[gradsusr] irrotational wind vector at 250 hPa

[Moustabchir Rachid]

Hi everybody!
I want to plot irrotational wind vector at 250 hPa using grads or opengrads
like in this link:
http://www.atmo.arizona.edu/~tgalarneau/ncar/map/2012AUG1-15/charts/07-PVIR.html
Could you please help me with this?
Other questions: what is the utility of the irrotational wind vector?
Thanks
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