[gradsusr] Determine of Polar Vortex

[Abdolreza Kashki]

Dear all,
How can I calculate(determine) polar vortex boder using the potential vorticity gradient in
isentropic surface in Grads?
Best Regards
kashki

Abdolreza Kashki 
Ph.D. Student of Climatology
Department of Physical Geography
Faculty of Planing & Geography Science 
University of Isfahan  
Isfahan, Iran
Postcode:81746-73441
Tel:+98(311)7933119
Fax:+98(311)7932257
Cell:+989151731453
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