General requirements
Encoding format
- Participating Centres are expected to produce output fields in GRIB2 data format and with consistent units. For more information please refer to Guide to the WMO Table Driven Code Form.
Units
- All fields shall use units as defined in WMO GRIB2 manual
Output frequency
- The model output steps should be between 1 and 12 hour
Missing values
- Bitmaps shall be used to indicate missing values (i.e. Soil temperature (st) shall be coded using a bitmap, because there is no model output over the sea)
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.
GRIB2 encoding
ECMWF encoding/decoding 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
WAVE-GRIB2 checking tool
Tool for checking input file compliance with the agreed GRIB2 format; any other additional checks (geometry, basic data quality i.e. allowed extremes check etc)? TBD
Encoding details
The complete WMO compliant GRIB2 definitions of all wave parameters are linked in the page Wave verification
General encoding details
| Section 1 | |||
|---|---|---|---|
| Octets | Key name | Value | Description |
| 6-7 | centre | WMO Manual 306 - WMO Codes | |
| ECMWF | 98 | ECMWF (RSMC) | |
| UKMO | 74 | UK Meteorological Office, Bracknell (RSMC) | |
| ... | |||
| 8-9 | subCentre | TBD | If centre is not sufficient |
| 10 | tablesVersion | 19 | latest WMO table |
| 20 | productionStatusOfProcessedData | 0 | Operational products |
| 1 | Operational test products | ||
| 21 | typeOfProcessedData | 1 | Forecast products |
| TBD | Any other products (e.g. analysis)? | ||
| 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)) |
Example of the validity type encoding
This example describes an instantaneous field valid on 20120103 00Z from a forecast starting on 20120101 00Z (time step=48 hours) .
- In bold, the values are fixed. They are meant to be used by all partners
- In green, these are the values for the example.
| 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 |
Examples of the required GRIB2 files
parameter | Sample file |
|---|---|
10 metre U wind component | WAVE_165.128_1x1.grb2 |
10 metre V wind component | WAVE_166.128_1x1.grb2 |
| Mean wave period based on second moment | WAVE_221.140_1x1.grb2 |
| Mean wave direction | WAVE_230.140_1x1.grb2 |
| Peak wave period | WAVE_231.140_1x1.grb2 |
Significant height of combined wind waves and swell | WAVE_229.140_1x1.grb2 |