The CCTM is a three-dimensional Eulerian air quality model. With an Eulerian perspective, models simulate the chemistry, transport, and other processes that affect pollutant concentrations within a specific geographical location. In this way, air parcels are transported into and out of the modeled location. In contrast to this, Lagrangian models, such as the Industrial Source Complex (ISC) dispersion model, follow the path of individual air parcels as they are transported to different geographical locations. With an Eulerian model structure, the extent of the area of interest (or domain) and the characteristics of the three-dimensional computational grid structure that represent this domain must be specified. The domain is also sometimes referred to as the grid or grid domain. The domain is divided into individual three-dimensional grid cells. A horizontal grid specification sets the x and y dimensions of each grid cell for the entire domain. All grid cells in a specified domain will have the same horizontal resolution. The vertical resolution of each grid cell depends on the vertical layer specification and the selected coordinate system. Therefore, the physical vertical extent of individual grid cells may vary over space and time.
This chapter describes how to define new horizontal grids, vertical layers, and chemical mechanisms in CMAQ. These specifications apply to multiple programs in the CMAQ model including ICON, BCON, JPROC, and CCTM. When configuring new simulations users must define the location, extent, and structure of the horizontal and vertical grids and the chemical mechanism for representing pollutant chemical transformations. CMAQ contains several default options for these parameters that can be used as templates for setting up new configurations. Before deciding to create new definitions for these, see if the existing options are sufficient for your model simulation. If a predefined choice is not appropriate, then follow the steps described in this section to create a new definition.
Once this information has been established for the horizontal grids, vertical layers, and chemical mechanisms, go to Chapter 8 to learn how to develop new model executables for running a simulation.
The choice of coordinate system, or map projection, for CMAQ is governed by the I/O API. While the grid resolution, horizontal, and vertical extent are determined by the meteorology configuration that is input to the model, the I/O API can be used to perform grid transformations to generate meteorology inputs to CMAQ on a number of different coordinate systems. While MM5 supports the Lambert Conformal, Polar Stereographic, and Mercator coordinate systems and thus directly supports these systems for CMAQ, the I/O API can expand these choices using its grid-to-grid transformation routines. The I/O API ultimately determines which coordinate systems CMAQ supports. The I/O API supports the following coordinate systems and can be used to perform grid-to-grid transformations between them:
While MM5 only supports a subset of these coordinate systems, the I/O API can be used to transform the MM5 meteorology into any of the systems that it supports. Therefore the limited number of projections output by MM5 is expanded through the use of the I/O API. Generally the same meteorology used to drive CMAQ is used to generate emissions for the model. The input meteorology, emissions, and CMAQ must all be configured for the same coordinate system.