Multiple Particle Concentration Calculation

To further expand upon the single-particle calculation discussed in the previous section, we will increase the particle number release rate until the simulation looks more realistic. If you have not completed the previous section, please go back and review that section before proceeding.

1. We ended the last section viewing the hourly concentration contribution of one particle as it was transported and dispersed along its path. Lets start by reducing the number of output frames to every three hours rather than hourly. Open the Concentration / Setup Run / Grids menu and change the last line from 01 01 00 to 01 03 00. Save to close all the setup menus. Then open the Advanced / Configuration Setup / Concentration / Release Number limits (Menu #4) and change the Particles Released per Cycle from 1 to 100. Save to close and exit all menus.


2. Now Run Model, open the contour display menu and when complete, view each of the four frames (at 20Z, 23Z, 02Z, and 05Z). Note at the inital time, all the particles are close together and the pattern is continuous, but as the particles spread out due to turbulence and vertical wind shear, the results quickly degrade and by the last frame it almost appears that each of the 100 particles are individually visible.


3. Run this exercise again for 1000 particles and 10000 particles: only the 10,000 particle simulation shows a continuous concentration pattern. Remember in the original example calculation we only released 2500 particles over one hour, a considerably smaller particle release rate, but the concentration pattern was very smooth. This is because we computed a 12 hour average concentration, rather than instantaneous concentrations. The model needs to sample a sufficient number of particles over the averaging interval and concentration grid size to insure robust results. The appropriate number can be determined by trial-and-error as the rate when the air concentration results are no longer sensitive to the particle release rate.


4. 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 and close all menus, and then run the model.


5. 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 defaults (perhaps increasing the zoom if desired) and Execute Display to open the graphic. In addition to the horizontal plane view, the vertical cross-section is shown in the lower panel, which shows very clearly 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.


6. Previously we found that the simulation results for a given particle release rate were sensitive to averaging time. A similar sensitivity exists with regard to the concentration grid size. To run a simulation with a finer resolution grid, open the Concentration / Setup Run / Grids menu to change the spacing of the grid from 0.05 to 0.005 in both latitude and longitude. One degree spacing is approximately 100 km, so 0.005 degree spacing is about half a kilometer. Note that a grid center specification of zero means that the concentration is centered over the release point. Save to close the menu and then run the model. Note this simulation will take a lot more time than the previous one with the 0.05 deg grid.


7. When the run completes, open the display contour menu. With all those nearby particles, the visual look of the plot can be improved by removing the closed contour about each fill color by changing the contour drawing options to none. The final graphic shows much more detail than the previous simulation, but again individual particles are visible.

The simulations with various particle number release rates suggested that more particles are required for more detailed simulations, but at a cost of increased computational time. This may become especially significant for continuous releases. There are other simulation options to reduce the particle number requirements which will be explored in the next section. For future reference, the control (cpart_control.txt) and namelist (cpart_setup.txt) files should be saved.