a question about NUDGING_SSH and other NUDGING opptions

[zhaoqian]

Hi all:
If the NUDGING_SSH flag is actived, where is the nudging of ssh performed in ROMS? I didn't find the code related.
I think ROMS only has the interface of nudging ssh, and we can't use this function directly. However, we can nudge M2,M3 and T directly. Am I right?
Thank for replying.

[arango]

Well, I need to clean and remove NUDGING_SSH option. This is a legacy code that I never cleaned. I tried that once and it was a very bad idea The nudging time scales are very nasty here and does not make sense in free-surface. Nudging of SSH is not possible in ROMS because it will be inconsistency with the vertically integrated continuity equation. It is only possible during assimilation of SSH in either OI or 4DVAR. Because of the nature of the cold restart assimilation to create a consistent initial conditions or ocean state. Nudging is a different beast because it will change volume and mass conservation. If you need such nudging, you need to do it via ubar and vbar for consistency with ROMS equations.

I have mentioned several times in this forum that the free-surface in ROMS is an statement of the vertical integrated continuity equation, which is a function of the vertically integrated velocity divergence. That is, ubar and vbar are the real prognostic variables in ROMS. The free-surface is kind of a diagnostic quantity from the 2D continuity equation. If you change zeta, you need to change ubar and vbar accordingly. Then, the very direct (and best) problem is to change only ubar and vbar. Check ROMS equations...

[zhaoqian]

Dear arrango:
Thank you for your work. I have updated the cppdefs.h and globaldefs.h files.