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

[Mohammadhossein Kharaghani]

​

Hi Ken
I'm really greatfull for your help
I had set useOBCSbalance=.TRUE. in data.obcs and I no longer had unstability problem, but the sea surface elevation quickly decreases , and while I have a tidal elevation of about 1 meter on the open boundaries, it decreases to order 0.001 m or even less in the domain;Therefore, I think this result has a problem.​
________________________________
From: Ken Hughes <kenneth.hughes at oregonstate.edu>
Sent: Tuesday, August 23, 2022 11:41 PM
To: mitgcm-support at mitgcm.org; Mohammadhossein Kharaghani
Subject: Re: [MITgcm-support] model unstability caused by increasing sea surface elevation(eta)


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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