Try using the scripts and formula from the below sources.

http://gradsaddict.blogspot.com/2013/05/script-thetags-defines-potential-and.html

and

http://gradsusr.org/pipermail/gradsusr/2010-January/010266.html

Hi Meenakshi,

For calculations, I suggest CDO instead of GrADS.

Best,

Thank you Sudhansu Sekhar Rath and M. Tufan Turp . I will try both the methods. Hope it resolves the problem.

For calculations, cdo

gives the best result rather than grads