[MITgcm-support] how to control obcs balance flow ?

[Martin Losch]

HI Li Zhi,

I agree with Matt, the best choice is to balance the fluxes during preprocessing. It is a bit tricky, because you need to take into account the exact geometry, i.e. the approrpiate dz, dxg, dyg, hFacS, hFacW. So don’t despair, if it doesn’t work the first time around. You also need double precision for the boundary files, as single precision can already give you some sea level change.

For completeness: The balancing that you asked about is described here: <http://mitgcm.org/public/r2_manual/latest/online_documents/node236.html>
<http://mitgcm.org/public/r2_manual/latest/online_documents/node236.html#SECTION00731560000000000000>

This method balances the net flow at each time step, which may not be what you want.

Martin


On Apr 23, 2014, at 7:30 AM, 李志远 <oceanlizy at 163.com> wrote:

> Thank you for your quick reply,  Matthew !
> I know that if open boundary flow in and flow out are not balanced , after a long time,the ssh will keep increasing or decreasing . So , in order to get a good ssh result ,
> open boundary flow in and flow out must be balanced . I can compute the total flow into/out  the model through each open boundary grid u ,v . I wonder if I can  adjust the u,v to make total flow into the model is absolutely equal to total flow out the model  at open boundary ? If so ,ssh will not keep increasing or decreasing .
> 
> thank you !
> YOURS  LI ZHI 
> BEST REGARDS !
> 
> 
> 
> 
> 
> 
> At 2014-04-19 00:00:01,mitgcm-support-request at mitgcm.org
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> >   1. Re: how to control obcs balance flow ? (Matthew Mazloff)
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> >----------------------------------------------------------------------
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> >Message: 1
> >Date: Thu, 17 Apr 2014 21:18:43 -0700
> >From: Matthew Mazloff <
> mmazloff at ucsd.edu
> >
> >To: <
> mitgcm-support at mitgcm.org
> >
> >Subject: Re: [MITgcm-support] how to control obcs balance flow ?
> >Message-ID: <
> 649BFBA3-CBC6-4105-802F-5A626BE61CE3 at ucsd.edu
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> >Content-Type: text/plain; charset="utf-8"
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> >Hello
> >
> >I correct OBCS as part of preprocessing and don't rely on ALLOW_OBCS_BALANCE.
> >
> >Basically you know the inflow/outflow that you want -- so correct your prescribed obcs to have that magnitude inflow.
> >
> >E.g. if you have a solution from another model that you are gonna make your southern boundary, first interpolate that field to your new model grid. Then make the net prescribed flow through a part of that (e.g. over each 1 degree section) to be the exact same as the original. This usually requires a perturbation to the velocity of about .001cm/s. But if you don't make that correction SSH will run away from you?.so its a vital correction.
> >
> >Matt
> >
> >
> >On Apr 16, 2014, at 7:39 PM, ??? <
> oceanlizy at 163.com
> > wrote:
> >
> >> From: Li zhiyuan <
> oceanlizy at 163.com
> >
> >> >To: MITgcm <
> mitgcm-support at mitgcm.org
> >
> >> >Subject:"how to control obcs balance flow ? "
> >> Dear MITgcm users:
> >>       my model has two open boundarys in south and east . I have "#define ALLOW_OBCS_BALANCE " in the OBCS_OPTIONS.h  , but the rusults revealed that  ssh is very high , even up to 2 m !!  I think it is because  the flow in and out  at open boundarys  are not balanced ,so ssh is anomalous. How to control  balance flow  at open boundarys ? 
> >> Thank you for your quick reply !
> >> Yours  Li zhiyuan
> >> Best regards  ! 
> >> 
> >> 
> >> 
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