[gradsusr] How to calculate ∂/∂(ln p) of a variable in grads

[lpasmanoranjan]

Dear Users,
I would like to calculate  thermal wind from geostrophic wind in grads.
Following is the formula:
Thermal Wind, Vt=  ∂Vg / ∂(ln p)
where Vg is the geostrophic wind, p is the pressure

I can't able to calculate the term  ∂ / ∂(ln p).

Any suggestions or help?
Thank you.
-- 
Kind Regards,
Mano
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