[MITgcm-support] How to simulate the geostrophic current?

[Martin Losch]

Hi Paul,

it's not clear to me what you want to do. The MITgcm implements the full so-called primitive equations (actually, it can also do the filtered Boussinesque Navier-Stokes equations). The dominant balance in large scale circulation simulations with the MITgcm (but also with other GCMs) will be the geostrophic balance.
What you are trying to do looks a bit like "diagnostic runs", where T/S are basically held fixed via restoring and the model computes some velocity field from that (as in the early papers by Bryan and Cox?). This will not give you a purely gestrophic flow field, as more terms of the balance are involved. For purely gestrophic balance, you'll have to turn of the advection of momentum, T,S, turn off the viscosity and diffusion. Maybe these parameters help:
tempStepping = .false., 
saltStepping = .false.,
momViscosity = .false.,
momAdvection = .false.,
momForcing = .false.,
and when there is no timestep in T/S, then you do not need the rbcs package, because T/S will not change.

I have no idea if this will do what you want, but it will give you a model run with T/S fixed in time and just the geostrophic balance left after the transients have faded (du/dt is not turned off!).

M.


On Mar 15, 2011, at 10:10 AM, miaocb wrote:

> Hi MITgcm users,
>     With prescribed temperature and salinity which varies horizontally, I need to diagnoise the corresponding geostrophic current.
> The geostrophic current could be computed via geostrophic balance: fv = 1/rho*dp/dx, fu = -1/rho*dp/dy. But the problem is how to
> implement it using MITgcm. 
>     The initial temperature and salinity are spedified in "data", and time step is set to 360 seconds.  During model run rbcs package is 
> used to relax temperature and salinity to their initial value. 
> hydrogThetaFile='theta_init.dat'
> hydrogSaltFile='salt_init.dat'
>     A 2D configuration with flat topography is used. I found that the geostrophic flow computed depends highly on the  relaxing timescale
> (tauRelaxT and tauRelaxS in "data.rbcs") and  viscosity cofficient viscAh in "data". Different advection schemes for temperature and salinity
> also give absolutely different results. I don't know how to set proper values for these parameters for such a situation.
>    Or are there any other things that should be considered ? I appreciate if someone could help me.
> 
> In my best simulation,  tauRelaxT and tauRelaxS  are both taken to be the interial period 2*pi/f0, and other parameters are listed as follows.
> 
> # Continuous equation parameters
> &PARM01
> tRefFile='tRef.dat',
> sRefFile='sRef.dat',
> viscA4=0.E12,
> viscAz=1.E-1,
> viscAh=400 !1.E0,
> no_slip_sides=.FALSE.,
> no_slip_bottom=.FALSE.,
> diffKhT=0.E0,
> diffKzT=0.E0,
> diffKhS=0.E0,
> diffKzS=0.E0,
> # tempAdvScheme=33,
> # saltAdvScheme=33,
> f0=8.000000E-05,
> beta=0.E-11,
> tAlpha=2.E-4,
> sBeta=7.4E-4,
> gravity=9.81,
> gBaro=9.81,
> rigidLid=.FALSE.,
> implicitFreeSurface=.TRUE.,
> eosType=LINEAR,
> hFacMin=0.2,
> nonHydrostatic=.FALSE.,
> readBinaryPrec=64,
> writeBinaryPrec=64,
> globalFiles=.TRUE.,
> saltStepping=.TRUE.,
> implicSurfPress=1.0, 
> implicDiv2DFlow=1.0, 
> nonlinFreeSurf=0,
> hFacInf=0.2,
> hFacSup=1.8,
> exactConserv=.TRUE.,
> useEnergyConservingCoriolis=.TRUE.,
> staggerTimeStep=.TRUE.,
> &
> 
> # Elliptic solver parameters
> &PARM02
> cg2dMaxIters=1000,
> cg2dTargetResidual=1.E-13,
> cg3dMaxIters=400,
> cg3dTargetResidual=1.E-13,
> &
> 
> # Time stepping parameters
> &PARM03
> startTime=0.0,
> endTime=8640000,
> deltaT=360.000000,
> abEps=0.02,
> pChkptFreq=0.,
> chkptFreq=0., 
> dumpFreq=3600.,
> &
> 
> # Gridding parameters
> &PARM04
> usingCartesianGrid=.TRUE.,
> usingSphericalPolarGrid=.FALSE.,
> delXfile='delX.dat',
> delY=1*5000.,
> delRfile='delZ.dat',
> &
> 
> # Input datasets
> &PARM05
> bathyFile='topo.dat',
> hydrogThetaFile='theta_init.dat'
> hydrogSaltFile='salt_init.dat'
> &
> 
> 
> Thanks,
> 
> Paul Miao
> 
> 
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