Problem with Fixed Grid Functions?

I have begun using ncl built in functions to calculate divergence and relative
vorticity from NCEP Reanalysis data (2.5x2.5). As it is on a fixed grid, I was
using uv2dvF and uv2vrF. The results came out to the right order of magnitude
but were not realistic. I did the analysis again using the Centered Finite Diff
functions and everything seemed to work.

Any ideas why the fixed grid functions were not working? I also want to
calculate divergent wind components (dv2uvf) but there does not seem to be a CFD
function for this (which is why I ask about the problem with the fixed grid
method).

My code for the fixed grid method is below.

Thanks for taking the time to look at this problem.
-Matthew

;===========================================================
  u = a_u->uwnd(:,:,:,:)
  v = a_v->vwnd(:,:,:,:)

  add_offset=202.660003662109
  scale_factor= .00999999977648258

  u_real=add_offset+u*scale_factor
  v_real=add_offset+v*scale_factor

  dv = uv2dvF (u_real,v_real) ; u,v ==> divergence
;===========================================================

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