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

Figures 1-3 show the baseline air stress fields (derived from NWS
analyses of surface barometric pressure which were augmented by all AIDJEX
data; see Appendix) compared with NWS predictions made 24, 36, and 48 hours
preceding the day in each figure. We have chosen to present the driving
force using the predicted air stress. It can be seen that the conditions
are quite different for the three days. On 27 and 30 January the general
features of the actual air stress field are represented. On the 30th it
appears that the low pressure moving into the area is displaced, and so
the air stress does not yet appear within the region of interest; on 27
January the features observed appear even at the 48-hour prediction level,
although the magnitude in the northern part of the region is not modeled
correctly. On 30 January, when high winds were noted throughout the area,
the predicted winds show the correct pattern everywhere, and the 48-hour
prediction is actually better than either the 24- or the 36-hour prediction.
This, of course, is chance and not proof of a more reliable prediction.
In general, although the main features of the air stress field are represented
in these predictions, many of the details within the region are approximated
poorly.
To demonstrate the effect of these errors in air stress on the simulated
ice response, we performed the &mulation with each of the predicted air
stress fields. These calculations provide us with a direct means of seeing
how the input errors are transmitted to the simulated ice response.
Before turning to those results, we note that if these driving forces
were observed during times when ice conditions were light enough that ice
stress had no effect on the ice response (a time that occurs often in the
summer), it would be meaningful to learn, by means of a free-drift model,
how the errors were transmitted to the ice response. We have used such a
model with a set of driving forces and computed the free-drift or winddriven
ice velocities during the three days of interest. The results are
presented in Figures 4-6. In the past we have noted (Pritchard et al., 1977)
that free drift provides a poor prediction of the ice response during this
eight-day period. However, this example is not aimed at evaluating
the accuracy of the free-drift response, but at estimating the effect of
errors in the air stress during conditions when ice stress is unimportant.
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