source: lmdz_wrf/trunk/WRFV3/external/io_grib2/README.io_grib2 @ 354

io_grib2

Author: Todd Hutchinson
        WSI
        thutchinson@wsi.com

28 September 2005

I. Introduction

io_grib2 is a WRF module that allows for input and output of WRF data in GRIB
version 2 format.

Why would anyone use GRIB2 output?  
  Data stored in GRIB2 format is compressed using advanced compression 
    (i.e., JPEG2000), so output files are much smaller than in other formats 
    such as netCDF and grib version 1.  Better compression is achieved with 
    larger grids.

II. Prerequisites

  In order to compile and link WRF using the grib2 format, one system library
  is required:
    libjasper.a:  http://www.ece.uvic.ca/~mdadams/jasper/

  The library should be installed in a system accessible place (i.e., 
       /usr/lib).  The paths to the jasper library and include files must be 
       specified via environment variables prior to running ./configure .  
       Please use the following environment variables:  
         JASPERLIB  Path to jasper library files (libjasperlib.a)
         JASPERINC  Path to jasper include files
       Note that if these environment variables are not defined before 
       ./configure is run, the Grib2 I/O package will not be built.  

III. Running WRF with grib2 input/output
  1. Set namelist entry or entries io_form_<x> to 10
  2. Run WRF as you normally would, for example:
     cd test/em_real
     ./real.exe
     ./wrf.exe

IV. Quilting
  If you are running WRF using MPI, you may use a seperate processor for
    quilting just as is done with netCDF.  Simply set the namelist variable
    nio_tasks_per_group to 1 (or more).

V. Details
  1. grib2map.tbl file
     io_grib2 makes use of a table for encoding the WRF data into GRIB2 
        format.  The table that io_grib2 uses is contained in the 
        WRF run directory, and is called grib2map.tbl.  This file is read at
        run-time by io_grib2.  Settings in this file are used to encode the 
        corresponding parameters in the output files.  You may modify 
        this file to suit your needs.

      The format of the table is described within the file itself.

VI. Comparison between file formats.  
  1. The following table shows the file sizes and run times for netcdf, grib1, 
     and grib2 format for a 6 hour simlation of the WRF jun11 test case.  The 
     domain size in this case is 91x82 by 28 levels.  Data was output every 
     3 hours (for a total of 3 output times).  The simulations were run on 
     NCAR's bluesky system.

                    |  Size (MB)  | WRF Time(m:s)
            ------------------------------------
            NETCDF  |   38.40     |     19:31   |
            GRIB1   |   13.42     |     21:22   |
            GRIB2   |    4.33     |     22:36   |
            ------------------------------------

VII. New namelist variables for Grib2 I/O:  
  The grib2 namelist variables may be optionally used to control 
  Grib2 I/O.  They are defined in new namelist "&grib2".  Note that the 
  &grib2 namelist must be included in file "namelist.input" even if empty.  
  (This annoyance is ultimately due to problems with old OSF1 Fortran90 
  compilers.)  Information on the variables is located in the WRFV2/run
  directory: README.namelist.