cdiff not working for z dim!?

[Thomas Spengler]

Dear all,

I am trying to calculate vertical gradients using the 'cdiff' function.

For example for my variabte t (temperature) I write:

ga-> set z 1 21
LEV set to 1 1000
ga-> define dt=cdiff(t,z)

and get the following result/error message:

Error from CDIFF:  Specified dimension non varying
Operation Error:  Error from cdiff function
  Error ocurred at column 1
DEFINE error:  Invalid expression.

when trying to use lev as the vertical dim:

ga-> define dt=cdiff(t,lev)
Error from CDIFF:  Invalid dimension argument
  2nd argument must be X, Y, Z, or T
Operation Error:  Error from cdiff function
  Error ocurred at column 1
DEFINE error:  Invalid expression.

Would be great if someone has a suggestion on how to calculate vertical
differentials in GrADS.

Thanks for your help!

Cheers,
Thomas