Jökull - 01.01.2009, Blaðsíða 3

Langjökull, energy balance and degree-day models
September-October using stakes and wires drilled into
the glacier in April or May (Björnsson et al., 1998;
2003).
Air temperature has been recorded since 2001 at
two automatic weather stations (AWSs) situated in the
surroundings of the ice cap (28 and 11 km away from
the glacier): S300 at 300 m a. s. l. on Söðulhólar, and
S475 at 475 m a. s. l. north of the mountain Skjald-
breiður (Figure 1). The surroundings of these sites
comprise lava, sand, and glacial lakes.
Our glacio-meteorological study was carried out
on the south-facing outlet Hagafellsjökull (Figure 1).
Mass balance measurements were done at 8 points
evenly distributed along a profile ranging from 500
to 1450 m a. s. l. One AWS (at G500) was located in
the ablation area at 500 m a. s. l. close to the glacier
terminus, and another (G1100) close to the equilib-
rium line, at 1100 m a. s. l. The AWSs were operated
from April to October during each year of observa-
tions, fully covering the ablation season.
The AWSs measured each 10 minutes the incom-
ing (Qi) and reflected (Qo) solar radiation, incom-
ing (Ii) and outgoing (Io) long-wave radiation, wind
speed (u), wind direction (WD), air temperature (T ),
and relative humidity (r) at 2 m above the surface
(Figure 1 and Table 1). The surface elevation changes
due to melting (d) were measured with a 30 minutes
interval by a sonic echo sounder. The meteorological
instruments were mounted on a mast that followed the
melting surface, but the sonic echo sounder on a mast
drilled several metres into the glacier (Figure 1). Arti-
ficial ventilation of the temperature and humidity sen-
sors (Vaisala HMP35 in Table 1) was not required due
to the enduring wind blowing on the glacier. This ef-
fective natural ventilation has been confirmed by sev-
eral experiments both on the Vatnajökull glacier (Fig-
ure 1) and during calibration of instruments off the
glacier.
The instruments were calibrated in Reykjavík in
April and the beginning of September each year. The
AWSs were visited regularly to ensure that they were
functioning properly and to lower the sonic echo
sounder. There are gaps in the data at both AWSs
from 21May to 29May 2001 because the memory ca-
pacity was exceeded. The sonic echo sounders failed
to record the melting rate continuously after 26 June
2001 at G1100 and after 11 August 2001 at G500, and
the whole summer 2004 at both stations, but the cu-
mulative melting during those periods was measured
(Figure 2).
Air pressure was not measured on the glacier but
estimated at an elevation h from synoptic observations
at meteorological stations of the Icelandic Meteoro-
logical Office, according to the empirical formula:
P (h) = P (h0)
!
1 " 0.0065(h" h0)
T (h0) + T0
"5.25
(1)
where P (h0) and T (h0) are the air pressure and air
temperature observed at a station at elevation h0, and
T0 = 273.15K (e. g. Wallace and Hobbs, 1977). This
relationship has been applied successfully at various
locations on and around Vatnajökull. Vapour pressure
was calculated as e = r · es/100, where r is the mea-
sured relative humidity and
es = 611.213 exp
!
17.5043
T
T + 241.2
"
(2)
Table 1: Instruments and accuracy. Parameters are defined in the main text. – Nákvæmni mælitækja.
Observation Equipment Accuracy Operating range
T , r Vaisala HMP35 0.2!C, 2% –
WD , u R.M.Young 5!, 0.1 m s"1 –
Q, I Kipp & Zonen CNR1 3%, 3% 0.3–2.8 µm, 5–50 µm
d SR50 max(1 cm, 0.4%) –
JÖKULL No. 59 3