meltwater, evaporation of solid ice and sublimation have been neglected here. The short-wave radiation, and hence the albedo, is a very important term in the energy balance equation. This is the reason why a good measurement of the albedo is a mayor goal of this project which will be discussed below. Meteorological variables which have been gathered over Vatnajökull are important for the different terms in the energy balance equation. For each of these terms several parameterisations exist and at this moment I have used the simplest ones. Because of this, the resulting mass balance of Vatnajökull is quite unrealistic.

Albedo from satellite images

I will be using satellite images for obtaining the albedo over Vatnajökull. This is a difficult task as there are several complications which may produce large errors. Complicating factors are anisotropy of the reflected radiation, narrow to broad band conversion, calibration of the satellite recordings, detection of clouds and an atmospheric correction. Comparison with field measurements can reveal whether the satellite derived albedo is accurate. Unfortunately, different resolutions of satellite derived and field measurements, large spatial variations of the albedo and few available cloud-free images hamper this comparison. Remote sensing can also be helpful for calibrating the mass balance model because it makes the meltline visible over vast ranges. But because of the poor availability of good images the position of the meltline cannot be traced well throughout the melt season.

Radar from satellite

Another type of remote sensing, radar, may be more useful. This radar uses microwaves and these can penetrate clouds and light precipitation and do not depend on sunlight. Observations with radar are therefor nearly always possible and this feature makes radar suitable for monitoring dynamic processes such as glacier melt. Another advantage of radar is that it can detect the onset of melt over snow surfaces as well as the position of the snowline as has been suggested by recent research. Radar may be able to detect four "radar glacier zones" that seem to be caused by the absence/presence of meltwater and snow and that are dynamic in time. In another study static glacier facies zones seem have been detected with radar. The detection of these zones depends on the structure of the snow and ice and not on the presence of meltwater. The resulting radar image is a combination of two effects: facies zonation (static) and presence of meltwater (dynamic in time). All of these observations have not been supported by hard evidence yet and are not conclusive. Moreover, the mechanisms that cause the different radar returns are very poorly known. Yet radar is potentially useful for calibrating the mass balance model and further research on this topic is well worth the time. Reproduction and comparison with simultaneous field measurements seems to be important. Other types of micro-wave imaging may be useful as well.
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The IMAU Automatic Weather Stations on Breiðamerkurjökull

Jan de Wolde
Institute for Marine and Atmospheric research, Utrecht University
Princetonplein 5, 4584 CC Utrecht, The Netherlands.

Within the framework of the EU-TEMBA project, IMAU performed a detailed glacio-meteorological experiment on Vatnajökull (Iceland) in summer 1996, in co-operation with the Science Institute of the University of Iceland, the Institute for Meteorology and Geophysics of the University of Innsbruck and the Faculty of Earth Sciences of the Vrije Universiteit in Amsterdam. Several energy balance stations were installed all over the ice cap, tethered balloon measurements were performed and meteorological radio sondes were launched. The high-quality dataset obtained provides good opportunities to study the ice-atmosphere interaction during the melt season in great detail. However, longer records of observational values are also needed to obtain a good insight in the relation between mass balance and meteorological conditions.

Therefore, in autumn 1995, IMAU installed an Automatic Weather Station (AWS) on Breiðamerkurjökull, a southern outlet-glacier of Vatnajökull. This AWS was developed at IMAU to operate unattended for a considerable period of time. The instruments have very little power use (using batteries) and are robust, so they can operate in cold and humid conditions. Meteorological quantities measured at this station are: pressure, air temperature, relative humidity, wind speed, wind direction, global radiation and reflected radiation. Although the quality of the data obtained will always be less than that of manned stations, the station provides very useful data on the basic meteorological quantities, because of its continuous record for several years.

