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In progress. All details still to be updated..

General requirements

Encoding format

WMO GRIB2 data format.

Missing values

Bitmaps shall be used to indicate missing values.

Grid and resolution

Each partner will provide their eps data on a 0.25/0.25 regular lat-lon grid (1440x721 grid points, including poles).
First grid point is 90N 0E. The data will be archived first from west to east (OE to 358.5), and then from north to south (90N to 90S).
The related GRIB 2 meta data called scanning mode should equal to 0.
- Scanning mode 0
  i scans positively (west to east)
  i points are consecutive
  j scans negatively (north to south)
The fields have to be provided on...
Grids are defined in GRIB2 using an accuracy of 1/1000000 (one millionth) of a degree.

Data compression

CCSDS compression should be used

GRIB2 encoding

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

Encoding software

ECMWF makes available a GRIB decoding library (ecCodes) for the benefit of the WMO community. 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

General encoding (TBA)

Specific details which should be encoded in GRIB2 sections 1 and 4:

Section 1
Octets	Key name	Value	Description	Comments
6-7	centre	X	WMO Centre's code (table below)
8-9	subCentre	0	WMO Sub-Centre's code if needed (table below)
10	tablesVersion	36	The latest WMO table^*1
11	localTablesVersion	0	Local tables not used
20	productionStatusOfProcessedData	16	Weather Prediction Model Intercomparison Project ^*2	To use during production phase
		17	Weather Prediction Model Intercomparison Project ^*2	To use during testing phase
21	typeOfProcessedData	0	Analysis products
		1	Forecast products

*1 New WMO tables version 36 (containing also new WPMIP keys) will be officialy released by WMO later in 2025 (November)

*2 New codes for the WPMIP (part of GRIB 2 tables version 36) to be approved by WMO by August 2025 (Manual on Codes (WMO-No. 306), Volume I.2, GRIB code table 1.3)

Section 4
Octets		Value (Example perturbed)	Value (Example control)
8-9	productDefinitionTemplateNumber	1/11*	1/11*
13	backgroundProcess
14	generatingProcessIdentifier
35	typeOfEnsembleForecast	255 (missing)	255 (missing)
36	perturbationNumber	7	0
37	numberOfForecastsInEnsemble	51	51

(*) statistically processed (typeOfStatisticalProcessing is set up)

Section 5
Octets		Value	Description
10-11	dataRepresentationTemplateNumber	42	[Grid point data - CCSDS recommended lossless compression (grib2/tables/29/5.0.table) ]

WMO Centre codes

The codes below are based on WMO Manual 306 - WMO Codes. Each organization must be uniquely identified by its WMO code. If necessary to distinguish between two organizations using the same WMO code the Sub-Centre one which differs must be provided.

Organisation	Acronym	WMO Centre code	Centre acronym^*1
China Meteorological Administration, China	CMA		babj
European Centre for Medium-Range Weather, UK	ECMWF	98	ecmf
Japan Meteorological Agency, Japan	JMA	34	rjtd
National Centres for Environmental Prediction, USA	NCEP	7	kwbc
National Aeronautics and Space Administration	NASA		nasa

Remarks:

*1 in ECMWF's ecCodes package

Examples of the required GRIB2 files

parameter	Sample file	GRIB metadata dump^*1

*1 using "grib_dump -O" command from ecCodes package