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

Tue Aug 11 13:43:28 EDT 2009

Martin,

Thanks for including obcs_prescribe_read to allow OB?eta files.

My goal is to implement a boundary condition using eta and nonlinear 
free surface with nonhydrostatic code. In previous postings it has been 
said that we can not use nonlinear free surface and nonhydrostatic. 
Where is that limitation in the code?  I don't see anything in 
update_etah.F or solve_for_pressure.F that looks like a limitation.

Thanks,

David

On 08/11/09 10:51, Martin Losch wrote:
> 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
>>
>>
>> _______________________________________________
>> MITgcm-support mailing list
>> MITgcm-support at mitgcm.org
>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support

-- 
David A. Hebert
NRC Postdoc
Naval Research Lab
Stennis Space Center, MS 39529
david.hebert.ctr at nrlssc.navy.mil
Phone: (228) 688-5846
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mitgcm.org/pipermail/mitgcm-support/attachments/20090811/fe8cd345/attachment.htm>