[MITgcm-devel] implicit diffusion of advection schemes

[Martin Losch]

Dimitris,

I move this to a new thread, as it is a different topic:
In my experience, the implicit diffusion of DST3 (33) acts on very  
short scales and fronts (at least in the horizontal) are very well  
preserved. So I would not blame DST3, but rather KPP, as Chris  
suggested. If you use a centered scheme you'll have to add vertical  
diffusion explicitly which will mean that in total you will not have  
less but potentially more diffusion with a centered scheme.

If you want to reduce "background diffusion", you could also try  
Alistairs new scheme (advScheme=7?, "DST7") which is even less  
diffusive and still stable than DST3, or Jean-Michels implementation  
of Prather. Both schemes are extremely accurate.

Chris, if I remember Baylor correctly, the centered scheme does have  
some implicit diffusion, but it's just not systematic, whereas the  
more sophisticated schemes direct the implicit diffusion towards  
making the scheme stable, see here http://forge.csail.mit.edu/ 
pipermail/mitgcm-support/2007-February/004605.html

Martin

On 15 May 2007, at 21:30, chris hill wrote:

> The centered scheme has no implicit diffusion, you out it in  
> explicitly (otherwise its horribly dispersive i.e. grid noise  
> everywhere). I told you this last week (and probably the week before).
>
> chris
> Dimitris Menemenlis wrote:
>> Jinlun, we have been using 3rd order direct space time with flux  
>> limiter
>> http://mitgcm.org/sealion/online_documents/node78.html
>> for the cube runs.
>> Advection schemes are a big black box for me.  Do you know if  
>> central differencing is less diffusive than above scheme?
>> Dimitris
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