The GRIB files are created using the GRIBEX software. To archive the GRIB files in MARS, the GRIB headers should be compatible with a set of rules to ensure that the files are consistent with the way the archiving system is defined. Therefore, every modelling partner is expected to take into account the following rules to write the GRIB headers.

The GRIB headers have several sections:

Section 1 provides general information about the field. These are the general rules to encode this section for ENSEMBLES fields:

KSEC1(1): Version number of code table 2 that allows to identify the variables. Different versions are required because specific definitions are used for some variables in ENSEMBLES. The following values are expected to be used: 128, the reference table, will be used for all variables that are not specifically defined for ENSEMBLES (i.e., all but maximum and minimum 2-metre temperature, fluxes, total precipitation or total soil moisture); 190, the table for projects on seasonal to interannual ensemble forecasts, will be used for maximum and minimum 2-metre temperature, fluxes, total precipitation and total soil moisture; 172 for the monthly means of the fluxes (rate of accumulation), as they change units from Ws/m**2 to W/m**2 (see common list of variables for more information); 174, the non-ECMWF seasonal forecast data, which is used for EUROSIP, contains the codes for some special variables such as the Met Office near-surface temperature.
KSEC1(2): Originating centre as given by WMO code table 0. This number will identify the centre that developed and runs the model. A given centre may have different models or different versions of the same model that can be distinguished with the "method" and "system" attributes. The GRIB codes in Table 0 correspond to the values of the "origin" attribute in MARS described in the table below:

Institution	Origin	GRIB code
ECMWF	ecmf	98
Met Office	egrr	74
Météo-France	lfpw	85
CERFACS	crfc	239
IfM-Kiel	ifmk	246
INGV	ingv	235
INM	lemm	214

The codes for a large set of centres can be obtained from the CCCC WMO documentation.

KSEC1(3): Generating process identification number. It can be any number, with a meaning for you, between 1 and 254. ECMWF uses 1.
KSEC1(4) = 255, unless your grid is catalogued, in which case there is no need to describe it in Section 2.
KSEC1(5): Flag showing whether sections 2 and 3 are present, as specified in WMO code table 1. It usually takes the value 128, meaning that section 2 is included, while section 3 is omitted.
KSEC1(6): Parameter indicator as taken from the ECMWF local table version 2 specified in KSEC1(1).
KSEC1(7-9): Follow the recommendations given in WMO code table 3. Use KSEC1(7)=112 for surface temperature and soil moisture, 100 for pressure levels and 0 otherwise.
KSEC1(10-14): The MARS variable "date" is built up with the values of the variables KSEC1(10-14). It always corresponds to the starting date of the experiment, even in the case of an experiment initialized using the lagged average method (in this case the start date will be the date of the last ensemble member).
KSEC1(15): The time units indicator is specified in WMO Code Table 4. KSEC1(15) will preferably be 2 (days) or 12 (12 hours) to cope with the long runs. This will be valid provided that there are no 6-hourly values to be archived.
KSEC1(16-18): The time evolution of the experiment is described in KSEC1(16-17). For instantaneous values, KSEC1(16) has the time step (in the units defined in KSEC1(15)) and KSEC1(17) = 0. For maximum and minimum temperature and for accumulated variables, KSEC1(16) will contain the time of the last time step used in the computation, while KSEC1(17) = 0. It implies that these variables will be daily values valid for period between the time step indicated and the previous 24 hours, as specified in Table 190. This is the only way to accomodate the high values for the time step that are expected in the multi-annual integrations. In all cases KSEC1(18) = 10, to be able to cope with long integrations.
KSEC1(19): Use the number of values employed in the computation of an average, a maximum or a minimum, when the time range indicates an average, accumulation or other sort of operation. Note that, to cope with the length of the integrations, the time range value used for all the variables is 10, which does not allow to describe the number of values used in the case of maximum or minimum temperature (four 6-hourly values of 2-metre temperature are used to compute the variable). Instead, this is thoroughly described in the table version 2. Therefore set to zero.
KSEC1(22) = 98, sub-centre identifier. This corresponds to the centre where the data are archived.
KSEC1(23) = 0, no scaling factor is used.
KSEC1(24) = 1, ECMWF local use in Section 1.
KSEC1(25-36): Set to zero.
KSEC1(37) = 15, the ECMWF local use definition for ensemble seasonal forecast atmosphere data. This local use definition is decribed here.
KSEC1(38) = 11, the class number for ENSEMBLES.
KSEC1(39) = 9, the type for forecast products.
KSEC1(40) = 1220, the multi-model seasonal forecast stream. Multi-annual integrations will require a different stream to optimise the use of MARS.
KSEC1(41): The experiment number (4 digits) may be used in any suitable way to identify your experiment among those that you can carry out with a given version of a prediction system. Please, do not use number 1 as this will be employed in a future rearchiving of the data once the experiments are finished, just in case you want/need to rerun something. We suggest to start from a number larger than 1000 for stream 1 and 2000 for stream 2 simulations.
KSEC1(42): The ensemble member number starts from number 0 and takes values up to the number of ensembles members minus one.
KSEC1(43): The total number of ensemble members, which may depend of the experiment, although most of the time it will be 9. If you are not sure about the final ensemble size, set it to zero.
KSEC1(44): The system number should be used to distinguish between different prediction systems used by the same institution. For instance, the Met Office will have to choose a system number for GloSea and another one for DePreSys. We suggest to start from the number 1.
KSEC1(45): The method number distinguishes between different methods to produce forecasts. For instance, the Met Office will have to assign specific method numbers to the different model versions obtained with the perturbed parameter approach or ECMWF to distinguish between runs with and without stochastic physics.

Some changes are required to code the monthly mean fields:

KSEC1(16): Number of time steps in the units indicated in KSEC1(15), for the end of the month counted from the beginning of the integration. For instance, for a 31-, 30- and 28-day long integration, the value will be 744, 720 and 672 if KSEC1(15)=1 (hours), and 31, 30 and 28 if KSEC1(15)=2 (days). KSEC1(17) will still be 0.
KSEC1(19): Use the number of values used to compute the monthly mean. For instance, for the monthly mean of January, it should take the value 31 except for the monthly means computed using 6-hourly data (e.g., 2-metre temperature), for which it should be 124.
KSEC1(37) = 16, the ECMWF local use definition for ensemble seasonal forecast monthly mean atmosphere data. This local use definition is decribed here.
KSEC(39) = 80, the type for the forecast seasonal mean. You may also want to archive the forecast seasonal maximum (81), the forecast seasonal minimum (82) or the forecast seasonal standard deviation (83).
KSEC1(40) = 1221, the multi-model seasonal forecasts monthly mean stream.
KSEC1(43): Set to zero.
KSEC1(46): Verifying month or the month the forecast tries to reproduce, following the format YYYYMM. This is the value that identifies which period the monthly mean refers to.
KSEC1(47): Averaging period (e.g., 6-hour, 24-hours) or time interval of data (in hours) used to compute the monthly mean. Check the agreement with the values in KSEC1(19).
KSEC1(48): Forecast month, starting from 1.

Section 2 describes the grid. It is important to distinguish between grid point and spherical harmonic fields. Grid-point fields are usually written from North to South and from West to East. You will have to code your vertical coordinates too.

Section 4 describes the type of encoding. It is important that you check the following:

KSEC4(1): It is the number of values encoded. Any disagreement in this value will force a GRIBEX error.
KSEC4(2)=16, the number of bits used per value. Larger values can also be used.
KSEC4(3-11): Set to zero.

11.10.2006