computation of potential vorticity in GRADS

[Ilukpitiya Mudi S P Jayawardena]

Dear all

I want to compute potential vorticity in GRADS using NCEP/NCAR reanalysis data. The data set don't have theta surface data.

I can define theta ( Potential temperature ) but I don't know how to compute [d(theta)/d(p)] and p is for pressure.

I tried to use the commands
'define prs=lev'
'define z=prs'
" define dtheta=cdiff(theta,z)"

it does not work. It says
' Error from CDIFF: specification dimension non varying
 Operational error : Error from cdiff function
Error occurred at column 1
DEFINE error : Invalid expression.

Please help me

Thanks

Ilukpitiya