[MITgcm-support] SSH drift with sponge-OBCS & OBCSbalance

[yang zhibin]

Thanks a lot. I will try it. Have a nice day.




------------------ Original ------------------
From:  "Matthew Mazloff"<mmazloff at ucsd.edu>;
Date:  Sat, Apr 13, 2019 05:40 AM
To:  "mitgcm-support"<mitgcm-support at mitgcm.org>;

Subject:  Re: [MITgcm-support] SSH drift with sponge-OBCS & OBCSbalance



Hi Yang

You can try using:
C     balanceEmPmR    :: substract global mean of EmPmR at every time step
C     balanceQnet     :: substract global mean of Qnet at every time step


I have never used these flags so cannot offer any wisdom about them


Matt




On Apr 12, 2019, at 9:40 AM, yang zhibin <853245241 at qq.com> wrote:

Hi, Matt


Maybe you're right. Since I am running a climatological model. And the forcing is simply got by averaging different years. And so the foring is not zero for mean.
By the way, do you have some ideas on this problem? Should I minus a mean value before running? But it's also not easy to do that, becaues I am not use heatflux and freshwater water directly 
(I use bulk formula). Thanks again for your help.


Cheers,
Yang




------------------ Original ------------------
From:  "Matthew Mazloff"<mmazloff at ucsd.edu>;
Date:  Sat, Apr 13, 2019 00:21 AM
To:  "mitgcm-support"<mitgcm-support at mitgcm.org>;

Subject:  Re: [MITgcm-support] SSH drift with sponge-OBCS & OBCSbalance



Hi Yang

I have never seen SSH be unbalanced with 
OBCSbalance = true
How slow is the drift? Could it be associated with something else (e.g. freshwater or heat forcing)?


Regarding " have the bathy gradient normal to the wall = 0 in the sponge; turn off tangential velocity; making the corners in the sponge layer have depth 0”
These are not really for balance. These are to prevent a spurious wave guide, spurious mixing, and forced divergence at the boundary.


Matt





On Apr 12, 2019, at 8:49 AM, yang zhibin <853245241 at qq.com> wrote:

Hi all,


Recently I am running a regional model with OBCS. For the purpose of volum balance, I use OBCSbalance. However, there still exist SSH drift (though slow). 
I checked the previous maillist and found a similar question about five years ago (http://mailman.mitgcm.org/pipermail/mitgcm-support/2014-July/009288.html). Thanks to the reply of previous modeler on this question.
In that mail, Mattew gave a complex process to solve this problem (e.g. have the bathy gradient normal to the wall = 0 in the sponge; turn off tangential velocity; making the corners in the sponge layer have depth 0). I wonder if the zero tangential velocity could affect inner or the sponge layer? In the end, Mattew also suggest use RBCS  instead of OBCS. But could the volum balance without OBCSbalance?


Cheers,
Yang
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