Introduction

Writing grib-2 files can be quite painful with all the libraries involved. However, there is a type of grib which is relatively easy to write. In this format, a single grib2 record looks like

     (binary header)           variable number of bytes
     (big endian ieee data)    4*ndata bytes
     (binary trailer)          4 bytes, ascii '7777'

   $ wgrib2 -order raw -d 1 in.grb2 -grib_ieee out
   1:0:d=2008022101:APCP:surface:1 month fcst:
   $ ls out*
   out.grb  out.h  out.head  out.tail

The first line takes the first record of the file and creates

   out.grb        ieee grib2 file (one record)
   out.head       binary file with the header
   out.tail       binary file with the trailer
   out.h          C header

Making a simple grib file

   $ wgrib2 -d 1 in.grb2 -no_header -ieee ieee.bin
   1:0:d=2008022101:APCP:surface:1 month fcst:

   $ cat out.head ieee.bin out.tail > first.grb
   $ wgrib2 first.grb -grib_out second.grb
   1:0:d=2008022101:APCP:surface:1 month fcst:

   $ wgrib2 second.grb -set_date 2008010100 -set_var SNOD -grib_out third.grb
   1:0:d=2008010100:SNOD:surface:1 month fcst:

Making a grib file with C

unsigned char head[] = {
 71, 82, 73, 66, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 164, 234, 0, 0, 0, 21,
 1, 0, 7, 0, 1, 2, 1, 1, 7, 216, 4, 14, 0, 0, 0, 0, 1, 0, 0, 0,
 72, 3, 0, 0, 0, 41, 16, 0, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0, 0,
 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 144, 0, 0, 0, 73, 0, 0, 0, 0, 0,
 0, 0, 0, 5, 93, 74, 128, 0, 0, 0, 0, 48, 133, 93, 74, 128, 21, 79, 4, 96,
 0, 38, 37, 160, 0, 38, 37, 160, 0, 0, 0, 0, 34, 4, 0, 0, 0, 0, 3, 5,
 2, 0, 180, 0, 0, 0, 1, 0, 0, 0, 0, 100, 0, 0, 1, 134, 160, 255, 0, 0,
 0, 0, 0, 0, 0, 0, 12, 5, 0, 0, 41, 16, 0, 4, 1, 0, 0, 0, 6, 6,
 255, 0, 0, 164, 69, 7,};

unsigned char tail[4] = {55, 55, 55, 55};

#define NDATA 10512
#define SEC0 0
#define DISCIPLINE 6
#define EDITION 7
#define SEC1 16
#define CENTER 21
#define SUBCENTER 23
#define MASTERTABLE 25
#define LOCALTABLE 25
#define YEAR 28
#define MONTH 30
#define DAY 31
#define HOUR 32
#define MINUTE 33
#define SECOND 34
#define SEC3 37
#define SEC4 109
#define PRODUCTDEFTEMPLATENUM 116
#define PRODUCTDEFTEMPLATE 118
#define PRODUCTCATEGORY 118
#define PRODUCTNUMBER 118
#define SEC5 143
#define SEC6 155
#define SEC7 161

#include <stdio.h>
#include <stdlib.h>
#include "g2.5.h"

// Method #2 of writing a grib file
//
//  sample program to create a grib2 ieee file
//  to convert to jpeg file, type wgrib2 in_file -grib_out out_file
//
// can also use wgrib2 to change the variable name, level and time
//   by the set_* commands
//
// 4/2008 Wesley Ebisuzaki

int main() {
        FILE *grib;
        float r;
        char s[4], *t;
        int i;

        grib = fopen("test.grb", "wb");

// to change octet N of section 4 to the value of K
// add the following line:
//      head[SEC4+(N-1)] = K;
// the grib documentation starts numbering octets at 1
// C uses 0 as the base

        head[YEAR] = 2008 / 256;
        head[YEAR+1] = 2008 % 256;
        head[MONTH] = 2;
        head[DAY] = 3;
        head[HOUR] = 4;

        head[DISCIPLINE] = 0;
        head[PRODUCTCATEGORY] = 1;
        head[PRODUCTNUMBER] = 2;

        // write grib header

        i = fwrite(head, 1, sizeof(head), grib);
        printf("size of head = %d\n",i);

        // for missing values, use the value NaN
        // note: wgrib2 will convert 9.999e20 to missing
        // write out IEEE big-endian data (NDATA)

        // silly array -- needs to be in big-endian

        r = 1.0;
        t = (char *) &r;
        // byte swap on a little endian machine
        s[0] = t[3];
        s[1] = t[2];
        s[2] = t[1];
        s[3] = t[0];
        for (i = 0; i < NDATA; i++) {
            fwrite(s,1,4,grib);
        }

        // write grib trailer

        fwrite(tail,1,sizeof(tail),grib);
        return 0;
}

As with the first example, one should convert undefined grid points and the ieee packing to a more commonly used packing using "wgrib2 in.grb -grib_out out.grb".

In this example, in.grb2 happens to have the correct grid but wrong the center. If you examine in.grb2, you see that it comes from NCEP and you happen to be working in in Kiribati (196 in WMO table). That can be easily fixed.

   $ wgrib2 in.grb2 -center
   1:0:center=US National Weather Service - NCEP (WMC)
   $ wgrib2 in.grb2 -set_center 196  -grib_out third.grb2
   1:0:d=2008022101:APCP:surface:1 month fcst:
   $ wgrib2 third.grb2 -center
   1:0:center=Kiribati (NMC)

Getting the "template" grib2 file

The procedure depends on having a grib2 file with the correct grid. For common grids, you can often find a such a grib file. Sometimes you can find a grib1 file and then you can use cnvgrib to convert the file into grib2. As a last resort, one could take a grib1 file, convert it to the correct domain by the program copygb and then use cnvgrib. Of course, the last resort is the hardest because it requires knowing the grid description in the arcane "GDS" format. (You'll have to read the fortran code and know a bit about grib1.)

5/2010: wgrib2 can make regular lat-lon template files by the -lola option.

Undefined grid points

In grib, undefined grid points are usually specified by a bit mask. However, ieee floating point numbers can have a value of NaN or "not a number". The current WMO documentation doesn't specify how to interpret a NaN, so wgrib2 interprets a NaN as undefined. Seems logical, doesn't? Anyways I'll assume logic holds until WMO defines otherwise. However, writing a NaN can be difficult in some languages. So one can use a special number as the undefined value. If you use 9.999e20, you can fix the file by

   wgrib2 in.grb2 -grib_out out.grb2

   wgrib2 in.grb2 -undefine_val -999 -grib_out out.grb2

Other grib decoders

   wgrib2 in.grb2 -grib_out out.grb2

Warning