where Kˆ H

max is the domain maximum contravariant

horizontal diffusivity and β hdiff = 0.75 in the CCTM. The

contravariant eddy diffusivity is related to the Cartesian

counterpart as Kˆ 11 = Kˆ 22 = mKH . The KH formulation used

in CMAQ is described below.

Horizontal eddy diffusivity

The diffusion process must represent the effects of physical

diffusion on pollutant dispersion. Although our

understanding of horizontal turbulence is rather limited,

appropriate accounting of physically-based horizontal

diffusion is necessary. We can identify certain types of

nonphysical horizontal diffusion, such as the numerical

diffusion resulting from the inconsistency (i.e., errors in

higher-order expansion terms) in the advection scheme and

artificial diffusion resulting from the instantaneous dilution

of emissions and concentrations by the finite volume of the

Eulerian grid cells. Ideally, the driver meteorology model

should provide the eddy diffusivity for use in the CCTM,

and therefore, whenever appropriate, a pass-through

approach is preferred. However, the horizontal diffusion

process is sometimes omitted in meteorology models

because the numerical diffusion associated with the

advection was already large. The eddy diffusivity must be

estimated in the CCTM when it is not passed through from

the meteorology model.



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