[MITgcm-support] isotropic_cos_scaling and gmredi

[Jean-Michel Campin]

Hi Martin,

My impression regarding cosFac & Redi (isopycnal) diffusivity:
The diffusivity is set according to "physical" argument 
(the effect of non-resolved eddies, not dependent of the 
grid spacing, at least at coarse resolution),
and not numerical (i.e., make the model stable at this 
resolution), and therefore, we don't need and don't want cosFac.
And regarding GM transport, I think it would not be right to put 
cosFac in it. Just thinking of a kind of "front-relax" experiment,
with uniform slope, don't think you want to flaten the isopycnal 
much more slowly where the (coarse) grid model is finer (with a 
curious stream function).
I am not saying that in a kind of "eddy permitting" model, those
arguments still apply, but at coarse resolution, this seems 
relatively clear to me.
Hope this will help.

Cheers,
Jean-Michel

On Sat, Jan 13, 2007 at 11:17:36AM +0100, Martin Losch wrote:
> Hi,
> 
> I run a global model on an "isotropic" grid, that is, my dlat is  
> scaled with cos(latitude), that is, dlat = dlat(0)*cos(latitude), so  
> that my grid cells are nearly quadratic (dlon = dlon(0)*cos(latitude)  
> because of converging longitude circles). With the CPP-flag  
> ISOTROPIC_COS_SCALING one can achieve that the viscosity and  
> diffusivity are scaled so that, e.g. d^2\theta/dy^2 is discretized as  
> (roughly): d^2\theta* cos(latitude)^n / ( dlat(0)^2*cos(latitude)^2 )  
> where "n" is set in data (parameter cosPower), that is, the  
> diffusivity decreases with grid spacing.
> 
> Now, for the viscosity this is depreciated and one should rather use  
> the dimensionless parameters viscAhgrid, which scale automatically  
> with the grid spacing, or even the variable viscosity (Leigth,  
> Smagorinsky), so I have ISOTROPIC_COS_SCALING defined only GAD_OPTIONS.h
> 
> However, I don't want to use horizontal diffusion, but the GMREDI  
> package, so that tracers are mixed along isopycnals. I made a few  
> suprising observations, had a look at the code and could not find any  
> provisions for the ISOTROPIC_COS_SCALING in the gmredi package. This  
> is the first time that I have a closer look at the gmredi code so I  
> may have missed something, but I assume that the equivalent routines  
> to gad_diff_x/y.F are gmredi_x/ytransport.F
> I am wondering why we don't have the
> cosFacU
> in gmredi_xtransport.F and
> #ifdef ISOPYCNAL_COS_SCALING
> cosFacV
> #endif
> in gmredi_ytransport.F in analogy to gad_diff_x/y.F
> Is this simply not necessary? because the redi-tensor entries already  
> contain this factor, and I just didn't see it?
> 
> Puzzled,
> 
> Martin
> 
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