                       User Tables               Wesley Ebisuzaki
                       ---- ------

GRIB files describe the contents (example, zonal wind, temperature) by 
a parameter table and parameter number.  Since the size of the parameter
table is limited to 255 entries, groups are finding (1) that they want
to use their own parameter table and (2) large groups are finding that 
they have to use multiple parameter tables.  

The parameter table used by wgrib is determined by (v1.5.0 and later)

  use built-in tables
  use user-defined table if available
  use built-in NCEP-2 table (warning under certain conditions)

Alternatively, if wgrib is compiled with the "-D P_TABLE_FIRST" 
option, then the search order is,

  use user-defined table if available
  use built-in tables
  use built-in NCEP-2 table (warning under certain conditions)

[This is the preferred search order for foreign-language parameter tables.]


The name of the user-defined table is searched for in 

  1) the environment variable "GRIBTAB"
  2) the environment variable "gribtab"
  3) the file gribtab

Defining an environment variable depends on the operating system
and on the shell.

  example
      export GRIBTAB=~/data/gribtab                   (bash)
      setenv GRIBTAB ~/data/gribtab                   (csh)
      GRIBTAB=$HOME/data/gribtab ; export GRIBTAB     (sh)

      set GRIBTAB=c:\gribtab                          (windows)


The format of the GRIBTAB file is

-----------------------------------------------------------------------
-1:CENTER:SUBCENTER:PARAMETER_TABLE
0:name:comment
1:name:comment
...
255:name:comment

-1:CENTER:SUBCENTER:PARAMETER_TABLE
0:name:comment
1:name:comment
...
255:name:comment

  (etc.)
-----------------------------------------------------------------------

"CENTER" is number between 0 and 255 with -1 being a wildcard.
"SUBCENTER" is number between 0 and 255 with -1 being a wildcard.
"PARAMETER_TABLE" is number between 0 and 255 with -1 being a wildcard.
"name" is a string with the variable name "TMP".  
"comment" is a string such describing the variable such as "temperature [K]"
Any missing parameter lines will be given a generic name and comment.

Each GRIB record contains the center (PDS octet 5), subcenter (PDS octet 26)
and parameter_table (PDS octet 4).  The names and comments from the
first section to match the center-subcenter-parameter_name in the PDS
are used.  Currently the subcenter are not used in matching the
built-in tables.  This will probably change when sub-centers start using
their own parameter tables.

                                Policy

The wgrib code is easily modified to add new built-in parameter tables.
New tables will be added under the following conditions: a reasonable
number of users, I don't have to type it in and the table is reasonably 
stable.  (Rules subject to change.)

