[MITgcm-support] Varying deltaTtracer in a barotropic model

[Patmore, Ryan D.]

Hi all,

I have picked up on some strange time stepping behaviour and I was wondering if someone can enlighten me. I am running a barotropic channel model with fixed temperature and salinity.  I have set
saltStepping=.FALSE.,
tempStepping=.FALSE.,
and I have also checked the monitoring output and as expected the temperature and salinity of the model do not change throughout the model run.

My problem is as follows. If I compare model runs with synchronous time stepping, i.e.

deltaTmom = deltaTfreesurf= deltaTtracer

and asynchronous time stepping, i.e.

deltaTmom = deltaTfreesurf = a * deltaTtracer,  where  a < 1,

 the synchronous setup uses a considerable amount more walltime than the asynchronous setup. The results in both cases are also different. If I was running a baroclinic model then this would be expected, but my model is barotropic and thus I expected no tracer operations to occur. Does anyone know why changing deltaTtracer in a barotropic run has any effect on the results?

An example data file is below.

Any suggestions will be much appreciated,
Ryan D. Patmore

#
# Model parameters
# Continuous equation parameters
 &PARM01
# ########################################
# boundaries
 bottomDragQuadratic=2.5e-3,
 no_slip_sides=.FALSE.,
 no_slip_bottom=.FALSE.,
# #########################################
# viscosities
 viscAz=1.E-2,
 useFullLeith=.TRUE.,
 viscC2Leith=2.0,
 viscC4Leith=0.2,
# #########################################
# misc param, eq-state & gravity & f
 selectCoriMap = 1,
 f0 = -1.27393e-4,
 beta = 1.1144e-11,
 gravity=9.81,
 readBinaryPrec=64,
 useSingleCPUio = .TRUE.,
# #########################################
# misc flags
 saltStepping=.FALSE.,
 tempStepping=.FALSE.,
 staggerTimeStep=.TRUE.,
 implicitFreeSurface=.TRUE.,
 implicitDiffusion=.TRUE.,
 implicitViscosity=.TRUE.,
 rigidLid=.FALSE.,
 nonlinFreeSurf= 4,
 exactConserv = .TRUE.,
 metricTerms = .TRUE.,
# #########################################
# topog params
 hFacMin=0.2,
 hFacMinDz=20.0,
# #########################################
 &

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

# Time stepping parameters
&PARM03
 niter0=0,
 endTime=1581120000.0,
 deltaTmom=900.,
 deltaTtracer=900.,
 deltaTfreesurf=900.,
 abEps=0.1,
 dumpInitAndLast=.TRUE.,
 dumpFreq=0,
 pChkPtFreq=31622400.0,
 chkPtFreq=2635200.0,
 monitorFreq=2635200.0,
# pickupSuff='ckptA',
##########################################
 &

# Gridding parameters
 &PARM04
 usingCartesianGrid=.TRUE.,
 usingSphericalPolarGrid=.FALSE.,
 ygOrigin=-75.0,
 delX= 288*25e3,
 delY= 288*25e3,
 delZ= 20.0,      21.11083314,   22.28336379,   23.52101874,   24.82741509,
   26.20637086,   27.66191612,   29.19830477,   30.820027  ,   32.53182237,
   34.33869368,   36.24592163,   38.25908017,   40.38405288,   42.62705009,
   44.99462708,   47.49370322,   50.13158219,   52.91597333,   55.85501416,
   58.9572942 ,   62.23188   ,   65.68834173,   69.33678107,   73.18786078,
   77.25283583,   81.54358634,   86.07265223,   90.85326995,   95.8994111 ,
  101.22582329,  106.84807324,  112.78259227,  119.04672431,  125.65877663,
  132.63807329,  140.00501165,  147.78112198,  155.98913035,  164.65302512,
  173.79812695,  183.45116289,  193.64034444,  204.39545001,  215.74791197,
  227.73090848,  240.37946047,  253.730534  ,  267.82314826,  282.69848968,


##########################################
 &

 &PARM05
 bathyFile='bathy.bin',
 zonalWindFile='wind.bin',
 &
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