[MITgcm-support] L4rdt

[David Ferreira]

Martin,
I put a note in the tag-index about the L4rdt fix !
Surely, everybody reads it regularly...
Anyway, the viscosity at the corner points were 4 times those at the 
center points, so
the "effective" viscosity was larger than intended. And indeed, with the 
new code, it will
probably be necessary to pump up a bit viscA4grid.
That said, I don't think that L4 was changed by the code modification, 
only L4rdt.
Are you sure there is not something else going on ?
Cheers,
david


Martin Losch wrote:
> Hi David,
>
> I do think that your fixing of L4rdt (in revision 1.38 of 
> mom_calc_visc.F) makes sense, but I thought I'll tell you a funny 
> story about the consequences: I just spent a day trying to figure out 
> why suddenly my usually stable runs started to explode after my recent 
> update.
> Since I am still fiddling with the (formerly very instable) seaice 
> solver, I was initally really frustated that apparently the solver 
> still doesn't produce stable results. But then I realized that the 
> blow-up started far away from the ice in the deep ocean (and near an 
> open boundary *and* near a tile edge/corner, so there were many things 
> that I tried to track down).
>
> I was not following the discussion about L4rdt (or forgot about it), 
> but then I found out that the change of L4rdt reduces the size of L4 
> (I am using a constant viscA4grid) by more than 37% near the equator 
> (and even moreso for the vorticity points. This is a good explanation 
> for my problems; I am currently checking if reverting to the old L4rdt 
> makes the problem go away (won't see the results today).
>
> I think it's funny that I stumbled over this, but I know of at least 
> one user who will stumble over this too (and will never know what 
> happened), so shouldn't there be a statement about this change on the 
> MITgcm-support list? (or maybe there is one and I missed it?)
>
> Martin
>
>
>
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