Creating a grid in CMAQ simply involves adding a few lines of text to the GRIDDESC file. Using a combination of the file format documentation in Chapter 4 and existing grid definitions as examples, you define new grids for CMAQ by adding a coordinate and grid description to the GRIDDESC file. Set the GRID_NAME environment variable in the CMAQ run scripts to point to the name of the new grid. The most common situation for creating a new CMAQ grid definition is encountered when you receive meteorology and/or emissions data that have not yet been modeled with CMAQ. If the meteorology data are raw MM5 outputs, by running them through MCIP, you will automatically generate a GRIDDESC file that can be input directly to both CMAQ and SMOKE. If the meteorology data are already in the CMAQ format and you do not have access to the GRIDDESC file generated by MCIP, you can determine the grid definition of the input meteorology (and emissions) by using the netCDF utility ncdump to view the header of one of the gridded binary input files.

All of the grid definition information required by CMAQ is contained in the headers of gridded meteorology and emissions netCDF input files. If you receive emissions and/or meteorology for a CMAQ simulation you can determine the coordinate and grid definitions by viewing the header of any of these files. Use a text editor to add the coordinate and grid definitions to a GRIDDESC file.