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 parameter rules

Units

• All fields shall use units as defined in GRIB Edition 2. The exact definition for each parameter is linked in Parameter page

Output frequency, levels for an/fc

• all details are available in Parameter page

Accumulations

• All accumulations shall start from the beginning of the forecast
• The initial state (forecast step zero) must not be included (as it should be 0 value field by definition)

Parameters archived since previous post-processing

• Some parameters (2m tmin/tmax, 10m wind gust) shall be archived with varying length of time range intervals:
  0-1/1-2/2-3/4-5/5-6/6-9/9-12/../27-30
  Find further in this page GRIB2 files examples including header outputs etc

Fluxes

• The flux sign convention is positive downwards.
• Positive evaporation represents condensation.

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 provide data on their native grid.
• Grids are defined in GRIB2 using an accuracy of 1/1000000 (one millionth) of a degree.

Ensemble type of outputs

• Ensemble member numbering must start from 1

Model levels

• Model level numbering must start from 1

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/1.17.0
> load grib_api 1.17.0 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)

Ensemble 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) 

Examples of encoding for various parameter validity types

Instantaneous

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.

Accumulations

This example represents a  54 hour accumulation of a field produced by a forecast started on 20120101 00Z.

• In bold, the values are fixed. They are meant to be used by all partners
• In green, these are the values for the example.

Maximum since the previous post-processing

This example represents a  maximum since the previous post-processing of a field valid on 20120103 00Z produced by a forecast started on 20120101 00Z . In this example, the maximum is the 6-hourly maximum in the time interval: steps 42 to 48.

• In bold, the values are fixed. They are meant to be used by all partners
• In green, these are the values for the example.

Minimum since the previous post-processing

This example represents a  minimum since the previous post-processing of a field valid on 20120103 00Z produced by a forecast started on 20120101 00Z . In this example, the minimum is the 6-hourly minimum in the time interval: steps 42 to 48.

• In bold, the values are fixed. They are meant to be used by all partners
• In green, these are the values for the example.