[MITgcm-support] Aligning hFacW, UVEL, dyG

[Martin Losch]

Hi Andrea,

for OBCS, the relevant velocity point is always inward of the tracer boundary point that you have specified. Because on a C-grid the velocity points are staggered, for a western boundary this means that for a boundary points at i, the prescribed velocity is a UVEL(i+1,….). For an eastern boundary at i it is UVEL(i,….). It’s the same for the hFacWs: for a western boundary at i this is the flux that you are interested in: FLX(i+1,j,k) = hFacW(i+1,j,k)*UVEL(i+1,j,k)*dz*dyG(i+1,j). FLX(i,j,k) = hFacW(i,j,k)*UVEL(i,j,k)*dz*dyG(i,j) is already outside of the domain. (Internally, you have FLX(i,j,k)=F(i+1,j,k))

Martin

> On 13 Mar 2017, at 21:43, Cimatoribus Andrea <andrea.cimatoribus at epfl.ch> wrote:
> 
> Dear all,
> a question possibly already discussed in the past, but I cannot find the answer.
> 
> For doing particle tracking in tracmass I am computing the advective volume flux, say in the zonal direction at the western face of (i,j) cell, as:
>  Uflx(i,j) = UVEL(i,j)*dZ*dyG(i,j)*hFacW(i,j)
> (omitting k index), as discussed also in the ml in the past 
> http://mitgcm.org/pipermail/mitgcm-support/2012-September/007945.html
> http://mitgcm.org/pipermail/mitgcm-support/2014-April/009091.html
> 
> This seems to be working fine, also for computing OB fluxes, but if I compare UVEL with hFacW, the first non-zero u-velocity (starting from the west) always has a hFacW=0 at the same i-index. I do not seem to be able to make sense of this mismatch, looking at the grid indexing in GRID.h or in the manual. I was wondering if anyone could clarify this point to me.
> 
> Thank you very much.
> Andrea
> 
> -- 
> Andrea Cimatoribus
> postdoctoral researcher
> EPFL ENAC IIE ECOL
> https://people.epfl.ch/andrea.cimatoribus
> 
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