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This icon generates the gridded input data and the AVAILABLE file needed to run the FLEXPART Lagrangian dispersion model. The input forecast or analysis fields are either automatically retrieved from ECMWF's MARS archive or read from the specified input file. For help on editing icons, please see Editing Icons.

The macro language equivalent is flexpart_prepare().

What data is available?

FLEXPART Prepare is currently only able to handle data from the operational ECMWF forecasts/analyses 2008, 4 June onwards.

Input data fields

The following is for information only - FLEXPART Prepare handles these details internally.

FLEXPART requires input fields on a regular latitude-longitude grid in GRIB format. The input data must contain five three-dimensional fields: the two horizontal wind components, vertical velocity, temperature and specific humidity. Additional surface fields (e.g. fluxes) are also needed. The three-dimensional input data has to be available on ECMWF model (i.e. η) levels defined by a hybrid vertical coordinate system. An important restriction is that all the data fields used within a FLEXPART run must have the same domain size, resolution, number of levels, etc.

All the required fields, with one exception, can be retrieved from ECMWF's MARS archive. The only exception is vertical velocity because FLEXPART needs the following field for its computations: \( \dot \eta \frac{\partial \eta}{\partial p} \)

Since only \( \dot \eta \) is archived in MARS the full product needs to be computed during the data preparation process (FLEXPART Prepare does it for you).

Input filenames

FLEXPART requires all the input GRIB files to be in the same folder using the following file naming convention: ENyymmddhh. The same directory has to contain the AVAILABLE file as well (it is automatically generated by FLEXPART Prepare).

The FLEXPART Prepare Editor

All the date parameters used for FLEXPART Prepare is given in YYYYMMDD format. Relative dates are allowed: e.g. -1 means yesterday, 0 means today, etc.                   

Prepare Mode

Specifies the data preparation mode. The possible values are: forecast and period.

In forecast mode the selected steps of a given forecast can be used for data generation. If the mode is set to period a period with a start and end date and constant time-step can be defined. In this case FLEXTRA Prepare tries to retrieve analysis fields from MARS whenever it is possible (for dates in the past) and uses forecast fields otherwise (for dates in the future).

Input Source

Specifies the source of the input GRIB data. The possible values are: mars and file.The default value is mars.

If the input source is mars the input GRIB data is retrieved from the MARS archive. When Prepare Mode is forecast the input source can also be set to file. In this case the GRIB file specified in Input File will be used as input data.

Input File

Specifies the full path to the file containing the input GRIB data. Available when Prepare Mode is forecast and  Input Source is file. The input file must contain the following fields:

Field typeParametersForecast steps (defined in Step)
surface fields

["sd","msl","tcc","10u","10v","2t","2d","sp", "z","lsm","lcc","mcc","hcc","skt", "stl1","swvl1"]

surface fluxes["lsp","cp","sshf","ewss","nsss","ssr"]    all + additional steps (see the note below)
model level fields["u","v","t","q","etadot"]all
surface analysis["sdor","cvl","cvh","sr"]  required for the valid date of step 0

The surface fluxes are accumulated fields and for the de-accumulation process they also require the step preceding the first step. We have a special case when the first step is 0 because in this case we need two additional steps but from the previous model run! E.g. for a 0 UTC model run when we use 3 hourly steps  we need the fluxes from step=6 and step=3 of the 18 UTC run on the previous day.


Specifies the run date of the forecast. Available when Prepare Mode is forecast.


Specifies the run time of the forecast in hours. Available when Prepare Mode is forecast.


Specifies the forecast steps in hours. Here a list of values is given. Available when Prepare Mode is forecast.

Period Start Date

Specifies the start date of the period. Available when Prepare Mode is period.

Period Start Time

Specifies the start time of the period. Available when Prepare Mode is period.

Period End Date

Specifies the end date of the period. Available when Prepare Mode is period.

Period End Time

Specifies the end time of the period. Available when Prepare Mode is period.

Period Step

Specifies the time step of the period in hours. The allowed values are as follows:  3 or 6. Available when Prepare Mode is period. The default value is 3.

Grid Interpolation

Specifies if the input GRIB fields need to be interpolated onto a target grid specified by Area and Grid. The possible values are on/off. Available when Input source is file. The default value is: on.


Specifies the area of the output grid in south/west/north/east format. The default value is  -90/-179/90/180.

Please note that to make global domains work with FLEXPART the western border must be set to one gridcell east of 180. E.g. if the east-west grid resolution is 1 degree area should be set to -90/-179/90/180 etc.


Specifies the resolution of the output grid in dx/dy format, where dx is the grid increment in east-west direction, while dy is the grid increment in north-south direction (both in units of degrees). The default value is: 1/1.

Top Level

Only data on and below this model level will be used to generate the FLEXPART input fields. This level can be specified either as a model level or as a pressure value. In the latter case FLEXPART Prepare will use the data retrieved for the first date to determine the topmost model level. The default value of this parameter is 1, which means that all the model levels will be used if Top Level Units is set to model_levels.

Top Level Units

Specifies the units of the value of Top Level. The allowed values are model_levels or hPa. The default value is model_levels. 

Reuse Input

If this parameter is set on FLEXPART Prepare checks the existence of the data files to be generated and if they are already in place no new data is retrieved and processed. If it is off all the fields are always retrieved and processed and the existing data files are overwritten. The same happens to the AVAILABLE file. The existence of a FLEXPART input GRIB file is checked by using the file name and a set of ecCodes keys from the first message in the file. These keys are as follows: date, time, stepRange, gridType, iDirectionIncrement, jDirectionIncrement, latitudeOfFirstGridPoint, latitudeOfLastGridPoint, longitudeOfFirstGridPoint, longitudeOfLastGridPoint.

Output Path

Specifies the output directory (can be a relative or absolute path) where the GRIB files and the AVAILABLE file will be generated. If this directory does not exist Metview will create it. The output GRIB files have the following naming convention: ENyymmddhh.