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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. For more information 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
- 10 metre U wind component
- 10 metre V wind component
- Mean wave direction
- Mean zero-crossing wave period
- Peak wave period
- Significant height of combined wind waves and swellRequired GRIB2 definitions
General encoding
Specific details which should be encoded in GRIB2 sections 1 and 4:
| 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)) |
...
| 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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