GRIB2 usage at NCEP

1
GRIB2 usage at NCEP

• Dr. V. KRISHNA KUMAR
• BOI VUONG
• NOAA/NWS/NCEP/Central Operations

NCEP WWB GRIB2 Seminar 15 June 2011
Room 707, WWB Camp Springs, MD
2
Overview

• GRIB2 Sections NCEP
• GRIB2 Documentation
• GRIB2 in NCEP Operations Encoding Decoding
  routines, NCEP Utilities (cnvgrib, degrib2,
  overdateg2, overmodelg2, copygb2,wgrib2..)
• GRIB2 Section 7 Data Section
• GRIB2 Packing Options
• How to write GRIB2 data directly? Some
  specific examples

3
Overview

• GRIB2 Encoding routines
• GRIB2 Decoding routines
• Declaration of Derived type gribfield
• How to use routine getgb2?
• How to use routine putgb2?
• GRIB2 read and write examples
• Summary

4
GRIB1 Sections

• 0 Indicator Section
• - GRIB (octet 1-4 32bits), length of
  message (octet 5-724bits), edition number -1
  (octet 88bits)
• - 1 octet 1byte 8bits
• - octets are numbered 1, 2, 3, etc., Bit
  positions within octets are from bit 1 to bit 8.
• - bit 1 most significant bit bit 8
  least significant)
• 1 Product Definition Section (PDS)
• - Length of PDS, parameter table version
  number, ID of center, generating process ID, grid
  identification (Table B), Ref time, parameter
  and units, level, valid time
• 2 Grid Definition Section (GDS) optional
• - Length of sec., sec. no, defn of grid surf
  and geometry of data values within the surface
• 3 Bit-Map Section (BMS) optional
• - Length of sec., sec. no, indication of
  presence or absence of data at each of the grid
  points, as applicable example sst
• 4 Binary Data Section (BDS)
• - Length of sec., sec. no, data values
• 5 End Section 7777

5
GRIB2 Sections

• 0 Indicator Section
• - GRIB, length, version,
  discipline (16 octets long)
• Section 1 Identification
• - Ref time, orig center, table
  versions, (length of the section is expressed as
  the first four octets (32 bits)
  -the section number in the fifth octet and octets
  beyond 21 are reserved for future use)
• 2 Local Use Section optional
  
• 3 Grid Definition Section
  
• 4 Product Definition Section
  
• - parameter, level, valid time
  
  
• 5 Data Representation Section
  
• - packing algorithm used, info
  needed to unpack
  
• 6 Bit Map Section
  
• 7 Data Section
  
• -packed data
• 8 End Section 7777

6
GRIB2 Documentation

• NCEP GRIB2 Documentation (internet - www)
• http//www.nco.ncep.noaa.gov/pmb/docs/grib2/
• NCEP GRIB2 Resources (Detailed Documentation)
  http//www.nco.ncep.noaa.gov/sib/grib2/
• GRIB2 specification from WMO contains template
  definitions and table entries http//www.wmo.ch/w
  eb/www/DPS/grib-2.html
• Local Tables
• http//www.nco.ncep.noaa.gov/pmb/docs/grib2
  
• NCEP source files and utilities on the web for
  decoding and encoding data in GRIB2 format
• http//www.nco.ncep.noaa.gov/pmb/codes/GRIB2/
  
• NCEP wgrib2 resources
• http//www.cpc.ncep.noaa.gov/products/wesley
  /wgrib2/
• See Wesleys PowerPoint presentation
  4/6/2011 at NCEP under Documentation
• for all the pertinent details on wgrib2

7
GRIB2 Significant Features

• One GRIB2 message can contain multiple fields.
  - Sequences of GRIB sections 2 to
  7, sections 3 to 7 or sections 4 to 7 may be
  repeated within a single GRIB message.
• - Once a section is repeated all the
  following sections must appear in order.
• - A section remains in effect until
  redefined.
• - Could be used for vector fields such
  as u v components, or ensemble members
• - IEEE single precision floating point
  representation used instead of standard IBM
  representation. The representation occupies four
  octets.
• GDS, PDS, DRS, and DS are all template driven.
• Octet assignments within a section are
  based on the Template specified.
• Parameters are defined by three values
• Discipline (specified in Section 0)
• Category (PDS)
• Parameter number (PDS).

