20030613: Grid definition? query in GEMPAK

John,

Your problem lies in your syntax. To produce the magnitude
of the wind as you wrote:
sqrt(avn_urel**2+avn_vrel**2)

The GEMPAK syntax would be:
sqrt(sum(mul(avn_urel,avn_urel),mul(avn_vrel,avn_vrel)))

Luckily, this is more easily done by taking the magnitude
of the constituted vector from avn_urel and avn_vrel:

mag(vecr(avn_urel,avn_vrel))

Steve Chiswell


>From: John Merrill <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200306131814.h5DIEbLd001292

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>Hello,
>   I'm using .gem files prepared from the Reanlaysis CD-ROM, monthly 
>average fields. The variable names differ from
>the usual ones, of course, but I encounter a difficulty in calculating 
>even a simple scalar. At glevel=200 I want to plot
>iostachs; the components of the wind here are avn_urel, avn_vrel (avn 
>for 'average,' I suppose). I tried to compute a new
>quantity by setting gfunc=sqrt(avn_urel**2+avn_vrel**2)//avn_s, was 
>dismayed to get the response that "requested grid
>avn_urel**2 in (specifications) cannot be found"; then, of course "The 
>requested scalar/vector cannot be computed." You'd
>be correct assuming I've not defined/computed grids previously.... 
>Thanks.     John
>
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> name="200ZSrestore"
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>!
>! Restore File : 200 Height and Isotachs
>!
>! Log:
>! J. Merrill            7/02   Initial creation as 500_msl composite
>! J. Merrill            6/32   Revise for height/isotach only
>!
>garea   20;85;60;150
>LATLON  8/2/2//15;15
>MAP     8/1/2
>GVCORD= pres             !pres
>GLEVEL  200              !200
>SKIP    0                !0  
>SCALE   -1               !0
>gfunc avn_urel//urel   !avn_vrel//vrel
>gfunc   hght             !sped
>TYPE    c                !c
>CONTUR  2/2              !2/2
>CINT    6                !10
>LINE    32/1/3           !32/2/2
>TITLE   32/-1/~ 200 hPa Height and Isotachs, April 2001 Mean
>TEXT    2/1/3/sw
>border  //3
>CLEAR   yes                
>
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