[MITgcm-support] flux limiters and timestep advice...

[Jean-Michel Campin]

Hi Jody,

>From your "data" file, it looks like you would need to turn on
staggerTimeStep (setting staggerTimeStep=.TRUE.,).

We added an error message a year ago (on Apr 08, 2008) for
this case:
> tail -2 STDERR.0000
> (PID.TID 0000.0001) ** WARNING ** GAD_CHECK: potentially unstable time-stepping (Internal Wave)
> (PID.TID 0000.0001) ** WARNING ** GAD_CHECK: need "staggerTimeStep=.TRUE." in "data", nml PARM01

Can you confirm that you see this error message in STDERR.##### 
or does your code is older than that ?

Thanks,
Jean-Michel

On Thu, Apr 09, 2009 at 11:27:49AM -0700, Klymak Jody wrote:
>
> Hi all,
>
> I'm running relatively high-resolution 2-D simulations of flow over  
> topography.  I was relatively happy using 2-nd order centered advection. 
> There was some gridscale noise, but the diffusivity tended to erase it 
> and the major features look fine.  Now I am being a little more fussy and 
> trying to balance energy with dissipation \epsilon = viscAz*(du/dz)^2 and 
> find that my dissipation is scale dependent such that fine-scale models 
> dissipate more than coarse scale.  My supposition is that this is due to 
> the extra gridscale noise.
>
> So I switched to a superbee flux limiting scheme (tempAdvScheme=77).  It 
> certainly removed much of the gridscale noise yet the resolved  
> dissipative features remain the same.
>
> However, after a while I start to get low-mode noise in the solution.   
> When I was using centered 2nd-order I could run 12.5 s timesteps w/o  
> difficulty.  If I run superbee I have to reduce this to 2 s, and still  
> about 20 h I start getting low-mode grid-scale noise.  The noise seems  
> largest near the edges of my topography, and extends through the whole  
> water column.
>
> Please see: http://web.uvic.ca/~jklymak/SuperbeeProblems.pdf
>
> FWIW, these are not huge signals, but completely destroy my energy  
> budgets.
>
> Perhaps my solution is simply to go down to 1-s timesteps (that will be 
> what I will do for now). However, I was curious if there were other  
> suggestions. Should I decrease just the barotropic time step?  Anything 
> else I've set up incorrectly in the attached?
>
> Thanks for any thoughts.
>
> Cheers,  Jody
>


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> --
> Jody Klymak
> http://web.uvic.ca/~jklymak/
>
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