No subject

small (~10^-8 of the tracer difference between 2 adjacent cells),
so adding a little bit of diffusion is helping.

Now, I checked back the code, and I am not sure that the way the CFL is
computed is perfectly right (I would prefer to see the same expression
as in gad_som_adv_x,y,r.F routines) and this could have an effect
in your case if the vertical resolution is non-uniform (or with
topography and partial-cell). So here is my questions:
1) do you get larger negative conc with 77 than with 33 ?
2) do you have non-uniform vertical resolution ? and if yes,
   what is the max or min ratio of delR(k)/delR(k+1) ?

Cheers,
Jean-Michel

On Tue, Dec 23, 2014 at 10:55:47AM +0000, Roland Young wrote:
> > I think it's probably just normal advection noise.  See, for example,
> > http://mitgcm.org/public/r2_manual/latest/online_documents/node72.html
> > http://mitgcm.org/public/r2_manual/latest/online_documents/node81.html
> 
> I think you're right, but I'm still having trouble with this (using
> scheme 33) as I'm getting negative concentrations that are too large
> to be removed by redistribution of the negative cells to nearby
> postive cells. I was wondering whether the temperature advection
> scheme should be the same as the tracer advection scheme? Currently
> temperature advection is the default (scheme 2).
> 
> > Typically advection schemes with flux limiters, as is 77, should
> > remove or minimize this type of overshoot, but maybe not perfectly
> > so.  You could try using 33 or 7, see if it helps get rid of
> > undershoot.
> 
> I've heard good things about scheme 7 but as I recall it requires
> OLx=OLy=4, so will probably slow down my runs considerably as I'm
> using tiles quite thin in latitude to make the zonal filter
> efficient (another story...). Is 7 that much better than the other
> schemes that it's worth trying in any case?
> 
> Are any of the other tracer options useful here? This is what I have
> currently set
> 
> PTRACERS_SOM_Advection = /* tracer uses SOM advection scheme */
>                   F
> PTRACERS_ImplVertAdv = /* implicit vert. advection flag */
>                   F
> PTRACERS_MultiDimAdv = /* tracer uses Multi-Dim advection */
>                   T
> PTRACERS_AdamsBashGtr = /* apply AB on tracer tendency */
>                   F
> PTRACERS_AdamsBash_Tr = /* apply AB on passive tracer */
>                   F
> PTRACERS_diffKh = /* Laplacian Diffusivity */
>                 0.000000000000000E+00
> PTRACERS_diffK4 = /* Biharmonic Diffusivity */
>                 0.000000000000000E+00
> PTRACERS_diffKrNr = /* Vertical Diffusivity */
>    33 @  0.000000000000000E+00              /* K =  1: 33 */
> PTRACERS_useGMRedi = /* apply GM-Redi */
>                   F
> PTRACERS_useDWNSLP = /* apply DOWN-SLOPE Flow */
>                   F
> PTRACERS_useKPP = /* apply KPP scheme */
>                   F
> 
> I tried a bit of horizontal diffusion of tracers (coefficient
> corresponding to an advection courant number around 0.01) and this
> improved it for a while by reducing the number of cells with
> negative tracer concentrations, so may be an option if it can be
> made stronger without smoothing the tracer fields too much.
> 
> Roland
> 
> 
> 
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