Abstracts on presentations held at the
WORKSHOP on VATNAJÖKULL
Skaftafell, Iceland, 20-24 June 1998
European Science Foundation
European Ice Sheet Modelling Initiative (EISMINT)
Abstracts on presentations
BACK
Basic characteristics of Vatnajökull: geometry, mass balance, flow, hydrology, subglacial geology.
Helgi Björnsson
Science Institute, University of Iceland
Dunhaga 3, 107 Reykjavik, Iceland
hb@raunvis.hi.is
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.
Hydrology
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.
REFERENCES
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
Helmut.Rott@uibk.ac.at
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
glare@raunvis.hi.is
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.
References:
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.
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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.
fp@raunvis.hi.is
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.
INTRODUCTION
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.
CONCLUSION
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.
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Basal processes and conditions beneath ice masses with deformable beds.
Tavi Murray
School of Geography, University of Leeds, LS2 9JT, UK
tavi@geography.leeds.ac.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 Glacier | 48 - 57 | 3.0 x 109- 3.1 x 1010 | 77 |
| Bakaninbreen | 16.6 - 87.5 | 1.1 x 1010 - 4.3 x 1010 | 5.8 - 28.8 |
| Falljökull | 7.3 - 29.8 | 6.0 x 108 - 2.4 x 109 | 125 |
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 |
|---|
| Trapridge | 1.7 x 1010 - 8.0 x 1010 | 30 | 48 - 57 | 77 | Blake,1992,Clarke, 1987 |
|---|
| Bakaninbreen | 1.1 x 1010 - 4.3 x 1010 | | 17 - 87 | 26 | Porter&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
H.Jiskoot@geography.leeds.ac.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
mtg@raunvis.hi.is
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 15-20°C 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
gudfinna@vaw.baum.ethz.ch
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
hilmar@vaw.baum.ethz.ch
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
marshall@geop.ubc.ca
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
j.oerlemans@fys.ruu.nl
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
Friedrich.Obleitner@uibk.ac.at
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 (64o11.5´N, 16o25.5´W, 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.
smec@geo.vu.nl
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
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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
B.Denby@fys.ruu.nl
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
m.s.deruijterdewildt@phys.uu.nl
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.
J.deWolde@phys.uu.nl
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
hb@raunvis.hi.is
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
tj@vedur.is
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.