Jökull - 01.01.2005, Blaðsíða 139

Reviewed research article
Mass balance of Vatnajökull outlet glaciers
reconstructed back to 1958
L. A. Rasmussen
Department of Earth and Space Sciences, University of Washington
Seattle, Washington, 98195-1310, USA
LAR@geophys.washington.edu
Abstract — A simple model using upper-air meteorological variables in the US National Centers for Environ-
mental Prediction and US National Center for Atmospheric Research (NCEP-NCAR) Reanalysis database is
used to model seasonal components of mass balance of five Vatnajökull outlet glaciers. The model was devel-
oped for glaciers in North America and has been applied to glaciers in Norway and Sweden. Over the period
of observations of mass balance between 1991 and 2001, the model had percentage r2 ranging from 41 to 93
for winter balance bw and from 55 to 84 for summer balance bs. Sensitivity to +1◦C warming ranged from
-0.82 to -1.26 meters per year water equivalent (m/a w.e.), due mainly to increased ablation and secondarily
to shift of precipitation from snow to rain. Sensitivity to 10 percent increase in precipitation was about +0.16
m/a w.e. The model, calibrated over the period of observations, was used to extend the mass balance series
over 1958–2003. In this series, the biggest shift in net balance bn at all five glaciers is between the means of
1958–1994 and 1995–2003. The 5-glacier average of the shift in bn was -0.66, due to shifts of -0.12 in bw and
-0.54 in bs, all in m/a w.e.
INTRODUCTION
Vatnajökull, the largest mass of ice outside of Antarc-
tica and Greenland, has been the subject of much in-
terest and research, which is superbly summarized by
Williams et al. (1997). Mass balance determinations
since 1991 for five outlet glaciers are given in Björns-
son et al. (2002). Using meteorological observations
made on the ice in 1996, deRuyter deWildt et al.
(2003) applied an energy balance model. deWoul and
Hock (2005) applied a degree-day model using daily
observations of temperature at nearby climate stations
and linear regression between precipitation observa-
tions there and accumulation on the glacier. Both of
those models were applied to the five glaciers consid-
ered here.
The mass balance model presented here uses
upper-air variables in a database that is global in
extent, has 6-hour temporal resolution, is free from
missing observations, and is maintained as an inte-
gral part of a major scientific enterprise. The model
was developed with data from South Cascade Glacier
(48.4◦N, 121.1◦W), first for bw (Rasmussen and Con-
way, 2001), then for bs (Rasmussen and Conway,
2003). It was successfully applied to glaciers in
Alaska (Rasmussen and Conway, 2004) and in Scan-
dinavia (Rasmussen and Conway, 2005).
US National Centers for Environmental Prediction
and US National Center for Atmospheric Research
(NCEP-NCAR) Reanalysis data (Kalnay et al. 1996,
Kistler et al. 2001) gives values of meteorological
variables at many levels in the atmosphere at 10,512
gridpoints spanning the entire globe, at integral mul-
tiples of 2.5◦ in both latitude and longitude. Data for
1958–2003 at three standard levels were downloaded
in November 2004 from East Anglia University (web-
site: http://www.cru.eau.ac.uk/cru/data/ncep). Sea-
sonal variation of upper-air conditions at the Reanaly-
sis gridpoint nearest Vatnajökull is shown in Figure 1.
JÖKULL No. 55 139