[MITgcm-devel] some thoughts on Pressure-Coordinate development

[Martin Losch]

Hi Jean-Michel,

Thanks for this list. I have very few and probably insignificant comments, see below,
Martin

> On 12. Jul 2021, at 20:54, Jean-Michel Campin <jmc at mit.edu> wrote:
> 
> Hi,
> 
> Just to have a record somewhere to list what I have in mind regarding future developments
> related to pressure-coordinate and to get some feedbacks. This is also separated from
> PR #334, branch "pcoord_fixes".
> 
> 1) Implement Mass-coordinate
> -> needed to allow gravity to vary with height
> -> also more natural to deal with atmospheric loading in Ocean using Mass-coord:
>    since Mass-Coord can be defined as Mass of water (liq + solid ?) above.
> -> more natural for non-boussinesq Non-Hydrostatic formulation
> Suggestion:
>  a) add Atmos & Ocean in Mass-coord formulations
>  b) keep option of Atmos in P-coord ; remove option for Ocean in P-coord ?
I have no problem with replacing p-coords by mass coords, assuming that it is very similar in structure etc. It’s also something “new”, that would be easier to publish.
> Comment:
>  would be easier to use conversion factor (such as: mass2rUnit/rUnit2mass or wUnit2rVel/rVel2wUnit)
>  everywhere in the code instead of having explicit conversion factor with rhoConst and gravity
>  so that it would work for both P-coord and Mass-coord
currently, mass2rUnit = gravity for p-coords. The conversion factor is rhoConst*gravity in some cases (i.e. when ever it is not possible to use the isomorphism, for example in ggl90 or with qsw in apply_forcing_t). Would you have a mass2rUnit(k) = rhoConst*gravity(k) if usingMassCoord?
> 
> 2) Estimating the depth in Ocean in P-coord or Mass-coord:
> -> currently, use rhoConst*gravity: this is not very accurate (less accurate than anelastic 
>    formulation) and not very satisfying to rely on a boussinesq reference density in a 
>    non-boussinesq formulation.
> -> most accurate way would be to use PhiHyd, as done, e.g., in pkg/atm_phys ; however this
>    might be tricky when using some approximation (e.g., Lin Free-Surf) and with bathymetry.
> -> compromise solution: use a conversion factor that only depends on vertical index k:
>    this is robust and not less accurate than anelastic formulation.
Not sure if I understand how it would work with phiHyd (as this is a changing variable). I do think, that the full non-linear free surface should be the “default” mode. As a linear free surface is already an approximation, is it really important to have the best approximation of depth in this case?
> 
> 3) Allow to provide bathymetry (in meter) as input file:
> -> this option exist for Atmos in P-coord, with some code converting orography ("topoFile",
>    in meter) to reference surface pressure for model partial-cell settings.
> -> would be nice to have this option available also for Ocean in P-coord.
> -> this might change if we update the way the solve_for_pressure works, solving
>    for increment of bottom pressure instead of the full bottom pressure anomaly,
>    since the need to use a reference bottom pressure might not be necessary anymore.
> Cheers,
> Jean-Michel
> _______________________________________________
> MITgcm-devel mailing list
> MITgcm-devel at mitgcm.org
> http://mailman.mitgcm.org/mailman/listinfo/mitgcm-devel

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 1665 bytes
Desc: not available
URL: <http://mailman.mitgcm.org/pipermail/mitgcm-devel/attachments/20210713/fc9c9108/attachment.p7s>