8
GRIB2 Significant Features contd.

• All longitudes MUST be in degrees East and be
  between 0 and 360 degrees. No negatives. No
  degrees West. No change in latitude definitions
  900 to 900.
• GDS and BMS no longer optional. They must be
  included. Only Section 2 (Local Use) is optional.
• Latitudes, Longitudes are in units of
  micro-degrees (10 6 degrees). Includes distance
  between grid points for Lat/Lon grids (GRIB1 was
• milli-degrees)
• All other grid point increments are in
  millimeters (GRIB1 was in meters)
• Floating point values are now stored in IEEE
  format (GRIB1 was IBM 360/195 format HDS)

9
SECTION 0  0 2 60463
10
(No Transcript)
11
SECTION 1  7 0 2 1 1 2010 11 9 6 0 0 0 1 
Contains  0  Local Sections  and  1  data fields.
12
SECTION 3  0 65160 0 0 0  GRID TEMPLATE 3. 0  
6 0 0 0 0 0 0 360 181 0 0 90000000 0 48 -90000000
359000000 1000000 1000000 0
13
PRODUCT TEMPLATE 4. 0   3 5 2 0 96 0 0 1 36 100
0 100000 255 0 0  TEXT HGT      1000 mb valid
at  36 hr after 20101109060000
14
DRS TEMPLATE 5. 3   -979087360 0 1 11 0 1 0 0 0
5124 0 4 1 1 32 5 1 2
15
GRIB2 in NCEP operations

• Encoding Routines
• GRIB2 message can contain gridded fields for many
  parameters on a number of different grids.
• Start a new GRIB2 message with a call to
• (1) call GRIBCREATE encodes sections 0 and 1
  at the beginning of message.
• (2) call ADDLOCAL adds a Local Use Section
  (Section 2) optional
• (3) call ADDGRID encodes a grid definition
  into Section 3. Defines the geometry of the
• data values
  in the fields that follow it. ADDGRID can be
  called
• again to
  change the grid definition describing subsequent
  data fields.
• (4) call ADDFIELD adds each data field which
  adds Sections 4, 5, 6, and 7 to the message.
• (5) call GRIBEND adds the final section 8 to
  the message which updates the length of
• the
  message. GRIBEND is required for each GRIB2
  message.
• Boi will demonstrate these routines with examples
  in the latter half of this seminar

16
GRIB2 in NCEP operations contd.

• Decoding Routines
• call GB_INFO finds out how many Local Use
  sections and data fields are contained in a given
  GRIB2 message. Also returns the number of octets
  of the largest Local Use section in the message.
  Ensures the dimensionality of the input array of
  GETLOCAL
• call GETLOCAL returns the requested occurrence
  of Section 2 from a given GRIB2 message.
• call GF_GETFLD gets all information pertaining
  to the nth data field in the message returns
  all the unpacked values for each Section and
  Template in a f90 type gribfield with options
  to unpack the Bit-map (if applicable) and the
  data values or just return the field description
  information.
• call GF_FREE(gfld) frees up memory held by
  gfld as many elements in derived type gribfield
  are dynamically allocated when decoded.
• Example
• use grib_mod
• type(gribfield) gfld
• call gf_getfld(cgrib,lengrib,1,.true.,.fa
  lse.,gfld,ierr)
• print,Num of grid points
  ,gfldngrdpts
• print,First data value,gfldfld(1)
• call gf_free(gfld)

17
GRIB2 in NCEP operations contd.

• Create GRIB2 Message
• call PUTGB2 if a variable of derived type of
  gribfield already exists, this
• subroutine can be used to pack it up and write it
  to a file Boi will demonstrate this
• NCEP GRIB2 Utilities
• /grb2index grib2file grib2indexfile
• - creates a GRIB2 index file
• /cnvgrib h (packing options 1 gt 2,
  2gt1, 2gt2)
• -converts all fields in a file between GRIB1
  and GRIB2
• /degrib2 grib2file
• -dumps the meta-data for each GRIB2 field in
  grib2file
• - basically prints contents of each element in
  derived type gribfield
• /wgrib2 grib2file
• Grads for GRIB2
• sh convert_grib2_to_grads.sh
• !/bin/sh
• set -xa
• file_listls grib2.t00z.wafs_grbf06.45
  gfs.t00z.wafs_grb45f06.grib2

18
GRIB2 in NCEP Operations
/nwprod/util/exec/cnvgrib -h Usage
cnvgrib -h -g12-g21-g22 -m
-p0-p2-p31-p32-p40-p41-p40000-p40010
ingribfile outgribfile cnvgrib version
cnvgrib-1.1.1 Must use one of the following
options -g12 converts GRIB1 to GRIB2
-g21 converts GRIB2 to GRIB1 -g22
converts GRIB2 to GRIB2 (used to change packing
option) Optional packing options (for use with
-g12 and -g22 only) -p0 simple packing
-p2 complex packing -p31 complex pack
with 1st order diffs -p32 complex pack
with 2nd order diffs -p40 JPEG2000
encoding -p41 PNG encoding Other Optional
options -m Use missing values instead
of bitmap (valid with -p2, -p31 or
-p32 options only)
19
GRIB2 in NCEP Operations

