source: trunk/WRF.COMMON/WRFV2/external/io_grib1/README.io_grib1 @ 3567

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1io_grib1
2
3Author: Todd Hutchinson
4        WSI
5        thutchinson@wsi.com
6
75 August 2004
88 February 2005 - Updated by Todd Hutchinson.  Section V was updated to
9                  clarify the description of the decimal scale factor field
10                  in the gribmap file.
1112 July 2005    - Updated by Todd Hutchinson.  GRIB version 1 input capability
12                  was added to WRF. All sections in this README were updated to
13                  add information relevant for grib input capability.
14
15
16I. Introduction
17
18io_grib1 is a WRF module that allows for input and output of WRF data in GRIB
19   version 1 format.
20
21Why would anyone use GRIB output? 
22  GRIB stores data in a compressed format so output files are much smaller
23    than in other formats such as netCDF.  In addition, encoding in GRIB format
24    is very efficient and can be faster than writing out netCDF formatted
25    data, especially for large grids.  (See Section V for more on performance).
26
27II. Running WRF
28  1. Set namelist entry or entries io_form_<x> to 5
29  2. Run WRF as you normally would, for example:
30     cd test/em_real
31     ./wrf.exe
32
33IV. Examining GRIB output with wgrib
34  1. wgrib is installed within external/io_grib1
35     (See http://wesley.wwb.noaa.gov/wgrib.html for more information.)
36  2. Define the GRIBTAB environment variable:
37        For bourne shell users:
38          export GRIBTAB=<your WRF dir>/run/gribmap.txt
39        For csh users
40          setenv GRIBTAB <your WRF dir>/run/gribmap.txt
41  3. To get a listing of the records in the GRIB data:
42        <your WRF dir>/external/io_grib1/wgrib wrfout_d01_000000
43
44IV. Quilting
45  If you are running WRF using MPI, you may use a seperate processor for
46    quilting just as is done with netCDF.  Simply set the namelist variable
47    nio_tasks_per_group to 1 (or more).
48
49V. Details
50  1. gribmap.txt file
51     io_grib1 makes use of a GRIB table for encoding the WRF data into GRIB
52        format.  The GRIB table that io_grib1 uses is contained in the
53        WRF run directory, and is called gribmap.txt.  This file is read at
54        run-time by io_grib1.  Settings in this file are used to encode the
55        corresponding parameters in the GRIB output files.  You may modify
56        this file to suit your needs.
57
58      The file has the following format:
59
60      the first line of a table (note: there may be several tables in a
61                                 gribmap.txt file)
62      -1:CENTER:SUBCENTER:PARAMETER_TABLE_VERSION
63          "-1" is an indicator for the start of a table
64          "CENTER" is generating center (PDS 5, i.e., for NCEP, this is 7)
65          "SUBCENTER" is the generating sub-center (PDS 26)
66          "PARAMETER_TABLE_VERSION" is the parameter table version (PDS 4)
67
68      Subsequent lines:
69      GRIBID:<PARAMETER>:<DESCRIPTION>:<WRF VAR>:<DECSCL>
70          "GRIBID" is the GRIB parameter id (PDS 9).
71          "PARAMETER" is the abbreviated parameter name.   
72          "DESCRIPTION" is a description of the parameter.
73          "WRF VAR" is a comma-seperated list of WRF variables that will be
74                    encoded as with PARAMETER described in the line and with
75                    the CENTER, SUBCENTER and PARAMETER_TABLE_VERSION listed
76                    at the beginning of the table.  WRF VAR must use
77                    the WRF variable names that are defined in the "DNAME"
78                    column in the WRF Registry.
79          "DECSCL" is the decimal scale factor.  It is the number of digits
80                    to the right of the decimal point that will be saved in
81                    the output data.  This number may be less than zero, in
82                    which case, the precision of data will be truncated the
83                    specified number of digits to the left of the decimal
84                    point.
