1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Modelling Ice Sheet Dynamics by Satellite-Derived Topography 31
Another use for high-resolution topography is as an input dataset for modelling the
surface mass balance of an ice mass. As mentioned earlier, this is often undertaken using a
positive degree day model, with the temperature parameterized as a function of position and
elevation, with the latter being the most sensitive parameter due to the effect of the adiabatic
lapse rate (the change in temperature as a function of altitude through the atmosphere).
Improvements in both the spatial resolution and vertical accuracy of DEMs of the GrIS
have had a substantial impact on estimates of mass loss by ablation. In a comparison of
the use of two DEMs (an older one derived chiefly from terrestrial measurements and one
derived from SRA data), a difference of 20% in ablation was obtained (van de Wal and
Ekholm, 1996). In another study resolution was found to influence ablation due to the
smoothing effect on valleys where greater ablation takes place due to their lower elevation
(Reeh and Starzer, 1996). Although the mean elevation may be the same, due to the binary
behaviour of the positive degree day model (melting can only take place if the temperature is
above 0 ◦ C), the amount of melt calculated may not be. Thus the desirable DEM resolution
must be sufficient to resolve glaciated valleys, typically of about 5–10 km in width. A
comparison between a 2.5 km DEM and a more recent 1 km DEM of Greenland indicated
that there was no difference in ablation estimated using these two models which suggests
that resolution-induced errors in estimating ablation are no longer important for the GrIS at
least (Bamber et al., 2001). Vertical accuracy in some marginal areas may still be, however.
2.5 Conclusion
Two quite different but complementary approaches to obtaining accurate topography over
the large, homogeneous expanses of the Antarctic and Greenland ice sheets have been
presented. Some examples of the datasets generated by the two methods and the application
of these data to problems in spatial modelling of the ice sheets have been discussed. Accurate
topography has been shown to be valuable both directly and indirectly for validating and
calibrating several types of numerical ice sheet models. In particular, DEMs have been used
to constrain thermo-mechanical models and test the reliability of the ice motion they predict.
This latter application resulted from using the DEM directly in a simple finite-difference
model, which traced and integrated the ice flux down a flow line to produce, with the benefit
of surface accumulation data, an independent, global estimate of the velocity field. Ice sheet
scale DEMs have also been used as the key input dataset for modelling the surface mass
balance of the Greenland ice sheet, which is crucial for accurately determining its net mass
balance and hence its contribution to sea level rise.
Limitations still exist in the current generation of satellite-derived DEMs, however. This
is particularly true for the Antarctic ice sheet where there are still no accurate data for the
central region south of 81.5 ◦ (the latitudinal limit of ERS-1 and -2); one-fifth of the ice sheet
is still relatively poorly mapped. There are also areas around the coast of Greenland and
along the TransAntarctic mountains where there is sparse radar altimeter and/or InSAR
coverage. In January 2003, the first of a series of satellite missions with the primary
objective of studying the cryosphere was launched. ICESat is a NASA-funded satellite
that carries a dual-frequency laser. The inclination of the orbit, combined with the pointing
capability of the laser, should provide valuable data over those parts of Antarctica and
Greenland not reached by the current suite of sensors. Further into the future, the European