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:

3) Time component and reforecasts

4) Examples of GRIB2 files