8.6 Simultaneous Multiple Grids


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Due to computational time restraints, most of the examples in the tutorial use only one meteorology grid and one concentration grid at a time. However, it is simple to configure the model to use multiple grids for each. In a previous example, when multiple meteorology grids were defined, the particle was transported and dispersed using the finest resolution grid available at the particle's location. When multiple concentration grids are defined, a particle contributes mass to all grids defined at the particle's location.

  1. In the previous section, we determined that a release of 7000 particles was sufficient for the CAPTEX simulation. However if we change the resolution of the concentration grid to a smaller value, more particles will be required. Use the test inputs option from the previous section to determine the particle release rate for a 0.05 deg concentration grid. Note that if the particle number set is sufficient, no message will be generated. So it is necessary to reduce the particle release rate prior to testing the inputs. The result of the test should show that 54,000 particles are required. But if we are only interested in the initial transport phase. Change the run duration from 68 h to 25 h and rerun the test and you should find that only 7,000 particles are required. Also it is best to work with only one grid at a time to determine which inputs limit the calculation.

  2. To insure that we use the same initial configuration, retrieve the previously saved input files captex_control.txt and captex_setup.txt. This configuration defines one meteorology input file and one concentration output file. To speedup the computations, reduce the run duration to 25 h and set the particle release number to 10,000. If time is not a constraint, then run the simulation at least 43 h or for the full duration (68 h) with all particles so that the results can be used in the next excercise.

  3. To create additional concentration grids press the Concentration / Setup Run / Pollutant, Deposition, and Grids tab to open the grid selection menu. Enter the numeric value of 2 under the grids column to define a total of two concentration grids and then select the Grid 2 radio-button to open the concentration grid input menu

  4. The menu is initially populated with the Grid #1 values. To create a finer resolution grid that is only applied near the release location rather than the whole domain, move the grid center slightly west to -82.5, decrease the spatial resolution from 0.25 to 0.05 degrees, decrease the span to 5.0 in both latitude and longitude, and give it a unique file name such as hysplit2FG.bin. This fine grid needs only to be active for the first 12 hours and can be turned off at 26 06. Now save the results to close all the input menus and rerun the model.

  5. When the run has completed open the Concentration / Display / Concentration / Contours menu and select the FG input file. To standardize the image between different display options, select the rings and map center check boxes and change the ring distance to 50 km.

  6. The resulting image after 6 hours for the second output period 2100-0000 can be compared with the 0.25 degree resolution result. At this near source distance, realistic simulations require finer concentration grids.

This example demonstrated the use of multiple air concentration grids. Multiple grids can be used to optimize computational resources because the model will dynamically adjust the integration time step at the particle's location to be consistent with the local grid resolution.

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