GRIB2 encoding

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

UERRA-GRIB2 checking tool

The so called tigge_check tool is a part of GRIB-API package. It should be used to validate all GRIB2 files prepared for UERRA (the option -u). The tool is checking all encoding details so that only fully compliant UERRA files following exactly required definitions would pass. It can check also allowed value ranges for each parameter if used with the option -v. These ranges have not been fully defined for all UERRA parameters yet e.g. for radiation fluxes. Sometimes warnings about values exceeding expected limits do not mean error in the checked data but only model specific correct values. 

Examples of tigge_check usage:

> module load grib_api/1.16.0
> load grib_api 1.16.0 Library (GRIB_API_MODE, GRIB_API_VERSION, GRIB_API_INCLUDE, GRIB_API_LIB, GRIB_API_INCLUDE_DIR, GRIB_API_LIB_DIR, EC_FFLAGS, EC_CFLAGS, EC_CXXFLAGS, EC_FLDFLAGS, EC_CLDFLAGS, EC_CXXLDFLAGS)
> 
# example with one data value ranges warning and one encoding error
> tigge_check -u -w -v *.grib2
> warning: an.enda.tcw.sfc.0.fp.grib2, field 1 [total_column_water_sfc]: total_column_water_sfc maximum value 31.5638 is not in [50,150]
  => [31.5638,150]
> fc.oper.2t.sfc.2.grib2 , field 1 [temperature_hl]: invalid height level 3

GRIB2 encoding details

The complete WMO compliant GRIB2 definitions of all UERRA parameters are linked here.

The additional generally required encoding UERRA details are listed below:

Deterministic system (both analysis and forecast products)

Ensembe DA system (both analysis and forecast products)

Examples of UERRA compliant GRIB2 files

*disregard data values in this grib (some step/level values were created artificially as real data has not been available yet)