Beyond defining the appropriate domains to use for processing digitized land cover data,

additional considerations are needed regarding the computational methods for processing of the

data. Due to the fact that the width of a sector increases with distance from the measurement

site, the land cover further from the site would receive a higher effective weight than land cover

closest to the site if a direct area-weighted averaging approach were used to calculate an

effective surface roughness. An inverse-distance weighting is recommended for determining

surface roughness from digitized land cover data in order to adjust for this factor, since the

length of an arc (across a sector) is proportional to the distance from the center. In addition, a

geometric mean is recommended for calculating the effective surface roughness due to the fact

that the AERMOD formulations are dependent on the ln(zo). Note that the arithmetic average of

the ln(zo) is mathematically equivalent to the geometric mean of zo. Since the Bowen ratio

represents the ratio between sensible heat flux and latent heat flux, the use of a geometric mean

is also recommended for calculating effective values of Bowen ratio. Geometric means are more

appropriate for calculating “average” values of ratios; for example, the “average” for Bowen

ratios of 0.5 and 2.0 should be 1.0, which is accomplished with the use of a geometric mean. A

simple arithmetic average is recommended for calculating effective values of albedo.

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