[MITgcm-devel] changes in momentum diagnostics

[Jean-Michel Campin]

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