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

[Klymak Jody]

Hi Jean-Michel,

On Apr 9, 2009, at  11:58 AM, Jean-Michel Campin wrote:

>
>> From your "data" file, it looks like you would need to turn on
> staggerTimeStep (setting staggerTimeStep=.TRUE.,).
>
> Can you confirm that you see this error message in STDERR.#####
> or does your code is older than that ?

Yes, my code is older than that!

I'm sorry to be too lazy to upgrade the code, but last time I tried I  
couldn't get things working and I'm impatient. I'll try to set aside  
the time in the near future.

I set staggerTimeStep=.TRUE., and as advertised I can run the 12.5 s  
timesteps w/o the instabilties.

(I don't think that this requirement is discussed in the manual at http://mitgcm.org/sealion/online_documents/manual.pdf 
, though having the error code is a good idea).

Thanks a lot,

Jody





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