Jökull - 01.07.2003, Side 3

Reviewed research article
A calibrated mass balance model for Vatnajökull, Iceland
Martijn de Ruyter de Wildt , Johannes Oerlemans and Helgi Björnsson

Institute for Marine and Atmospheric Research, Universiteit Utrecht, Utrecht, The Netherlands;
m.s.deruijterdewildt@phys.uu.nl

Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland; hb@raunvis.hi.is
Abstract – Vatnajökull (Iceland) is the largest ice cap in the world where the energy and mass balance have
been studied with good spatial and temporal resolution. In this paper we use these data to analyze the energy
balance and to construct a calibrated and spatially distributed mass balance model. The incoming longwave
radiation is best modeled as a function of meteorological variables in the free atmosphere just above the rel-
atively thin katabatic layer, instead of those at the 2 m level. The ratio of changes in the 2 m temperature to
changes in the free atmospheric temperature (the climate sensitivity) is smaller than 1. Therefore, when the bulk
method is used to compute the turbulent fluxes, the 2 m temperature must be explicitly calculated. Otherwise
the sensitivity of Vatnajökull to climatic change would be overestimated. When the model is forced with data
from a permanent weather station not on the ice cap, it reproduces the observed mass balance reasonably well.
Horizontal precipitation gradients over Vatnajökull are large, which results in a strongly varying sensitivity to
external temperature changes over the ice cap. The mass balance and its sensitivity is thus highly dependent
on local climatic conditions. For a temperature increase of 1 K and a simultaneous precipitation increase of
5.3%, the mean specific mass balance of Vatnajökull decreases by 0.56 m w.e.
INTRODUCTION
Many authors have studied the energy and mass bal-
ance of glaciers. In earlier studies single points on
a glacier were studied (e.g., Ambach, 1963; Munro
and Davies, 1978). However, the mass balance de-
pends strongly on altitude which is why others stud-
ied the energy balance along glacier transects (e.g.,
Braithwaite and Olesen, 1990; Munro, 1990; Greuell
et al., 1997) and modeled mass balance gradients and
sensitivities (e.g., Ambach and Kuhn, 1985; Van de
Wal and Oerlemans, 1994; Jóhannesson, 1997). Only
a few authors have studied the surface of a glacier
or ice cap in a three-dimensional way (e.g., Arnold
et al., 1996, Oerlemans et al., 1999). Oerlemans et
al. (1999) carried out a glacio-meteorological exper-
iment on Vatnajökull (Iceland), which is the first ice
cap where the melt process has been observed with
good spatial and temporal resolution. In this work
we use data from this experiment to analyze the en-
ergy balance and present a calibrated mass balance
model which is based on a calculation of the sur-
face energy balance. We force this model with data
from meteorological stations that are close to but not
on the ice cap. For these stations long meteorologi-
cal records are available with which we can calculate
the mean specific mass balance over the last decades
and study the sensitivity of Vatnajökull to external
climatic change. Precipitation over Vatnajökull is
largely unknown, which is why we use this quantity
to tune the model to the mass balance data. The mass
balance is evaluated at a number of sites distributed
across the ice cap. A Digital Elevation Model (DEM)
with a horizontal resolution of 500 m is used to obtain
the altitude and other geographical features for these
points (Figure 1). The mean specific mass balance is
then calculated with an interpolation scheme that was
especially developed for this purpose.
Vatnajökull (Figure 1) is located in southeastern
Iceland and is one of the largest temperate ice caps
in the world (8200 km ). The altitude ranges from
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