[MITgcm-support] SSH drift and obcs again

[Chris Hill]

Hi Tom,

  When you turn off norther edges obcs (ii) do you put a wall at the 
northern edge?

Chris
Thomas Haine wrote:
> Folks, 
> 
> I've been able to isolate the SSH drift/obc issue I reported a few weeks
> ago to a specific part of the code. My expertise is dwindling as I get
> deeper in, so maybe someone else can help:
> 
> Configuration: checkpoint57p_post, GFDL ic., linear free sfc, seaice
> off, evap and precip off, obc sponge off, obc orlanski off. Realistic
> Denmark Strait bathy. at 2km resolution and open bdys at (parts of) all
> 4 walls. I've corrected the OBCS_BALANCE code to zero the net transport
> divergence so eta won't drift. (N.B., the existing OBCS_BALANCE code has
> several bugs - when I get all this sorted out I'll provide a cleaned up
> version).
> 
> 
> i) What works: obcs switched on at N & S boundary with net barotropic
> inflow at the north and outflow at the south. I see no drift in eta to
> machine precision and small O(1e-14) cg2d Sum(rhs). Looks good.
> 
> ii) What doesn't work: as above but switch off northern obc (in
> data.obcs). Now eta drifts, cg2d rhs is O(1) and w diverges after 2
> steps. Looks bad!
> 
> iii) Also doesn't work (dies at step 2): 
> as i) except: pkg/obcs FALSE,
> as i) except: N obc closed and E or W obc open.
> 
> iv) Also works: As i) except with solid walls at all edges in my bathy
> file and pkg/obcs off. Reassuring.
> 
> The pressure solver source is vastly different in i) and ii) at the
> first solve. This leaves little room for the problem: it's somewhere in
> cg2d and related code in relation to obcs. Any ideas where?
> 
> Thanks! Tom.
> 
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