In this section we will explore the possible causes of trajectory error. In the first calculation a forward trajectory will be compared to a backward trajectory started at the termination point of the forward trajectory.
- Start by retrieving the information into the trajectory setup menu from the previously save control file traj_base_control.txt, but change the starting height from 10 m to 750 m, also rename the output file from tdump to tdump_fwrd, then save the changes and run the model. Select the Delete file then Run if the existing SETUP.CFG file prompt appears. If your trajectory matches the trajectory shown, save the control file as traj_fwrd_control.txt.
- Now we need to setup a new trajectory calculation to start from the last point of the previous trajectory. To do this you need to open the output file tdump_fwrd and note the information on the last record in the file that indicates the time 83 9 28 13 and location 37.467 -75.259 325.9 of the last point. Small differences in your end point position compared to the one shown here are expected and are caused by changes in compilers and computational platforms (AMD vs INTEL chip).
- Then open the trajectory setup menu and enter that information as the start time and location. Two additional changes are required: select the back radio-button, which changes the run duration to a negative 68 hours. Also change the output file from tdump_fwrd to tdump_back and then save, run the model, and display the result.
- If your result matches the trajectory trajectory shown, save the control file as traj_back_control.txt. Note the title label differences between the two calculations, the location of the source points, and the direction of time along the abscissa of the vertical trajectory projection.
- The two trajectories look very similar. To compare them directly, we can use the trajectory display program to superimpose the forward and backward trajectories. Open the
trajectory display menu and in the first entry line of input endpoints enter the two file names using a plus symbol (without spaces) between the two file names: tdump_back+tdump_fwrd. Select Meters-agl as the vertical coordinate in the display menu to see the results in height rather than pressure units. The superimposed trajectories are so close that they almost appear to be identical.
We can also examine the end of the backward trajectory output file to determine exact ending position 39.891 -84.185 736.7 and comparing it with the original starting location 39.90 -84.22 750.0 suggesting that the computational integration error is about 1 km horizontally and about 13 m vertically after a 136 hour (2 x 68) integration time.