The availability of high resolution digitized land cover databases provides an opportunity to

apply systematic procedures to determine surface characteristics based on an objective analysis

of the gridded land cover data across a domain. A proper analysis of such data must take into

consideration the relationship between surface characteristics and the meteorological

measurements on which the surface characteristics will be applied. While the following

discussion offers specific recommendations regarding the methods for determining surface

characteristics from digitized land cover data, the general principles on which these

recommendations are based are also applicable to determining surface characteristics from other

sources of non-digitized land use and land cover data.

Based on model formulations and model sensitivities, the relationship between the surface

roughness upwind of the measurement site and the measured wind speeds is generally the most

important consideration. The effective surface roughness length should be based on an upwind

distance that captures the net influence of surface roughness elements on the measured wind

speeds needed to properly characterize the magnitude of mechanical turbulence in the approach

flow. A number of studies have examined the response of the atmosphere to abrupt changes in

the surface roughness, and provide some insight into the relationship between measured winds

and surface roughness [e.g., Blom and Warenta (1969), Businger (1986), Högström and

Högström (1978), Horst and Weil (1994), Irwin (1978), Rao, et al. (1974), and Taylor (1969)].

Such changes in surface roughness result in the development of an internal boundary layer (IBL)

which grows with distance downwind of the roughness change, and defines the layer influenced

by the transition in surface roughness. The size and structure of the IBL is very complex, even

for idealized cases of uniform roughness upwind and downwind of the transition. The IBL is

also affected by the magnitude and direction of the roughness change and the stability of the

upstream flow. The IBL generally grows more slowly for stable conditions than for neutral or

unstable approach flow, and will also tend to grow more slowly for rough-to-smooth transitions

than for smooth-to-rough transitions. The relationship between surface roughness and measured

 

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