[MITgcm-support] model unstability caused by increasing sea surface elevation(eta)

[Ken Hughes]

Hi MohammadHossein,


You might have unbalanced fluxes on the open boundaries. Specifically, 
you haven't set useOBCSbalance=.TRUE. in data.obcs.


Recently, I had a regional model whose sea surface height changed by 
100m over one month! The lack of balancing was the cause.


Ken Hughes


On 2022-08-23 11:43 a.m., Mohammadhossein Kharaghani wrote:
>
> [This email originated from outside of OSU. Use caution with links and 
> attachments.]
>
> ​Hello MITgcm community
>
> I have a simple model with three open boundaries in the north, east 
> and west, and in this model that Tide is given to the model as a time 
> series (including 8 components) up to 30 hours (more than one 
> period).There is no problem in the topography and input data of open 
> boundaries. However, sea surface elevation(eta) in my model rises 
> quickly and greatly at all points from the start of running, thus, the 
> model quickly becomes unstable. Griding and dimensions of the model 
> are as follows:
>
> The grid in the horizontal direction is equal to ten meters and in the 
> vertical direction it has 14 layers of 0.5, 1 and 2 meters from the 
> surface to the floor. The range of the model is as 1.18 km x 0.8 km 
> and the maximum depth is 15 meters.
>
> Best regards
>
> MohammadHossein
>
>
> *Data file:*
>
>  &PARM01
>
>  Tref = 21.084700, 21.020412, 20.977085, 20.948000, 20.928308, 
> 20.914621, 20.896999, 20.885466, 20.877001, 20.870974, 20.864302, 
> 20.859730, 20.857693, 20.853001,
>
>  sRef= 41.913029, 41.911030, 41.909740, 41.908958, 41.908516, 
> 41.908287, 41.908123, 41.908012, 41.907799, 41.907497 41.907089, 
> 41.906998, 41.906975, 41.906933,
>
>  viscAz=1.E-3,
>
>  viscAh=1.E-2,
>
>  no_slip_sides=.FALSE.,
>
>  no_slip_bottom=.FALSE.,
>
>  diffKhT=1.E-2,
>
>  diffKzT=1.E-3,
>
>  diffKhS=1.E-2,
>
>  diffKzS=1.E-3,
>
> # saltAdvScheme=4,
>
>  f0=0.0,
>
>  beta=0.E-11,
>
>  eosType='LINEAR',
>
>  tAlpha=2.E-4,
>
>  sBeta =0.E-4,
>
>  gravity=9.81,
>
>  implicitFreeSurface=.TRUE.,
>
>  rigidLid=.FALSE.
>
>  implicitDiffusion=.FALSE.,
>
>  exactConserv=.TRUE.,
>
>  nonHydrostatic=.FALSE.,
>
>  hFacMin=1.0,
>
>  implicSurfPress=1.0,
>
>  implicDiv2DFlow=1.0,
>
>  nonlinFreeSurf=1,
>
>  hFacInf=0.2,
>
>  hFacSup=1.8,
>
>  saltStepping = .FALSE.,
>
>  tempStepping = .FALSE.,
>
> #- not safe to use globalFiles in multi-processors runs
>
> #globalFiles=.TRUE.,
>
>  readBinaryPrec=64,
>
>  writeBinaryPrec=64,
>
>  writeStatePrec=64,
>
>  &
>
> # Elliptic solver parameters
>
>  &PARM02
>
>  cg2dMaxIters=500,
>
>  cg2dTargetResidual=1.E-13,
>
>  cg3dMaxIters=400,
>
>  cg3dTargetResidual=1.E-13,
>
>  &
>
> # Time stepping parameters
>
>  &PARM03
>
>  nIter0=0,
>
>  nTimeSteps=108000,
>
>  deltaT=1.0,
>
>  pChkptFreq=3600.,
>
>  chkptFreq=0.,
>
>  dumpFreq=3600,
>
>  monitorFreq=1.,
>
>  monitorSelect=2,
>
> # for time dependent open boundary conditions, activate the following 
> 3 lines:
>
>    periodicExternalForcing=.TRUE.,
>
>    externForcingPeriod=3600.,
>
>    externForcingCycle =108000.,
>
>  &
>
> # Gridding parameters
>
>  &PARM04
>
>  usingCartesianGrid=.TRUE.,
>
>  dySpacing=10.,
>
>  dxSpacing=10.,
>
>  delZ=0.5,0.5,0.5,0.5,0.5,0.5,1.0,1.0,1.0,1.0,2.0,2.0,2.0,2.0,
>
>  &
>
> # Input datasets
>
>  &PARM05
>
>  bathyFile='TopoKharg4',
>
>  &
>
>
> *Data.obcs file*
>
> # Open boundaries
>
>  &OBCS_PARM01
>
>  OB_Jnorth =   118*-1,
>
>  OB_Iwest  =   80*1,
>
>  OB_Ieast  =   80*-1,
>
>  useOBCSsponge = .TRUE.,
>
> # This flag check and equalise topography across open boundaries:
>
>  OBCSfixTopo = .FALSE.,
>
>  useOBCSprescribe= .TRUE.,
>
>   OBNuFile='OBNut.iwkharg_obcs',
>
>   OBNvFile='OBNvt.iwkharg_obcs',
>
>   OBWuFile='OBWut.iwkharg_obcs',
>
>   OBWvFile='OBWvt.iwkharg_obcs',
>
>   OBEuFile='OBEut.iwkharg_obcs',
>
>   OBEvFile='OBEvt.iwkharg_obcs',
>
>   OBEetaFile='OBEeta.iwkharg_obcs',
>
>   OBNetaFile='OBNeta.iwkharg_obcs',
>
>   OBWetaFile='OBWeta.iwkharg_obcs',
>
>   OBCS_monitorFreq= 1.,
>
>   OBCS_monSelect = 1,
>
>  &
>
>  &OBCS_PARM03
>
>   spongeThickness= 10 ,
>
>   Urelaxobcsinner= 1000,
>
>   Vrelaxobcsinner= 1000,
>
>   Urelaxobcsbound= 100,
>
>   Vrelaxobcsbound= 100,
>
>  &​
>
>
>
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