Models to estimate pollution dispersion and wind flow in cities (both at the city-scale and above) require a parametrical description of the urban canopy. For instance, two key parameters are the aerodynamic roughness length z(sub 0) and the zero-plane displacement height z(sub d), which are related, amongst others, to the surface drag coefficient, the scale and intensity of turbulence, the depth of the roughness sub-layer and the wind speed profile. The calculation of z(sub 0) and z(sub d), however, is not straightforward. The classical way to estimate them in open terrain is based on the measurement of wind profile data from a tall mast or, less accurately, on the inference from published roughness values for similar terrain elsewhere (Davenport, 1960; Davenport et al., 2000). Both methods, however, are very difficult to apply to cities, due to the considerable height where wind measurements should be taken (well above the urban canopy) and to the irregularities of urban texture. A promising alternative that has become available in recent years, due to increasing computing resources and the availability of high-resolution 3-D databases in urban areas, is based on the calculation of z(sub 0) and z(sub d) from the analysis and measure of the city geometry (urban morphometry). This method is reviewed for instance in Grimmond and Oke (1999), where values are calculated using different formulas and then compared with the results of field measurements. Urban morphometry opens up a new range of parameters that can easily be calculated in urban areas and used as input for meso-scale and urban dispersion models. This paper reviews a number of them and shows how they could be calculated from urban Digital Elevation Models (DEM) using image-processing techniques. It builds up on the recent work by Ratti et al. 2000, extending the number of case studies cities: London, Toulouse, Berlin, Salt Lake City and Los Angeles.
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