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[GEMPAK #YKO-952227]: Wind decoding problem for grib2 grid 227

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  • Subject: [GEMPAK #YKO-952227]: Wind decoding problem for grib2 grid 227
  • Date: Thu, 05 Feb 2015 12:40:43 -0700
Hi Art,

I decoded the file and am plotting it with GDSTREAM, I see the odd contouring 
that you mention, and after playing with IJSKIP and STREAM values I can produce 
a streamline plot that looks "okay".

from "phelp stream"
  
     STREAM    lines/arrows/stop/slow/scale
  
     The STREAM parameter controls several parameters dealing with
     the overall streamline calculation and display.  "Lines" is a
     real number multiplier which controls the number (density) of
     streamlines drawn (default 1.0); "arrows" is a real number
     multiplier which controls the number of arrowheads displayed
     (default 1.5*lines); "stop" is a real number multiplier which
     controls how close a streamline comes to another streamline
     before drawing is discontinued (default 0.5); "slow" is a
     real number multiplier which controls the minimum vector speed
     threshold for stopping a streamline (default 0.67); "scale" is
     a real number multiplier which controls how much the input vector
     field is scaled prior to streamline calculation (default 0.33).

I tried IJSKIP=10 and STREAM=1////2 and that seemed to minimize the unusual 
contouring, but did not eliminate it entirely.  It may be that you need to play 
with these values yourself to find a visually coherent plot.

Checking the values in GDLIST and plotting them with GDPLOT2, it seems that the 
contouring is due to the relatively small wind speeds in the area, a lot of sub 
5 knot winds at 10 meter height make it difficult to produce a visually 
appealing streamline field.  

So I think the data are correct, and the odd contouring a result of the low 
wind speeds and dense grid spacing.

What do you think?

-Michael



> We're trying to use ARW and NMM winds from NCEP grids that appear to be on an 
> equivalent
> of the 227 grid but they seem to not be properly decoded. Attached is an 
> example ARW grib2
> wind field and gif streamline display (decoded with gempak 7.0.3 and 
> displayed with gempak 6.6.0).
> Any ideas what might be wrong?
> 
> Thanks... Art
> 
> --
> Arthur A. Person
> Research Assistant, System Administrator
> Penn State Department of Meteorology
> email: address@hidden, phone: 814-863-1563
> 
> 
> 

Ticket Details
===================
Ticket ID: YKO-952227
Department: Support GEMPAK
Priority: Normal
Status: Open