• /nwprod/util/sorc/overmodelg2.fd
• more sample.script
• THIS SCRIPT READS A FORECAST FILE (UNIT 11),
  REPLACES MODEL INDENTIFIER IN
• PRODUCT TEMPLATE 4.x FROM A ID FILE UNIT 51.
• STANDARD INPUT IS A ID (2-DIGIT INTEGER), FOR
  EXAMPLE 96 (GFS Model)
• set -x
• export XLFRTEOPTS"unit_varsyes" Allow
  overriding default names.
• export XLFUNIT_11gfs.t00z.pgrb2f12.2p5deg
• export XLFUNIT_51gfs.t00z.pgrb2f12.2p5deg.new
• overmodelg2 ltlt EOF
• 98
• EOF
• /nwprod/util/sorc/overdateg2.fd
• THIS SCRIPT READS A FORECAST FILE (UNIT 11),
  MODIFIES REFERENCE TIME YEAR MONTH
• DATE AND HOUR IN SECTION 1 AND IN PRODUCT
  TEMPLATE 4.x TO REPLACE INTERNAL
• REFERENCE TIME FROM A DATE IN FILE UNIT 51.
• STANDARD INPUT IS A YYYYMMDDHH (10-DIGIT
  INTEGER), FOR EXAMPLE 2010090112
• set -x

20
GRIB2 in NCEP Operations

• /nwprod/util/sorc/copygb2.fd
• NAME
• copygb2 - copies GRIB2 files
• SYNOPSIS
• copygb2 -g "kgdtn kgdt" -i "ip
  ipopts"
• -k "kpdtn kpdt" -v "uparms" -X
• -B mapgrib -b mapindex -A "ltgt
  mapthreshold" -K "mapkpds"
• -M "mask"mergegrib -m
  mergeindex -a
• then either
• grib2in index2in grib2out
• or
• -x grib2in grib2out
• DESCRIPTION
• The command copygb2 copies all or part of
  one GRIB2 file to another
• GRIB2 file, interpolating if necessary.
  Unless otherwise directed
• (-x option), the GRIB2 index file is also
  used to speed the reading.
• The fields are interpolated to an output
  grid if specified (-g option).
• The interpolation type defaults to bilinear
  but may be specified directly
• (-i option). The copying may be limited to
  specific fields (-k option).

21
GRIB2 in NCEP Operations

• export grid_new"30 6 0 0 0 0 0 0 1073 689
  20191999 238445999 8 25000000 265000000 5079406
  5079406 0 64 25000000 25000000 -90000000 0 0"
• export grib_inrtma2p5.t12z.2dvaranl_ndfd.1field.
  grb2
• Create a grib2 index file of the input grib
  file
• /nwprod/util/exec/grb2index grib_in
  grib_in_ix
• export grib_indexgrib_in_ix
• /nwprod/util/exec/copygb2 -g "grid_new" -i0
  grib_in grib_index rtma5p0.t12z.2dvaranl_ndf
  d.1field.grb2
• echo "Max Min value from the original 2p5 grid "
• /nwprod/util/exec/wgrib2 -grid -max -min
  rtma2p5.t12z.2dvaranl_ndfd.1field.grb2
• echo "Max Min value from the converted 5p0 grid
  "
• /nwprod/util/exec/wgrib2 -grid -max -min
  rtma5p0.t12z.2dvaranl_ndfd.1field.grb2
• echo " Max Min from the current operational 5
  degree grid "
• /nwprod/util/exec/wgrib2 -grid -max -min
  rtma.t12z.2dvaranl_ndfd.1field.grb2
• exit

• Script to convert a RTMA 2p5 grib2 file to a
  5p0 grib2 file
• Author V. Krishna Kumar October 5, 2010
  NCO/Systems Integration Branch
• set -x
• clean the output file prior to running this
  script
• I created a single field grib2 file from the
  original production grib2 file using
• wgrib2 -d 1 in_grb2file -grib out_grb2file
• rm rtma5p0.t12z.2dvaranl_ndfd.1field.grb2
• HGT - Conversion from 2p5 to 5
• Input file rtma2p5.t12z.2dvaranl_ndfd.1field.
  grb2
• Output file rtma5p0.t12z.2dvaranl_ndfd.1field.
  grb2
• 2p5 Grid template 3.30 6 0 0 0 0 0 0 2145
  1377 20191999 238445999 8 25000000 265000000
  2539703 2539703 0 64 25000000 25000000 -90000000
  0

