[MITgcm-support] Re: OBCS

[Matt Palmer]

Just in reply to Jakes email,

I have been using the OBCS package and found it to be incompatible with 
the C-D scheme.

Matt.

Martin Losch wrote:

> 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
>>>
>
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