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General requirements
Encoding format
- Participating Centres are expected to produce output fields in WMO GRIB2 data format and with consistent units. All data must be encoded following the below rules in the paragraph Encoding details. For more general information about GRIB2 format please refer to WMO Manual on Codes .
Missing values
- Bitmaps shall be used to indicate missing values.
Grid and resolution
- The fields have to be provided on a regular latitude‐longitude grid at the resolution that is best matching the native resolution of the direct model output.
- Grids are defined in GRIB2 using an accuracy of 1/1000000 (one millionth) of a degree.
- If several limited areas will be provided they should be merged into one larger area on a regular lat-lon grid, with a missing value indicator in the grid-points not covered.
GRIB2 encoding
All data must be encoded following the required rules described below to achieve fully compliant LC-WFV 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. EcCodes provides a simplified access to GRIB2 headers through a set of virtual keys documented here
For more general information about GRIB2 format please refer to WMO Manual on Codes .
Encoding details
Parameters
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| Section 1 | |||||
|---|---|---|---|---|---|
| Octets | Key name | Value | Description | ||
| 6-7 | centre | X | WMO Centre's code (table below) | ||
| 8-9 | subCentre | X | WMO Sub-Centre's code if needed (table below) | ||
| 10 | tablesVersion | 19 | The latest WMO table | ||
| 20 | productionStatusOfProcessedData | 0 | Production data | ||
| 1 | Test data | ||||
| 21 | typeOfProcessedData | 0 | Analysis products | ||
| 1 | Forecast products | ||||
| Section 4 | |||
|---|---|---|---|
| Octets | Key name | Value | Description |
| 8-9 | productDefinitionTemplateNumber | 0 | Analysis and Forecast products (instantaneous) (Analysis or forecast at a horizontal level or in a horizontal layer at a point in time (grib2/tables/17/4.0.table)) |
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| Time ( instantaneous ) | |||||
|---|---|---|---|---|---|
| Start of forecast ( section 1 ) | |||||
Octets | values | Meaning of values | Grib_api values | ||
| 12 | Significance of Reference Time | 1 | Start of forecast | ||
| 13-14 | Year (four digits) | 2012 | year of start of forecast | dataDate=20120101 dataTime=0
| |
| 15 | Month | 01 | month of start of forecast | ||
| 16 | Day | 01 | day of start of forecast | ||
| 17 | Hour | 00 | hour of start of forecast | ||
| section 4 e.g. template 4.1 | |||||
| 18 | Indicator of unit of time range | 1 | Hour | ||
| 19-22 | Forecast time | 48 | Forecast step | stepType=instant, step=48 | |
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parameter | Sample file |
|---|---|
10 metre U wind component | wave.10u.grib2 |
10 metre V wind component | wave.10v.grib2 |
| Mean wave period based on second moment | wave.mp2.grib2 |
| Mean wave direction | wave.mwd.grib2 |
| Peak wave period | wave.pp1d.grib2 |
Significant height of combined wind waves and swell | wave.swh.grib2 |
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