[gradsusr] LFC calculation

[Charles Seman]

Following Jeff Duda's idea for getting the LFC...

I'm not sure if this would work, but perhaps you could use "defval" to 
re-define a GrADS 2D LFC field initially defined to "undef"...

here is some GrADS pseudo-code for a single time-level, NOT TESTED, that 
you may be able to adapt?

'open 'your_GrADS_ctl_file

* find "nx" and "ny"...

'query file'
say
say result
rec5 = sublin(result,5)
nx = subwrd(rec5,3) ; ny = subwrd(rec5,6) ; nz = subwrd(rec5,9) ; nt = 
subwrd(rec5,12)

'set x 1 'nx
'set y 1 'ny
'set z 1'
'set t 1'

* initialize LFC field variable to "undef" using 
"a_GrADS_ctl_file_2D_var" from "your_GrADS_ctl_file"...

* define maskoutfield = -1 everywhere to use maskout() define a new 
"LFC" field = undef everywhere

'maskoutfield = const(a_GrADS_ctl_file_2D_var,-1,-a)'
'undef = maskout(a_GrADS_ctl_file_2D_var,maskoutfield)'
'LFC = undef'
'set x 1' ; 'set y 1' ; 'd undef' ; undef_value = subwrd(result,4)

  yy=1
while ( yy <= ny )
  xx=1
while ( xx <= nx )
  'set x 'xx
  'set y 'yy

*  load a sounding for this (x,y) point and
*  use some GrADS code to calculate LFC
*  note, LFC may not exist and if it doesn't
*  don't redefine LFC variable at this point (remains undefined)...

  LFC_value = undef_value ; initialize LFC_value for this (x,y) point to 
"undef" value

...do calculations to update "LFC_value" if possible

  if ( LFC_value != undef_value )

*  use "defval" to redefine LFC for this point in 2D field using LFC_value

    'set defval LFC 'xx' 'yy' 'LFC_value

  endif

  xx=xx+1
endwhile
  yy=yy+1
endwhile

I hope this helps... please let me know if you have any questions on the 
above GrADS pseudo-code...

Chuck


Jeffrey Duda wrote:
> Unless you're using OpenGrads, which might allow for defining a variable 
> by grid point, you probably can't compute LFC height in any direct way.  
> If you were somehow able to compute a vertical parcel path at each 
> gridpoint, however, you could then use the fndlvl command 
> (http://www.iges.org/grads/gadoc/gradfuncfndlvl.html) to find the first 
> vertical level at which the parcel temperature exceeds the environmental 
> temperature.  It's the first part that I can't think of a way to do.
> 
> If you are dealing with WRF output and you have the netCDF files, you 
> can run ARWpost or WPP on them to get LFC calculated. Otherwise you 
> might try using some other graphics software.  Sorry I can't be of more 
> help.
> 
> Jeff Duda
> 
> On Wed, Apr 20, 2011 at 9:39 AM, Meredith Croke <mcroke at airdat.com 
> <mailto:mcroke at airdat.com>> wrote:
> 
>     Hi Everyone,
> 
>     I’m looking to plot a contour plot of LFC height, similar to the
>     graphic on the SPC Mesoscale Analysis page, my control file does not
>     output LFC directly,  is there anyway I can calculate this in grads?
> 
>      
> 
>     Thanks for your help.
> 
>      
> 
>     Meredith
> 
> 
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> 
> 
> -- 
> Jeff Duda
> Iowa State University
> Meteorology Graduate Student
> 3134 Agronomy Hall
> www.meteor.iastate.edu/~jdduda <http://www.meteor.iastate.edu/~jdduda>
> 
> 
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-- 

Please note that Charles.Seman at noaa.gov should be considered my NOAA
email address, not cjs at gfdl.noaa.gov.

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  Charles Seman                                Charles.Seman at noaa.gov
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