1 Spatial Modelling of the Terrestrial Environment - Georeferencial

28 Spatial Modelling of the Terrestrial Environment
b
Figure 2.8
(cont.)
too soft downstream in the model. The introduction of thermodynamics in experiments
3 and 4 appears to partially remove this effect and the agreement in central Greenland is
relatively good, suggesting that the ice rheology and thermodynamics (which are coupled)
have been reasonably well prescribed for the summit region and northern half of the ice
sheet. However, the incorporation of thermodynamics appears to have created a slight
difference between the northern and southern halves of the ice sheet with the model generally
under-estimating velocities, especially toward the margins, for northern Greenland (ice too
stiff ) and over-estimating them in the south (ice too soft). Furthermore, the thermodynamic
simulations have generated several extensive regions of slow flow (white areas) and show
as much variability as the isothermal cases. It should be noted that as a consequence of
uncertainties in the in situ value for the ice viscosity, it is reasonable to scale U d by up
to a factor 2 to obtain better agreement. Thus, what is important to note in Figure 2.9 is
not the absolute difference but gradients in the difference as these cannot be ‘corrected’
by a global scaling factor applied to U d . Areas of high variability in greyscale indicate,
therefore, a poor representation of the pattern of flow in the model. In general, there is