The interaction of trajectories with the underlying terrain can be examined in more detail by forcing the trajectory to intersect the ground. The easiest way to approach this is by computing an isobaric trajectory starting just upwind of some large terrain feature. Although this is a rather artificial configuration, it is not unrealistic that a trajectory, using any of the vertical motion options discussed in the previous section, might intersect the ground under certain meteorological conditions.
- Start by opening the trajectory setup menu and configuring a forward isobaric calculation to start on 83 09 01 00 for a duration of 72 hours from 45N 125W 500 m-AGL, a location just off the west coast of the U.S. Name the output file fdump and find the global reanalysis file RP198309.gbl.
- To see the terrain below the trajectory, open the Advanced / Configuration Setup / Trajectory tab and open menu #6, set the Terrain Height checkbox and Save to close each menu and then go ahead and run the model.
- Now open the Trajectory / Display and select the Meters-AGL radio-button. Press Execute Display and you will see the trajectory intersect the mountains about 24 hours downwind. Subsequently, the trajectory continues to rise with the terrain and then proceed at its new pressure level.
- For reference, save the trajectory input files as traj_isob_control.txt and traj_isob_setup.txt.
- Open the Trajectory / Utilities / Simple Listing menu to show the contents of the fdump endpoints file and copy the last endpoint position 49.573 -93.315 967.8 to the Trajectory Setup menu; your endpoint position may differ slightly. You can also use Notepad to view the endpoints file. Note the time, 83 09 04 00 and enter it into the starting time field. Set the backward radio-button, rename the output file to bdump, save, and then run the trajectory model.
- Display the backward trajectory graphic where you can see the back trajectory intersect the terrain and continue at that level rather than descending. This can be more clearly seen by combining the trajectory endpoints files fdump+bdump in the display menu to show both trajectories on the same image. In this particular case the backward trajectory is not reversible because there is no way for the backward model calculation to know which point along the forward calculation first intersected the terrain.
Although under most circumstances trajectories are fully reversible, the results shown here suggest that when trajectories intersect the ground information is lost. This may be a more common problem than is commonly recognized, such as when computing backward trajectories associated with ground-level sampling. This may be of particular concern in areas where typical upwind sectors are also associated with elevated terrain (the U.S. East coast).