[MITgcm-devel] changes in pkg/layers/layers_fluxcalc.F

[Ryan Abernathey]

>
> I run a simple 2-D (y-z) test with the updated pkg/layers and the layers
> output (VH & Hs) are different, even where the bottom is flat and all
> hFac = 1 through the full column.
> I look into your changes in layers_fluxcalc.F and the code line 303-304
> seems wrong to me: When hFacS=1, I expect to find the same weighting as
> before
> (MapFact, 1-MapFact) but instead I am getting (2-MapFact, MapFact-1).
> Can you confirm this (I might have missed something) ?
>

What you say appears to be true. Currently I have
 mfac = 1.0 - hFacS(i,j,k+1,bi,bj)*(MapFact(kk) - 1.0)
but it seems like instead it should be
 mfac = 1.0 + hFacS(i,j,k+1,bi,bj)*(MapFact(kk) - 1.0)
i.e. a simple sign error.
This seems like such a big mistake that there is no way the code should
have worked at all. But I have been using it successfully for months. I
will have a closer look this afternoon and try to sort out what is going on.


> The second thing is that I am not sure that we want to consider an hFacC
> weighted averaged when computing the temperature/salt/rho at the
> velocity location. In the case where the model uses the default
> 2nd order centered  advection scheme, the previous layers averaging (no
> hFacC
> weight) was close to the model advection ; but with the new layers
> averaging
> (with hFacC weight) this is no longer the case.
> In other words, I am not convinced that this hFacC weighted averaged is
> more accurate for pkg/layers diagnostics.
> I am curious to know why your made this choice (hFacC weight).
>

I see your point here too. My thinking was, when there is topography in the
bordering cell, the previous averaging would simply interpolate using the
zero in that cell, producing a highly spurious value at the velocity point.
So there is no doubt that the previous averaging was wrong in this case.
The hFacC weighting was a way of dealing with this by de-weighting the
neighbor cell, eventually to zero for full topography. But I think you are
saying it would be better to just use the masks here.


>
> Cheers,
> Jean-Michel
>
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