4.4 Mixed Layer Trajectories




With a mixed layer depth of 1716 m AGL estimated in the previous section, it would be reasonable to bracket the transport layer with trajectories starting at 10, 750, and 1500 m AGL.

  1. Start with the previous section's basic trajectory calculation and then open the Trajectory / Setup menu and enter 3 for the Number of Starting Locations prior to pressing the Setup Starting Locations button. Then replace the 10 meter height in the 2nd and 3rd lines with 750 and 1500. If the 2nd and 3rd lines do not have the same starting latitude and longitude, these also need to be changed to match the 1st entry line. Select OK and then Save to close the Trajectory Setup menu.

  2. As before, in the previous trajectory example, run the model, then open the trajectory display menu which will create a graphic showing the three trajectories, which provide a much better representation of surface concentration distribution. However, the more southern measured concentrations still cannot be explained through these trajectories. Save the control file information using the name traj_lev3_control.txt

  3. Before proceeding further with the case analysis, there is another way to let the model automatically select a height within the boundary layer. Open the Advanced / Configuration Setup / Trajectory menu. This menu creates a namelist file called SETUP.CFG which contains a list of internal HYSPLIT variable names and their default values. This menu permits changes to those defaults.

  4. Select submenu (2) to define MSL/AGL units and then select the fraction of the mixed layer radio-button. This option changes the interpretation of the starting heights defined in the trajectory setup menu from heights above ground level (the default), heights above mean-sea-level, or in this case to a fraction of the boundary layer height. In this situation, the absolute height is computed internally and may change day-to-day. After saving each of the open configuration menus, save the configuration using the name traj_mixd_setup.txt.

  5. Go back to the trajectory setup menu, replace the 3 locations with 1 and then open the starting locations menu and replace the height field with 0.5. This will cause the trajectory to start at a height of one half the mixed layer depth.

  6. After saving, proceed to the Trajectory / Run Model menu, where this time a new menu will appear to confirm that the model should be run using the SETUP.CFG namelist file. Select the Run using SETUP file button to continue with the calculation. The other button options exist in case the SETUP file was left over from a previous simulation and may not be needed. After the run completes, open the trajectory display menu where this 0.5 simulation result is very similar to the previous 750 m trajectory.

Most of the samplers that detected tracer were to the right of the low-level trajectory. Because wind directions generally veer with height, this result is consistent with the idea that the bulk of the tracer was transported at higher levels above ground but still within the boundary layer.