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Abstract
We use a conceptual model to investigate how randomly varying building heights within a city affect the atmospheric drag forces and the aerodynamic roughness length of the city. The model is based on the assumptions regarding wake spreading and mutual sheltering effects proposed by Raupach (Boundary-Layer Meteorol 60:375-395, 1992). It is applied both to canopies having uniform building heights and to those having the same building density and mean height, but with variability about the mean. For each simulated urban area, a correction is determined, due to height variability, to the shear stress predicted for the uniform building height case. It is found that u ^sub *^/u ^sub *R^, where u ^sub *^ is the friction velocity and u ^sub *R^ is the friction velocity from the uniform building height case, is expressed well as an algebraic function of λ and σ^sub h^/h ^sub m^, where λ is the frontal area index, σ ^sub h^ is the standard deviation of the building height, and h ^sub m^ is the mean building height. The simulations also resulted in a simple algebraic relation for z ^sub 0^/z ^sub 0R^ as a function of λ and σ ^sub h^/h ^sub m^, where z ^sub 0^ is the aerodynamic roughness length and z ^sub 0R^ is z ^sub 0^ found from the original Raupach formulation for a uniform canopy. Model results are in keeping with those of several previous studies.[PUBLICATION ABSTRACT]