numerical integration across the science processes and some

input/output sequences. The driver checks data consistency

for a run scenario (input files, run time, and grid/coordinate

information) and calls for the initialization routines (INIT).

INIT initializes pollutant initial concentrations for gaseous

species in molar mixing ratio units (ppm) and aerosol

species in density units (μg m-3), the same as the output units

of CMAQ. The pair of couple/decouple routines (COUPLE)

are instituted to convert the concentration in density units

(ϕ i ) to mixing ratio for the vertical diffusion and chemistry.

Because the molar mixing ratio used in chemistry is just a

constant multiplication of the ratio of molecular weight of

gas species to air as in m i = q i (Mair / Mi ) , no further

conversion is necessary. These unit conversion calls limit

the interchange between groups of process modules using

different concentration units. The driver structure of the

current CCTM is given in Figure 2. A synchronization time


step (Δ tsync ) is used to ensure the global stability of the

numerical integration at the advection time step, which is

based on a Courant number limit. Nesting requires finer

synchronization time steps for the fine grid domain. In the

CCTM, the process synchronization time steps are

represented as integer seconds due to a limitation of Models-

3 I/O API that can only handle integer seconds for I/O data.

All the needed data are appropriately interpolated based on

the synchronization time step. For maintaining numerical

stability and for other reasons, an individual process module

may have its own internal time steps. In general, each

science process module uses the synchronization time step

as the input time step of required environmental data. The

global output time steps can be set differently from the

synchronization time step. Usually, the output time step is

set to be one hour, but sub-hourly output down to the

synchronization time step is possible.



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