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