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[GEMPAK #ROY-668075]: Question regarding relative vorticity calculation in GEMPAK

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  • Subject: [GEMPAK #ROY-668075]: Question regarding relative vorticity calculation in GEMPAK
  • Date: Thu, 18 Jan 2018 09:04:29 -0700
> I am just starting to learn about GEMPAK's enormous capabilities and I
> had a question regarding the calculation of relative vorticity. In the
> documentation here -
> 
> https://www.unidata.ucar.edu/software/gempak/man/parm/apxB.html
> 
> you have vorticity defined as -
> 
> VOR ( V ) = DDX ( v ) - DDY ( u )
> 
> but in the code comments you have it defined as -
> 
> *                                    *
> *     VOR ( V ) = DDX(v) - DDY(u) - v*{(mx/my)*[d(my)/dx]} +        *
> *                   + u*{(my/mx)*[d(mx)/dy]}

It appears that you are comparing the calculation for vorticity from apxB.html 
to a code comment rather than the actual code that calculated vorticity.   The 
commented formula includes second-order components which are not included in 
the calculation (guessing here, since I've not dug through code to find where 
this is from - that's where a reference would help, source code file name and 
line number). 

I do not have a test script to calculate PV from reanalysis data, I do now know 
what exactly your reanalysis data are - already in GEMPAK grid form or is it in 
grib2 (or something else?) If the data exists as a GEMPAK file then you are 
good to go and can just use the PVOR grid parameter

from https://www.unidata.ucar.edu/software/gempak/man/parm/apxB.html 

PVOR  Potential vorticity in a layer
      PVOR ( S, V ) = - GRAVTY * AVOR ( VLAV (V) ) * LDF ( THTA ) /
                          ( 100 * LDF ( PRES ) )
                   The 100 converts millibars to Pascals.
                   Units are Kelvins / meters / Pascals / seconds**3
                   (note that GRAVTY is included).  PVOR works on a layer
                   in PRES or THTA coordinates.  In isobaric coordinates,
                   the scalar operand, S, is THTA, THTE, or THES.  In
                   isentropic coordinates, the scalar operand, S, is
                   PRES.  Multiplying by 10**6 gives standard PV units.

Ticket Details
===================
Ticket ID: ROY-668075
Department: Support GEMPAK
Priority: Normal
Status: Open
===================
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