[MITgcm-support] Aligning hFacW, UVEL, dyG

[Cimatoribus Andrea]

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