[MITgcm-support] Logarithmic decrease in Eta without OBCS/EXF

Wed Jul 9 09:23:11 EDT 2014

Dear all,

I am trying to study regional ocean dynamics in the Faroe-Shetland Channel using the MITgcm, with model boundary/initialisation forced from HYCOM data and an atmospheric forcing from WRF. I have been trying to systematically work up to this (though in reality leapfrogging back and forth somewhat), and along the way have an encountered a problem whereby the default model (without OBCS and EXF packages running, so doubly-periodic boundaries) has a decreasing and solely negative sea surface height anomaly. To me this is worrying as it implies that the model is losing water, which it should not in its default state with periodic boundaries. In building up the model I have ran the model many times in various manners to try and isolate the problem. I have come up with the following progression below highlighting the nature of my problem. All runs have been for 2 weeks model time = 1209600s, and run on a 288x288 spherical grid (approx. 2km resolution) with 35 vertical levels and a linear free surface, except where stated.

1)      In .data: bathyFile given, hydrogTheta/SaltFile not given, tRef/sRef taken as initialisation at 10°/35.0. No other forcings/initialisations. No OBCS or EXF.

o   Result [Good]: No decrease in eta, constant throughout at 0m.

o   Notes: As it should be with no initialisation and a constant temperature/salinity basin.

2)      In .data: Same as 1), except that I have set a constant north-eastwards wind forcing at 2Pa (high wind stress). Model run time is 6h = 21600s. No OBCS or EXF.

o   Result [Good]: Eta output has positive-negative range from -0.35m to 0.25m, dynstat_eta_mean is of the order 0.007m after 2 weeks.

o   Notes: ncview shows convergence/divergence of water on the various coastlines given in the bathymetry. Only issue is short model run time might be masking a potential later loss of eta throughout the domain.

3)      In .data: bathyFile and hydrogTheta/SaltFile given (tRef/sRef set 10°/35.0 for consistency). Temperature and salinity set as passive tracers (tAlpha =sBeta = 0). No other forcings/initialisations. No OBCS or EXF.

o   Result [Bad?]: After about 2.5 days of highly varying Eta (max/min ranges from +/- 1m) difference between max/min Eta decreases so that the mean Eta stabilises (with very small standard deviation) to around -0.07m without further dropping.

o   Notes: After the period of 'instability' the mean Eta stabilises - but shouldn't it stabilise back to zero Eta? Should there even be a period of instability/variance in Eta given that the Temperature and salinity themselves are just passive tracers?

4)      Same as 3), except temperature is now an active tracer (tAlpha = 2.E-4).

o   Result [Bad]: After 2 weeks of running, eta_mean has dropped to -0.85m, and is dropping logarithmically.

o   Notes: After period of volatility lasting the first day, the logarithmic drop in eta is approximately a 0.7m drop in eta every tenfold increase in time (i.e. eta_mean drops by 0.7m as we go from 10E4 to 10E5 seconds).

5)      Same as 3), except now temperature and salinity are both active tracers (tAlpha = 2E.-4, sBeta = 7.4E-4).

o   Result [Bad]: Similar result to 4), except the logarithmic drop in eta_min is smaller.

6)      Same as 5), except now added in initial current files, uVelInitFile and vVelInitFile. [This is the configuration I have given at the bottom of this email]

o   Result [Bad]: Similar to 3,4), except the logarithmic drop in eta_min is smaller, around 0.5m per tenfold increase in time.

7)      Same as 6) except running with nonlinFreeSurf=4, select_rStar=2.

o   Result [Bad]: Same outcome as 6.

I have done a number of other runs with various flags turned on and off, but I felt these to be the most relevant. I have also performed runs with OBCS and EXF turned on, and the loss in eta is much, much larger (over a two week run with OBCS, EXF on I lost close to 45m of water). However based on the mailing list I am aware that this loss could be due to net divergence at the model boundaries with OBCS, so I somewhat less concerned given that I may be able to use OBCS_BALANCE or Orlanski schemes to 'stem the flow'. However if there is loss in the default case I'm a bit more concerned that something is fundamentally going wrong.

I would appreciate any solutions, help, guidance, or just ideas (which are inherently more useful than banging one's head on a keyboard).

Thanks very much,

Arjav

Input file ./data:

# Model parameters

# Continuous equation parameters
&PARM01
# Tracer advection
tempAdvScheme=33,
tempImplVertAdv=.FALSE.,
tempVertAdvScheme=33,
saltAdvScheme=33,
saltImplVertAdv=.FALSE.,
saltVertAdvScheme=33,
# Tracer diffusion
implicitDiffusion=.FALSE.,
diffKhT=1.45E-7,
diffKhS=1.E-8,
diffKzT=1.45E-7,
diffKzS=1.E-8,
# Viscosity Parameters
implicitViscosity=.FALSE.,
viscAz=1.E-2,
viscAh=5.E2,
# Equation of state and reference parameters
eosType='LINEAR',
tAlpha=2.E-4,
sBeta=7.4E-4,
tRef=35*10.,
sRef=35*35.,
# Basic physics parameters
rotationPeriod=86400.,
gravity=9.81,
# Boundary conditions
no_slip_sides=.FALSE.,
no_slip_bottom=.TRUE.,
# Sea surface flags
rigidLid=.FALSE.,
implicitFreeSurface=.TRUE.,
nonlinFreeSurf=0,
exactConserv=.TRUE.,
# Solution and I/O parameters
nonHydrostatic=.FALSE.,
readBinaryPrec=64,
# Miscellaneous
# implicitIntGravWave=.TRUE.,
staggerTimeStep=.TRUE.,
&

# Elliptic solver parameters
&PARM02
cg2dMaxIters=1000,
cg2dTargetResidual=1.E-13,
&

# Time stepping parameters
&PARM03
startTime=0.,
# endTime = 60s x 60m x 24h (=86400s) x 2wks = 1209600s
endTime=1209600.,
deltaT=6.0,
dumpFreq=3600.,
# deltaTmom=12.0,
# deltaTtracer=12.0,
abEps=0.1,
# pChkptFreq=1382400.,
chkptFreq=0.0,
# monitorFreq = Order(dumpFreq)
monitorFreq=1800.,
monitorSelect=3,
&

# Gridding parameters
&PARM04
usingSphericalPolarGrid=.TRUE.,
xgOrigin=-9.32,
ygOrigin=57.93,
delX=288*0.0370370370370363,
delY=288*0.0178571428571459,
delZ= 1.18,         1.40,      1.65,      1.96,      2.32,      2.74,      3.24,
                3.83,      4.53,      5.36,      6.34,      7.50,      8.87,      10.49,
                12.41,    14.68,    17.36,    20.53,    24.29,    28.73,    33.98,
                40.19,    47.54,    56.23,    66.51,    78.67,    93.05,    110.06,
                130.18,  153.98,  182.13,  215.42,
                254.80,  301.38,  356.48,
&
&PARM05
bathyFile='../../topog.fsc288',
hydrogThetaFile='../../theta.init',
hydrogSaltFile='../../salinity.init',
uVelInitFile='../../u.init',
vVelInitFile='../../v.init',
# zonalWindFile=,
# meridWindFile=,
&

----------------------------------------------------
Arjav Trivedi

PhD Student
Space and Atmospheric Physics Group
Imperial College London
Prince Consort Road
South Kensington, SW7 2AZ

T: +44 (0) 20 7594 1402
E: arjav.trivedi09 at imperial.ac.uk<mailto:arjav.trivedi09 at imperial.ac.uk>
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