[MITgcm-support] SSH drift and obcs again

[Martin Losch]

Hi Tom,
my guess is, that you need to satisfy BOTH of the following:
1. you need you corner points to have consistent u and v velocities, so 
that they do not introduce vertical velocitites due to horizontal 
divergences.
2. At the same time you need your total flow to balance
Have you tried this:
After specifying consistent velocities at the corners, compute the 
total balance without changing the corner velocities, that is, you take 
these points into account, when you compute the imbalance, but when you 
compute average boundary velocity corrections, you don't take into 
account the area occupied by the corner points and you don't correct 
the corner points.

I wonder if this can be included in the code in general way ...

Martin

On Oct 26, 2005, at 9:32 PM, Thomas Haine wrote:

> OK, I've made a little more progress. The obc corners are the problem:
>
> i) I can get zero cg2d_b and eta drift if I have obcs on opposite
> boundaries (N&S or E&W). This requires me to put a solid wall on the
> boundaries where I don't specify obcs. (Incidentally, I have zero eta
> drift with N&S obcs and periodic E/W bcs but *not* vice versa. This I
> don't understand but presumably it depends on my specific
> configuration). So, obcs with no obc corners works OK - meaning I can
> correct for net total convergence across the obcs accurately.
>
> ii) I can get zero cg2d_b and eta drift with an obc corner (I've tried 
> 1
> and 3 corners) only if I put zero depth at each corner. I.e. adding a
> single-cell island to block the obc corner works.
>
> I've checked that: my obc forcing files have consistent speeds at the
> corners (as Martin's message of 24 October) and that my bathy file has
> no gradient at the corners.
>
> So, how should I calculate the total convergence of fluid across my 
> obcs
> when the obcs have a corner?  If I can calculate this exactly then I 
> can
> correct for it and guarantee zero eta drift.
>
> Thanks, Tom.
> -- 
> --------------------------------------------
>  Thomas W. N. Haine
>
>  Associate Professor of Physical Oceanography,
>  Department of Earth & Planetary Sciences,
>  329 Olin Hall, The Johns Hopkins University,
>  Baltimore, MD 21218, USA.
>  Tel : 410 516 7048,   Fax : 410 516 7933
>  Thomas.Haine at jhu.edu
>  http://www.jhu.edu/~eps/faculty/haine
> --------------------------------------------
>
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