[MITgcm-support] SSH problems with nonlinFreeSurf and Star

[Stanislav Martyanov]

Hello everyone!

Having tested my several MITgcm configurations (details here:
http://mailman.mitgcm.org/pipermail/mitgcm-support/2020-April/012462.html)
I encountered an issue with the model results. For unknown reasons the sea
level has high oscillations and drops (up to  minus 3-6 m) which can not be
explained by the prescribed external forcing and OBCS (they are OK, SSH =
plus/minus 1 m at the boundary). Also, delR=2 m in the upper layers, and
minimum depth in the domain is 5 m. No tides, no sponge.

At each time step the model used SURF_ADJUSTMENT...

Below are some parameters from the data file used for the reference run (№
0):
 &PARM01
 hFacMin = 0.2,
 hFacMinDr = 0.5,
 hFacInf = 0.2,
 hFacSup = 2.0,
 vectorInvariantMomentum = .TRUE.,
 highOrderVorticity = .TRUE.,
 rigidLid = .FALSE.,
 implicitFreeSurface = .TRUE.,
 nonlinFreeSurf = 4,
 select_rStar = 0,
 exactConserv = .TRUE.,
 useRealFreshWaterFlux = .TRUE.,
 staggerTimeStep = .TRUE.,
 multiDimAdvection = .TRUE.,
 implicitViscosity = .TRUE.,
 implicitDiffusion = .TRUE.,
 viscAhGrid = 0.0,
 viscAhGridMax = 0.5,
 viscC2leith = 1.0,
 viscC2leithD = 1.0,
 useFullLeith = .TRUE.,
 viscAr = 1.0e-4,
 no_slip_sides = .FALSE.,
 no_slip_bottom = .TRUE.,
 bottomDragQuadratic = 0.0025,
 tempAdvScheme = 33,
 saltAdvScheme = 33,
 diffKhT = 0.0,
 diffKhS= 0.0,
 diffKrT = 1.0e-4,
 diffKrS = 1.0e-4,
 useSingleCpuIO = .TRUE.,
 &
 forcing_In_AB = .FALSE.,
 momDissip_In_AB = .FALSE.,
 cAdjFreq = -1.0,
 usingCurvilinearGrid = .TRUE.,

Having spotted such erroneous results in STDOUT statistics, I started
tuning some model's functionality to figure out what happened. Some
conclusions for the subsequent runs:
1) I turn off the river runoff everywhere in OBCS. The problem remained.
2) + I removed eta boundary conditions from data.obcs.  The problem
remained.
3) + I set useOBCS = .FALSE. in data.pkg, thus closed all the open
boundaries, suspecting them. But the problem remained.
4) I returned to the initial settings (run № 0, see above), but set:
nonlinFreeSurf
= 0 and  linFSConserveTr = .TRUE., and the problem vanished, no
SURF_ADJUSTMENT in the STDOUT any more!
5) I returned to the initial settings (run № 0, see above), but set:
nonlinFreeSurf
= 4 and  select_rStar = 2, , and get the followng message from one of tiles:
 fail at i,j=  44  39 ; rStarFacC,H,eta =  0.199701  5.000000E+00
-4.001496E+00
WARNING: r*FacC < hFacInf at       1 pts : bi,bj,Thid,Iter=   1   1   1
  350
STOP in CALC_R_STAR : too SMALL rStarFac[C,W,S] !

Could you please help me to figure out:
a)  If I want to use non-linear free suface but do not want to use rStar,
may I set  select_rStar=0 and nonlinFreeSurf=4 simultaniously? In the
verification 'internal_wave' I found nonlinFreeSurf=3 and select_rStar=0.
So, for select_rStar=0 the parameter  nonlinFreeSurf=0 is not a must?
b) When selecting nonlinFreeSurf=4 and  select_rStar=2 the model crushes
complaining  about too SMALL rStarFac[C,W,S]. My vertical resolution is too
small (2 m) or I should change the hFacMin or hFacDr somehow?
c) Where does the key DISABLE_RSTAR_CODE come from? It is used in many
checks throughout the code, but I could not find where it is defined.
Should I change it if I do not want to use rStar? Or using just
select_rStar=0  is enough?
d) Does the parameter hFacInf works only in the case of nonlinFreeSurf !=
0, keeping the upper layer alive? What if I use nonlinFreeSurf= 0 and at
some moment  -eta > dr(1)? The upper layer thickness may become 0 when
nonlinFreeSurf=0?

Thanks!

Regards,
Stanislav Martyanov
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