22
GRIB MESSAGE  1  starts at 1  SECTION 0  0 2
60463  SECTION 1  7 0 2 1 1 2010 11 9 6 0 0 0
1  Contains  0  Local Sections  and  1  data
fields.  FIELD  1  SECTION 0  0 2  SECTION
1  7 0 2 1 1 2010 11 9 6 0 0 0 1  SECTION 3  0
65160 0 0 0  GRID TEMPLATE 3. 0   6 0 0 0 0 0 0
360 181 0 0 90000000 0 48 -90000000 359000000
1000000 1000000 0  NO Optional List Defining
Number of Data Points.  PRODUCT TEMPLATE 4. 0  
3 5 2 0 96 0 0 1 36 100 0 100000 255 0 0  TEXT
HGT      1000 mb valid at  36 hr after
20101109060000  NO Optional Vertical
Coordinate List.  Num. of Data Points  
65160     NO BIT-MAP  DRS TEMPLATE 5. 3  
-979087360 0 1 11 0 1 0 0 0 5124 0 4 1 1 32 5 1
2 Data ValuesMIN        -525.79998779
AVE          70.36888123 MAX        
313.10000610
GRIB MESSAGE  2  starts at 60464  SECTION 0  0
2 59574  SECTION 1  7 0 2 1 1 2010 11 9 6 0 0 0
1  Contains  0  Local Sections  and  1  data
fields.  FIELD  1  SECTION 0  0 2  SECTION
1  7 0 2 1 1 2010 11 9 6 0 0 0 1  SECTION 3  0
65160 0 0 0  GRID TEMPLATE 3. 0   6 0 0 0 0 0 0
360 181 0 0 90000000 0 48 -90000000 359000000
1000000 1000000 0  NO Optional List Defining
Number of Data Points.  PRODUCT TEMPLATE 4. 0  
3 5 2 0 96 0 0 1 36 100 0 97500 255 0 0  TEXT
HGT      975 mb valid at  36 hr after
20101109060000  NO Optional Vertical
Coordinate List.  Num. of Data Points  
65160     NO BIT-MAP  DRS TEMPLATE 5. 3  
-984977408 0 1 11 0 1 0 0 0 5130 0 4 1 1 32 5 1
2 Data ValuesMIN        -323.89999390
AVE         278.48535156 MAX        
514.00000000
23
GRIB MESSAGE  149  starts at 7050617  SECTION
0  0 2 93572  SECTION 1  7 0 2 1 1 2010 11 9 6
0 0 0 1  Contains  0  Local Sections  and  2 
data fields.  FIELD  1  SECTION 0  0 2 
SECTION 1  7 0 2 1 1 2010 11 9 6 0 0 0 1 
SECTION 3  0 65160 0 0 0  GRID TEMPLATE 3. 0  
6 0 0 0 0 0 0 360 181 0 0 90000000 0 48 -90000000
359000000 1000000 1000000 0  NO Optional List
Defining Number of Data Points.  PRODUCT
TEMPLATE 4. 0   2 2 2 0 96 0 0 1 36 100 0 100000
255 0 0  TEXT U GRD    1000 mb valid at  36 hr
after 20101109060000  NO Optional Vertical
Coordinate List.  Num. of Data Points  
65160     NO BIT-MAP  DRS TEMPLATE 5. 3  
-1014333440 0 1 9 0 1 0 0 0 4835 0 4 1 1 32 5 1
2 Data ValuesMIN         -27.70000076
AVE            .24823742 MAX         
27.20000076
  FIELD  2  SECTION 3  0 65160 0 0 0  GRID
TEMPLATE 3. 0   6 0 0 0 0 0 0 360 181 0 0
90000000 0 48 -90000000 359000000 1000000
1000000 0  NO Optional List Defining Number of
Data Points.  PRODUCT TEMPLATE 4. 0   2 3 2 0
96 0 0 1 36 100 0 100000 255 0 0  TEXT V GRD   
1000 mb valid at  36 hr after 20101109060000 
NO Optional Vertical Coordinate List.  Num. of
Data Points   65160     NO BIT-MAP  DRS
TEMPLATE 5. 3   -1015873536 0 1 9 0 1 0 0 0 4934
0 4 1 1 16 5 1 2 Data ValuesMIN        
-24.30000114 AVE            .03596115
MAX          30.39999962 GRIB MESSAGE  150 
starts at 7144189
24
GRIB2 Section 7 Data Section

• Data coded with the minimum number of bits
  necessary to provide the accuracy.
• Simple scaling data multiplied by an
  appropriate power of 10 before forming the
  non-negative differences, and then using the
  binary scaling to select the precision of the
  transmitted value.
• Data coded in the form of non-negative scaled
  differences from a reference value of the whole
  field plus, if applicable, a local reference
  value. (reference value is normally the minimum
  value of the data set which is represented).