85                       Examples:
86                         -2 : Data will be encoded at the hundreds level,
87                              i.e., 102425.231 will be encoded as 102400
88                          3 : Data will be encoded at the thousandths level,
89                              i.e., 102425.231243 will be encoded as 1024.231
90      Example:
91           1:PRES:Pressure [Pa]:P,PSFC:1
92             In this example, GRIB parameter 1, abbreviated PRES, is
93                pressure in pascals.  WRF variables P and PSFC (as listed in
94                the WRF Registry) will be encoded as PRES.  The data will have
95                one decimals of precision, i.e., 101323.1 Pa.
96           62:NCPCP:Large scale precipitation [kg/m^2]:NCPCP:2
97             In this example, GRIB parameter 62, abbreviated NCPCP, is
98                Large-scale precipitation.  The WRF variables NCPCP (as listed
99                in the WRF Registry) will be encoded as NCPCP.  The data will
100                have two decimals of precision, i.e., 1.23 kg/m^2 (i.e.,
101                1.23 mm).
102
103      Note:
104          For GRIB encoding, parameters 1-127 are the standard parameters
105            described by the WMO GRIB convention.  Interpretation of parameters
106            128-254 vary depending on the setting of the CENTER, SUBCENTER,
107            and PARAMETER_TABLE_VERSION.  In the default gribmap.txt file,
108            the CENTER and SUBCENTER are both set to be 255.  Five tables
109            with PARAMETER_TABLE_VERSION's varying between 2 and 6 are
110            specified in the default gribmap.txt file.
111
112  2. Variable dimensions.
113      A GRIB file is a set of records of data.  Each record is a 2-dimensional,
114        horizontal field, i.e., temperature on a pressure surface,
115        In the case of the WRF Eulerian mass model, we have variables on Eta
116        coordinates, so, we have, i.e., temperature on the 0.995 Eta level.
117      WRF outputs 3, 2 and 1-dimensional fields and point data.  3-dimensional
118        (i.e., u-component of wind) fields are stored in GRIB as a series of
119        horizontal 2-dimensional fields.  WRF horizontal 2-d fields are stored
120        simply as horizontal 2-d fields (i.e., surface pressure). 
121        One-dimensional fields with dimension varying in the vertical (i.e.,
122        the vertical coordinate ZN), are stored as a set of 1x1 grids valid
123        at the center of the grid.  So, the third vertical level (ZN) is
124        encoded as a 1x1 grid, valid at the center of the grid.  Point data
125        is also stored as a 1x1 grid valid at the center of the grid.
126     
127VI. Performance
128   GRIB output has proven to be very fast, as compared to netcdf output.
129
130   Listed below is a comparison of file size and run-time for io_grib1 and
131   io_netcdf.  WRF was run twice, once with GRIB output and once with netcdf
132   output.  In both cases, 32 CPUS (on 16 machines) were dedicated to
133   model integration, and one was CPU dedicated to I/O and quilting (using
134   nio_tasks_per_group = 1).  The run had a domain with 360x485 grid points
135  (12 km spacing) and 10-minute output.  The model was run out to 3 hours.
136
137   Shown is the file size for one-output time, the total model run-time
138   and the time for I/O as listed in the WRF output.  In addition, the
139   additional time that is required for a time-step just after model output
140   is listed.
141
142   Format    File Size    Run time     I/O time   additional time after output
143   netCDF     368 MB       719 s         1.26 s            3.3 s
144   GRIB v1    109 MB       519 s         0.28 s            1.9 s
145
146
147VII. Testing
148   We (at WSI) have tested io_grib1 with WRF running on linux (redhat v 9.0). 
149   We typically use the intel fortran compiler (v8.0) and the gnu C compiler,
150   so, that configuration has the most testing.  We have also done limited
151   testing with the Portland Group Compiler on linux.  We have not tested any
152   other platforms or compilers.
153
154io_grib1
155
156Author: Todd Hutchinson
157        WSI
158        thutchinson@wsi.com
159
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