[MITgcm-support] Strips on the surface

[Martin Losch]

Hi Kateryna,

I would try to increase hFacMin to a least 0.1. hFacMin=0.001 means that your thinnest grid cell is 0.3m*0.001=0.3mm thick so that even with your small time step would allow only < 3cm/s (CFL wdt/dz < .5) of vertical displacement. The model may be stable, because only a few grid cells are affected, but this can cause noise.

You have free slip everywhere, I suggest to have no slip turned on at either bottom or lateral walls, so that energy can be dissipated.

More accurate advection schemes (than 77) are 33 and 7, but they too will only mask problems. With these schemes you can turn off explicit diffusion (if required for stability) (diffK?? = 0.)

Martin

Unrelated: 
- You might want to check out a different time stepping scheme that does not damp surface waves as much (but I guess you need to find a solution for the non-hydrostatic code to work with this). See <http://mitgcm.org/public/r2_manual/latest/online_documents/node41.html> and verification/internal_wave/input:
# Crank-Nickelson
 implicSurfPress=0.5,
 implicDiv2DFlow=0.5,
# only for nonlinear free surface
# nonlinFreeSurf=4,
# hFacInf=0.2,
# hFacSup=1.8,
# saves some time if you are not interested in salinity at all.
 saltStepping=.FALSE.,

On May 18, 2012, at 8:58 PM, Kateryna Terletska wrote:

> 
> Dear MITgcm users,
> 
> I run MIT model to investigate 3D internal seiches in elongated lakes. I run parallel version of MIT on 32 nodes. 
> But I obtain some strange  strips on the surface (picture is in the attachment). Increasing of viscosity and decreasing of dt didn't worked.
> Also some vertical strips occur when seiches undergoes strong interaction with the bottom topography (it looks like instability). Picture of temperature in vertical cross section is also attached.
> Can you give some ideas to overcome this problem?    
> 
> Thanks 
> Kateryna.
> 
> 
> My data file is
> 
> # ====================
> # | Model parameters |
> # ====================
> #
> # Continuous equation parameters
>  &PARM01
>  tRef=100*20.,
>  sRef=100*0.,
>  viscAh=8.E-4,
>  viscAz=8.E-4,
>  bottomDragLinear=0.E-4,
>  no_slip_sides=.FALSE.,
>  no_slip_bottom=.FALSE.,
>  diffKhT=8.E-4,
>  diffKzT=8.E-4,
>  diffKhS=8.E-4,
>  diffKzS=8.E-4,
>  f0=0.e-4,
>  beta=0.E-11,
>  eosType='LINEAR',
>  rigidLid=.FALSE.,
>  implicitFreeSurface=.TRUE.,
>  gravity =9.81,
>  nonHydrostatic=.TRUE.,
>  readBinaryPrec=64,
>  staggerTimeStep=.TRUE.,
>  hFacMin= 0.001,
> #- not safe to use globalFiles in multi-processors runs
>  useSingleCpuIO =.TRUE.,
>  tempAdvScheme=77,
>  &
> 
> # Elliptic solver parameters
>  &PARM02
>  cg2dMaxIters=300,
>  cg2dTargetResidual=1.E-13,
>  cg3dMaxIters=20,
>  cg3dTargetResidual=1.E-8,
>  &
> 
> # Time stepping parameters
>  &PARM03
>  nIter0=0,
>  nTimeSteps=900000,
>  deltaT=0.005
>  abEps=0.1,
>  pChkptFreq=50.,
>  chkptFreq=0.0,
>  dumpFreq=20.,
>  monitorSelect=2,
>  monitorFreq=1.,
>  &
> 
> # Gridding parameters
>  &PARM04
>  usingCartesianGrid=.TRUE.,
>  delX=1000*5.0,
>  delY=200*5.0,
> # dYspacing=5,
>  delZ=100*0.3,
>  &
> 
> # Input datasets
>  &PARM05
>  bathyFile='topog.lake',
>  hydrogThetaFile='Temp.init',
>  &
> 
> 
> 
> <strips_on_elevation.jpg><Vertical_temperature.jpg>_______________________________________________
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