25
GRIB2 Section 7 Data Section contd.

• Complex packing for grid-point values split the
  whole set of scaled data values into groups on
  which local references (local minima) are removed
  pre-processing may be needed on the scaled
  data.
• Complex packing for spectral data.
• The original data value Y can be recovered with
  the formula
• Y10DR(X1X2)2E
• where EBinary scale factor, DDecimal
  scale factor, RReference value of the whole
  field, X10, X2Scaled (encoded) value.

26
GRIB2 Packing Methods

• GRIB2_com Group packing DR Template 5.2
• GRIB2_1od Group packing with 1st order
  differencing Template 5.3
• GRIB2_2od Group packing with 2nd order
  differencing Template 5.3
• Image compression algorithms within the GRIB2
  standard.
• Both treat grid point data as a grey scale image
  (single component)
• JPEG_2000 (Part-1 standard) (ISO/IEC 15444-1)
• http//www.jpeg.org/JPEG2000.html
• PNG (Portable Network Graphics image compression
  format)
• http//www.libpng.org/pub/png/.

27
How to write GRIB2 directly? An example to
convert APCP data into GRIB2

• The degrib2 diagnostic was generated from APCP
  GRIB2 output and the source code is in
• hostname
• c2n6.ncep.noaa.gov
• pwd
• /nco/save/sib/wx12kk/degrib2_analysis/sorc
• Input binary data file XMRG2417
• run script sh runGRIB2.sh
• output GRIB2417_grib2
• Usage HEtoGrib2_exec ltinput XMRG DIRgt ltXMRG
  file namegt ltoutput GRIB DIRgt ltoutput GRIB2 file
  namegt
• For full details, see the source code
  HEtoGrib2.f90
• call write_nesdis_grib2(gnesdis,nx,ny,nflds,
• varname,typeproc,isign,
• rtime(1),rtime(2),rtime(3),rtime(4),rtime(
  5),rtime(6),
• ityped,g2Path, accumHour, modelId,
  subCenter)
• An example of converting an ASCII data to GRIB2
  format from scratch
• http//www.nco.ncep.noaa.gov/sib/grib2/examples/dr
  ap2g2/

28
GRIB2 Encoding Routines

• The following routines are needed to write a
  GRIB2 message
• Gribcreate (gribcreate.f)
• This routine must be called first to
  initialize a new grib2 message. It uses to
• encode and pack section 0 and section 1
  and it encodes at the beginning of GRIB message
  (first 8-bytes) with
• GRIB - - 0 2, bytes (5 and 6) reserved
  for future use, byte 7 is for discipline, byte 8
  is for GRIB Edition 2 and
• leaves space for length of GRIB in bytes
  9 to 16
• call gribcreate(cgrib,max
  pts,listsec0,listsec1,ierr)
• For example GRIB2 section 0 and section 1
• -GRIB2 section 0 - Indicator Section
• listsec0(1)0 Discipline
  (see Code Table 0.0) ) is meteorological
  products
• listsec0(2)2 GRIB Edition
  Number (currently 2)
• -GRIB2 section 1 - Identification Section

29

• GRIB2 Encoding Routines (continued)
• Addlocal (addlocal.f) Local use
  section. It is optional
• is used to add a Local Use Section 2
  to GRIB2 message
• call addlocal (cgrib,lcgrib,csec2,lcse
  c2,ierr)
• Addgrid (addgrid.f)
• is used to add a Grid Definition
  Section (grid geometry of the data values in the
  field) to a GRIB2 message.
• call addgrid (cgrib, lcgrib, igds,
  igdstmpl, igdstmplen, ideflist, idefnum, ierr)
• For example Global Lat/Lon (1.25 degree)
  41760 data points (288x145) GRIB2 section 3 -
  Grid Definition Section 3.0
• igds(1)0
  0 is source of grid definition (see Code
  Table 3.0)
• igds(2)288145
  Number of grid points in the defined grid
• igds(3)0
  Number of octets needed for each additional
  grid points definition
• igds(4)0
  Interpolation of list for optional points
  definition (Code Table 3.11)
• igds(5)0
  Lat/Lon grid - Grid Definition Template
  Number 0 (Code Table 3.1)
• igdstmpl(1) 6
  Earth assumed spherical with radius
  6,371,229.0m
• igdstmpl(2) 0
  Scale factor of radius

