[MITgcm-support] heavy diffusion and water source

[Patrick Rosendahl]

Hello Martin,
thanks for the help.

I am working with periodic boundaries.

A) I have added useRealFreshWaterFlux, select_rStar, nonlinFreeSurf and 
hFac* to the data file (the rest was already set as suggested, also in 
my CPP_OPTIONS.h). But I still get +2.5e-5 m sealevel rise after t=40000s.
I guess it is because I am using a 1-pixel model in the XZ plane. When I 
am forcing vVel(1) = vVel(2) everything is good:

-------------------------------------integr_continuity.F
#ifdef EXACT_CONSERV
       IF (exactConserv) THEN

C--   Compute the Divergence of The Barotropic Flow :

C-    force duplicated values, because we are working with a slice only
       IF ( sNy.eq.1 ) THEN
         DO k=1,Nr
           DO i=1,sNx+1
             vFld(i,2,k,bi,bj) = vFld(i,1,k,bi,bj)
           ENDDO
         ENDDO
       ENDIF
-------------------------------------integr_continuity.F

However, still wondering why the level rises... Maybe the level is 
computed at another point in the code with a different formula. What I 
mean is this situation:
"The formula" might be partially evaluated before and after the model 
duplicates the vVel-values itself. Then the values are inconsistent and 
the model "thinks" there was a rise before the timestep already - hence 
it conserves it.


B) It turned out that the "diffusivity" was rather fluxes due to rising 
sea level.

