source: lmdz_wrf/WRFV3/external/io_grib1/README.io_grib1 @ 1

io_grib1

Author: Todd Hutchinson
        WSI
        thutchinson@wsi.com

5 August 2004
8 February 2005 - Updated by Todd Hutchinson.  Section V was updated to 
                  clarify the description of the decimal scale factor field 
                  in the gribmap file.
12 July 2005    - Updated by Todd Hutchinson.  GRIB version 1 input capability 
                  was added to WRF. All sections in this README were updated to
                  add information relevant for grib input capability.


I. Introduction

io_grib1 is a WRF module that allows for input and output of WRF data in GRIB 
   version 1 format.

Why would anyone use GRIB output?  
  GRIB stores data in a compressed format so output files are much smaller 
    than in other formats such as netCDF.  In addition, encoding in GRIB format
    is very efficient and can be faster than writing out netCDF formatted 
    data, especially for large grids.  (See Section V for more on performance).

II. Running WRF
  1. Set namelist entry or entries io_form_<x> to 5
  2. Run WRF as you normally would, for example:
     cd test/em_real
     ./wrf.exe

IV. Examining GRIB output with wgrib
  1. wgrib is installed within external/io_grib1 
     (See http://wesley.wwb.noaa.gov/wgrib.html for more information.)
  2. Define the GRIBTAB environment variable:
        For bourne shell users:
          export GRIBTAB=<your WRF dir>/run/gribmap.txt
        For csh users
          setenv GRIBTAB <your WRF dir>/run/gribmap.txt
  3. To get a listing of the records in the GRIB data:
        <your WRF dir>/external/io_grib1/wgrib wrfout_d01_000000

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. gribmap.txt file
     io_grib1 makes use of a GRIB table for encoding the WRF data into GRIB 
        format.  The GRIB table that io_grib1 uses is contained in the 
        WRF run directory, and is called gribmap.txt.  This file is read at
        run-time by io_grib1.  Settings in this file are used to encode the 
        corresponding parameters in the GRIB output files.  You may modify 
        this file to suit your needs.

      The file has the following format:

      the first line of a table (note: there may be several tables in a 
                                 gribmap.txt file)
      -1:CENTER:SUBCENTER:PARAMETER_TABLE_VERSION
          "-1" is an indicator for the start of a table
          "CENTER" is generating center (PDS 5, i.e., for NCEP, this is 7)
          "SUBCENTER" is the generating sub-center (PDS 26)
          "PARAMETER_TABLE_VERSION" is the parameter table version (PDS 4)

      Subsequent lines:
      GRIBID:<PARAMETER>:<DESCRIPTION>:<WRF VAR>:<DECSCL>
          "GRIBID" is the GRIB parameter id (PDS 9).
          "PARAMETER" is the abbreviated parameter name.   
          "DESCRIPTION" is a description of the parameter.
          "WRF VAR" is a comma-seperated list of WRF variables that will be
                    encoded as with PARAMETER described in the line and with 
                    the CENTER, SUBCENTER and PARAMETER_TABLE_VERSION listed 
                    at the beginning of the table.  WRF VAR must use 
                    the WRF variable names that are defined in the "DNAME" 
                    column in the WRF Registry.
          "DECSCL" is the decimal scale factor.  It is the number of digits
                    to the right of the decimal point that will be saved in
                    the output data.  This number may be less than zero, in 
                    which case, the precision of data will be truncated the 
                    specified number of digits to the left of the decimal 
                    point. 
                       Examples:
                         -2 : Data will be encoded at the hundreds level, 
                              i.e., 102425.231 will be encoded as 102400
                          3 : Data will be encoded at the thousandths level, 
                              i.e., 102425.231243 will be encoded as 1024.231
      Example:
           1:PRES:Pressure [Pa]:P,PSFC:1
             In this example, GRIB parameter 1, abbreviated PRES, is 
                pressure in pascals.  WRF variables P and PSFC (as listed in
                the WRF Registry) will be encoded as PRES.  The data will have
                one decimals of precision, i.e., 101323.1 Pa.
           62:NCPCP:Large scale precipitation [kg/m^2]:NCPCP:2
             In this example, GRIB parameter 62, abbreviated NCPCP, is 
                Large-scale precipitation.  The WRF variables NCPCP (as listed 
                in the WRF Registry) will be encoded as NCPCP.  The data will 
                have two decimals of precision, i.e., 1.23 kg/m^2 (i.e., 
                1.23 mm).

      Note:
          For GRIB encoding, parameters 1-127 are the standard parameters
            described by the WMO GRIB convention.  Interpretation of parameters
            128-254 vary depending on the setting of the CENTER, SUBCENTER, 
            and PARAMETER_TABLE_VERSION.  In the default gribmap.txt file,
            the CENTER and SUBCENTER are both set to be 255.  Five tables
            with PARAMETER_TABLE_VERSION's varying between 2 and 6 are 
            specified in the default gribmap.txt file.

  2. Variable dimensions.
      A GRIB file is a set of records of data.  Each record is a 2-dimensional,
        horizontal field, i.e., temperature on a pressure surface,
        In the case of the WRF Eulerian mass model, we have variables on Eta 
        coordinates, so, we have, i.e., temperature on the 0.995 Eta level.
      WRF outputs 3, 2 and 1-dimensional fields and point data.  3-dimensional 
        (i.e., u-component of wind) fields are stored in GRIB as a series of 
        horizontal 2-dimensional fields.  WRF horizontal 2-d fields are stored 
        simply as horizontal 2-d fields (i.e., surface pressure).  
        One-dimensional fields with dimension varying in the vertical (i.e., 
        the vertical coordinate ZN), are stored as a set of 1x1 grids valid
        at the center of the grid.  So, the third vertical level (ZN) is 
        encoded as a 1x1 grid, valid at the center of the grid.  Point data
        is also stored as a 1x1 grid valid at the center of the grid.
      
VI. Performance
   GRIB output has proven to be very fast, as compared to netcdf output.

   Listed below is a comparison of file size and run-time for io_grib1 and
   io_netcdf.  WRF was run twice, once with GRIB output and once with netcdf
   output.  In both cases, 32 CPUS (on 16 machines) were dedicated to 
   model integration, and one was CPU dedicated to I/O and quilting (using 
   nio_tasks_per_group = 1).  The run had a domain with 360x485 grid points 
  (12 km spacing) and 10-minute output.  The model was run out to 3 hours.

   Shown is the file size for one-output time, the total model run-time 
   and the time for I/O as listed in the WRF output.  In addition, the 
   additional time that is required for a time-step just after model output
   is listed.

   Format    File Size    Run time     I/O time   additional time after output
   netCDF     368 MB       719 s         1.26 s            3.3 s
   GRIB v1    109 MB       519 s         0.28 s            1.9 s


VII. Testing
   We (at WSI) have tested io_grib1 with WRF running on linux (redhat v 9.0).  
   We typically use the intel fortran compiler (v8.0) and the gnu C compiler, 
   so, that configuration has the most testing.  We have also done limited
   testing with the Portland Group Compiler on linux.  We have not tested any
   other platforms or compilers.

io_grib1

Author: Todd Hutchinson
        WSI
        thutchinson@wsi.com