Geoscience Reference
In-Depth Information
patterns from the other experiments. Results from the PRECON experiment point
out the importance of specifying the initial ice thickness. In the control experiment,
ice is much thicker than 2 m over most of the domain. Starting with an overly thin
ice cover in the PRCON experiment leads to an unrealistically large ice loss. The
reason is that less total energy is needed to completely melt the thin ice. When the
ice dynamics are turned off (NODYN) we see the role of ice motion in the develop-
ment of the feature. With no dynamics to move the ice offshore, the spatial pattern
of observed ice loss cannot be reproduced.
Removing the ice albedo feedback (NOIALB) gives a September ice field that is
more extensive and more compact, owing to reduced summer melt. This is in accord
with observations, which suggest the importance of unusually early melt and con-
sequent albedo reductions in the development of the ice anomaly. Without the early
opening of coastal leads and polynyas seen in the observations (the NOANOM
experiment), a smaller ice anomaly is generated, and the ice edge is more compact
than for the control experiment.
A good example of an ice sheet application is the study by Ettema et al. (
2009
),
who used RACMO (version RACMO2/GR) to study the surface mass balance of
the Greenland Ice Sheet for the 1957-2007 period. The model was run at an
11 km resolution for a domain covering the Greenland Ice Sheet and adjacent
seas. For their application, RACMO2 was coupled to a physical snow model.
The atmospheric part of the model was forced at the lateral boundaries and the
sea surface by ECMWF data. Looking at the ice sheet as a whole, subtracting
total modeled mass loss as averaged over the 1958-2007 period represented by
runoff (248 Gt yr
−1
) and evaporation/sublimation (26 Gt yr
−1
) from total mass
gain averaged over the same period from snowfall (697 Gt yr
−1
) and rainfall (46
Gt yr
−1
) yields an average positive net surface mass balance for the ice sheet of
469 ± 41 Gt yr
−1
. The surface mass balance trend over the 1958-2007 period is
positive over the interior and negative along coastal regions (
Figure 9.17
). Their
results suggest that considerably more mass accumulates on the Greenland Ice
Sheet than previously thought, adjusting earlier estimates upward by as much as
63 percent. It is stressed that surface mass balance does not include mass loss
by iceberg calving. Iceberg calving is a major source of mass loss. As had been
discussed in
Chapter 8
, the total mass balance for the Greenland Ice Sheet is at
present decidedly negative.
9.7
Numerical Weather Prediction Models
Almost any study of observed variability of the Arctic atmospheric circulation
makes use of output from NWP models. From the preceding discussion, NWP out-
put is also commonly used as lateral forcing for regional climate models or to pro-
vide wind forcing for sea ice and coupled ice-ocean models. Output from GCMs is
also commonly verified against output from NWP models. We hence tend to think
of NWP output as “truth.” The assumption, however, is incorrect. It is better to think
