[MITgcm-support] stagger grid and obcs

[Jean-Michel Campin]

Hi Yu-Kun,

You could find some documentation in MITgcm manual regarding OBC positions
(section 6.3.1.4).

In your simple example, c1 & c3 will be tracer point OB, respectively Western 
& Eastern OB (and the model tracer value will be overwritten by pkg/obcs); 
same for tangential velocity component v1 and v3.
Regarding normal component, the true OB values are u2 (Western OB) and u3 (Eastern OB),
but for historic reason, the OB conditionis is also applied to u1 and u4, but this
should not have any effect on the solution (to test this, we can set OBCS_uvApplyFac=0
and check that the results are the same as with the default setting: OBCS_uvApplyFac=1).

And regarding model output, in the output file you will find at the OB location
the OB value that pkg/obcs imposes/overwrites.

Finally, regarding how "halo" regions are filled, there are some ways to disable 
the X or Y periodicity of the domain (when using pkg/obcs at both ends), but:
a) without pkg/exch2: it's not well maintained (and not tested) and not sure if 
 it works well.
b) it does works with pkg/exch2 but involves some careful settings.
c) it's not necessary to prevent "periodic domain" rules (even with OB at both
  ends) and it works nicely with the default "periodic domain" assumption.

Cheers,
Jean-Michel

On Fri, Apr 22, 2016 at 10:34:30PM +0800, ?????? wrote:
> Dear MITgcm user,
> 
> 
> Although I've asked related questions about stagger grids here, I am still a little confused about them.
> 
> 
> Here let's make a concrete example in x direction only.  Assume that there are 3 zonal grids (Nx=3), then the zonal velocity points and mass points are laid out as:
>       u1        c1        u2        c2        u3        c3        u4
> 
> 
> When OBCS is off, periodic OB is used so that u1=u4.  In this case, we only need to prepare 3 zonal velocity values and 3 tracer values for u(1-3) and c(1-3).
> 
> 
> When OBCS is on, OBwU is applied to u1 and u2 (Martin told me this is very important) and OBeU is similarly applied to u3 and u4.  Then the effective values of u is only 2 (Nx-1), which means that the one column of initial u field (Nx=3) will be overwritten by OBu.  I'm a little confused whether the outer most of initial velocity fields should be exactly the same as those of OBs?  If yes, then the effective velocity in the model interior should be Nx-1 and the output velocity field should contain at least one slice of OB velocity.  If true, which OB should be output? East or west?
> 
> 
> Can anyone clarify these details?  Thanks in advance.
> 
> ------------------
>  Best regards 
>  
> Yu-Kun Qian (??????) 
> Center for Monsoon and Environment Research 
>  Department of Atmospheric Sciences
> School of Environmental Science and Engineering 
>  Sun Yat-sen University 
> No. 135 Xingang West Road, Haizhu District 
> Guangzhou, 510275, P.R. China 
> Tel; 020-84115227 
> Email: qianyk at mail3.sysu.edu.cn

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