7.6 Display Particle Positions


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Although the air concentration calculation results are typically represented by a grid cell integral representing a pre-defined layer in the atmosphere, it is possible to display snapshots (an instantaneous view) of the individual particle positions in three-dimensional space using the simulation parameters of the previous example and rerunning the simulation with additional output. If not continuing from the previous section, retrieve cpart_control.txt and cpart_setup.txt.

  1. To illustrate the nature of the particle-trajectory calculation, we can output the individual particle positions in addition to the air concentrations by opening the Advanced / Configuration Setup / Concentration / Input and Output of PARTICLE FILES menu #9 and change the First Output and Repeat Interval from 0 to 3. This will create a special output file called PARDUMP which will contain all the particle information every three hours. This is a more complex file than the ASCII trajectory endpoints file. It is binary and in addition to the particle position, it contains all the information needed to restart the model. Save to exit. Too speed up the calculation, temporarily increase the size of the concentration grid to 0.05 or 0.50, as those results are not needed for the particle display. Then save to close all menus and run the model.

  2. A different program is used to display the contents of the particle endpoints file. Press the Concentration / Display / Particle menu tab to open the particle display menu. Accept the cross-section (X-Sect) defaults, perhaps increasing the zoom if desired, and Execute Display to open the graphic and advance to the last frame (26th at 05Z). In addition to the horizontal plane view, the vertical cross-section is shown in the lower panel. Other display options include plane or vertical projection views as well as map projection optimized for global particle distributions. To speed up the creation and display of the graphics, try plotting every 5th particle.

  3. There is an alternative output for particles that shows the trajectory of the mass-weighted centroid position of all the particles as a function of time. The output is similar to a single particle trajectory but the trajectory path refects both the mean transport and turbulence. This center-of-mass trajectory option can be turned on in the Advanced / Configuration Setup / Concentration / Output CENTER-OF-MASS Trajectory menu #12. For this example enter the output file name cmtraj.txt. Rerun the concentration model and then open the Trajectory Display menu. The format of cmtraj.txt is identical to all other trajectory output files. Enter the endpoints file name, optimize the display with hourly output, height units for display, and minimal zoom. The resulting trajectory is very similar to the trajectory in the previous example.

  4. If you have Python installed on your system you can also show the center-of-mass particle distribution along the trajectory. This option will be discussed in much more detail in later section on alternative display options. For this output, open the Trajectory Display menu again and select the Python tab. Leave all the other options at their default values and the display will show increasingly larger ellipses representing the growth of the standard deviation of the particle distribution along the trajectory.

The particle display results clearly show why a single particle trajectory is insufficient to simulate the pollutant distribution. Particles that mixed to the upper-levels of the boundary layer travel at faster speeds. Winds aloft also tend to be more clockwise than those at the surface. Center-of-mass trajectories may be a suitable display alternative when the pollutant distribution remains continuous.

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