This page contains a few rules on the UERRA parameters. A complete list of all the parameters and GRIB2 encoding is given here.
UERRA parameters rules
Units
- All fields shall use units as defined in GRIB Edition 2
Accumulations
- All accumulations shall start from the beginning of the forecast
- The initial state (forecast step zero) shall be a part of all forecasts, all members, etc.
- The values of accumulated fields shall be set to zero for t=0
Instantaneous fields
- Instantaneous fields shall be archived with a frequency defined here for each group of parameters and forecast type.
Parameters archived since previous post-processing
- Some parameters (2m tmin/tmax, 10m wind gust) shall be archived with varying lenght of time range intervals (0-1/1-2/2-3/4-5/5-6/6-9/9-12/../27-30 as per here)
Fluxes
- The flux sign convention will be positive downwards.
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
- Each partner will provide their eps data on their native grid.
- Grids are defined in GRIB2 using an accuracy of 1/1000000 (one millionth) of a degree.
Types of forecasts
- The Control Forecast must be number 0,
- The number of members is identical to the number of all EPS members plus the Control Forecast.
GRIB2 encoding
ECMWF encoding/decoding Software
ECMWF makes available a GRIB decoding library (grib_api) for the benefit of the WMO community. grib_api can be freely downloaded from here and the documentation can be found here. The grib_api provides a simplified access to GRIB2 headers through a set of virtual keys documented here.
Encoding
1) UERRA production status of processed data
Octet 36 of section 1 of a GRIB2 message contains the Production status of processed data. The WMO has added two values to table 1.3 Production status of data:
- 8: UERRA operational products
- 9: UERRA test products
The value 9 should be used when testing and 8 for operational data.
2) Control and perturbed forecasts
Control and perturbed forecasts are identified in section 1 and 4. The following tables explains how to code them in GRIB2:
| Section 1 | |||
|---|---|---|---|
| Octets | Example perturbed | Example control | |
| 21 | typeOfProcessedData | =4 | =3 |
| Ensemble (section 4, template 4.61) | |||
|---|---|---|---|
| Octets | Example perturbed | Example control | |
| 35 | type of ensemble forecast | = 255 (missing) | = 255 (missing) |
| 36 | perturbation number | = 7 | = 0 |
| 37 | number of forecasts in ensemble | = 51 | = 51 |
3) Time component and reforecasts
4) Examples of GRIB2 files
| real-time | re-forecast | |
| Daily average | Control Perturbed | Control Perturbed |
| Accumulations | Control Perturbed | Control Perturbed |
| Instantaneous | Control Perturbed | Control Perturbed |
| Maximum over the last 6 hours | Control Perturbed | Control Perturbed |
| Minimum over the last 6 hours | Control Perturbed | Control Perturbed |