[MITgcm-devel] changes in momentum diagnostics

[Jean-Michel Campin]

Hi,
Also opened a github issue #214 in case you prefer to 
leave comments there.
Jean-Michel

On Tue, Mar 12, 2019 at 10:12:46AM -0400, Jean-Michel Campin wrote:
> Hi,
> 
> I am working on adding/filling diagnostics for the implicit bottom-drag 
> option (currently only implemented: selectImplicitDrag=2), since the current
> implementation is missing all relevant diagnostics part.
> 
> And there are 2 significant changes I suggest to make:
> 
> 1) instead of having 2 (u & v) 3-D diagnostics for the bottom-drag/friction tendencies
>  I propose to have only 2 (for u & v components) 2-D diagnostics for bottom stress.
>  This would simplify vertically integrated flow analysis ; bottom drag tendencies
>  would still be accounted for inside Um_Diss,Vm_Diss and if one wants to recover
>  them, it should not be difficult (apart from variation of grid-cell thickness
>  when using z*) to do if from the bottom stress.
> 
> 2) And regarding pressure gradient, I suggest to modified current diagnostics
>   Um_dPHdx,Vm_dPHdy to also include surface pressure gradient and NH pressure
>   gradient contribution.
>  These 2 (currently missing) contributions are linear, so not difficult to
>  add back. However: 
>   a) for a quick look at momentum budget, it would be nicer to get all the
>   group of terms directly (without the need to compute a gradient and
>   output of additional 2-D var PHI_SURF, different from g*ETAN !) and close 
>   the budget. Following the same idea, there would be also 2 more 3-D
>   diags for implicit viscosity/bottom-friction tendencies (but this is
>   less of a breaking change).
>   b) if I am interested in thickness weighted momentum budget (using recently
>   added diagnostics option "h" in fileFlags 3rd position), I don't get the
>   correct weights if done off-line afterwards.
>   My impression is that with tidal forcing being added to more simulations,
>   this thickness weighted option could become more useful.
> 
> Any suggestion ? Comments ?
> 
> Cheers,
> Jean-Michel
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