[MITgcm-devel] seaice velocity mask

[gael forget]

Dear all,

I have been looking at the seaice model and its adjoint recently, and  
my present focus
is on the dynamics. I use the next generation ECCO grid (global lat- 
lon-cap, 1 degree
resolution). I also run tests with the lab_sea verification experiment.

My questions have to do with the masking of seaice velocities.
Below I attach two figures that, for some arbitrary model time average,
show the zonal ice velocity field (computed from SIuice/SIvice) with
SEAICE_maskRHS=.FALSE. (fig 1) and with SEAICE_maskRHS=.TRUE.
(fig 2). The two runs are otherwise similar, and use evp. The two runs
have the "bug fix" that I checked in earlier today.

As shown by fig 1, if I do not use SEAICE_maskRHS, there are "virtual  
ice velocities"
(e.g. in the tropics) in the diangostic fields that look silly. With  
regard to diagnostics (at least)
I would argue that  "virtual ice velocities" velocities should be  
masked out. For sure I would not
show a field like fig. 1 to a user and call it an estimate of ice  
velocities. Any objection?

Yet switching SEAICE_maskRHS to .TRUE. affects not only the diagnostics
but also the model solution (not shown) which is puzzling (suspicious?).
My guess is that the "virtual ice velocities" affect the model solution
through the computation of strain rates at the open ocean/ice boundary.

In that respect, my (likely naive) assumption would have been that  
SEAICE_maskRHS=.TRUE.
would be default option... but Dimitris mentioned to me that it is  
never used and that it may be
wrong/unsafe. Should I avoid using it? If this is the case, shouldn't  
the option be removed (or fixed)?
If we do need those virtual velocities during the model computation,  
then I would appreciate
some explanations. Why is it so? What are they? Are they meant to be  
a first guess somehow?
Do they imply an adequate boundary condition at the open ocean ice  
edge somehow?

Thanks in advance,
Gael










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