[MITgcm-devel] diagnostics of pkg/shelfice drag

[Jean-Michel Campin]

Hi,

I am going to start to work on making the ice-shelf drag term fully implicit.
Not planning to have a new run-time switch, but just to use current
 selectImplicitDrag.GE.2 to solve implicitly for shelfice drag term.
It will be very similar to (and will use some pieces of) the implicit bottom
drag that was introduced in Nov 2016 and modified earlier this year (PR #219,
started in March 2019 and merged in Aug) to fix the bottom-drag diagnostics.

One issue here is the diagnostic of shelfice drag:
For the implicit case, it requires to store the drag coeff ; this means
that changing the diagnostic from a 3-D one (momentum tendency from shelfice drag)
into a 2-D one (stress at the bottom of the ice-shelf) is more efficient and
has less memory footprint.
Therefore, for the implicit case, I am planing to only implement a 2-D diagnostics 
of shelfice bottom stress.

Regarding the explicit case, we have several options:
1) just keep the current 3-D diag of drag tendency.
2) change it to a 2-D ice-shelf stress ; it means that the same diagnostic
 can be used in both cases (explicit or implicit) with the same content.
3) have both diags (2-D and 3-D) available,
I would prefer (2), which is what has been implemented for bottom stress.
But could decide on any of the 3 options if there is a good motivation for it.

Feedbacks are welcome.
Cheers,
Jean-Michel