[MITgcm-support] Re: MITgcm-support Digest, Vol 74, Issue 10

[Martin Losch]

Marcello,

I have used your question as a motivation to actually include code to  
obcs_prescribe_read that reads OB?eta from files (I did not touch the  
Orlanski bit). When you update your local copy the new code should  
appear. However, as you can imagine it is not fully tested, so I would  
be grateful if you could provide feedback about possible problems, in  
case you are going to use this option.

Martin

On Aug 11, 2009, at 8:46 AM, Martin Losch wrote:

> Hi Marcello,
> sorry for the incomplete documentation of obcs (I just checked,  
> there is actually something useful thanks to Patrick. It used to be  
> much worse).
> On Aug 10, 2009, at 7:23 PM, Marcello Gatimu Magaldi wrote:
>
>> I went through the code. Please correct me if I am wrong (very  
>> likely) but it
>> seems to me that:
>> 1) obcs for the ssh are only defined if NONLIN_FRSURF is defined
> No, OB?eta are always defined, but for the linear free surface they  
> are always zero.
>> 2) even if NONLIN_FRSURF is defined OBEeta and similars are:
>>  a) put to zero if neither orlanski nor obcs_prescribe are defined
>>  b) not even assigned to a value if orlanski or obcs_prescribe are  
>> defined
>>
> I guess, originally obcs_calc.F was meant to be a template for  
> specific code for specific experiments; that's why all OB* variable  
> are assigned these weird values, so if you believe that OB?eta  
> should be different from zero in your case, you can define this  
> value in obcs_calc. When we included the useOBCSprescribe option  
> (apply values read from a file), we did not care about the non- 
> linear free surface, so this part of the code is missing (and no  
> warning about that either, that should be added). If you are  
> interested in extending the existing code (and even share this  
> contribution) to do that we would be more than happy. It shouldn't  
> be hard to do.
> if you try to run obcs with a non-linear free surface and Orlanski  
> you'll find that the model stops with  an error saying that this  
> combination is not implemented, so that case it dealt with (although  
> not in a satifactory way). Again if you would like to improve the  
> code here, that would be great, but that's probably a bit more  
> complicated. Than adding eta to the "obcs_prescribe_read" code.
>
> I have even more incomplete code stubs for obcs: Stevens (1990)  
> boundary conditions (used e.g. in MOM3 and higher), which are a mix  
> of radiation and prescribed boundary conditions. If you are  
> interested in that, I can check it in although it's not yet complete  
> (only east and west boundaries and NO special nonlinear free surface  
> treatment, may actually not be necessary), and you can help  
> finishing it.
>
>> Now I was wondering if this is done on purpose. Don't we always  
>> need boundary
>> conditions for eta? I am confused since the MITgcm remains  
>> different from
>> the models cited above because it always solves an elliptic  
>> equation for
>> ssh. The other models employ the time-splitting technique and they  
>> always
>> are in need of obcs for ssh and for BAROTROPIC vertically-averaged  
>> velocities.
>> In these cases, ob conditions reflecting the hyperbolicity of the  
>> equations
>> solved such as characteristic method or flather conditions can be  
>> employed.
>> Cannot we do the same for MITgcm?
> As far as I understand, the elliptic pressure solver (you'll find it  
> in solve_for_pressure) for the LINEAR free surface does not use eta  
> on the open boundaries, because it sets cg2d_b/x=0 on open  
> boundaries in the same way as on closed boundaries. I am not sure  
> but this implies no flow across the boundary by surface elevation.  
> This makes sense for closed boundaries; for open bcs it only means  
> that there is no flow across or along boundaries due to surface  
> elevation. The non-linear contribution is treated as a correction to  
> the linear free surface and is applied separately in the correction  
> step of the "pressure correction method" that is used in the MITgcm.
>
> Martin
>
>
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