The extent of the horizontal grid used in CMAQ is limited by the size of the domain of the input meteorology. A combination of the CMAQ meteorology processor, MCIP, and the I/O API are used to prepare the input meteorology for the CCTM and contain utilities for windowing subsets of the data, performing grid transformations, and aggregating/interpolating between grid cell resolutions. Choosing the appropriate horizontal grid scale and extent for your simulation is largely dependent on the issues that you want to address through the modeling; practical consideration should also be paid to the relationship between grid size, output file size, and execution times.
In early versions of CMAQ (pre version 4.3), Fortan include files defined the horizontal configuration. The COORD.EXT file contained data statements for defining the horizontal coordinate and grid to be used for CMAQ. CMAQ used these include files at compilation to create executables that were hard-wired to a specific grid configuration. Starting with CMAQ version 4.3, CMAQ developers transitioned to a dynamic horizontal grid configuration that could be defined at execution, forgoing the need to recompile the program each time a user wanted to simulate a new grid. Horizontal grids are now configured in CMAQ through the GRIDDESC file (Section 4.1.1). CMAQ horizontal grids are selected in the “horizontal grid definition” section of the CMAQ run scripts by setting the GRIDDESC and GRID_NAME environment variables to point to an existing grid definition file and to one of the grids defined in the file, respectively. Horizontal grids can be selected from either predefined grids contained in the distributed GRIDDESC file or by adding/modifying horizontal grids through the GRIDDESC file.
CMAQ is distributed with a GRIDDESC file that contains definitions for two different coordinate systems and several different grids on these coordinate systems. While the tutorial dataset packaged with CMAQ only uses two of these grids (M_32_99TUT02 and M_08_99TUT02), any of these predefined grids are appropriate for setting up CMAQ simulations. For the CCTM, which requires gridded meteorology and emissions as input, the only viable grid choice is one that coincides with available input datasets. For the other meteorology-independent processors, any of the grid definitions listed in the distributed GRIDDESC file are options. To select one of the predefined horizontal grids, set the GRID_NAME environment variable in the applicable run script to the name of the grid that you would like to model.