The recommended upwind distance for processing land cover data to determine the effective

surface roughness for input to AERMET is 1 kilometer relative to the meteorological tower

location. This recommended distance is considered a reasonable balance of the complex factors

cited in the discussion above. If land cover varies significantly by direction, then surface

roughness should be determined based on sector. However, the width of the sectors should be no

smaller than a 30-degree arc. Further information on the definition of sectors for surface

roughness is provided in the AERMET user’s guide (EPA, 2004a). Exceptions to the

recommended default distance of 1 kilometer for surface roughness may be considered on a

case-by-case basis for applications involving site-specific wind speed measurements taken at

heights well above 10m, in situations with significant discontinuities in land cover just beyond

the recommended 1 kilometer upwind distance, or for sites with significant terrain

discontinuities (e.g., the top of a mesa or a narrow, steep valley). Another factor that may need

to be considered in some cases for determining an effective surface roughness length is the

potential contribution of nearby terrain or other significant surface expression, not reflected in

the land cover data, to the generation of mechanical turbulence. Use of a non-default distance

for surface roughness estimation, or modification of surface roughness estimates to account for

terrain/surface-expression effects, should be documented and justified in a modeling protocol

submitted to the appropriate reviewing authority prior to conducting the modeling analysis.

 

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