Case Study Base Configuration

Now that we have successfully configured an air concentration calculation and compared the results with the ground-level sampling data from CAPTEX release #2, we will experiment with different dispersion parameters and turbulence settings to determine their effect upon the calculation results. You should have saved the previous CONTROL and SETUP.CFG settings. Retrieve captex_control.txt into the Setup Run menu and captex_setup.txt into the Concentration Configuration menu. This will be the starting point for all the case study customizations.

1. To run through many model combinations as quickly as possible and limit the complexity of the analysis, we will only compare the model predictions to the first complete aircraft sampling pass about 200 km downwind from the tracer release point. See the CAPTEX report for a complete description of all the data. This first pass occurred around 0300 UTC on September 26th. Open the Concentration / Setup Run menu and change the run duration from 68 hours to 13 hours, enough time to take the calculation through 0600 on the 26th.


2. To reduce the number of graphical output frames, open the Concentration Grid #1 menu and change the sampling start time from 83 09 25 18 00 to 83 09 26 03 00. This insures that the binary concentration output file will only contain one 3-hour average air concentration period, covering the last three hours of the simulation. Press Save to close all menus and then press the Run Model tab.


3. When the run has completed, press the Display / Concentration / Contours menu tab to open the graphics menu. Make the same changes as in previous examples, that is the concentration multiplier should be 1.0E+12 and the output units will be pg. Three additional changes are required.
   
   
   • First, force the map center further to the northeast, rather than over the tracer release point, by selecting the map center Set checkbox and entering a new position of 41.0 -82.5. In this way the map is centered over the plume.
   
   
   • Second, because we have a sense of the range of potential concentrations, we will set the contours by checking the UserSet radiobutton and entering the following seven contours using a plus symbol as a delimiter: 50000+20000+10000+5000+2000+1000+500. The field window is too small to show all values at the same time, but you can use the cursor to view the whole string.
   
   
   • Third, increase the zoom slider bar to 80.
   
   
   • Then Execute Display to open the graphic


4. In the trajectory section, we started a backward trajectory from the maximum concentration point defined in the 914 m MSL aircraft sampling flight. This file is in the DATEM format. This aircraft pass managed to cover the entire horizontal extent of the plume. The concentration plotting program we have been using, concplot, has a command line option (-q) to plot the values from simple data files (ID LAT LON VALUE), but the data points must be reformatted. The measured concentrations from this flight have been copied to data_case.txt which you should either create manually or copy it from the tutorial directory to the HYSPLIT /working directory.


5. Unfortunately, there is no GUI option to set the data plot command line option, but when Execute Display is pressed, the GUI script automatically creates a concplot batch file (or .sh in unix) with all the command line options set in the GUI. Open concplot.bat in notepad and replace one of the non-set switches (-:) with the string -qdata_case.txt and save to a unique name such as conc_case_plot.bat. Now from Windows Explorer or from the command line, run the concplot plot batch file and then open the output file, concplot.ps, to see the model predictions and measurements.


6. The large red square in the center of the plot shows the grid cell (to scale) with the maximum concentration. The 25 km resolution of the concentration grid may be appropriate for the full 68 hour simulation, but it is too coarse to examine the model prediction just 200 km downwind. Open the Setup Run / Grids menu and change the resolution from 0.25 to 0.05 degrees. Save to exit, then Run Model, but when it completes, instead of opening the Display / Concentration GUI, just run the previously saved conc_case_plot.bat file and open concplot.ps to see the new model prediction.


7. To examine the effect of using the puff parameterization instead of the default 3D particle, open the Advanced / Configuration Setup / Concentration menu #3 to switch from 3D particle to Top-Hat-Horizontal Particle-Vertical mode and also menu #4 to change the particle release number from 50000 to 5000. Fewer puffs than particles are required for the same simulation. Save to exit, run the model and the conc_case_plot.bat file and open concplot.ps to see the top-hat puff prediction.


8. Follow the same process to configure the Gaussian-horizontal particle-vertical puff simulation. Now the graphic will show a slightly wider tracer plume because of the larger scan radius used in the Gaussian puff calculation compared with the previous top-hat puff computation. The particle/puff positions are the same in both calculations. Now save the control file settings to conc_case_control.txt and the namelist file parameters to conc_case_setup.txt.

We've found that for this particular case, the model's plume simulation 200 km downwind matched the position and width of the aircraft measured plume. Although there were differences in the model layer depths as well as averaging times, the magnitude of the concentration predictions were very similar to the observations. Matching the sampling times and layer depths more closely with the measurements is left as an exercise for the reader. Note the batch file short-cut introduced here can be used to add the surface measurement data to the 68 hour CAPTEX animation. This is another exercise left to the reader.