Dear Ryo,
A million thanks for the detailed explanation. It was very helpful to me and maybe to the other users of Ferret as well.
It is clear that it is nearly impossible to get the correct w velocities using the data remapped on a non-native model grid. In my case, the SODA data was already remapped from the varying tripolar Arakawa-B grids to a uniform 1/2x1/2 Mercator coordinate horizontal grid. So, I may have to check with the SODA team whether they can generate the w field using the original model outputs. I will keep you posted.
with best regards
Satheesh