3-D Potential Vorticity

[jrleeman]

Calculate the 3D potential vorticity. The equivalent of PVR3 in GEMPAK.

From GEMPAK Docs:

PVR3  3-D Potential vorticity for a level
      PVR3 ( S, V ) = - CONSTANT * 3D-VOR ( V ) * 3D-GRAD ( S ) 
                   Units are Kelvins / meters / Pascals / seconds**3
		   PVR3 works on a level in PRES or HGHT coordinates.  
                   The constant used is GRAVTY in PRES and is inversly
                   related to DDEN (density) in HGHT.  The scalar 
                   operand, S, is THTA, THTE, THWK, or THES. The 
                   vector, V, has to be GEO or WND.  Multiplying 
                   by 10**6 gives standard PV units.