[gradsusr] Grads vertical derivative error with cdiff

[Aishwarya]

Hi,

I'm trying to find vertical derivative of geopotential (geopt). I tried d
cdiff(geopt,z), i find the error :
Error from CDIFF:  Specified dimension non varying
Operation Error:  Error from cdiff function
  Error ocurred at column 1
DISPLAY error:  Invalid expression
  Expression = cdiff(u,z)

Any help would be great!

Thank you,
-------
*Aishwarya,
Graduate Research Assistant,
Atmospheric Sciences,
University of Arizona,Tucson.*
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