[MITgcm-support] surface treatment and resolution dependentinstability?

[Jody Klymak]

Hi Yu-Kun Qian,

> On 27 Apr 2017, at  0:37 AM, 钱钰坤 <qianyk at mail3.sysu.edu.cn> wrote:
> 
> Hi Jody,
> 
> One possible reason may be that as your grid gets larger, you need to increase the viscosity to prevent energy accumulating at grid scale.

Yeah, I tried that, and it does allow the runs to go longer.  But its a barotropic problem that is “fixed" at the same viscosities when I switch to 

rigidLid=.FALSE.,
implicitFreeSurface=.TRUE.,

I’m not clear why the free-surface treatment affects the barotropic stability negatively as we go to larger grid cells.

> PS: Previous experience tells me that if you turn on rigidLid (rigidLid=.TRUE.), the overall time for the integration will increase significantly (compared to rigidLid=.FALSE.).  I've known idea why this should be the case.

I think it just converges faster.   I *did* find Dukowicz and Smith, (JGR, 1994) that talks about the difference between the two methods, and they indicate faster convergence.  But I admit that I have not fully digested the material yet.

Thanks,   Jody

> 
> ------------------
> Best regards 
> 
> Yu-Kun Qian (钱钰坤) 
> Center for Monsoon and Environment Research
> Department of Atmospheric Sciences
> School of Environmental Science and Engineering 
> Sun Yat-sen University 
> No. 135 Xingang West Road, Haizhu District 
> Guangzhou, 510275, P.R. China 
> Tel; 020-84115227 
> Email: qianyk at mail3.sysu.edu.cn <mailto:qianyk at mail2.sysu.edu.cn>
>  
>  
>  
>  
> ------------------ 原始邮件 ------------------
> 发件人: "Jody Klymak"<jklymak at uvic.ca>;
> 发送时间: 2017年4月27日(星期四) 上午6:25
> 收件人: "mitgcm-support"<mitgcm-support at mitgcm.org>;
> 主题: [MITgcm-support] surface treatment and resolution dependentinstability?
>  
> 
> Hi all,
> 
> Just trying to educate myself…
> 
> I am running a doubly-periodic domain with some the only external forcing a body force applied to keep the flow moving. The domain is 4000 m deep, with dz=10 m.  Hydrostatic simulations...
> 
> I had as my surface conditions:
> 
> rigidLid=.TRUE.,
> implicitFreeSurface=.FALSE.,
> exactConserv=.FALSE.
> 
> and this ran fine at 100-m horizontal resolution, and 1000-m.  
> 
> Without changing anything about the simulations, except moving to 2000-m, the solution would blow up after a few time steps.  In particular I noticed that `dynstat_eta_mean` would start increasing (or decreasing) before any of the CFL criteria seemed to be going bad.   With that clue in mind I found that:
> 
> rigidLid=.FALSE.,
> implicitFreeSurface=.TRUE.,
> exactConserv=.FALSE.
> 
> seems stable.
> 
> 1) Is this instability something thats known?  It seems non-intuitive to me that as dx,dy gets larger the instability grows.
> 2) What are the consequences of rigid lid versus implicit free surface? Is there a good primer on choosing these?  I’m ashamed to say that I have simply plugged along w/ the same values for years w/o really questioning them.
> 
> Thanks a lot,   Jody
> 
> --
> Jody Klymak    
> http://web.uvic.ca/~jklymak/
> 
> 
> 
> 
> 
> 
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--
Jody Klymak    
http://web.uvic.ca/~jklymak/





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