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) Deterministic and ensemble data assimilation forecasts
Two types of datasets are expected for UERRA - outputs from deterministic and ensemble data assimiliation systems. How they should be encoded in GRIB2 is shown below.
Control and perturbed forecasts are identified in section 1 and 4. The following tables explains how to code them in GRIB2:
Both systems - deterministic and ensemble DA (analysis or forecast products)
| Section 1 | |||
|---|---|---|---|
| Octets | Analysis products | Forecast products | |
| 21 | typeOfProcessedData | 0 | 1 |
Deterministic system (both analysis and forecast products)
| Section 4 | |||
|---|---|---|---|
| Octets | Analysis and Forecast products (instantaneous) | Analysis and Forecast products (statisticially processed = accumulated/min/max values ) | |
| 8-9 | productDefinitionTemplateNumber | 0 Analysis or forecast at a horizontal level or in a horizontal layer at a point in time (grib2/tables/17/4.0.table)) | 8 (Average, accumulation, extreme values or other statistically processed values at a horizontal level or in a horizontal layer in a continuous or non-continuous time interval (grib2/tables/17/4.0.table)) |
Ensembe DA system (both analysis and forecast products)
| Section 4 | |||
|---|---|---|---|
| Octets | Analysis and Forecast products (instantaneous) | Analysis and Forecast products (statisticially processed = accumulated/min/max values ) | |
| 8-9 | productDefinitionTemplateNumber | 1 (Individual ensemble forecast, control and perturbed, at a horizontal level or in a horizontal layer at a point in time (grib2/tables/17/4.0.table)) | 11 (Individual ensemble forecast, control and perturbed, at a horizontal level or in a horizontal layer, in a continuous or non-continuous time interval (grib2/tables/17/4.0.table)) |
| Ensemble (section 4, template 4.61) | ||
|---|---|---|
| Octets | Example for 2nd eps member of ensemble with 4 members (0/1/2/3) | |
| 35 | type of ensemble forecast | 255 (missing) |
| 36 | perturbation number | 2 |
| 37 | number of forecasts in ensemble | 4 |
3) Time component of GRIB2 definition
Examples of GRIB2 files
| deterministic | ensemble DA | |
| Accumulations | ||
| Instantaneous | ||
| Maximum since previous post-processing | ||
| Minimum since previous post-processing | ||
| Parameter on soil level | ||
| Parameter on soil layer |