Jökull - 01.01.2013, Page 100

H. Ágústsson et al.
Figure 5. Mean simulated precipitation [m] at 3 km resolution during winter (Sept.–April, left), as well as an-
nually (April–April, right) for 2006–2011. Also shown are the ice cap outlines, locations of observation sites
and contour lines of topography in the atmospheric model at 100 m intervals. Note the difference in the colour
scales. Meðaltal vetrarúrkomu [m] (sept.–apríl, til vinstri) og ársúrkomu (apríl–apríl, til hægri) 2006–2011,
í 3 km víðum reiknimöskvum. Jafnframt eru sýndar útlínur jökla, 100 m hæðarlínur reiknilíkansins og stað-
setningar mælistöðva. Athugið að litakvarði er ekki sá sami á báðum myndum.
DISCUSSION
Figure 3 compares the elevation gradients in winter
mass balance within the caldera plateau of Mýrdals-
jökull with data from similar elevations on the north-
ern part of Hofsjökull (Sátujökull transect) and from
the Breiðamerkurjökull outlet glacier from Vatna-
jökull. Important differences are observed, which can
be related to differences in the shape of the predomi-
nant orography. At Mýrdalsjökull ice cap, the great-
est orographic uplift of the impinging airflow occurs
immediately above the top of the southeastern slopes
of the ice cap, with far less uplift occuring over the
plateau itself. Hence, the maximum in precipitation is
expected near the southeast extremity of the plateau,
but not at the highest elevation, as is in fact indicated
by the simulated precipitation (Figure 5). The com-
parative transects on Breiðamerkurjökull and Hofs-
jökull display relatively uniform and steep slopes in
the elevation intervals considered and hence the air-
flow there will in general experience greater and more
uniform orographic uplift than over the Mýrdalsjökull
plateau, resulting in the general increase in observed
winter mass balance with elevation.
At the elevation of the ice cap plateau (1350–
1500 m a.s.l.), solid as well as liquid precipitation can
be expected for most of the year. Winter rainfall will
refreeze inside the snow pack but rain and melt dur-
ing summer and early autumn will be lost as seep-
age through the snow pack. In fact, already by May,
the temperature of the winter layer has been close to
or at 0◦C (Figure 2). It is not clear how early in au-
tumn, rainfall can be expected to refreeze within the
snow pack, but in November 2012 the snow pack was
below freezing (by inspection) but unfortunately no
cores have been extracted in the early autumn. During
the core drilling in May, numerous and up to several
cm thick ice-lenses are found in the lowest part of the
winter layer, i.e. in snow deposited during the begin-
ning of the accumulation season. This suggests that
refreezing of rain starts quite early in the autumn. Fur-
thermore, if the mean daily temperature at both Vík in
Mýrdalur and Vatnsskarðshólar during 2006–2011 is
extrapolated to the height of the plateau using the wet
adiabatic lapse rate of 0.65◦C per 100 m, it can be es-
timated that the mean temperature there goes below
0◦C no later than at the end of September.
100 JÖKULL No. 63, 2013