[gradsusr] vertical derivative

[Shing]

hi

i want to take vertical derivative of humidity in the isobaric coordinates.
i used the function cdiff:


define dq=cdiff(q,z)

but got error message saying
error from cdiff: specific dimension non varying
operation error: error from cdiff function
error occurred at column 1
define error: invalid expression

it works fine when i take horizontal derivatives, such as dq=cdiff(q,x) and
dq=cdiff(q,y)

is it not the way to do vertical derivative in grads?

thank you very much!

shing
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