Abstract
It is generally assumed that the
mass-balance gradient on glaciers is more or less conserved
under climatic change. In studies of the dynamic response
of glaciers to climatic change, one of the following
assumptions is normally made: (i) the mass-balance
perturbation is independent of altitude or (ii) the
mass-balance profile does not change - it simply
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shifts up
and down. Observational evidence for such an approach is
not convincing; on some glaciers the inter-annual changes in
mass balance seem to be independent of altitude, on others
not at all. Moreover, it is questionable whether inter-annual
variation can be "projected" on different climatic states.
To see what a physical approach might contribute, we
developed an altitude-dependent mass-balance model. It is
based on the energy balance of the ice/snow surface, where
precipitation is included in a parameterized form and
numerical integrations are done through an entire balance
year (with a 30 min time step). Atmospheric temperature,
snowfall, and atmospheric transmissivity for solar radiation
are all dependent on altitude, so a mass-balance profile can
be calculated. Slope and exposure of the ice/snow surface
are taken into account (and the effects of these parameters
studied). In general, the calculations were done for 100 m
elevation intervals.
Climatological data from the Sonnblick Observatory
(Austria; 3106 m a.s.l.) and from Vent (2000 m a.s.l.; Oetztal
Alps, Austria) served as input for a number of runs.
Simulation of the mass-balance profiles for Hinterseisferner
(north-easterly exposure) and Kesselwandferner (southeasterly
exposure) yields reasonable results. The larger
balance gradient on Kesselwandferner is produced by the
model, so exposure appears to be an important factor here.
Sensitivity of mass-balance profiles to shading effects,
different slope, and exposure are systematically studied.
Another section deals with the sensitivity to climatic change.
Perturbations of air temperature, cloudiness, albedo, and
precipitation are imposed to see their effects on the
mass-balance profiles. The results clearly show that, in
general, mass-balance perturbations depend strongly on
altitude. They generally increase down-glacier, and are not
always symmetric about the reference state.
For typical climatic conditions in the Alps, we found
that a 1 K temperature change leads to a change in
equilibrium-line altitude of 130 m. Three factors contribute
to this large value: turbulent heat flux, longwave radiation
from the atmosphere, and fraction of precipitation falling as
snow. Here, the albedo feed-back increases the sensitivity in
a significant way.
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