[MITgcm-support] Re: OBCS

[Alistair Adcroft]

I've recently been sorting out the interaction between OBCs and NLFS. 
There is a missing OBCS_APPLY_ETA:

RCS file: /u/gcmpack/MITgcm/model/src/integr_continuity.F,v
retrieving revision 1.9
diff -r1.9 integr_continuity.F
168a169,171
 >        IF ( useOBCS )
 >      &    CALL OBCS_APPLY_ETA( bi, bj, etaN, myThid )
 >

which I will check in shortly but otherwise everything does work. There 
are no missing exchanges - this must have been fixed by Patrick.

With the above patch, NLFS and z* work with OBCS at 1 and -1 (i.e. the 
first points in the domain) which I prefer and there are no rediculous 
values coming out of the pressure solver. Note that topography must 
always be flat at OBCs (i.e. at H(I_OBW)=H(OWB+1)). There is no need to 
have a false wall in topography when you put OBCs at 1,-1; the OBCs 
override any periodicity that would otherwise appear. I haven't checked 
but a false wall might be necesary if the OBCs are interior to the 
computational domain (e.g. at 2,-2).

A.
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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-- 
Dr Alistair Adcroft            http://www.mit.edu/~adcroft
MIT Climate Modeling Initiative        tel: (617) 253-5938
EAPS 54-1624,  77 Massachusetts Ave,  Cambridge,  MA,  USA