30

• 
  GRIB2 Encoding Routines (continued)
• Addfield (addfield.f)
• This routine uses to add and packs up
  Production Definition Section (section 4), Data
  Representation
• Section (section 5), Bit-Map section (section
  6) and Data Section (section 7) to a GRIB2
  message.
• call addfield (cgrib, lcgrib, ipdsnum,
  ipdstmpl, ipdstmplen, coordlist, numcoord,
  idrsnum, idrstmpl,
• idrstmplen, fld,
  ngrdpts, ibmap, bmap, ierr)
• For example GRIB2 section 4 - Product
  Definition Template Number 4.0 (Analysis of
  Forecast at a
• 
  horizontal level or in a horizontal
  layer at a point in time)
• ipdsnum0 0 is Product
  Definition Template Number 4.0 ( see Code Table
  4.0)
• numcoord0 Number of values
  in array coordlist
• coordlist0.0 Array
  containing floating point values intended to
  document the vertical
• 
  discretization associated to model data on hybrid
  coordinate vertical levels
• ipdstmpl(1) 3 Meteorological
  products, Mass Category
• ipdstmpl(2) 5 Geopotential
  Height (HGT) parameter number 5 (see Table
  4.2-0-3)

31

• 
  GRIB2 Encoding
  Routines (continued)
• -Data Representation Section (section 5)
• For example -GRIB2 section 5 - Data
  Representation Section
• idrsnum 40
  Data Representation Template Number 40 ( see Code
  Table 5.0 )
• 
  (40 is Grid Point Data - JPEG2000
  Compression)
• idrstmpl(1) 1236599024 Reference
  value (R) (IEEE 32-bit floating-point value)
• idrstmpl(2) 0
  Binary scale factor (E)
• idrstmpl(3) 2
  Decimal scale factor (D)
• idrstmpl(4) 18
  Number of bits required to hold the resulting
  scaled and referenced data values.
• idrstmpl(5) 0
  Type of original field values
• idrstmpl(6) 0
  Type of Compression used. (0 for Lossless)
• idrstmpl(7) 255
  Missing Target compression ratio
• -GRIB2 section 6 - Bit Map Section

32

• 
  
  GRIB2 Encoding Routines
  (continued)
• Gribend (gribend.f)
• This routine finalizes a GRIB2 message after
  all grids and fields have been added. It adds
• the End Section (7777) to the end of the
  GRIB2 message and calculates the length of GRIB2
  message
• and stores it in the section 0 in (octet 9
  16)
• call gribend (cgrib, lcgrib,
  lengrib, ierr)

33
GRIB2 Decoding Routines

• To Read a GRIB2 message with call routines are
  following
• Skgb (skgb.f)
• is used to search a file for the next
  GRIB _ _ _ 2 (8-btyes) message in GRIB2 file.
• If found, the length of message is
  decoded from byte 9 to 16. (see Indicator
  section 0)
• call skgb(ifl1,iseek,msk1,lskip,lgrib
  )
• Gb_info (gb_info.f)
• is used to search through a GRIB2 message
  to find out how many local use
• sections and data fields found in the a
  given GRIB2 message
• call gb_info (cgrib, lcgrib,
  listsec0, listsec1, numfields, numlocal,
  maxlocal, ierr)
• Gf_getfld (gf_getfld.f)
• is used to get all information of grid
  definition (section 3), product template
• (section 4), Bit-map (if applicable), and
  the unpack data values for a given data
• field. All information returned is store
  in a derive type variable gfld
• call gf_getfld (cgrib, lcgrib,
  ifldnum, unpack, expand, gfld, ierr)
• Note
• gfld is a type of gribfield which is
  defined in module grib_mod (see grib_mod.f
• for declaration of derive type gribfield)

34
Declaration of Derived type gribfieldin Module
grib_mod

• type gribfield          integer
  version,discipline          integer,pointer,dimen
  sion() idsect          integer
  idsectlen          character(len1),pointer,dimen
  sion() local          integer locallen 
          integer ifldnum          integer
  griddef,ngrdpts          integer
  numoct_opt,interp_opt,num_opt         
  integer,pointer,dimension() list_opt       
    integer igdtnum,igdtlen         
  integer,pointer,dimension() igdtmpl         
  integer ipdtnum,ipdtlen         
  integer,pointer,dimension() ipdtmpl         
  integer num_coord          real,pointer,dimens
  ion() coord_list          integer
  ndpts,idrtnum,idrtlen         
  integer,pointer,dimension() idrtmpl         
  logical unpacked          logical
  expanded          integer ibmap         
  logical1,pointer,dimension() bmap         
  real,pointer,dimension() fld      end type
  gribfield
• If variable gfld is declared of type
  gribfield.
• For example
• Type (gribfield) gfld
• The variable gfld would have the
  following components
• gfldversion GRIB edition number (
  currently 2 )

