[MITgcm-support] Re: OBCS

[Martin Losch]

Hi Jake,

thanks for your answer.
I do have the open boundaries on the outermost grid-points (so only 1's 
and -1's in the data.obcs), so that the corners are part of both open 
boundaries (both zonal and meridional ones). The physical solution 
indeed does not look affected by these large |w|-values in the corners. 
I don't use the CD-scheme. I think all exchanges are now correct in the 
current code. At least I have run an experiment with 4 tile vs 1 tile 
and they all gave the same results.

I do use the passive tracer package, though, for which there are no 
open boundaries implemented. I am worried about the passive tracers 
being affected by the strange w-values on the boundaries. But I guess 
my real problem is the fact, that passive tracers are treated with 
periodic boundaries while U,V,T,S are not. So I'll have to do something 
about that.

Martin

On Oct 25, 2004, at 3:30 PM, Jake Gebbie wrote:

>
> Hi Martin:  My memory is fading at this point, but here's my 
> assessment.
>
> If you're setting the o.b. points to be the outermost grid-points in 
> the
> domain, then the vertical velocity field (or sea surface height) at 
> those
> points don't affect the interior solution. In other words, those points
> don't get used in the global pressure solver. But the large or strange
> values should be only on the open boundaries and not in the interior.
>
> So, to be clear, what are the coordinates of the "corner" versus the 
> open
> boundary coordinates?
>
> One other idea: I believe the CD scheme and open boundaries are
> incompatible (can anyone else back me up?). Is CD turned off?
>
> Dan Lea has also found that there was a missing exchange in the OB
> routines. Hopefully, it has found its way into the main trunk.
>
> Cheers,
> --Jake
>
>
>
>
>
> On Mon, 25 Oct 2004, Martin Losch wrote:
>
>> Hi,
>>
>> I have made an obseration  with the OBCS package, and I can't quite
>> figure out what it is, mayby someone can help:
>>
>> I have a box with 4 open boundaries, where I prescribe U and V (and T
>> and S). From looking at obcs_apply_uv (which is called from dynamics
>> and momentum_correction_step, so before integr_continuity), I would
>> think that u(1,:) = u(1-1,:)=u_obcs(east),
>> u(nx,:)=u(nx+1,:)=u_obcs(west), v(:,1)=v(:,1-1)=u_obcs(south), and
>> v(:,ny)=v(:,ny+1)=v_obcs(north). So at the corners, dw/dz = 
>> du/dx+dv/dy
>> = 0 and w(z) = const.
>> But that's not what I observe. There is a vertical profile (nearly
>> linear in 3 of 4 corners). This w a the corners is more or less
>> consistent (smooth transition) with surrounding points at the southern
>> points (iy=1), but very singular at the at the northern points
>> (iy=sNy). In fact, w is large (factor 100 larger than interior) and
>> positive at (ix=1,iy=sNy) and large and negative at (ix=sNx,iy=sNy).
>>
>> This looks very much like a bug to me, but before I spend too much 
>> time
>> on hunting it down, I would like to know if anyone has made similar
>> observations. Maybe there's a simple explanation, why at these corners
>> w has to be so strange ...
>>
>> Martin
>>