A canopy model of mean winds through urban areas

Abstract

SUMMARYAn urban canopy model is developed for spatially averaged mean winds within and above urban areas. The urban roughness elements are represented as a canopy-element drag carefully formulated in terms of morphological parameters of the building arrays and a mean sectional drag coefficient for a single building. Turbulent stresses are represented using a mixing-length model, with a mixing length that depends upon the density of the canopy and distance from the ground, which captures processes known to occur in canopies. The urban canopy model is sufficiently simple that it can be implemented in numerical weather-prediction models.The urban canopy model compares well with wind tunnel measurements of the mean wind profile through a homogeneous canopy of cubical roughness elements and with measurements of the effective roughness length of cubical roughness elements. These comparisons give confidence that the basic approach of a canopy model can be extended from fine-scale vegetation canopies to the canopies of large-scale roughness elements that characterize urban areas.The urban canopy model is also used to investigate the adjustment to inhomogeneous canopies. The canonical case of adjustment of a rural boundary layer to a uniform urban canopy shows that the winds within the urban canopy adjust after a distance x 0 = 3L c ln K, where L c is the canopy drag length-scale, which characterizes the canopy-element drag, and ln K depends weakly on canopy parameters and varies between about 0.5 and 2. Thus the density and shape of buildings within a radius x 0 only determine the local canopy winds. In this sense x 0 gives a dynamical definition of the size of a neighbourhood.The urban canopy model compares well with observations of the deceleration of the wind associated with adjustment of a rural boundary layer to a canopy of cubical roughness elements, but only when the sectional drag coefficient is taken to be somewhat larger than expected. We attribute this discrepancy to displacement of streamlines around the large-scale urban roughness elements, which yields a stress that decelerates the wind. A challenge for future research is to incorporate this additional 'dispersive stress' into the urban canopy model.

Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers

“…The roughness length results, in figure 8(a), closely follow the skin friction trends as a function of both the solidities, as shown in table 1. The drag-peak location herein is partially in contrast with previous studies, for which it was found at λ P ≡ λ F ≈ 0.15 (Hagishima et al 2009;Leonardi & Castro 2010;Kanda et al 2004) and λ P ≡ λ F ≈ 0.16 (Santiago et al 2008;Coceal & Belcher 2004). These differences are perhaps not surprising giving the high uncertainty in the fitting procedure which results in the visible scatter of the data for different studies in figure 8(a), even when values of similar frontal and plan solidity are considered.…”

“…These studies include both numerical and physical experiments (Cheng & Castro 2002b;Coceal & Belcher 2004;Kanda et al 2004;Cheng et al 2007;Hagishima et al 2009;Santiago et al 2008;Leonardi & Castro 2010 among various others). Open symbols in figure 2(a) show the cases examined in these studies.…”

Trade-offs and responsiveness of the single-layer urban canopy parametrization in WRF: An offline evaluation using the MOSCEM optimization algorithm and field observations

“…These two parameters are now removed and the attenuation coefficient is derived from an urban morphology relation (Table II, Eq. 21) using available parameters (Coceal and Belcher, 2004;Di Sabatino et al, 2008).…”

Influence of urban morphology on air flow over building arrays

“…They have led to the development of the few urban canopy models that are available today. These models usually assume spatially averaged velocity profiles, adopting an approach similar to that used for flow over vegetation canopies (Finnigan, 2000;Coceal and Belcher, 2004), and, in some cases, even a single spatially averaged canopy velocity (U C , see Bentham and Britter, 2003). These properties depend strongly on the local geometry and existing models generally relate them to the mean building height (H b ) and the lambda parameters (λ p , and λ f , see above).…”