A first analysis of the AWS-data demonstrated several interesting features. Temperatures at 2m above the surface are relative high: during winter months the temperature drops below zero every now and then, but for most of the time temperatures are above freezing point. Melt rates are large at Breiðamerkurjökull and the ice cap can only exist due to enormous amounts of precipitation. Relative humidity values are always high (60% - 100%). A remarkable persistent katabatic wind is observed during the whole year: a downslope flow was observed in more than 90% of the observing period. Only during wintertime some variation in winddirection was observed. After snowfall, the surface albedo on the glacier snout is around 0.8, but as soon as the snow has disappeared, albedo values decrease to 0.2-0.4. In summer 1997, albedo values appeared to lower than in summer 1996, which might be caused by volcanic ashes on ice due to the eruption at Grímsvötn in autumn 1996. Further data analysis will aim at the surface energy balance throughout the year. Calculated melt rates will be compared to observational values. Also data obtained form a second AWS will be involved, that is operational since spring 1998 at higher elevation at Breiðamerkurjökull.
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Historic fluctuations of the outlets of Vatnajökull and climate change, 900-1930 AD

Helgi Björnsson
Science Institute, University of Iceland
Dunhaga 3, 107 Reykjavik, Iceland

At the time of the settlement of Iceland (874 A.D.) and up to the thirteenth century the climate is believed to have been similar to that of the warm period from 1920 to 1960. At that time the glaciers were smaller than they are today. Some of the largest outlets from Vatnajökull such as Breidamerkurjökull and Tungnirjökull were 10-15 km shorter than at present. In the fourteenth century the climate gradually became colder but the most drastic deterioration in climate set in during the "Little Ice Age" from 1600 to 1920. Then the average air temperature was probably 3 to 4'C lower than during the Climatic Optimum after the Pleistocene. The most extensive glacier advance culiminated for the steeper glaciers in the 1750's and in 1850 to 1890 for the broad lobes from the plateau ice caps. Breidamerkurjökull reached its maximum extent in 1894 only 256 m short of the sea-shore. During the "Little Ice Age" cultivated land and farms were over-run by ice at Breidamerkursandur.

A general recession of the glaciers set in during the 1890's and became quite rapid after 1930, but began to slow down during the 1960's. Since 1890 the largest outlets of Vatnajökull have retreated as much as 2-3 km and the volume of the ice cap has decreased by the order of 5 to 10%. The reduction of the smaller outlet glaciers has been catastrophic. Hoffellsjökull lost one third of its volume in the period 1890 to 1936. On Breidamerkursandur an area of 52 km2 became deglaciated during the period 1894 to 1968. Districts cultivated by farmers in the 12th century are exposed again. Glaciers fed from large accumulation areas which lie well above the firn line have retreated later than outlets which were more drastically affected by the rise in the firn line. The glacier recession has had important hydrological effects. The reduction in the volume of glaciers in Iceland during the first half of the century amounts to a specific discharge from glaciers of the order of 20 1/s km2. For the higher located ice caps such as Hofsjökull and Langjökull the values are 10 and 15 I/s kM2, respectively. A change towards equilibrium in glacier mass balance would reduce the svecific discharge from these ice caps by 15-20%, with profound consequences for electrical power production.

The general recession of the glaciers has been interrupted by local short-lived advances of individual glaciers. The general retreat of glaciers has slowed down in Iceland since the 1960's. The trend towards cooler summers since the 1940's has begun to affect the glacier snouts.
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Glacier variations in Iceland 1930-1995

Tómas Jóhannesson1) Oddur Sigurdsson2)
1) Veðurstofa Íslands (Icelandic Meteorological Office).
Bústaðavegi 9, IS-150 Reykjavík, Iceland
2) Orkustofnun (National Energy Authority)
Grensásvegi 9, IS-108 Reykjavík

Glacier variations in Iceland since 1930 show a clear response to variations in climate during this period. Most non-surging glaciers retreated strongly during the early half of the monitoring period, following the warm climate between 1930 and 1940.

A cooling climate after 1940 led to a slowing of the retreat and many glaciers started to advance around 1970. A warming of the climate since about 1985 has led to an increased number of retreating glaciers in recent years. The variations of non-surging glaciers in Iceland since 1930 appear to be caused primarily by variations in temperature as there are no long term variations in precipitation over this period. Variations of surge-type glaciers are dominated by the surge events. Climate variations do, nevertheless, seem to play a role in observed long term variations of some surge-type glaciers although they are more difficult to interpret than for the non-surging glaciers.