Jökull - 01.07.2003, Side 12

de Ruyter de Wildt et al.
For the 2 m windspeed () the picture is differ-
ent. For some stations where the katabatic forcing is
strong (U3, A4, U8, U9 and R4),


 is 0.2 to
0.3. However, the uncertainties are large for these sta-
tions and even larger for the other stations, which dis-
play no significant sensitivities. We therefore do not
develop a parameterization for . Instead we use the
mean wind speed in the free atmosphere, as measured
by the radiosondes. For U3, R2 and R4 this results in
a mean wind speed that is much too low, while for I6 it
is much too high (Table 2). The former three stations
lie at the very margin of the ice cap and experience
strong katabatic winds. They represent, however, only
a very small part of Vatnajökull. I6 is situated near
nunataks that probably provide shelter from the wind.
Table 2. Differences between mean observed and
mean simulated daily mean values of 2 m temp-
erature (
) and windspeed (
) during
the 1996 experiment. – Meðaltalsmismun-
ur mælds og reiknaðs dagsmeðaltals loft-
hita og vindhraða í 2 m hæð sumarið 1996.
Station Altitude (m)
 (K)
 (m/s)
U3 165 0.12 -1.57
A4 279 0.23 -0.44
A5 381 0.38 0.52
I6 715 1.17 1.15
R6 820 1.19 0.04
R4 830 0.27 -1.15
U9 870 1.13 -0.38
R2 1100 0.60 -1.30
R5 1140 0.30 -0.18
U8 1210 0.10 -0.29
U7 1530 0.46 0.54
R3 1715 0.02 0.36
Cloudiness
Cloudiness was observed every three hours close to
U2 in the south and less regularly near R5 in the north.
Mean values of cloudiness for these two stations and
for the stations in Hornafjörður and KIR are shown in
Table 3. More information about the mean cloudiness
can be gained by assuming the cloud observations of
station U2 to be valid for all stations and then compar-
ing mean calculated with mean observed global radi-
ation. The correspondence is good for most stations
(Figure 6).
Table 3. Mean observed cloudiness (< >) for several
locations and periods. – Meðaltal skýjahulu á ýmsum
athugunarstöðvum.
Station Altitude <n> summer <n> summer
(m) 1996 1966–1996
U2 50 0.750
R5 1140 0.694
Hornafj. 0 0.803 0.808
KIR 35 0.764 0.764
Figure 6. Mean observed (circles) and simulated
(crosses) values of the global radiation over Vatna-
jökull as a function of altitude. In order to minimize
errors in the observations, measurements correspond-
ing to high solar zenith angles (>80Æ) were not used
for this comparison. – Mæld (hringir) og reiknuð
(krossar) sólgeislun í mismunandi hæð á Vatnajökli.
Some stations have large deviations, which are
most likely caused by an erroneous cloud factor.
Other factors in equation 2 cannot produce such large
deviations. For U9, U10, R1, R3 and R6 the cal-
10 JÖKULL No. 52, 2003