[MITgcm-devel] changing OBC in multiDim advection

[Jean-Michel Campin]

Hi,

I am planning to change the OBC in multiDim advection (gad_advection.F):
Currently: 
1) calculate "future" OB values (obcs_calc) at the beginning
  of the time step.
2) after updating the local tracer field (in gad_advection.F) with
 X-advective flux, we apply the "future" OB value before computing 
 fluxes in Y-direction.

There is no clear reason to use, at this stage, the "future" OB value
rather than the current OB value (i.e., corresponding to the current time step).
And as a matter of fact, I prefer the alternative way (use current OB value)
to compute all explicit contribution, and it's also much simpler.

One might think that using value at current-time + deltaT/2 would be 
more accurate (would be more complicated for sure), but it's not necessary 
the case if other than horizontal advection (e.g., forcing, vertical 
processes) dominate the evolution of tracer near OB regions.

I propose to change step-2 to use the current OB value (instead of future 
one). This will change the results when multi-dim is used for a tracer 
(active or passive) which has time-varying OBC.

I did not see any "OBCS_APPLY" calls within "seaice_advection.F", 
and propose, later on, to implement the new method (using "current 
OB values") in seaice_advection.F.

Is it OK for everybody ?

Cheers,
Jean-Michel