Aerodynamic properties of urban areas derived from analysis of surface form

Author(s): Grimmond, C.S.B., Oke, T.R.

Source: Journal of Applied Meteorology; Sep99, Vol. 38 Issue 9, p1262, 31p.

Abstract: Several methods to determine the aerodynamic characteristics of a site through analysis of its surface form (morphometry) are considered in relation to cities. The measures discussed include zero-plane displacement length (z_d), roughness length (z₀), depth of the roughness sublayer (z_r) and aerodynamic conductance (g_aM). A sensitivity analysis is conducted on seven formulae to estimate z_d, and nine for z₀, covering a wide range of probable urban roughness densities. Geographic information systems developed for eleven sites in seven North American cities, are used to characterize their morphometry - the height, shape, three-dimensional area and spatial distribution of their roughness elements (buildings and trees). Most of the sites are in residential suburbs but one is industrial and two are near city centers. This descriptive survey of urban geometric form is used, together with the morphometric formulae, to derive the apparent aerodynamic characteristics of the sites. The resulting estimates of z_d and z₀ are compared with values obtained from analysis of wind and turbulence observations. The latter are obtained from a survey of approximately sixty field, and fourteen laboratory, studies of real and scale model cities. Despite the comprehensive nature of the survey very few studies are found to be acceptable, and their scatter is large, hence they do not provide a standard against which to test the morphometric algorithms. Further, the data show only weak relations between measured z_d and z₀ and roughness density. The relative merits of morphometric and wind-based estimates of aerodynamic parameters are discussed. Recommendations are made concerning the choice of method to estimate z_d and z₀ in urban areas, and their most likely magnitude.