Grimmond, C.S.B., Oke, T.R.
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 (zd), roughness length (z0), depth of the roughness sublayer (zr) and aerodynamic conductance (gaM). A sensitivity analysis is conducted on seven formulae to estimate zd, 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 zd 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 zd 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 zd and z0 in urban areas, and their most likely magnitude.