[MITgcm-support] SSH drift and obcs again

[Martin Losch]

Hi Tom,

if you turn of the nothern obcs (or the entire package) without closing 
your boundaries, the model by default assumes cyclic/periodic boundary 
conditions, so I am not surprised that your case ii, iii, if you use 
the same bathymetry file.

As for iv, I always had the feeling that there is something fishy going 
on at the corners. Many values are prescribed twice, first for the 
zonal boundaries then for the meridional boundaries in obcs_apply_ts. 
Also, at the corner points there should the horizontal velocities 
should have no divergence, otherwise you always prescribe a non-zero 
vertical velocity there, which could be the source of your problems? 
For example, assume a south-west corner at 1,1. Then for each layer
first
u(1,1) = obsu(1)
v(1,2) = obsv(1)
v(1,1) = obsv(1)
and then
u(2,1)=obwu(1)
u(1,1)=obwu(1)
v(1,1)=obwv(1)
In order to ensure non-divergence (and thus zero vertical velocities), 
you must have at all levels
u(2,1)=u(1,1) => obwu(1) = obsu(1)
v(1,2)=v(1,1) => obwv(1) = obsv(1)
which is not automatically guaranteed in the code, and I don't see how 
that can be done in a general way (because the corner could be anywhere 
in the domain). So you'll have to take care of that when you create the 
boundary conditions values.
However, I am sure that you've already thought about that already, ...

Martin

On Oct 22, 2005, at 10:23 PM, Thomas Haine wrote:

> No, there's no wall in my bathy file although most of the northern
> domain is blocked.
>
> Tom.
>
> On Fri, 2005-10-21 at 13:12 -0700, Chris Hill wrote:
>> Hi Tom,
>>
>>   When you turn off norther edges obcs (ii) do you put a wall at the
>> northern edge?
>>
>> Chris
>> Thomas Haine wrote:
>>> Folks,
>>>
>>> I've been able to isolate the SSH drift/obc issue I reported a few 
>>> weeks
>>> ago to a specific part of the code. My expertise is dwindling as I 
>>> get
>>> deeper in, so maybe someone else can help:
>>>
>>> Configuration: checkpoint57p_post, GFDL ic., linear free sfc, seaice
>>> off, evap and precip off, obc sponge off, obc orlanski off. Realistic
>>> Denmark Strait bathy. at 2km resolution and open bdys at (parts of) 
>>> all
>>> 4 walls. I've corrected the OBCS_BALANCE code to zero the net 
>>> transport
>>> divergence so eta won't drift. (N.B., the existing OBCS_BALANCE code 
>>> has
>>> several bugs - when I get all this sorted out I'll provide a cleaned 
>>> up
>>> version).
>>>
>>>
>>> i) What works: obcs switched on at N & S boundary with net barotropic
>>> inflow at the north and outflow at the south. I see no drift in eta 
>>> to
>>> machine precision and small O(1e-14) cg2d Sum(rhs). Looks good.
>>>
>>> ii) What doesn't work: as above but switch off northern obc (in
>>> data.obcs). Now eta drifts, cg2d rhs is O(1) and w diverges after 2
>>> steps. Looks bad!
>>>
>>> iii) Also doesn't work (dies at step 2):
>>> as i) except: pkg/obcs FALSE,
>>> as i) except: N obc closed and E or W obc open.
>>>
>>> iv) Also works: As i) except with solid walls at all edges in my 
>>> bathy
>>> file and pkg/obcs off. Reassuring.
>>>
>>> The pressure solver source is vastly different in i) and ii) at the
>>> first solve. This leaves little room for the problem: it's somewhere 
>>> in
>>> cg2d and related code in relation to obcs. Any ideas where?
>>>
>>> Thanks! Tom.
>>>
>>> _______________________________________________
>>> MITgcm-support mailing list
>>> MITgcm-support at mitgcm.org
>>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>>>
>>
>>
> -- 
> --------------------------------------------
>  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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