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 an ensemble forecast on pressure levels:
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 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" ;
}
- In addition to the air parcel time, Met.3D requires the two time attributes
trajectory_starttimeandforecast_inittimethat 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. - The
start_lon,start_latandstart_isobaricvariables are optional and define the grid from which the trajectory bundle was started (used for WCB detection). - Similarly, the
time_intervalanddelta_pressure_per_time_intervalvariables are optional and define pre-computed values for WCB detection. Contact us if you’re interested.