Cheers,
Patrick

Martin Losch wrote:
> Patrick,
> not sure what's happening, but according to your data.pkg you are NOT 
> using OBCS (so no "open boundaries"), probably you mean periodic boudaries?
> 
> A) exact conservation of volume (no sea level rise for a closed system) 
> is achieved like this:
> 1. in CPP_OPTIONS.h: #define EXACT_CONSERV (that's the default)
> 2. in data: exactConserv = .true. (not default for backward 
> compatibility, I guess), and
>  useRealFreshWaterFlux=.TRUE., if you have surface fresh water forcing 
> (salinity restoring or EmPmR)
> if that's not sufficient, then you could try the non-linear free surface 
> (may not really exactly work with OBCS, if that's required):
> 3. in CPP_OPTIONS.h: #define NONLIN_FRSURF (not default)
> 4. in data (taken from verification/global_ocean.cs32x15/input/data:  
> select_rStar=2, nonlinFreeSurf=4, hFacInf=0.2, hFacSup=2.0, please look 
> up the meaning of the parameters in the documentation
> 
> B) AdvScheme=2 is the simplest scheme (2nd order central differences) 
> and should not be stable without additional diffusion, but that's not 
> your problem, I guess. You use 33 for temp and saltAdvScheme, this 
> scheme should be used with staggerTimeStep=.true., otherwise you'll get 
> a lot of vertical mixing (by static instability?), often discussed in 
> the email-list, but never quite understood (at least not by myself), 
> this mixing will affect your ptracers regardless of their advection 
> scheme. Setting staggerTimeStep=.true. should fix your problem. I'd then 
> go with 33 for ptracers as well; there is numerical diffusion in that 
> scheme, but it's small and directed towards stabilizing the scheme.
> 
> Martin
> On 2 Nov 2008, at 02:38, Patrick Rosendahl wrote:
> 
>> Hello,
>>
>> I have a flow in wind direction (constant tx) with open boundaries. 
>> And I have 2 problems:
>>
>> A) The sealevel rises very slowly like 1e-6 in 100s. I fixed this by 
>> setting hDivFlow(i,j)=0.0 and same in integrate_for_w(). This kept my 
>> eta=w=0. But this is not a clean method. V is zero as it should. Any 
>> thoughts on this?
>>
>> B) There is a lot of vertical diffusivity in the ptracers, although 
>> the transport is only in x-direction. I have set the diffusivity to 
>> 0.0 - no help. I am using advectionScheme=2, since it appears to have 
>> the least diffusivity. Why is there so much diffusivity?
>>
>> Thanks,
>> Patrick
>>
>> ----------------------------------------------------------------------------------------- 
>>
>> DATA:
>>
>> # ====================
>> # | Model parameters |
>> # ====================
>> #
>> # Continuous equation parameters
>>  &PARM01
>>  tRef=51*20.0,
>>  sRef=51*0.0,
>>  viscAh=1.3E-6,
>>  viscAz=1.3E-6,
>>  no_slip_sides=.FALSE.,
>>  no_slip_bottom=.TRUE.,
>>  diffKhT=1.4E-7,
>>  diffKzT=1.4E-7,
>>  diffKhS=1.1E-9,
>>  diffKzS=1.1E-9,
>>  f0=0.0,
>>  sBeta =0.,
>>  gravity=9.81,
>>  rhoConst=1000.0,
>>  rhoNil=1000.0,
>> # heatCapacity_Cp=3900.0,
>>  implicitDiffusion=.TRUE.,
>>  implicitViscosity=.TRUE.,
>>  eosType='LINEAR',
>>  readBinaryPrec=32,
>> # hFacMin=0.2,
>> # hFacInf=0.2,
>>  tempAdvScheme=33,
>>  saltAdvScheme=33,
>>
>>  nonHydrostatic=.FALSE.,
>> # nonlinFreeSurf=3,
>>  exactConserv=.TRUE.,
>>  implicitFreeSurface=.TRUE.,
>>  rigidLid=.FALSE.,
>>  &
>>
>> # Elliptic solver parameters
>>  &PARM02
>>  cg2dMaxIters=200,
>>  cg2dTargetResidual=1.E-13,
>>  cg3dMaxIters=10,
>>  cg3dTargetResidual=1.E-9,
>>  &
>>
>> # Time stepping parameters
>>  &PARM03
>>  nIter0=0,
>>  nTimeSteps=1000,
>>  deltaT=10.0,
>>  abEps=0.1,
>>  pChkptFreq=500000.0,
>>  chkptFreq=500000.0,
>>  dumpFreq=100.0,
>>  monitorFreq=500000.0,
>> # outputTypesInclusive=.TRUE.,
>>  &
>>
>> # Gridding parameters
>>  &PARM04
>>  usingCartesianGrid=.TRUE.,
>>  usingCylindricalGrid=.FALSE.,
>>  usingCurvilinearGrid=.FALSE.,
>>  delX=20*20.0,
>>  delY=1*20.0,
>>  delZ=51*0.2,
>>  &
>>
>> # Input datasets
>>  &PARM05
>>  zonalWindFile='windstressx.const_1.000.bin.real*4',
>>  bathyFile='depth.open_10.0.bin.real*4',
>>  &
>>
>> ----------------------------------------------------------------------------------------- 
>>
>> DATA.PTRACERS:
>>
>>  &PTRACERS_PARM01
>> # PTRACERS_dumpFreq,
>> # PTRACERS_taveFreq,
>> # PTRACERS_monitorFreq,
>>  PTRACERS_advScheme(1)=2,
>>  PTRACERS_advScheme(2)=2,
>> # PTRACERS_ImplVertAdv,
>> PTRACERS_diffKh(1)=0.0,
>> PTRACERS_diffKh(2)=0.0,
>> PTRACERS_diffK4(1)=0.0,
>> PTRACERS_diffK4(2)=0.0,
>> PTRACERS_diffKr(1)=0.0,
>> PTRACERS_diffKr(2)=0.0,
>> # PTRACERS_ref,
>> # PTRACERS_useGMRedi=.FALSE.,
>> # PTRACERS_useKPP=.FALSE.,
>> # PTRACERS_Iter0,
>>  PTRACERS_numInUse=2,
>> # PTRACERS_initialFile,
>> # PTRACERS_useRecords,
>>  PTRACERS_names(1)='tracer1',
>>  PTRACERS_names(2)='tracer2',
>>  PTRACERS_long_names(1)='tracer1',
>>  PTRACERS_long_names(2)='tracer2',
>>  PTRACERS_units(1)='ppm',
>>  PTRACERS_units(2)='ppm',
>> # PTRACERS_timeave_mnc,
>>  PTRACERS_snapshot_mnc=.TRUE.,
>>  PTRACERS_monitor_mnc=.TRUE.,
>>  PTRACERS_pickup_write_mnc=.TRUE.,
>> #     &     PTRACERS_pickup_read_mnc
>>  &
>>
>> ----------------------------------------------------------------------------------------- 
>>
>> DATA.PKG:
>>
>> # Packages
>>  &PACKAGES
>>  useMNC=.TRUE.,
>> # useOBCS=.TRUE.,
>> # useKPP=.TRUE.,
>> # useMY82=.TRUE.,
>>  usePTRACERS=.TRUE.,
>>  &
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