[MITgcm-devel] Re: [MITgcm-support] obcs_apply_ptracer

[Dimitris Menemenlis]

Martin, as a general comment for this tracer obcs mask problem, we already had a 
very similar discussion thread last July because of seaice:
http://forge.csail.mit.edu/pipermail/mitgcm-devel/2007-July/002898.html
In particular, the following message describes why resetting T/S to zero at 
boundaries caused sea-ice to blow up:
http://forge.csail.mit.edu/pipermail/mitgcm-devel/2007-July/002900.html
similar to the tracer package blowing up when tracer value is set to zero.

> I am trying to write code for the check of topography as suggested on the
> support list. Basically the check works better than expected: 
> verification/seaice_obcs is set up as a bad example with non-zero topography
> across the open boundaries.

I don't understand why one should not be able to specify boundary conditions at 
locations where there is a gradient of topography across the open boundary: 
seaice_obcs appears to work OK.  If it is a problem, however, then it would be 
preferable for the obcs package to modify the bathymetry as needed outside the 
open boundaries rather than issue a warning and stop.  The seaice_obcs is carved 
out of lab_sea experiment.  You are almost guaranteed that this 
gradient-condition will occur for any arbitrary calving of a regional domain 
from a larger integration.  Would that be a difficult modification?  If you tell 
me exactly what is needed I can try my hand at it (but blindly because I do not 
fully understand how obcs works).

> (also is has some advection/timestepping options that lead to a warning, also
> not a good example, unless you want to test this check).

Ooops!  What exactly is the advection/timestepping problem?  I need to make sure 
that I am not using it for our ongoing Arctic tests (sigh!) or for the 
cubed-sphere integrations.

> Do you want to fix this experiment, or would you like me to do it? Having
> said that, I'd rather not, because I am very likely to miss important points
> of this experiment.

Again, if possible, I would prefer to fix pkg/obcs to handle arbitrary 
bathymetry (by changing it outside the obcs region) rather than change the 
bathymetry of seaice_obcs.

D.