William M Connolley wrote:
> On Sat, 11 Aug 2007, Eric Swanson wrote:
> > Having been educated as a mechanical engineer, I learned a bit about
> > structures and stresses. I've also done a small bit of work with
> > fatigue failure. That said, I think it's obvious that ice does
> > exhibit tensile strength, all thought how that applies to sea-ice, I
> > can't yet say.
>
> Ice certainly has a tensile strength. Where it gets confusing (in terms of
> large
> scales, model or reality) is that you shouldn't think of it as one floe; its
> always an assembly of floes. Thats where both the "viscous" bit comes from,
> and
> the zero tensile strength (actually I think you'd find that a homogenous solid
> piece of ice 1m think has almost no tensile strength (measured by the wind
> whether it had leads or not; the same ice, on scales of 1m, would be strong).
>
> -W.
Thanks. With a bit of googling, I found some reports on the subject,
including Hunke and Dukowicz, (1997), which I think you pointed to.
They do mention that the viscous approach includes the thickness of
the sea-ice in the calculations. Others have pointed out the low
tensile strength of ice, but that wasn't what I was trying to deal
with as much as the possible difference between first year and multi-
year ice in terms of motion. Using the viscous model, it would appear
obvious that the first year ice should tend to move about much more
easily than multi-year ice .
Other reading brought out the scale problem which Michael discussed.
The physical strength of the ice is a factor at micron levels, yet the
movement and stresses are evident over hundreds of kilometers. One
example given showed fractures in the ice cap which produced features
evident over nearly the whole extent of the ice field. Ice rafting
and pressure ridges change the character of the ice as well and are
more likely to be seen in multi-year ice compared with first year
ice. Michael should know that the scale of the model in terms of the
resolution (or grid spacing) would change the level of detail that
would be appropriate for the sea-ice model as well, since the
treatment of the sea-ice as an aggregate of individual floes probably
will not be appropriate when the average floe size is near that of the
grid size. I doubt that's yet a problem, BTW.
However, if the sea-ice becomes thinner as time goes on, the likely
result might be smaller floes, as storms and waves would tend to break
up the larger floes. That would make motion easier and might be
represented as a reduced viscosity. As it is, the model by Hibler is
said to use a viscosity that varied with the stress tensor. I do not
yet know whether that variation also includes ice thickness, but I
think it should. As noted by Hunke and Dukowicz, the viscosity used
in the VP models extend over several orders of magnitude, which is
part of their motivation in offering their improved model, since their
use of an elastic response speeds the computations considerably,
especially for short term transient situations.
Perhaps we will witness another strong storm moving across the Arctic
sea-ice this season, while the ice is near minimum. Such an event
could prove extremely interesting to the curious, since we wouldn't
need a model to see changes.
E. S.
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