5.4 Multiple Trajectories in Time

The original trajectory calculation started at the beginning of the tracer release, but conditions can change with time and space. We can investigate both of these options to see if it improves the simulation with respect to the ground-level concentration pattern.

1. Start by configuring the simulation for the mid-boundary layer (600 m AGL) trajectory by retrieving traj_fwrd_control.txt into the trajectory / setup run menu. The menu buttons will not reflect the changes in the options after a retrieve operation. Save and reopen the menu to see all the changes.


2. Then go to the Advanced / Configuration Setup / Trajectory tab, press the Reset button, and then open menu #3 multiple trajectories in time. Set the restart interval to 1 hour, which will cause the model to start a new trajectory every hour from the starting point and time. The second entry, the trajectory Restart duration, should be set to 4 hours, which will cause the model to only start new trajectories for four hours regardless of the duration of the run.


3. Save all the changes and then run the model, open the trajectory display menu to view the result. Use pressure as the vertical coordinate to match the image. The four trajectories, one hour apart, span the tracer release period from 17 to 20 UTC. Each successive trajectory is further south and west, indicating that changes to the flow field during this short 4 hour period could account for at least some of the tracer observed over Pennsylvania.


4. Because we already know that a single trajectory level and time is insufficient to capture the plume structure, we can combine the multiple starting heights with starting trajectories in-time. Start with the same trajectory setup used for the three mixed layer trajectories (or retrieve traj_lev3_control.txt) and then save the changes.


5. Now open the configuration menu #3 again and change the interval to 6, the duration to 24, and the number of levels to 3, consistent with the three starting heights. What happens in this variation is not intuitive. The input parameters have the same meaning as before, but instead of starting 3 new trajectories every 6 hours at the initial starting location, new trajectories are started at the end of each of all the previous trajectories. The calculation result illustrates how changing wind direction with height within the mixed layer can account for much of the observed tracer dispersion. Slight differences between your results and the illustration can be expected.

The primary result shown in this section was that the surface tracer concentration distribution could be explained in large part by the changing wind directions with height and time within the mixed layer.