[gembud] Fw: Decoding HPC PWPF GRIB2 files

Received this from the HPC in regards to
http://www.unidata.ucar.edu/mailing_lists/archives/gembud/2011/msg00016.html

-Jeff



Hello,

There are known problems with the GEMPAK nagrib2 decoder for proper GRIB2 
messages containing probabilities of exceeding thresholds for some parameters, 
specifically snow accumulations in the case of HPC's new experimental 
probabilistic winter precipitation forecasts (PWPF).  The problem lies in the 
conversion of the threshold value stored in the GRIB2 PDS to a character string 
interpolated into the parameter name.  For snowfall, GRIB2 requires units of 
meters, requiring a scaling factor of 4 for the threshold, resulting in values 
too small to appear as nonzero in the parameter name.  Hence, all the 
exceedance probability grids are assigned the same parameter name.  Since HPC 
is producing valid GRIB2 messages, we plan no changes to our product 
generation.  This problem does not occur for freezing rain.  Here is a 
suggested work-around for GEMPAK users (I admit, I have not actually tried it.):


  1.. Break the monolithic grid file into individual GRIB2 messages using 
wgrib2 (http://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/) 
  2.. Run each GRIB2 message separately through nagrib2, producing separate 
GEMPAK grid files.

  3.. Run gddiag to change the parameter names and write the grids to a single 
GEMPAK grid file.

When running nagrib2, set the G2TBLS as follows:  G2TBLS   = 
g2varlcl.tbl;g2varlcl.tbl, where g2varlcl.tbl is a file having these contents:

!
! Snowfall (thresholds in m) exceedance probabilities:
!
!D# CT# ID# PDT#
000 001 029 009 Snowfall Prob > 1 inch           0->1                 PS--      
       0  -9999.00
!
!  Freezing rain (thresholds in mm) accumulation exceedance probabilities:
!
000 001 225 009 ZR Accum Prob > .01 inch         0->1                 PZ--      
       0  -9999.00


I hope this information is helpful.

Keith
=============================

david.novak@xxxxxxxx wrote: 
Not sure if you're on the gembud mailing list, but saw this post about the snow 
probs. I don't think it was resolved.

------------------------------------------------------------------------------

        Subject: [gembud] adding info to tables 
        From: Jeff Lake - Admin <admin@xxxxxxxxxxxxxxxxxxxx> 
        Date: Sun, 16 Jan 2011 07:47:17 -0500 
        To: gembud@xxxxxxxxxxxxxxxx 
        To: gembud@xxxxxxxxxxxxxxxx 

  good morning, 
  I'm trying to muddle my way creating images using the experimental snow 
probability gribs from the HPC 
  ftp://ftp.hpc.ncep.noaa.gov/pwpf/conus/pwpf_24hr/ 

  using dcgrib2 to convert to gem I get the following 

  [2213] 110116/1224[GB 1]  No GEMPAK parameter name defined for this grid. 
  [2213] 110116/1224[DECODE_GRIB2 -34] Could not determine parameter name 0 1 
29 9 [0] 
  [2213] 110116/1224[GB 1]  No GEMPAK parameter name defined for this grid. 
  [2213] 110116/1224[DECODE_GRIB2 -34] Could not determine parameter name 0 1 
29 9 [0] 
  [2213] 110116/1224[GB 1]  No GEMPAK parameter name defined for this grid. 
  [2213] 110116/1224[DECODE_GRIB2 -34] Could not determine parameter name 0 1 
29 9 [0] 
  [2213] 110116/1224[GB 1]  No GEMPAK parameter name defined for this grid. 
  [2213] 110116/1224[DECODE_GRIB2 -34] Could not determine parameter name 0 1 
29 9 [0] 
  [2213] 110116/1224[GB 1]  No GEMPAK parameter name defined for this grid. 
  [2213] 110116/1224[DECODE_GRIB2 -34] Could not determine parameter name 0 1 
29 9 [0] 
  [2213] 110116/1224[GB 1]  No GEMPAK parameter name defined for this grid. 
  [2213] 110116/1224[DECODE_GRIB2 -34] Could not determine parameter name 0 1 
29 9 [0] 
  [2213] 110116/1224[GB 1]  No GEMPAK parameter name defined for this grid. 
  [2213] 110116/1224[DECODE_GRIB2 -34] Could not determine parameter name 0 1 
29 9 [0] 
  [2213] 110116/1224[DC 5]  Normal termination. 
  [2213] 110116/1224[DC 2]  Number of bulletins read and processed: 7 
  [2213] 110116/1224[DC 6]  Shutting down. 

  looking at that, studying the 'tables/grid/g2varswmo2.tbl' using DeGRIB I 
added the following to the g2varswmo2.tbl 

  000 001 029 009 24 hr Prob of Total snowfall     %                    
ProbASNOW24      ?  -9999.00 

  well now no errors in the log, it does create the gem file 
  but when I do a gdinfo on it... the output file is empty 
  so something isn't correct 
  do I need to add the definition somewhere else ?? 

  thanks 
  -Jeff 

  _______________________________________________ 
  gembud mailing list 
  gembud@xxxxxxxxxxxxxxxx 
  For list information or to unsubscribe,  visit: 
http://www.unidata.ucar.edu/mailing_lists/