Encoding details

General requirements

Encoding format

• WMO GRIB2 data format.

Missing values

• Bitmaps shall be used to indicate missing values.

Grid and resolution

• Data should be provided on a 1.25/1.25 regular lat-lon grid (288x145 grid points, including poles).
• First grid point is 90N 0E. The data will be archived first from west to east (OE to 358.75), and then from north to south (90N to 90S).
  • The related GRIB 2 key called scanning mode  must be equal to 0 then

  • Scanning mode 0 i scans positively (west to east) i points are consecutive j scans negatively (north to south)

• Grids are defined in GRIB2 using an accuracy of 1/1000000 (one millionth) of a degree.

GRIB2 encoding

All data must be encoded following the required rules described below to achieve fully compliant LC-GCR format from all centres.

Encoding software

ECMWF makes available a GRIB decoding library (ecCodes) for the benefit of the WMO community. The ecCodes can be freely downloaded from here and the documentation can be found here.

For more general information about GRIB2 format please refer to WMO Manual on Codes.

Encoding details

Data compression

The CCSDS compression should be used. More information can be found in What is CCSDS/AEC packing and how I can set it - ecCodes GRIB FAQ

General encoding

Specific details which should be encoded in GRIB 2 sections.

Data encoding GRIB2 sections example below is valid for:

• centre ECMWF
• Time-mean total precipitation rate (paramId 235055)
  • monthly mean of daily means for January 1991

*1 New WMO tables version 36 and higher containing  new LC-GCR keys

*2 New codes for the LC-GCR (part of GRIB 2 tables version 35) approved by WMO in January 2025 (Manual on Codes (WMO-No. 306), Volume I.2, GRIB code table 1.3)

Model version

Each model is identified in ecCodes for given centre and model version from the following keys:

• centre (WMO centre code)
• backgroundProcess (uniquely defined for each model, see table below)
• generatingProcessIdentifier (uniquely defined for each model version, see table below)

For example model=CRA-40 is encoded using:

• centre=38
• backgroundProcess=1
• generatingProcessIdentifier=1

Model version encoding

Remarks:

*1 in ECMWF's ecCodes package

*2 LC-GCR model names are correctly recognised in ecCodes v.2.48.0 and higher

Examples of LC-GCR GRIB2 files

• Parameters
• examples based on the ERA5/ECMWF data
  • meta data of the encoded fields are fully compliant to the required standard
    • grib-check v.0.0.9 or higher must be used to verify LC-GCR GRIB2 encoding

Remarks

• ecCodes Version 2.48.0 (and higher) should be used to display correctly LC-GCR related meta data 

Example of encoding script (TBU)

The package includes conversion script, input data (ERA5, GRIB1) and outputs after conversion to LC-GCR GRIB2.

• encoding_example.tar.gz (TBU once the latest grib-check is out with eccodes 2.48.2 bundled in)

# Example of ERA5 GRIB1 monthly average fields conversion to LC-GCR compliant GRIB2
#   requirements:
#     eccodes 2.48.2 or higher recommended (2.48.0 minimum)
#     grib-check version 0.0.9 or higher (https://github.com/ecmwf/grib-check)
# run:
#   ksh convert.ksh 2>&1 | tee convert.out
# remarks:
#   grib_filter rules must be modified for given model!
#     search for xxx marking such places

$ tar tvf encoding_example.tar
drwxr-x--- marm/ma           0 2026-09-07 13:01 encoding_example/
drwxr-x--- marm/ma           0 2026-09-07 12:44 encoding_example/log/
-rw-r----- marm/ma         748 2026-09-07 12:37 encoding_example/log/grib_check.ecmf.19910101.fc.sfc.out
-rw-r----- marm/ma         748 2026-09-07 12:37 encoding_example/log/grib_check.ecmf.19910101.an.sfc.out
-rw-r----- marm/ma       18036 2026-09-07 12:52 encoding_example/log/ecmf.19910101.an.sfc.dump
-rw-r----- marm/ma       18095 2026-09-07 12:52 encoding_example/log/ecmf.19910101.fc.sfc.dump
drwxr-x--- marm/ma           0 2026-09-04 14:02 encoding_example/data/
-rw-r----- marm/ma      501552 2026-09-04 09:44 encoding_example/data/lcgcr.199101.grib1
drwxr-x--- marm/ma           0 2026-09-07 12:06 encoding_example/out/
-rw-r----- marm/ma      101185 2026-09-07 12:52 encoding_example/out/ecmf.19910101.fc.sfc.grib2
-rw-r----- marm/ma      102828 2026-09-07 12:52 encoding_example/out/ecmf.19910101.an.sfc.grib2
-rw-r----- marm/ma        3867 2026-09-07 12:52 encoding_example/convert.out
-rw-r----- marm/ma        5383 2026-09-07 12:52 encoding_example/convert.ksh