Vertical eddy diffusivity

Again, the driver meteorology model ideally should provide

the vertical eddy diffusivity for use in a CTM and therefore,

whenever appropriate, a pass-through approach is preferred.

28

To provide a default ability to link to generic meteorological

data, an eddy diffusivity parameterization is provided with

the CCTM. The contravariant vertical eddy diffusivity is

related to the components of the diffusivity tensor in the

Cartesian coordinates as:

where Kzz is the eddy diffusivity in the geometric height

coordinate. One of the problems with first-order closure is

finding a rational basis for the eddy diffusivity

parameterization. Only routinely measured or modelresolvable

meteorological variables are used to explicitly

specify a K-profile. Except for the cases with complex

topography the effects of the horizontal gradient terms in

Eq. (28) are small, and therefore, neglected. The set of eddy

diffusivity formulations used in CMAQ is an extension of

those used in RADM, which is a vertically integrated

version of Brost and Wyngaard [62]. Refer to Chang et al.

[10], Hass et al. [63], and Byun and Dennis [64] for the

details.

Dry deposition

In Eulerian AQMs, the deposition process affects the

concentrations in the lowest layer as a boundary flux

condition. Considering the deposition process as the

diffusion flux at the bottom of the model, we can relate the

boundary condition in the generalized coordinate system to

that of the Cartesian coordinate system as

 

 

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