Jökull - 01.01.2009, Síða 9

Langjökull, energy balance and degree-day models
station according to the 2004 SPOT5 images. Albedo
below 0.1 has also been observed at glacier outlets of
the nearby Vatnajökull ice cap (e. g. Lister, 1959; Rei-
jmer et al., 1999; Guðmundsson et al., 2005), but is
in that case mainly related to exposed volcanic tephra.
At G1100, albedo declined gradually through the ab-
lation seasons from 0.8–0.9 to 0.5–0.6, but jumped
rapidly down to 0.4 when the previous year’s sum-
mer surface was exposed around the middle of Au-
gust 2001 and 2003 and the beginning of September
2004. Due to increased turbulent fluxes and reduced
solar radiation, the relative contribution of net radi-
ation decreased steadily throughout the summers in
spite of the declining albedo. Frequent passage of
low-pressure systems over Iceland results in Septem-
ber being windier than the summer months of June to
August. Highmelt rates were producedby eddy fluxes
during the warm September months of 2001 and 2002
(Figure 3a-b).
All the energy components supplied to the melting
increased downglacier (Figure 5), the net long-wave
radiation due to higher cloud cover and the net short-
wave radiation as a result of lower albedo. Turbulent
fluxes were maintained by down slope glacier winds
and high air temperatures; wind directional constan-
cies were 0.6 (in G500) and 0.5 (in G1100) where 0
means wind blowing equally from all directions and 1
wind solely down the steepest slope (e. g. Björnsson et
al., 2005). The relative contribution of the net radia-
tion components to melting was on average!60–70%
in the ablation area (Figure 5b).
The performance of degree-day models
Daily melting was calculated with the degree-models
(DDM1 and DDM2) at stakes and the two weather
stations on the Hagafellsjökull outlet 2001–2005 (Fig-
ure 1), using the ddf -parameters in Table 3. Although
not describing the physicalmelt processes, the degree-
day models simulated annual and seasonal variations
in the ablation at G500 and G1100 reasonably (Figure
6). The most successful degree-day predictions were
obtained by applying temperature observations away
from the glacier (TS) using the constant wet adiabatic
lapse rate ($), rather than temperatures observed on
the glacier itself (TG). Lang (1968) concluded the
same by investigating a small drainage basin of the
Aletschgletscher glacier, Switzerland. This is shown
in our data by correlations and residuals of both daily
melting rates obtained with Eq. 5 (Table 4 and Figure
4c) and annual summer balance (Figure 6d-e). This
Figure 5. Variation of weather pa-
rameters, observed winter (bW )
and summer (bS) balances as
well as the energy budget dur-
ing days of melting (Mc # 0),
along the profile in Figure 1. The
values are averages over the en-
tire ablation season 2004. ELA
and GT: altitude of the equilib-
rium line and the glacier ter-
minus, respectively, in the year
2004. The lower x-axis in (b)
shows the power supplied for
melting and the upper the corre-
sponding water equivalent units.
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JÖKULL No. 59 9