CMAQ consists of aset of core programs that are needed to perform a basic air quality model simulation. Figure 5.1 shows these core programs and how they relate to each other. Utility programs such as PROCAN and PDM are excluded from this diagram; these programs are discussed later in this chapter. The lines between the boxes represent one or more files passing between the programs, and show the dependencies among the programs. The blue boxes represent programs that are not part of the CMAQ distribution package but supply data necessary for an air quality simulation. The yellow boxes represent the basic CMAQ preprocessors, MCIP, ICON, BCON, and JPROC. The red box represents the CMAQ chemistry-transport model, CCTM, the Eulerian air quality modeling component of CMAQ. Data flows between the CMAQ programs are represented in Figure 5.1 by arrows. The red arrows illustrate the flow of data from the CMAQ preprocessors to the CCTM. The green arrows show the data feedbacks from the CCTM to create initial and boundary conditions for nested simulations. The black arrow illustrates the connection between the meteorology model and MCIP. Finally, the blue arrow shows that the output from MCIP can be used to drive an emissions model, such as SMOKE.
Along with these programs, CMAQ also contains several utilities and two code libraries, STENEX and PARIO, to implement source specific and diagnostic features in CMAQ and for parallel applications of the model.
The Emissions Model and Meteorology Model (Figure 5.1) are described in detail in Chapters7 and 8. An emissions model is required to convert annual, county-level emissions estimates to gridded, hourly emissions formatted for CMAQ. The SMOKE and CONCEPT models are two programs available for preparing emissions for CMAQ. Meteorology models, such as MM5 and WRF-ARW, generate gridded meteorology for input to both CMAQ and the emissions models.
MCIP is the first program in the CMAQ distribution package that a user should run when setting up a new simulation. It is used to preprocess the data from a meteorology model for CMAQ; the GRIDDESC file output from MCIP is needed to define the modeling grid.
ICON creates a binary netCDF initial conditions file for input to the CCTM. With the option of creating initial conditions from either a text file of vertical concentration profiles or from an existing CCTM output file, ICON outputs initial conditions data that are configured for a specific modeling grid and chemical parameterization.
BCON creates a binary netCDF lateral boundary conditions file for input to the CCTM. With the option of creating initial conditions from either a text file of vertical concentration profiles or from an existing CCTM output file, BCON outputs boundary conditions data that are configured for a specific modeling grid and chemical parameterization. If derived from an existing CCTM or larger-scale (e.g., global-scale) output file, BCON produces dynamic boundary conditions that vary in time and space. When derived from vertical concentration profiles, BCON produces static boundary conditions for input to the CCTM.
JPROC converts physical information about photoreactive molecules into clear-sky photolysis rate look up tables for input to the CCTM.
The CCTM is run last in the sequence of programs. All of the other CMAQ programs, emissions, and meteorology models are used to prepare the inputs to the CCTM. By using data that are synchronized for a particular modeling time period, model grid, vertical layer configuration, and chemical parameterization, the CCTM produces hourly estimates of pollutant concentrations, wet and dry deposition rates, and visibility metrics.
In addition to the core programs shown in Figure 5.1, CMAQ also includes utilities and libraries for utilizing some of the special features in CMAQ and setting up the CCTM for multi-processor computing. CMAQ includes the PROCAN utility, for preparing process analysis simulations, CHEMMECH, the CMAQ chemical mechanism compiler, and the Plume Dynamics Model (PDM), for subgrid-scale representation of selected point sources (also known as plume-in-grid or “PinG”). The Stencil Exchange library (STENEX) is a module that the CCTM uses to control the communication between processors in a multiprocessor computing environment. Similarly, the CCTM uses the PARIO library to synchronize the reading and writing of information across and between multiple processors.
In the remaining sections of this chapter, we provide detailed descriptions of these programs, utilities and libraries, in alphabetical order. Information about the third party libraries used by CMAQ, such as the IO API, netCDF, and MPICH is available in Chapter 2.