[MITgcm-support] Freshwater OBCS problem

[Dustin Carroll]

Hello,

I have a model set up to simulate circulation in a fjord. I am using OBCS with my western boundary being forced by the tides (with a 100km sponge layer). The eastern boundary is a small channel through the topography / bathymetry. Through the eastern boundary I am forcing fresh water into the fjord with a u vel of 0.5 m/s.

The initial salinity prescribed to the model grid varies between 32 and 35, increasing with depth.

I am having a problem with my model blowing up in the first time step. I seems that anytime I turn on the freshwater eastern boundary I get NEGATIVE SALINITY warnings and the cg2d doesn't converge and I get NaN's. Is there anything else I have to consider when having such a strong salinity gradient at a OBCS? 

I have tried setting freshwater flux to true, increased my cg2d iters / lowered my timestep, and tried both 33 and 77 advection schemes. I am using EXF for forcing my boundary conditions.

Thanks,

# ====================
# | Model parameters |
# ====================
#
# Continuous equation parameters
 &PARM01
 viscAhGrid=1.E-2,
 viscAhGridMax=1,
 viscAhGridMin=5.E-3,
 viscA4Grid=1.E-3,
 viscAz=1.E-5,
 bottomDragQuadratic=2.E-3,
 no_slip_sides=.FALSE.,
 no_slip_bottom=.FALSE.,
 diffKzT=1.E-5,
 diffKzS=1.E-5,
 tempAdvScheme=77,
 saltAdvScheme=77,
 f0=143.E-6,
 beta=417.E-14,
 eosType='MDJWF',
 rigidLid=.FALSE.,
 implicitFreeSurface=.TRUE.,
 implicitViscosity=.TRUE.,
 implicitDiffusion=.TRUE., 	 	
 staggerTimeStep=.TRUE.,
 hFacMin=0.2,
 nonHydrostatic=.TRUE.,

 rhoConstFresh=1000.,

 exactConserv=.TRUE.,
 useRealFreshWaterFlux=.TRUE.,

 readBinaryPrec=64,
 writeBinaryPrec=32,
 &

# Elliptic solver parameters
 &PARM02
 cg2dMaxIters=1200,
 cg2dTargetResidual=1.E-13,
 cg3dMaxIters=50,
 cg3dTargetResidual=1.E-8,
 &

# Time stepping parameters
 &PARM03
 nIter0=0,
 nTimeSteps=28800,
 deltaT=10,
 abEps=0.1,
 pChkptFreq=0,
 chkptFreq=0,
 dumpFreq=21600,
 monitorSelect=2,
 monitorFreq=21600.,
 periodicExternalForcing=.FALSE.,
 externForcingPeriod=3600.,
 externForcingCycle=5184000.,
 &

# Gridding parameters
 &PARM04
 usingCartesianGrid=.TRUE.,
 delXfile='delX.bin',
 delYfile='delY.bin',
 delRfile='delZ.bin',
 &

# Input datasets
 &PARM05
 bathyFile='bathy.bin',
 hydrogThetaFile='temp.bin',
 hydrogSaltFile='salt.bin',
 zonalWindFile='',
 meridWindFile='',
 &