Jökull - 01.01.2005, Qupperneq 162

Þröstur Þorsteinsson et al
pare measured velocities to theoretical values. Signif-
icant bedrock topography, large melting rates, heavy
crevassing (Figure 10), and thin ice made exact calcu-
lations of surface velocities very difficult. Conven-
tional methods that treat bedrock as a perturbation
(Thorsteinsson et al., 2003), or use averaging over
several ice thicknesses (Paterson, 1994) cannot be ap-
plied. In this type of landscape, full-blown numerical
modeling is probably needed to get realistic velocity
fields.
However, it is clear that the velocity increase ob-
served at MD11 has to be related to sliding. The same
explanation is valid for the other locations as well.
Whether sliding was a contributing factor already in
April is impossible to verify, from the data we ob-
tained.
Acknowledgements
The success of a field campaign like ours depends on
assistance from skilled people familiar with the survey
region. We especially wish to thank Benni, Hansína
and their staff at Arcanum (http://snow.is) and the res-
cue squad at Vík in Mýrdal, Grétar and Þórður, for
snowmobiles, navigation and assistance. We thank
Eyrún and family at Sólheimahjáleiga for providing
wonderful food and good accommodations. Helgi
Björnsson and Finnur Pálsson at the Institute for Earth
Sciences, University of Iceland, were extremely help-
ful in providing us with their data and sharing their
experience. Finally, but most importantly, we thank
Björn Oddsson, S. O’Neel (UNAVCO GPS special-
ist), Gísli Árni Gíslason, Pétur Már Sveinsson, and
Elín Helga Þórarinsdóttir, for assistance in the field.
ÁGRIP
Ísjármælingar, mælingar á hreyfingu yfirborðs með
GPS og mælingar á bráðnun voru gerðar á um 16
km2 svæði á NV-Mýrdalsjökli árið 2003. Úfið lands-
lag er undir jöklinum á þessu svæði, þar sem hæðar-
munur reyndist 185 m og ísþykktin 112–304m. Mikil
bráðnun mældist yfir sumarið en yfirborð jökulsins
lækkaði um 4–10 m frá 22. apríl til 18. september.
Erfitt reyndist að finna rétta lengdarskala fyrir tengsl
ísþykktar og yfirborðshalla við hraða á yfirborði vegna
fárra mælinga á hraða og hve botn var ósléttur.
HEIMILDIR
Björnsson, H., F. Pálsson and M. T. Guðmundsson 2000.
Surface and bedrock topography of the Mýrdalsjökull
ice cap, Iceland. Jökull 49, 29–46.
Matsouka, K., Th. Thorsteinsson, H. Björnsson, E. D.
Waddington. Radio-wave scattering from within tem-
perate glacier ice, Myrdalsjökull, Iceland. J. Glaciol-
ogy (submitted).
Paterson, W. S. B. 1994. The Physics of Glaciers. Perga-
mon, 3rd edition, 480 pp.
Thorsteinsson, Th., C. F. Raymond, G. H. Gudmundsson,
R. A. Bindschadler, P. Vornberger and I. Joughin 2003.
Bed topography and lubrication inferred from surface
measurements on fast-flowing ice streams. J. Glaciol-
ogy 49, 481–490.
162 JÖKULL No. 55, 2005