35

• Declaration of Derived Type Gribfield
• (continued)
• gflddiscipline Message Discipline ( see Code
  Table 0.0)
• gfldidsect() Contains the entries in the
  Identification section 1
• gfldidrtmpl() Contains the data values for
  the specified Data Representation Template
• gfldidsectlen Number of elements in
  gfldidsect().
•   gfldlocal() Pointer to character array
  containing contents                     of Local
  Section 2, if included
• gfldlocallen length of array gfldlocal()
• etc
• See routine (gribmod.f) in documentation block
  for more components of
• variable gfld is declared of type gribfield

36
How to use routine getgb2?

• Getgb2 (getgb2.f)
• is used to search for selected fields
  and unpack from a GRIB2 message
• Call getgb2 (lugb,lugi,j,jdisc, jids,
  jpdtn, jpdt, jgdtn,jgdt, unpack, k, gfld, iret)
• Where
• j INTEGER NUMBER OF
  FIELDS TO SKIP ( J0 search from beginning)
• jdisc   GRIB2 DISCIPLINE NUMBER
  OF REQUESTED FIELD                  ( IF
  -1, ACCEPT ANY DISCIPLINE)   jids  
  INTEGER ARRAY OF VALUES IN THE IDENTIFICATION
  SECTION                 (-9999 FOR
  WILDCARD)  jpdtn     INTEGER PRODUCT
  DEFINITION TEMPLATE NUMBER (N)                 
  ( IF -1, DON'T BOTHER MATCHING PDT - ACCEPT
  ANY )  jpdt       INTEGER ARRAY OF VALUES
  DEFINING THE PRODUCT
• DEFINITION TEMPLATE
  4.N OF THE FIELD FOR WHICH
• TO SEARCH (-9999
  FOR WILDCARD)  jgdtn INTEGER GRID
  DEFINITION TEMPLATE NUMBER (M)                 
  ( IF -1, DON'T BOTHER MATCHING GDT - ACCEPT
  ANY )  jgdt    INTEGER ARRAY OF VALUES
  DEFINING THE GRID DEFINITION                 
  TEMPLATE 3.M OF THE FIELD FOR WHICH TO SEARCH 
                  (-9999 FOR WILDCARD)
• unpack LOGICAL VALUE INDICATING
  WHETHER TO UNPACK BITMAP/DATA
• .TRUE.
  UNPACK BITMAP AND DATA VALUES
• .FALSE. DO
  NOT UNPACK BITMAP AND DATA VALUES
• k INTEGER FIELD
  NUMBER UNPACKED
• gfld Derived type
  gribfield ( defined in module grib_mod )

37

• How to Use
  Getgb2?
• An example of using getgb2 is used to search for
  field (Temperature at 100 mb level) by
• product definition template.
• Initialize all variables to default -9999 or -1
  for wildcard
• J 0 Search from the
  beginning of GRIB2 file
• DJDSC-1 Discipline number
• JIDS-9999 Identification Section
• JGDTN-1 Grid Definition Template
  Number
• JGDT-9999 Grid Definition Template
• JPDTN-1 Product Definition
  Template Number
• JPDT-9999 Product Definition
  Template
• Search selected field by product definition
  template number
• and the values of product definition template 4.n
  of the field
• JPDTN 0 (search for production
  template 4.0)

38

• How to use
  Getgb2? (continued)
• How do we get the information of product template
  4.0 for field Temperature at 100mb from
• GRIB2 message ?
• We can use utility degrib2 to display the
  content of GRIB2 message
• degrib2 grib2file
• The content of GRIB2 file is displayed below
• GRIB MESSAGE 2 starts at 49405
• SECTION 0 0 2 17175
• SECTION 1 7 0 2 1 1 2010 12 13 0 0 0 0 1
• Contains 0 Local Sections and 1 data fields.
• FIELD 1
• SECTION 0 0 2
• SECTION 1 7 0 2 1 1 2010 12 13 0 0 0 0 1
• SECTION 3 0 41760 0 0 0

