[MITgcm-support] obcs balance

[Martin Losch]

Dear 名 徐,

the routine where the open boundary conditions are set is  
obcs_calc.F. At the bottom of this routine, you'll find the code that  
does the balancing. I have never used that part of the code and now  
that I look at it, it seems strange that  the loop ranges of the loop  
for the calculation of the average flux (the first loop in each  
section), to be substracted from the velocity field in the second  
loop, range from I=1-Olx,sNx+Olx (for north/south) and J=1-Oly,sNy 
+Oly (for east/west).
Naively I would think that they should range only over the  
computational domain *without* the overlaps, so I=1,sNx and J=1,sNy,  
but I may have overlooked something.

So please try this, and let us know, this may be a bug in the code.

Alternatively you can balance your velocity fields offline, i.e.,  
before you prescribe them. That's what I normally do. This way you  
can make sure that your fields balance over the period that you want,  
e.g. a tidal cycle, a day, a week/month/year/decade (I guess it's a  
year in your case).

Martin

On 19 Dec 2008, at 03:23, 名 徐 wrote:

> Dear:
>      I had set exactConserv=.true. in data (PARM01),but it seems  
> that the result had not a major improvement.
>     Also ,I had the tests as you showed.The sea level hardly raised  
> if all  boundaries had been close or  did not prescript the  
> velocities normal to the boundaries.Meanwhile,I had another test.If  
> only the west  boundary velocities been prescribed,the sea level  
> hardly raised.But, If only the south  boundary velocities been  
> prescribed,the sea level  evidently raised.I think the larger flux  
> of volume caused the sea level raise,beacause the  south boundary  
> flux of volume is larger 15.6% than  the  west boundary flux of  
> volume.As the exp4 the open boundary flux of volume is small,so the  
> sea level hardly raised after ten years integrated.
>      Also ,I have a question: which routine apply the parameter  
> "obcs balance" or  which routine balance the boundary flux of volume ?
>
> Hi there,
>
> I've had a quick look through your files. There are a few things  
> you can
> try to narrow down the problem:
> 1. set exactConserv=.true. in data (PARM01); this should improve the
> conservation of volume.
> 2. close all of you boundaries (e.g. by creating a topography with  
> a wall at
> i=1 and j=1) and see what happens
> 3. with the open boundaries turned on, do not prescript the  
> velocities normal
> to the boundaries (the default should be zero, so that there should  
> not be any
> flux of volume through the boundaries).
>
> Martin
> PS. can you try to reproduce your problem with a smaller domain for  
> faster
> integrations (say a 10x20 or so box). that would make debugging  
> simpler for you
> and then we could try this on one of our machine and by that help  
> you much
> better.
>
> On 16 Dec 2008, at 10:53, 名 徐 wrote:
>
> > Thank you for your answer,I have sent the all files you need on  
> enclosure.
> >
> > 好玩贺卡等你发，邮箱贺卡全新上线！<data.zip>
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