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[GEMPAK #JYO-583263]: Re: 20060314: computing a parameter at a single location

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  • Subject: [GEMPAK #JYO-583263]: Re: 20060314: computing a parameter at a single location
  • Date: Tue, 14 Mar 2006 13:25:38 -0700
>Thanks Steve,
>
>The first solution you proposed is the kind of thing I was looking
>for...but GDPOINT wasn't listed at the site I usually use for GEMPAK
>reference. 

The contrib program lacked a $GEMPTXT/gdpoint.txt file which I 
created and will now be listed on:
http://www.unidata.ucar.edu/software/gempak/help_and_documentation/manual/programs/


>I'm still a bit confused at how to use the grad() diagnostic.
>Would I use it in GDPOINT or another program? How would I specify the two
>points to use to calculate the gradient? And perhaps most importantly,
>what would be the units of the return value (i.e. per degree, per grid
>point)? I apologize if these are simple issues, but I haven't used grid
>diagnostics much.

The GRAD() function computes a vector field from a grid. You can use this
in a GFUNC expression such as:
GFUNC = MAG(GRAD(thta))    (using thta as an example, any scalar can be used)
Or the components with
GFUNC = UR(GRAD(thta)) and GFUNC = VR(GRAD(thta))
  **note UR and VR are grid relative components, UN and VN are grid relative 
components
   (Your grid may not have the row/columns 

If you wanted to use 2 points in the gradient, you could compute the ddx and 
ddy, or grad
and compute an average gradient between the 2 points, or the actual gradient at 
any
point in between the 2 points.

The units of GRAD(s) will be that of the scalar quantity per meter.
For example, using GRAD(pmsl) yields mb/m. You can multiply this
to more convenient units as needed.


Steve Chiswell
Unidata User SUpport





Ticket Details
===================
Ticket ID: JYO-583263
Department: Support GEMPAK
Priority: Normal
Status: Closed