5.5 Multiple Trajectories in Space

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. Start with the mid-boundary layer (750 m AGL) trajectory configuration by retrieving traj_fwrd_control.txt.

1. Another way of analyzing the flow field important for the tracer transport is to compute trajectories over the whole domain. Open the trajectory setup menu and enter three starting locations, all at 750 m AGL: 38N 84W, 44N 74W, and 39N 83W. The first two points represent the lower left and upper right corner of the domain over which trajectories will be started. The difference between the first point and the third point represents the spacing of a virtual grid from which trajectories will be started. Also reduce the total run time to 6 hours. Save the changes and run the trajectory. If the message pops up that the SETUP.CFG namelist file found!, select Delete file then Run. The namelist file was leftover from the previous calculations and is not needed for this illustration.


2. Only three trajectories are shown when the standard model run menu tab is selected. To create a matrix grid of trajectories go to the Trajectory / Special Runs / Matrix tab and you will get a prompt asking if the input was properly setup with three sources defining the grid domain. When the model run is finished, open the trajectory display, and you should see an illustration of the flow field over the domain.


3. One element to remember from this section is that when the setup menu was configured for three locations the resulting control file only showed those three points. However, when the model was run through the special runs tab, it invoked a pre-processor step (program=latlon), that converted the three starting locations into a control file with 77 starting locations. Complex control files can also be created manually with any text editor.

The trajectory matrix configuration shown in this section is a quick way to visualize the flow regime by creating a large number of simultaneous trajectories.