[MITgcm-support] Nonhydrostatic mode and Kinetic energy

[Jody Klymak]

Hi Sergey, 

Why are you using a quadratic drag in this case, rather than no-slip?  Quadratic drag is just an approximation to the bottom friction, which I would expect to be quite poor if your topography is so steep that W~U near the seafloor.  

Cheers,  Jody

> On Nov 12, 2014, at  13:24 PM, Fancer Lancer <fancer.lancer at gmail.com> wrote:
> 
> Hello Jean-Michel,
> 
> If there is no routine, which uses KE including vertical velocity W, why does the monitor print the full KE?
> Anyway I am most interested in the bottom friction at the moment, since I'm trying to simulate some laboratory experiment, where the bottom velocity is measured.
> What I said before about the bottom friction and the impermeable boundary condition. If the bottom is flat, then KE doesn't have to take into account the vertical velocity, since it would be zero in the near bottom points due to the impermeable BC. If the bottom is uneven, then the only normal velocity is zero, therefore we need to take into account all velocity components. Otherwise MITgcm uses incomplete KE formulation for quadratic friction. I still think so even though the bottom drag is calculated only for the horizontal velocities, since KE is the energy of movement and there is the movement along Z-axis too.
> 
> In fact I don't completely understand what you mean under "KE scale like the vertical shear". If it means the vertical velocity, then I explained my opinion in the previous paragraph.
> 
> Sincerely,
> -Sergey
> 
> 
> 
> -------------------------------------
> Sergey V. Semin
> Post graduate course student
> - Applied Mathematics and Informatics, Institute of Radio-electronics and IT,
> NNSTU n.a. R.E.Alekseev
> phone: +61 401 654 292 <>, +7 920 031 888 1 <>
> e-mail: serge.v.semin at gmail.com <mailto:serge.v.semin at gmail.com>
> On Wed, Nov 12, 2014 at 11:41 PM, Jean-Michel Campin <jmc at ocean.mit.edu <mailto:jmc at ocean.mit.edu>> wrote:
> Hi Sergey,
> 
> Thanks for pointing to this problem. I don't think there is
> a problem in the code, but the documentation would need to be fixed.
> Note that the expression of KE diagnosed in the monitor:
>  pkg/monitor/mon_ke.F
> contains the W contribution.
> 
> The KE as computed in pkg/mom_common/mom_calc_ke.F is used
> a) for the quadratic bottom drags: without the W contribution, KE scale
>   like the vertical shear, so not sure W should be added.
> b) in some Reynolds scaling for horizontal viscosity (pkg/mom_common/mom_calc_visc.F):
>   all these viscosity calculations are using only the horizontal velocity,
>   so it seems more consistent to keep using KE without W contribution.
> c) in the vector-invariant momentum formulation:
>   here it's clear that KE should not include the W contribution
>   since the the vorticity part only deal with horizontal components of the velocity.
> 
> Cheers,
> Jean-Michel
> 
> On Mon, Nov 10, 2014 at 11:16:39AM +0400, Fancer Lancer wrote:
> > Hello MITgcm developers and users.
> >
> > I've discovered an inconsistency between the User's manual and the code.
> > There is a routine, which calculates KE at any k-level MOM_CALC_KE() it
> > resides the pkg/mom_common/mom_calc_ke.F . The manual states, that in the
> > nonhydrostatic mode it is supposed to be calculated as:
> > KE = 0.5*(u^2 + v^2 + e_{nh} w^2),
> > but there is no vertical velocity in the  MOM_CALC_KE() routine at all. On
> > the first glance (I'm not completely sure) this mistake should not affect
> > the bottom drag forces, since there is the impermeable condition on the
> > bottom. But there may be some other modules where KE is used and where w is
> > not zero.
> > Could someone clarify whether I'm right or wrong?
> >
> > Sincerely,
> > -Sergey
> >
> > -------------------------------------
> > Sergey V. Semin
> > Post graduate course student
> > - Applied Mathematics and Informatics, Institute of Radio-electronics and
> > IT,
> > NNSTU n.a. R.E.Alekseev, Russia
> 
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--
Jody Klymak    
http://web.uvic.ca/~jklymak/





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