wind speeds is even more complex in real world applications given the typically patchy nature of

the heterogeneity of surface roughness elements.

The recommended upwind distance for surface roughness should take into account the fact that

surface roughness effects in AERMOD are more important for stable atmospheric conditions

than for neutral/unstable conditions, and that meteorological monitoring sites are typically

characterized by open (low roughness) exposures in order to accommodate recommended siting

criteria (EPA, 2000). For typical measurement programs, including NWS stations, the reference

wind measurements will be taken for an anemometer height of approximately 10 meters above

ground. An upwind distance based on the recommended siting criterion of at least 10 times the

height of nearby obstacles (EPA, 2000), which would correspond to a distance of about 100m for

typical obstacles such as trees and 2-3 story buildings, is considered inadequate for this purpose.

However, the previous recommendation to use an upwind distance of 3 kilometers for surface

roughness is considered too large because the boundary layer up to typical measurement heights

of 10m will generally respond to changes in roughness length over much shorter distances.

Including land cover information across an upwind distance that is too large could misrepresent

the amount of mechanical turbulence present in the approach flow and bias model results,

especially for low-level releases.


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