[MITgcm-support] Non-Zero Meridional Velocity Kelvin Wave

[Bertrand Louis Rene Delorme]

Hi Martin,

Thanks a lot for your suggestion. It did decrease the amplitude of V by a factor of 10, which is extremely positive
(as Yu-Kun mentioned, we cannot expect to have a meridional velocity exactly equal to 0, but still it should be pretty small).

I guess that I have been confused by the grid specification in the model and let me know if I got that right now.

Let’s say that I want a domain with a north and bottom boundary, where I specify U, V, T and ETA as IC. 
First, I generate a Nx*(Ny+1)*(Nz+1) grid. 
My bathymetry file, Bat, is of size Nx*(Ny+1)*(Nz+1), with Bat(:,:,Nz+1)=0 the bottom boundary, and Bat(:,Ny+1,:)=0 the north wall (elsewhere Bat=-H).
Then, the U grid is (Nx+1)*(Ny+1)*(Nz+1), V grid is Nx*(Ny+2)*(Nz+1), ETA grid is Nx*(Ny+1) and T grid is Nx*(Ny+1)*(Nz+1).
But since I prescribed a north and bottom boundary, I guess that the Y(Ny+1) column and Z(Nz+1) row are not considered by the model, is that right?
Now, I need my IC to be set on the relevant grid, ie: U_IC on (Nx+1)*(Ny+1)*(Nz+1), V_IC on Nx*(Ny+2)*(Nz+1), ETA_IC on Nx*(Ny+1) and T_IC on Nx*(Ny+1)*(Nz+1).
Yet, when I do that, I don’t get the IC I expect from the model. It seems to me that the model is expecting all IC dataset to be Nx*(Ny+1)*(Nz+1).
Does that mean that we need to define the initial velocities at cell boundaries without considering the last (or first?) boundary of the domain?

Thanks again,
Bertrand



Le 17/11/2017 3:23 AM, « MITgcm-support au nom de Martin Losch » <mitgcm-support-bounces at mitgcm.org au nom de Martin.Losch at awi.de> a écrit :

    Hi Bertrand,
    
    I didn’t read the thread very carefully but I found this
    
    > I also checked my IC variables and those are perfectly symmetric about the equator (assuming that I am defining my initial U and V velocity at cell center, right?).
    
    Just to avoid confusion: the initial conditions are actually specified at the corresponding grid points. I.e. dynamic topograph Eta is a the center of the C-cell, Uini and Vini are at the U- and V- points of the grid cell. If you assumed that they are all co-located, that may be the problem?
    
    Martin
    
    
    > On 17. Nov 2017, at 03:58, Bertrand Louis Rene Delorme <bdelorme at stanford.edu> wrote:
    > 
    > A little update on my problem (note also that the plot of Vm_dPHdy_Surf in my previous message is wrong):
    > -          The full baroclinic pressure gradient force (ie: Vm_dPHdy-gravity*(ETAN(i) - ETAN(i-1) )/DYC)  and
    > the Coriolis force (in Vm_Advec or Vm_Cori -- same) are the only forces occurring in the v-momentum equation 
    > (AB_gV is non-zero but much smaller).
    > -          I plotted those at the first time step (see enclosed). We can see that they do seem to have opposite spatial structures.
    > Yet, looking at the difference of their amplitude ( ABS(f*u) - ABS(dPHI/dy) ), we can see that they do not balance
    > each other perfectly. The induced acceleration is consistent with the plot of V.
    >  
    > So, I managed to find where the nonzero values in V come from, but still struggle to know why.
    > If anyone has an idea, please let me know.
    >  
    > Thanks,
    > Bertrand
    >  
    
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