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

stresses appear to be relatively unaffected by errors in the air stress, even
at 48-hour prediction times.
It is not possible at the present time to ascertain how much effect on
the stress field comes from the boundary motion and how much from the air
stress driving force. We guess, however, that a significant portion of the
driving force that affects the stress comes from the boundary conditions and
that is why the stress field is relatively insensitive to errors in the
applied air stress field. The number of errors caused by air stress relative
to the number caused by boundary motion must, of course, be a significant
function of the ice strength. During the winter, when strengths are LO8
dyn cu-l, we have shown that the stress field feels the effect of the boundary
motion to a great degree. During the summer, since strength is reduced to
very low values and the stresses are contained within the yield surface, errors
on the stress field will be due primarily to errors in air stress. At intermediate
values the results lie between these two extremes.
The work of Coon et al. (1977) describes how the stress is an important
quantity in predicting ice conditions for operations. The stress, of course,
is a large-scale stress, but we conjecture that it is directly related to
forces acting between floes, and therefore to forces that would act on a ship
or structure. We have been able to simulate the stresses, and they show that
these stresses are relatively insensitive to errors in predicted barometric
pressure fields.
CONCLUSIONS
The AIDJEX ice model, in previous work, was used to hindcast conditions
observed during the AIDJEX main experiment, a time for which air stress fields
were prescribed accurately and boundary motions were given by the observed
motion of data buoys. In that test it was shown to simulate the ice dynamics
accurately when the ice stress is important.
In the present work we used the National Weather Service 24-, 36-, and
48-hour predicted surface barometric pressure maps to determine the air stress
fields. It was our desire to learn how much the solution accuracy is degraded
if these predicted air stresses are used to drive the model in a forecast mode.
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