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 her
The version of GRIB-API which can recognise all UERRA parameters is currently under development and should be released to some extent by the 1st half of June. Until then please use the version 1.15.0 which can identify UERRA products as per below (paragraph 1).
Encoding
1) UERRA production status of processed data
The WMO has added two values to table 1.3 Production status of data:
| Section 1 | |||
|---|---|---|---|
| Octets | Key name | UERRA operational products | UERRA test products |
| 20 | productionStatusOfProcessedData | 8 | 9 |
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 | Key name | Analysis products | Forecast products |
| 21 | typeOfProcessedData | 0 | 1 |
Deterministic system (both analysis and forecast products)
| Section 4 | |||
|---|---|---|---|
| Octets | Key name | 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 | Key name | 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 | |||
| AN | FC | AN | FC | |
| Accumulations | ||||
| Instantaneous | ||||
| Maximum since previous post-processing | ||||
| Minimum since previous post-processing | ||||
| Parameter on soil level | ||||
| Parameter on soil layer | ||||