Jökull - 01.01.2013, Síða 93

Mass balance of Mýrdalsjökull ice cap
atures are higher than at most other Icelandic ice caps,
and there is frequent rainfall during winter and high
rates of summer melting. The first mass balance ob-
servations of Mýrdalsjökull, described in Eyþórsson
(1945), report an annual snow layer thickness of 7.9 m
(not water equivalent, w. eq.) at 1300 m elevation in
the late summer of 1944, while Rist (1957) describes
observations giving a winter balance of 5.8 mwe (w.
eq.) at 1350 m a.s.l. in the early summer of 1955.
Here, we present and study systematic mass balance
measurements in the accumulation area at the caldera
plateau of Mýrdalsjökull that have been carried out by
volunteers of the Icelandic Glaciological Society. A
first estimate of the annual precipitation at the survey
sites is given, based on the sum of the measured winter
mass balance data and estimated summer precipita-
tion at the sites. The summer precipitation is inferred
from a comparison of the measured mass balance with
observations of precipitation at two nearby weather
stations. Such an estimate is relevant when e.g. pa-
rameterizing summer rain at the ice cap as well as
meltwater runoff. The estimate is similar to that based
on atmospheric simulations with a state of the art at-
mospheric model, validated with observations of the
winter balance at the survey sites. The simulated pre-
cipitation fields give a detailed estimate of the spatial
distribution and magnitude of precipitation falling on
the ice cap. Such simulated data is important for mod-
eling and understanding the mass balance of the ice
cap itself, as well as for providing input for coupled
glacial mass balance and ice flow models. No ablation
is expected during winter at the elevation of the sur-
vey sites, as is e.g. indicated by temperature profiles
through the winter snow pack in early spring. There-
fore the winter mass balance represents well, and is
here synonymous with, the actual precipitation falling
on the ice cap plateau. It is in fact a better measure of
orographic precipitation than point observations made
with traditional rain gauges (e.g. Rögnvaldsson et al.,
2007). The measured winter balance (accumulation)
is invaluable for studies of orographic precipitation
in general, and in particular for verifying the perfor-
mance of atmospheric models and improving precipi-
tation forecasts.
MASS BALANCE MEASUREMENTS
The mass balance of Mýrdalsjökull was observed at
three sites in 2001 (M0, M1 and M2). As differ-
ences in exact location are minimal, two of the survey
sites in 2001 are here considered to be the same as
where the balance has been observed annually since
the spring of 2007 (M1, M2 and M3, cf. Tables 1–2
and Figure 1).
M0 is the easternmost site and is located above the
eastward facing slopes of Kötlujökull outlet glacier. It
has the lowest elevation of the four sites, with the ice
cap rising 200 m above its location to the south and
100 m to the west. Survey sites M1, M2 and M3 are
located near ice-divides on the caldera plateau. Site
M1 is located in a relatively wide and shallow depres-
sion north of the southern edge of the plateau, where a
slight increase in the thickness of the snow pack may
be expected because of deposition due to snowdrift.
Survey site M2 is located a few kilometres south of the
northern edge of the plateau, where the ice cap is quite
flat. Finally, M3 is to the southwest, on the flat and
wide top of the ice cap. There, scouring due to snow-
drift is significant. As the plateau is relatively flat, the
limited number of survey sites is not considered to be
a problem for representing adequately the mass bal-
ance of the plateau. Previous studies from Langjökull
ice cap in Iceland show that a similar layout of survey
sites yields a good representation of the mass balance
in a region where spatial gradients in precipitation are
relatively simple (Pálsson et al., 2012).
The methods for measuring the mass balance are
the same as those employed on the other large ice
caps in Iceland, e.g. Hofsjökull since 1987 (Sigurðs-
son et al., 2002; Þorsteinsson, 2009) and Vatnajökull
since 1991 (Björnsson et al., 2002). During a one
day spring expedition, usually around 10 May, a snow
core is drilled through the winter snow layer us-
ing an engine driven drill. The density of the core
is measured to calculate the water equivalent of the
winter layer. Measurements of the core temperature
and visual observations of meltlayer incidence and
thickness, as well as grain size, are carried out on
each snow core. Visual inspection of dust content is
used together with observations of grain coarseness
to identify the previous year’s summer surface and
JÖKULL No. 63, 2013 93