20030710: Use of two diagnostic parameters

>From: John Merrill <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200307101905.h6AJ59Ld017477

>Hello,
>   I see PLCL defined as a diagnostic parameter in Appendix A, and would 
>like to map or plot this over an area using
>gridded fields, again Nogaps analysis/forecast or Reanalysis fields. 
>Using gdmap with gfunc=plcl and glevel=1000,
>for example, I get "Input grid plcl... not found." I've not tried 
>entering the formula after gfunc=, but am beginning to think
>that's how these specialized parameters are used - is that so? I've 
>never used gddiag, and don't have the impression that
>forcing the calculation of a field using this program is what's needed here.
>  Secondly, and related only in that this will be used with gridded 
>analysis fields, how does one use the LOWS (or
>HIGH) parameter to scan, e.g., PMSL, in my case with gvcord=none; 
>glevel=0? Perhaps LOWS(pmsl,5), indicating
>the field of interest and a 5 grid point "squradius", but does the 
>program then march through the grid and report extrema
>which are "further apart" then this distance? And what does it yield or 
>report - the value, the grid location or both?
>I'm tracking the progression of disturbances over time in a large area, 
>and wonder if I use of this parameter will simplify
>my work. Is this used with gdmap, with gdlist, or in some other way? 
>I've used carefully-selected limits in HILO in
>GDPLOT2, but could easily wade through a list of _all_ relative lows for 
>each synoptic time, using a script to loop through
>the times.       Thanks.       John
>

John,

Appendix A are parameters, not grid diagnostic functions which are in
Appendix B1 and B2.

However, I do have a function available which you can use as
GVCORD = PRES
GLEVEL = 1000
GFUNC = PLCL (PRES, TMPC, DWPC)

another way to use typical surface grids (from the NCEP models)
GVCORD = NONE
GLEVEL = 0
GFUNC = PLCL ( PRES, tmpc@2%HGHT, dwpc@2%HGHT)

The PLCL function is one I created on the way making a general CAPE function.

As for highs and Lows, you can plot these values on a map using gdplot2 like:
GDPFUN   = high(emsl,5)
TYPE     = p

The type P will plot the data value, or you can use markers etc.

The high and lows function will return -9999 for all grid points
except the relative maxima.

Steve Chiswell