Jökull - 01.01.2009, Blaðsíða 1

Reviewed research article
Comparison of energy balance and degree-day models of
summer ablation on the Langjökull ice cap, SW-Iceland
Sverrir Guðmundsson1, Helgi Björnsson1, Finnur Pálsson1, and Hannes H. Haraldsson2
1Institute of Earth Sciences, University of Iceland, Sturlugata 7, IS-101 Reykjavík, Iceland
2National Power Company of Iceland, Háaleitisbraut 68, 103 Reykjavík, Iceland
Abstract— Physical and empirical degree-day models are presented which simulate melting of the Langjökull
ice cap in SW-Iceland, during the ablation seasons 2001 to 2005. The models were derived and evaluated using
micro-meteorological observations on the glacier and tested with mass balance observations. The observed
daily melt rates were successfully simulated by energy balance calculations based on meteorological observa-
tions on the glacier. Air temperatures in the low-albedo surroundings of the glacier represent daily variations
in the incoming radiation flux better than the damped boundary layer temperature above the melting glacier.
Comparison of the empirical degree-day and physical energy balance models indicates that for a regional tem-
perature change less than 3 !C, the empirical degree-day model may provide realistic predictions of changes
in glacier ablation.
INTRODUCTION
In order to describe the relationship between climate
and the mass balance of glaciers, various physical
and empirical models have been presented that are
based on measurements obtained either on or away
from glaciers. The physical models provide direct es-
timates of the energy balance components but they
are often impractical due to lack of observations of
weather parameters and surface characteristics on the
glaciers (e. g. Lang, 1968; Van deWal, 1996). Empiri-
cal models describe statistical relations between melt-
ing and weather parameters. They are often based on
temperature alone that is easily observed and has of-
ten been recorded over decades. The temperature ob-
servations are either away from the glacier (e. g. Jó-
hannesson et al., 1995), or within the glacier boundary
layer (e. g. Braithwaite, 1995a).
The present paper is a contribution to the discus-
sion on melt models, presenting both physical bud-
get calculations of energy and empirical degree-day
models describing melting. The experimental site is
Hagafellsjökull, an outlet glacier of the Langjökull
ice cap in SW-Iceland (Figure 1). Radiation com-
ponents were measured directly in situ, and turbu-
lent fluxes were calculated using wind, air temper-
ature and humidity measured in the boundary layer.
We examine whether temperature records within the
glacier boundary layer or at ice free areas away from
the glacier signify better changes in glacial melting.
Based on physical models of the energy balance we
test the performance of degree-day melt models and
their applicability for predicting changes in melt rates
in response to changes in regional temperatures. The
degree-day models are also tested against mass bal-
ance observations at stakes that have not been used
for model calibration.
LOCATION AND OBSERVATIONS
Langjökull is the second largest ice cap in Iceland
(925 km2 in area), located in Southwest Iceland at
!64.7! N and!20.4!W (Figure 1). Elevations on the
glacier surface range from 450 to 1450 m above sea
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