Description

Since the CF-conventions only provide limited support for trajectory data (in particular no ensemble dimension), we’re currently using a custom NetCDF layout that follows the CF-versions. There are a few limitations:

  • There is only one time dimension per NetCDF file, which corresponds to the time of the particle positions along the trajectories. If you have a series of trajectory bundles started at different times (i.e., a bundle of trajectories for each forecast time step as used for WCB detection), you need one NetCDF file per trajectory bundle.

The following example contains a NetCDF header of a file containing trajectories computed from an ensemble forecast:

netcdf sometrajectorydata {
dimensions:
    time = 17 ;
    trajectory = 215332 ;
    ensemble = 51 ;
    start_lon = 101 ;
    start_lat = 41 ;
    start_isobaric = 52 ;
    time_interval = 8 ;
variables:
    double time(time) ;
            time:standard_name = "time" ;
            time:long_name = "time" ;
            time:units = "hours since 2012-10-19 06:00:00" ;
            time:trajectory_starttime = "2012-10-19 06:00:00" ;
            time:forecast_inittime = "2012-10-17 00:00:00" ;

    float lon(ensemble, trajectory, time) ;
            lon:standard_name = "longitude" ;
            lon:long_name = "longitude" ;
            lon:units = "degrees_east" ;
    float lat(ensemble, trajectory, time) ;
            lat:standard_name = "latitude" ;
            lat:long_name = "latitude" ;
            lat:units = "degrees_north" ;
    float pressure(ensemble, trajectory, time) ;
            pressure:standard_name = "air_pressure" ;
            pressure:long_name = "pressure" ;
            pressure:units = "hPa" ;
            pressure:positive = "down" ;
            pressure:axis = "Z" ;

    float temperature(ensemble, trajectory, time) ;
            temperature:standard_name = "temperature" ;
            temperature:long_name = "temperature" ;
            temperature:auxiliary_data = "yes" ;
            temperature:units = "K" ;

    float start_lon(start_lon) ;
            start_lon:long_name = "longitude of start grid" ;
            start_lon:units = "degrees_east" ;
    float start_lat(start_lat) ;
            start_lat:long_name = "latitude of start grid" ;
            start_lat:units = "degrees_north" ;
    float start_isobaric(start_isobaric) ;
            start_isobaric:long_name = "Isobaric surface of start grid" ;
            start_isobaric:units = "hPa" ;
            start_isobaric:positive = "down" ;
            start_isobaric:axistype = "pressure levels" ;

    float time_interval(time_interval) ;
            time_interval:long_name = "time interval" ;
            time_interval:units = "hours" ;
    float delta_pressure_per_time_interval(ensemble, trajectory, time_interval) ;
            delta_pressure_per_time_interval:long_name = "max. delta pressure of trajectory in time interval around start time" ;
            delta_pressure_per_time_interval:units = "hPa" ;
}

Required and optional data are:

  • Required: In addition to the air parcel time, Met.3D requires the two attributes trajectory_starttime and forecast_inittime for the time variable that define the time at which the trajectory was started, as well as the forecast initialisation/base time of the forecast data on which the trajectory was computed.
  • Required: The vertices/particle positions of the trajectories need to specified in the variables lon, lat, and pressure.
  • Optional: Additional variables traced along the trajectories (e.g., temperature in the example above) need to be identifyable by the attribute auxiliary_data = "yes".
    • You can put an arbitrary number of auxiliary trace variables into your file.
  • Optional: The start_lon, start_lat and start_isobaric variables are optional and define the grid from which the trajectory bundle was started (used for WCB detection).
  • Optional: Similarly, the time_interval and delta_pressure_per_time_interval variables are optional and define pre-computed values for WCB detection. Contact us if you’re interested.

Code

Reading NetCDF files with trajectory data is implemented in MTrajectoryReader:

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