The RIVM trajectory model, described in this report, enables calculation of a backward or forward trajectory. These trajectories are used to "follow" previous released air pollution in a backward mode or to "find" the origin of air pollution in a forward mode.
The trajectories are used in the smog forecasting and in the TREND-model. Presently 6-hourly ECMWF wind fields at 1000 and 850 hPa, with 3 degree x 3 degree latitude-longitude resolution are used. Wind fields with a different resolution in latitude-longitude can also be used after simple adjustments. An iterative method, described in Rosier et al. 1983, is applied to calculate the trajectories. Within limits, the user is free to choose the time step (1-, 2- or 6-hour), transport height, length, starting or arrival date and starting or arrival position of the trajectory.
The differences between the trajectories calculated with time steps of 1-, 2- and 6-hour were small. For the 96-hour trajectories at 1000 and 850 hPa the deviations were generally within 1 degree latitude and longitude i.e. 100-200 km. The trajectory calculated with the six-hour time step could be used without a great loss in accuracy compared to the calculations with the one-hour time step.
typical error in the trajectory path was 500 km, which is about 30% of a typical travel distance. However, close to quickly changing weather systems, such as cyclones, the error can be as large as the travel distance and makes the calculations unreliable. The error in the forecasted trajectory was found to be larger than the above error estimation due to larger uncertainties in the forecasted compared to the analyzed wind fields. A manual on how to run the model is also found in the report.