8.6 Simultaneous Multiple Grids

Due to computational time restraints, all 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. When multiple meteorology grids are defined a particle will be transported and dispersed using the finest resolution grid available at the particle's location and time. When multiple concentration grids are defined, a particle contributes mass to all grids defined at the particle's location.

1. 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, it is suggested that the particle number be reduced to 10,000 and the model run duration to 25h.


2. To create additional concentration grids press the 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


3. 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.


4. When the run has completed open the 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 and increase the zoom to 100%.


5. 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.


6. To add another meteorology data file into the computation, open the Concentration / Setup menu and press the Add Meteorology Files button to open a browse menu. Select the 9 km resolution WRF output file captex2_wrf09.bin. To complete this example more quickly, reduce the simulation time to 13 hours. Then save and rerun the simulation.


7. In this case, near the source, where the WRF data are defined the model will use the finer resolution WRF data and compared with the original NARR calculation, the results using WRF show much more horizontal dispersion. For particles that pass outside of the WRF domain, the model will use the NARR meteorological data.

This example demonstrated the use of multiple meteorology and 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.