[gradsusr] How to calculate the meridional curvature of zonal wind in ERA-Interim data

[shaoxuan]

Hi All,
I want to calculate the meridional curvature of the zonal wind velocity (Uyy) useng ERA-Interim Zonal Wind Data.
I find the GRADS code for calculate the gradient of one field variable in the webpage http://www.iges.org/grads/gadoc/gradfunccdiff.html .
However, I have not find the calculation code for the meridional curvature using GRADS. 
Does any one has any idea to help me figure it out? Thanks.
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