Jökull - 01.07.2003, Blaðsíða 20

de Ruyter de Wildt et al.
ously describe the state of that part of the boundary-
layer that generates  . The Greenland ice sheet is
so large that advection of relatively warm air from ice
free areas does not affect  . Greuell et al. (1997)
also used  and e and found that a value of 8 for m
is suitable for a site high in the accumulation area of
the Pasterze (Austria). On the tongue of that glacier,
however, it was not and they noted that this was pre-
sumably caused by the shallow katabatic layer over
the tongue. We find the same for Vatnajökull when and e are used in the equations 10 and 11 in stead
of  and e (Figure 10b). We then find a value
of 2 for m (with a residual standard deviation for !

of 0.028), which is not acceptable from a theoretical
point of view (Konzelmann et al., 1994). However, in
this case a more realistic value of 9 correspond less
well to the data (residual standard deviation for !

is 0.035). This means that the katabatic layer over
Vatnajökull is generally not thick enough to influence
 and that relatively warm air above the katabatic
layer plays a larger role.
When an expression for !
 is available, !
can
be determined by fitting the parameterization to data
that were measured when the sky was entirely over-
cast ( =1). The values that we find for the different
stations lie between 0.937 and 0.958 and do not dis-
play any variation with altitude. We therefore use the
mean value of 0.952 henceforth. This value is in line
with those found by Konzelmann et al. (1994), 0.952,
and by Greuell et al. (1997), 0.976. Next,  is deter-
mined from the data for which 0< <1 (for =0 and
=1, p has no influence). For all stations the same
value is found, namely 3, except for R5 where it has a
value of 4. For this station, however, only few data are
available. We therefore use a value of 3 throughout.
Acknowledgements
We would like to thank all the people involved in the
1996 Vatnajökull experiment, and especially Friedl
Obleitner, Finnur Pálsson, Paul Smeets and Jan de
Wolde for providing information about the data. We
also thank Trausti Jónsson from the Icelandic Mete-
orological Office for providing climatological data.
Several anonymous reviewers provided useful com-
ments. This work was funded by SRON (Space Re-
search Organization of the Netherlands) under project
number eo-030. The Vatnajökull experiment was sup-
ported by the European Union (Framework IV, con-
tract EV5V-CT95-0105), The National Power Com-
pany of Iceland and NWO (Netherlands Foundation
for Scientific Research) through GOA (Netherlands
Geosciences Foundation).
ÁGRIP
Reiknilíkan af afkomu Vatnajökuls
Mæligögn um orkubúskap í sex sjálfvirkum veður-
stöðvum og afkomu Vatnajökuls frá 1995–1996 eru
notuð til þess að stilla af líkön sem lýsa afkomu hans
með veðurþáttum frá veðurstöðvum utan jökulsins.
Líkanið er síðan notað til þess að líkja eftir afkomu
áranna 1965 til 1999 eftir mældu hitastigi og úrkomu
á Kirkjubæjarklaustri. Líkanið bendir til þess að við
hlýnun um 1ÆC, sem fylgdi 5,3% aukning í úrkomu,
myndi afkoma Vatnajökuls rýrna um 0,56 m að vatns-
gildi, jafndreift yfir jökulinn.
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18 JÖKULL No. 52, 2003