AIDJEX Bulletin #40 - Polar Science Center - University of Washington

Since free drift of ice is computed as a local response, it is relatively
simple to state the magnitude of the error that occurs under these conditions.
There is a one-to-one relationship between the orientation of the velocity and
the orientation of the air stress at a given magnitude of air stress. An
error in air stress orientation of one degree is therefore transmitted into a
one degree error in ice velocity orientation.
The errors in ice speed related to errors in air stress magnitude are not
as simple to describe, but since there is a quadratic relationship, there is.
again a direct transmittal of the errors. It must be pointed out that at certain
times, especially during 27 January when winds were low, the ocean
currents can have an appreciable effect on free-drift ice velocities. For that
time the ocean currents provide a larger input in many areas than does the air
stress. Since this quantity remains fixed when driving with either observed
or predicted air stress, we can expect the simulated results to be falrly
consistent, and this is observed. For 27 January the simulated results using
predicted air stresses, even out to times of 48 hours, are quite reasonable
in most of the region. However, near Banks Island the magnitude and orientation
are appreciably off as the prediction interval increases to 48 hours.
In Figure 5, where we show results for 30 January, it is seen that the
general response is again reasonably accurate. However, in this case the predicted
drift is not accurate between Barrow and Prudhoe Bay. Otherwise, the
results show relatively good qualitative agreement.
For 2 February (Fig. 6), when the winds come about and show a large
variation across the region, the variation is represented quite well with the
24-hour prediction and reasonably well with the 36-hour prediction, but the
correlation is degraded appreciably with the 48-hour prediction. This deviation
occurs because the location of the high region is wrong at the time.
We have chosen to simulate the winter conditions presented by Pritchard
et al. (1977) using the ice model with a squished teardrop yield surface and
a yield strength of lo* dyn em-’, because it produced the best representation
of ice velocity in the study performed for OCSEAP. We have modified the air
stress driving forces during the selected three days to see how much effect
the change has on the simulated ice response. Results are presented in
Figures 7-9.
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