[MITgcm-devel] diffusion in pkg/seaice

[Martin Losch]

Hi Gael,

to me, diffusion of sea ice is an unphysical concept: I cannot imaging any type of small scale mixing that can be represented by a down gradient coarse graining approach. The diffusion is only in there for numerical reasons (in advect/diffus) and the value of diff1=0.001 or 0.002 or whatever it is (note the beautiful variable name), and the double application of s/r diffus is probably the result of some long ago tuning exercise of Jinlun. I added the possibility of diffusion to the other advection for consistency (and in case they are needed). My personal choice for an advection scheme is 33 or 77, so stable flux-limited schemes that do not require any explicit diffusion, and based on that I would be happy to get rid of diffusion alltogether if one of these were default.

If it weren't for the adjoint ... as long as the 2nd order advection in advect is still required for some configurations, you need the diffusion. But I would be happy to 
1. get rid of diffus and diff1, replace it with a single call to seaice_diffusion and diffKhArea/Heff etc. (which all should default to 0 in analogy to diffKhT/S/Ptr)
2. get rid of the original non-flux form advection or at least change the default of the corresponding parameter so that it is not used be default
3. get rid of the min(dx,dy) factor, which is really unintuitive and non-conservative.
All of this is code that I never use, and that I would not advise to use either.

The only reason I did not do this yet is that I do not want to tune up the verification experiments (and possibly running systems). I suggest, if you start changing this, do it radically. At least change the defaults to something useful:
      SEAICEuseFluxForm  = .TRUE.
      diff1 = 0.
     advectionScheme = 77 (although this is debatable and I see why this may not be the best choice because of analogy)

Martin

On May 26, 2011, at 3:04 AM, Gael Forget wrote:

> Dear Martin and co,
> 
> with regard to seaice_diffusion.F, after discussing the issue with Jean Michel, I am preparing 
> to add a diffKh argument, whose units will be m/s2 (as a standard Laplacian diffusion) 
> and that will be used instead of DIFF1*MIN( _dxF(I,J,bi,bj), _dyF(I,J,bi,bj)).
> 
> Each tracer will be associated with a specific diffusivity (SEAICEdiffKhArea etc.) that will 
> be specified alongside the advection scheme in data.seaice, and whose default will be 0.
> 
> In the single-dimensional advection scheme case, when advect.F and diffus.F are
> used, I am not going to change anything for now. There are several options, and 
> I am looking for opinions. We could for example add a call to seaice_diffusion.F
> in seaice_advdiff.F with an "if diffKhXXX.NE.0 and diff1.EQ.0" condition. Would this work?
> 
> Cheers,
> Gael
> 
> 
> 
> 
> 
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