[MITgcm-support] dissipation and diffusion rates

[Martin Losch]

Hi Yangxin,

1) Um_Diss, etc is “guDissip” in the code, which contains only the explicit contributions. With implicit viscosity, you need the extra terms that are derived from VISrI_Um and Vm (the vertical diffusive fluxes of momentum, that are computed with an implicit method).
2) Um_Diss is the tendency due to dissipation, but VISrE/I_Um is the vertical flux (du/dz*area). To get to a tendency, you need to compute (VISrE_Um[k]-VISrE_Um[k+1])/volumeOfGridCell. 
3) the same is true for 'DFrE_TH' and ‘DFrI_TH’. They are vertical fluxes that already include the area. The units are correct to, as far as I can see, because the diffusion term (d/dz kappa dtheta/dz) should have degC/s, so kappa dtheta/dz has the unis degC/s * m, and kappa dtheta/dz * area then degC/s m^3, you need to do the same computation as for VISrE/I_Um to get the tendencies.

Martin

> On 27. Sep 2019, at 19:46, Yangxin He <y67he at uwaterloo.ca> wrote:
> 
> Hi there,
> 
> I am currently running a simple oceanic internal wave simulation. I have implemented Smagorinsky for horizontal viscosity, pp81 for the vertical viscosity and diffusion. I have the horizontal diffusion set to 0. Part of my data file is attached in the end.
> I would like to output dissipation/diffusion rates using the diagnostic package. I know I should have Um_Diss, Vm_Diss and Wm_Diss. This way my dissipation rate for the kinetic energy will be 
> Kt=U*(Um_Diss)+V*(Vm_Diss)+W*(Wm_Diss).
> pp81 can only be used if implicitViscosity and implicitDiffusion are on. Few questions here
> 1) From reading some of the previous posts, I got the impression that if implicitViscosity is on, I need to add more terms to my expression of Kt. Why?
> 
> 2) If 1) is true, I will need 'VISrE_Um', 'VISrI_Um', 'VISrE_Vm' and ''VISrI_Vm'', but their units are 'm^4/s^2' instead of 'm/s^2'. How do I understand this? and how can I add them to Kt?
> 
> 3) For the diffusion rate of the density equation, I will need both 'DFrE_TH' and 'DFrI_TH'? Their units are degC.m^3/s, but I am expecting degC/s. Does this need some scaling? or my understanding is not quite right?
> 
>  viscAz=1.E-5,
>  viscAh=0.,
>  no_slip_sides=.FALSE.,
>  no_slip_bottom=.FALSE.,
>  diffKhT=0.,
>  diffKzT=1.E-5,
> #new parameters for smag
>  viscC2Smag=2.0,
> # viscC4Smag=1.0,
>  f0=6.5E-5,
>  beta=0.,
>  eosType='LINEAR',
>  tAlpha=2.E-4,
>  sBeta =0.,
>  rhonil=1028.0,
>  rhoConst=1028.0,
>  gravity=9.81,
>  rigidLid=.TRUE.,
>  implicitFreeSurface=.FALSE.,
> # exactConserv=.TRUE.
>  nonHydrostatic=.FALSE.,
> # minimum cell fraction. This reduces steppiness from Jody
>  hFacMin=0.1,
> # implicSurfPress=0.5,
> # implicDiv2DFlow=0.5,
> # nonlinFreeSurf=3,
> # hFacInf=0.2,
> # hFacSup=1.8,
> #- not safe to use globalFiles in multi-processors runs
>  globalFiles=.TRUE.,
>  readBinaryPrec=64,
>  writeBinaryPrec=64,
>  writeStatePrec=64,
>  saltStepping=.FALSE.,
> #parameters for pp81
>  implicitDiffusion=.TRUE.,
>  implicitViscosity=.TRUE.,
> #change advection shceme
>  tempAdvScheme=33,
>  staggerTimeStep=.TRUE.,
>  &
> 
> 
> Thanks
> 
> Yangxin
> 
> 
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