Jökull - 01.07.2003, Page 14

de Ruyter de Wildt et al.
Figure 7. Mean observed (obs.) and calculated (cal.) components of the energy balance during the 1996 ex-
periment. Those stations are shown where all radiation components have been measured.  is the reflected
shortwave radiation,   the net longwave radiation,  the energy used to heat up the insulating layer, the melt energy and   the net radiation. Other symbols are explained in the text. ,  and  (if ap-
plicable) have not been measured directly, but the sum of these three modeled components should be equal to
the observed difference between  and  . – Mældir (obs.) og reiknaðir (cal.) orkuþættir sem bárust jökli
í ýmsum sjálfvirkum veðurstöðvum.  , sólgeislun sem fellur á jökul, , sólgeislun sem endurkastast frá
jöklinum,  , heildarjarðgeislun,,, varmaþættir frá hlýju og röku lofti sem berst inn yfir jökulinn, orka til upphitunar yfirborðslags, , heildarorka til leysingar, , heildargeislun.
Mean specific mass balance
We compute the mean specific mass balance ( )
with an interpolation scheme that has been especially
designed for this purpose. With this interpolation
scheme vertical gradients are taken into account, even
when the evaluation sites do not resolve changes in
altitude. For each grid point of the DEM we deter-
mine the  closest evaluation sites that do not differ
more than 500 m in altitude from the grid point. Then,
because of the limited height differences, a linear rela-
tion between mass balance and altitude can be found
for the  evaluation points. With this relation the
mass balance at the altitude of the gridpoint is calcu-
lated. In order to avoid discontinuities in the resulting
mass balance field, the contribution of each evalua-
tion point is weighted with the inverse of its distance
to the grid point. The algorithm works best when for
all parts of the ice cap, the entire altitudinal range is
represented by the evaluation points. In order to ful-
fill this condition we use 128 evaluation points, and
with  = 6 good results are obtained. Figure 8 shows
the annual precipitation and , interpolated over the
whole ice cap and averaged over the period 1965–
1999. Clearly most precipitation falls in the south
and southeast, which is due to predominant southerly
to easterly wind direction. Most precipitation falls
on top of Öræfajökull, the highest part of Vatnajök-
ull which lies in the south: almost 9 m w.e. annually.
12 JÖKULL No. 52, 2003