39
How to use routine putgb2?

• Putgb2 routine (putgb2.f)
• is used to pack a single field into a
  GRIB2 and writes out that message to the
• file associated with unit number.
• Call putgb2
  (lugb,gfld,iret)
• NOTE
• The information to be packed into the GRIB
  field is stored in a derived type variable, gfld.
• Gfld is of type gribfield, which is defined in
  module grib_mod, so users of this routine will
• need to include the line "USE GRIB_MOD" in
  their calling routine. Each component
• of the gribfield type is described in the
  subprogram documentation block of routine
  gribmod.f
• An example of using routine putgb2.f
• This program (g2_putgb2.f) reads an entire
  GRIB2 file and find selected fields from a file
  (data card)
• and uses routine (putgb2.f) to pack into a
  GRIB2 message and writes them out to a new GRIB2
  file.
• The content of data card is following
• GRIBIDS DESC' HGT 100 mb ',JPDTN 0 ,JPDT
  3 5 2 0 96 0 0 1 6 100 0 10000 255 0 0 /
• GRIBIDS DESC' HGT 100 mb ',JPDTN 0 ,JPDT
  3 5 2 0 96 0 0 1 6 100 0 15000 255 0 0 /

40
GRIB2 in NCEP Operations (continue)

• /nwprod/util/sorc/copygb2.fd
• The command copygb2 copies all or part of
  one GRIB2 file to another GRIB2 file,
  interpolating if necessary. Unless otherwise
  directed (-x option), the GRIB2 index file is
  also used to speed the reading. The fields are
  interpolated to an output grid if specified (-g
  option). The interpolation type defaults to
  bilinear but may be specified directly
  (-ioption).
• For example 1 Script to copy all NAM GRIB2
  (Regional CONUS - 40.63km - Lambert Conformal)
  file
• GRID TEMPLATE 3. 30
  6 0 0 0 0 0 0 185 129 12190000 226541000 8
  25000000 265000000
• 
  40635000 40635000 0 64
  25000000 25000000 0 0
• to NAM GRIB2
  (Regional CONUS High Resolution - 15km - Lambert
  Conformal)
• GRID TEMPLATE 3. 30
  6 0 0 0 0 0 0 344 224 19262000 238623000 8
  37500000 262000000 i
• 
  15000000 15000000 0 64
  30000000 60000000 0 0
• export grid2"30 6 0 0 0 0 0 0 344 224 19262000
  238623000 8 37500000 262000000
• 15000000 1500000 0 0
  64 30000000 60000000 0 0"
• export g1nam.t00z.awip3d72.tm00.grib2
• export x1nam.t00z.awip3d72.tm00.grib2i
• copygb2 -g "grid2" g1 x1
  nam.t00z.awip3d72.tm00.grib2.new
• For example 2 Script to extract selected field
  "MSLET Mean Sea Level" from NAM GRIB2
• (Regional CONUS 212 (40.63km -
  Lamber Conformal) file by using the information
• PRODUCT TEMPLATE
  4. 0 3 192 2 0 84 0 0 1 72 101 0 0 255 0 0

41
GRIB2 Reads and Writes Examples

• The program readwriteg2.f reads an entire GRIB2
  file and extract selected field(s) unpacked from
  GRIB2 messages and write it out to a new GRIB2
  file.
• The program readgrib2.f reads an entire GRIB2
  file and extract selected field(s) from data
  card and return packed GRIB2 message(s) to write
  them out to a new GRIB2 file.
• The program degrib2.f reads an entire GRIB2 file
  and display the content of an entire GRIB2 file.
• This program (g2_putgb2.f) reads an entire GRIB2
  file and find selected fields from a file (data
  card) and uses routine (putgb2.f) to pack into a
  GRIB2 message and writes them out to a new GRIB2
• The program drap2g2.f reads the Global D-region
  Absorption Prediction ASCII data
• and write the data in GRIB2 file
• The examples of these programs are available to
  download (tar file) from this url
• http//www.nco.ncep.noaa.gov/sib/grib2/examples/
• Reads and displays the content of GRIB2 files
• Reads and extracts selected fields from data card
  and write them out to a new file
• Reads and extracts selected fields from data card
  and write them out to a new file using putgb2

42
SUMMARY

• NCEP GRIB2 tools hopefully will provide guidance
  to all users to generate GRIB2 messages directly
• WMO has mandated all operational centers to
  transition to GRIB2 data for all the operational
  products
• Usage of CNVGRIB type of tools to convert from
  GRIB1 to GRIB2 should be minimized
• Generation of GRIB1 products take heavy storage
  and are also computationally expensive
• NCEPs parallel GRIB2 generation of NCEP POST
  using NCOs g2tmpl library (under development)
  for NEMS, NMMB and GFS show encouraging results
  and are very valuable for NCEP's codes to run
  more efficiently as well as occupy less disk and
  tape space
• NCO will continually improve the GRIB2 Template
  source code to incorporate all the Product
  Definition Templates.
• NCO will improve the existing GRIB2 tools in the
  future.