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19990601: Grib documentation

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  • Subject: 19990601: Grib documentation
  • Date: Tue, 1 Jun 1999 16:36:26 -0600

Deb,

Tom reminded me you were interested in documentation of the
grib identification used for NOAAport and NCEP.

The Grib documentation can be downloaded from:
ftp://nic.fb4.noaa.gov/pub/nws/nmc/docs/gribed1/
in word perfect or PDF format.

The various awips grid projection are defined as well as
the grid identification scheme.

In particular, I beleive you asked about the WMO identifier which is
detailed in section 5 of the files on the ftp server above, located
in Appendix A.


The Header that you find on the IDD and use for pqact.conf information
has the format:

TTAAii CCCC DDHHMM

Since the WMO header is not necessarily unique, we have the LDM tack on 
tha /mXXX identifier which uses the model id in the product definition
section (PDS) (described in section 1 of the manual, eg model 89 is ETA48.

From the above format:

T1 is : H, Y or Z
H grids include international exchange grids such as the thinned AVN, MRF
and ECMWF grids on the IDD.

Y and Z designators denote AWIPS grids.

T2 denotes the grid parameter- eg temp, height, moisture, wind, etc.

A1 is the grid identifier, eg Q = grid 211, defined in table A2.

A2 is the forecast time (for Y and Z designators in T1, the
letter designation has different time period meanings as shown in
Table A3.

The ii number is a level designator, as shown in table A4.


The CCCC for NCEP models is KWB_ where the 4th letter represents models
as shown in table A4. Note that many models still use KWBC historically
and have not transitioned to the 4th letter yet.


All of the awips grids are definied in the PDS section and can be cross
referenced by the letter in (), eg, grid 201 will be labled
with (A) in the table.

Since there are 255 parameters and many different vertical coordinates
used in the PDS block of the code, the WMO header cannot fully describe
the contenets of a grib product. We have several utilities, including
gribdump (in the decoders package) which will output the PDS information,
as well as programs like nagrib in gempak which can very verbosely output 
all the the octet values in the grib header portion.

If you have specific questions to the references I pointed out above
please let me know.

Steve Chiswell
Unidata User Support