[MITgcm-support] Thermal wind in ocean cause too large velocity?

[Martin Losch]

On top of Jody's very valid statement about adjustment processes, for the pure geostrophic balance I extract this from your information:

dx=100 (m), Lx=100*100=10km, dz=20m, Lz=1000m
rho(S=0,T=20,p=0)-rho(S=0,T=13,p=0) = -1.17 kg/m^3
thermal wind gives
dv/dz = - g/f/rho0 drho/dx = 9.81/1.e-4/1000 * 1.17 /10e3 m/s = 0.0115 m/s*(1/m)

integrating only over the first 5layers already gives dv/dz*100m = 1.15 m/s and with the full depth you'd get 11.5m/s, so I am not suprised by your 1.5m/s

Martin


On May 21, 2012, at 7:28 PM, Jody Klymak wrote:

> If you start your model with zero velocity and a large horizontal temperature gradient, it will undergo a Rossby adjustment first which can have large wave velocities associated with it, though 4 m/s seems large. The Coriolis force takes at least an inertial period to become important.  
> 
> If you want your model to be in geostrophic balance at startup, you need to specify the velocities in the initial conditions as well.  
> 
> Cheers,   Jody
> 
> 
> On May 21, 2012, at  4:38 AM, Chun-Yan Zhou wrote:
> 
>> Based on
>> The temperature difference in horizontal direction can cause large velocity because of thermal wind.
>> And I did a test where the initial temperature field changed form 20-13 degree along the X direction, no other forces are applied.
>> Large velocity is expected, but the maximum velocity is 4m/s, and the minimum is -4m/s, most of  the velocity is around 1.5m/s. Is the velocity should be so large?
>> Any suggestion is appreciated.
>> chunyan
>> 
>> # ====================
>> # | Model parameters |
>> # ====================
>> #
>> # Continuous equation parameters
>> &PARM01
>> tRef=20*20.,
>> sRef=20*35.,
>> viscAh=4.E-2,
>> viscAz=4.E-2,
>> no_slip_sides=.FALSE.,
>> no_slip_bottom=.FALSE.,
>> diffKhT=4.E-2,
>> diffKzT=4.E-2,
>> f0=0.E-4,
>> beta=0.E-11,
>> tAlpha=2.0E-4,
>> sBeta =0.,
>> tempAdvScheme=77,
>> gravity=10.,
>> rhoConst=1000.,
>> rhoNil=1000.,
>> heatCapacity_Cp=4000.,
>> #rigidLid=.TRUE.,
>> implicitFreeSurface=.TRUE.,
>> #exactConserv=.TRUE.,
>> eosType='LINEAR',
>> nonHydrostatic=.TRUE.,
>> saltStepping=.FALSE.,
>> staggerTimeStep=.TRUE.,
>> &
>> 
>> # Elliptic solver parameters
>> &PARM02
>> cg2dMaxIters=1000,
>> cg2dTargetResidual=1.E-9,
>> cg3dMaxIters=1000,
>> cg3dTargetResidual=1.E-9,
>> &
>> 
>> # Time stepping parameters
>> &PARM03
>> nIter0=0,
>> #endTime=43200.,
>> nTimeSteps=21600,
>> deltaT=10.,
>> abEps=0.1,
>> pChkptFreq=43200.,
>> chkptFreq=7200.,
>> dumpFreq=1800.,
>> monitorFreq=600.,
>> monitorSelect=1,
>> monitorFreq=1.,
>> &
>> 
>> # Gridding parameters
>> &PARM04
>> usingCartesianGrid=.TRUE.,
>> dXspacing=100.,
>> dYspacing=100.,
>> delZ=50*20.,
>> &
>> 
>> # Input datasets
>> &PARM05
>> hydrogThetaFile='T.bin',
>> &
>> %%%%%%%%%%%gendata.m
>> var=zeros(100,100,50);
>> for i=1:100
>>   var(i,:,:)=20-0.07*(i-1);
>> end
>> fid=fopen('T.bin','w','b'); fwrite(fid,var,'real*4'); fclose(fid);
>> 
>> 
>> 
>> Dear all,
>> 
>> 
>> 
>> 
>> The University of Dundee is a registered Scottish Charity, No: SC015096
>> 
>> 
>> _______________________________________________
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>> MITgcm-support at mitgcm.org
>> http://mitgcm.org/mailman/listinfo/mitgcm-support
> 
> --
> Jody Klymak    
> http://web.uvic.ca/~jklymak/
> 
> 
> 
> 
> 
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