Science process classes in the CCTM are identified in Eq.

(17). The only data dependencies among the CCTM science

modules are the trace species concentration fields (i.e.,

concentrations are the objects of science process operations)

and the model integration time step. To facilitate modularity

and to minimize data dependency in the CCTM, we store

concentrations in global memory while the environmental

input data are obtained from random-access files and

interpolated to the appropriate computational

(synchronization) time step. This realizes the characteristic

“thin-interface” structure of the CCTM:

Concentration and timing data are the only

arguments of module driver routines.

Environmental data are provided with a standard

I/O interface.

Model structure (i.e., grid and chemical

mechanism) data are passed through the shared

include files.

Standard physical constants are given in fixed

shared include files.

5.3 Grid and Nesting

CMAQ uses a regular (structured and uniform) horizontal

grid system for simplicity. To ensure accuracy of

simulations for desired areas at fine scales, a static grid

nesting technique is used. It involves the sequential

placement of multiple finer-scale meshes in desired regions

of the domain so as to provide increased spatial resolution

locally. The spatial resolution of the coarse grid is usually

an integer multiple of that of the fine grid. First, the coarse

grid solution is marched forward one time step. This

solution provides boundary and initial conditions (both


concentration and flux) to the fine grid solution that is

advanced at a smaller time step (usually an integer fraction

of the time step of coarse grid). It is customary to set the

time step ratio equal to the grid size ratio to retain the

numerical accuracy at the same order of approximation.

When the fine grid solution catches up with the coarse grid

solution, the former may or may not be used to update the

latter (i.e., two-way vs. one-way nesting). There are a few

shortcomings of using grid nesting. One is the tendency for

propagating dispersive waves to discontinuously change

their speeds upon passing from a coarse mesh to the next

finer mesh and to reflect off the boundaries of each nest due

to the mismatch across the mesh boundaries. CMAQ

includes a Lagrangian plume-in-grid (PinG) method to

resolve the spatial scale of plumes emanating from major

elevated point source emitters.



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