[MITgcm-support] obcs eta

[Matthew Mazloff]

Hi Mike,

U is set to be non-divergent on the open boundary (for a western or  
eastern boundary), and then eta on the boundary is determined by the  
divergence of V. This value of eta (and the pressure on the open  
boundary), however, is never used in the model.

Sorry I can't help more, I have no experience with Orlanski boundary  
conditions.

-Matt





On Oct 12, 2007, at 7:41 AM, Michael A. Spall wrote:

> I am running some nonhydrostatic calculations in a channel with
> the obcs package and surface cooling.  I specify an initially  
> geostrophically
> balanced state and impose u, v, T on inflow (western boundary)
> and use Orlanski on outflow (eastern boundary).  The obcs appear to
> work well, at least the fields look smooth approaching the outflow
> boundary.  However, the eta field changes by 10's of meters over only
> a few days, sometimes upward, sometimes downward.  The
> gradients in eta look good, and balance the geostrophic flow.
> I get the same result when I do a hydrostatic version of the run.
> It also seems impervious to changes in convergence parameters
> and horizontal/vertical resolution.
>
> I can't find how to specify eta on the inflow boundary.  How does the
> model calculate eta on the boundaries?  Is there a way to ensure that
> mass is conserved in the domain?  I defined ALLOW_OBCS_BALANCE,
> which sounded like it was supposed to balance mass, but this
> doesn't help.
>
> My data file and OBCS_OPTIONS.h files are below.  Any suggestions
> welcome.
>
> Thanks,
> Mike
>
>
> ========================================================
>
>
> # ====================
> # | Model parameters |
> # ====================
> #
> # Continuous equation parameters
> &PARM01
> tRef=25*1.,
> sRef=25*35.,
> viscAh=1.0,
> viscAz=1.E-5,
> no_slip_sides=.TRUE.,
> no_slip_bottom=.TRUE.,
> diffK4T=0.E5,
> diffKhT=1.0,
> diffKzT=1.E-5,
> diffK4S=0.E4,
> diffKhS=1.0,
> diffKzS=1.E-5,
> f0=1.e-4,
> beta=0.E-11,
> tAlpha=2.E-4,
> sBeta =0.E-4,
> rigidLid=.FALSE.,
> implicitFreeSurface=.TRUE.,
> exactConserv=.TRUE.,
> eosType='LINEAR',
> hFacMin=0.05,
> nonHydrostatic=.TRUE.,
> readBinaryPrec=64,
> bottomDragLinear=0.E-4,
> tempAdvScheme=33,
> useSingleCpuIO=.TRUE.,
> &
>
> # Elliptic solver parameters
> &PARM02
> cg2dMaxIters=1000,
> cg2dTargetResidual=1.E-13,
> cg3dMaxIters=50,
> cg3dTargetResidual=1.E-8,
> &
>
> # Time stepping parameters
> &PARM03
> niter0=0,
> endTime=864000.,
> deltaT=40.0,
> abEps=0.01,
> pChkptFreq=0.0,
> chkptFreq=0.0,
> dumpFreq=86400,
> monitorFreq=86400.,
> &
>
> # Gridding parameters
> &PARM04
> usingCartesianGrid=.TRUE.,
> usingSphericalPolarGrid=.FALSE.,
> dXspacing=200.,
> dYspacing=200.,
> delZ=25*20,
> &
>
> # Input datasets
> &PARM05
> bathyFile='topog.bin',
> surfQfile='Qnet.forcing',
> hydrogThetaFile='T.init',
> uVelInitFile='U.init',
> vVelInitFile='V.init',
> pSurfInitFile='Eta.init',
> &
> ===========================================
> C $Header: /u/gcmpack/MITgcm/pkg/obcs/OBCS_OPTIONS.h,v 1.5  
> 2005/10/10 05:53:49 mlosch Exp $
> C $Name:  $
>
> C CPP options file for OBCS package
> C
> C Use this file for selecting options within the OBCS package
> C
> C OBCS is enabled with ALLOW_OBCS in CPP_OPTIONS.h
>
> #ifndef OBCS_OPTIONS_H
> #define OBCS_OPTIONS_H
> #include "PACKAGES_CONFIG.h"
> #include "CPP_OPTIONS.h"
>
> #ifdef ALLOW_OBCS
>
> C Enable individual open boundaries
> #undef ALLOW_OBCS_NORTH
> #undef ALLOW_OBCS_SOUTH
> #define ALLOW_OBCS_EAST
> #define ALLOW_OBCS_WEST
>
> C This include hooks to the Orlanski Open Boundary Radiation code
> #define ALLOW_ORLANSKI
>
> C Enable OB values to be prescribed via external fields that are read
> C from a file
> #define ALLOW_OBCS_PRESCRIBE
>
> C This includes hooks to sponge layer treatment of uvel, vvel
> #undef ALLOW_OBCS_SPONGE
>
> C balance barotropic velocity
> #define ALLOW_OBCS_BALANCE
>
> #endif /* ALLOW_OBCS */
> #endif /* OBCS_OPTIONS_H */
>
>
> ===========================================
>
> -- 
> ==================================================
>
> Michael A. Spall
> Department of Physical Oceanography
> W.H.O.I. MS #21
> 360 Woods Hole Road
> Woods Hole, MA   02543
>
> mspall at whoi.edu
> (508) 289-3342 (office)
> (508) 457-2181 (fax)
>
> =================================================
>
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