[MITgcm-support] initial velocity field for staggered grid

[Martin Losch]

Hello,

the default boundary conditions for the MITgcm is double period, i.e. when you specify an initial u-velocity at i = 1 (which is a “lon_low”), then this is copied to u at i = Nx+1  (which is at “lon_high” = “lon_low” + Nx*dx, I am assuming a regular grid with constant dx for simplicity), analogous for the j-direction and v

If you don’t want periodic BC, then you need to have a wall (depth = 0) along i=1 or i = Nx (or both, but one is enough), or you need to use open boundary conditons. With the OBCS package you can specify u a i = 1 (more generally i=OB_Ieast) and i = Nx (OB_Iwest). These values overwrite the default periodic boundaries. Because of the staggered nature of the grid, the specified u will be used at i=OB_least and, more importantly, at OB_Ieast+1, and at (more important) OB_Iwest and OB_Iwest+1. The j-case is analogous. For T and S, the boudary values are at grid index OB_Ieast/west, but as you have found out, the position of the scalar points with these indices is between the velocity points OB_Ieast/west and OB_Ieast/west+1. If you want to be super accurate, you’ll have to take tha into account.

Martin

On Sep 24, 2014, at 7:19 AM, 钱钰坤 <qianyk at mail3.sysu.edu.cn> wrote:

> Hi all,
> 
> I setup a regional run using OBCS.  The initial conditions (ICs) includes U, V, T, and S.  I notice that the model is defined on C-grid (staggered grid).  Therefore, the ICs of T and S can be specified on cell grids (first longitude = lon_low + resolution / 2).  For zonal velocity U, however, they are defined on face grids  (first longitude = lon_low).  Do I need to prepare the face-grid IC for U which is different from cell-grid IC for T and S?  If yes, then I think that there should be Nx+1 U grids (one more than the cell grids Nx) but in fact IC of U contains only Nx grids (as example case shows).  Similar problem could be found for V (Ny).
> 
> I also notice that when specifying the OBCS, the west OBCS for T and S may be defined at lon_low + resolution / 2 while that for U should be defined at lon_low.  Has this to be taken into account when generating OBCS?  Does the OBCS at t0 exactly the same as IC? for different BCs? for different variables?
> 
> ------------------
> Best regards 
> 
> Yu-Kun Qian (钱钰坤) 
> Center for Monsoon and Environment Research
> Department of Atmospheric Sciences
> School of Environmental Science and Engineering 
> Sun Yat-sen University 
> No. 135 Xingang West Road, Haizhu District 
> Guangzhou, 510275, P.R. China 
> Tel; 020-84115227 
> Email: qianyk at mail3.sysu.edu.cn
>  
>  
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