Evaluation of Urban Local-Scale Aerodynamic Parameters: Implications for the Vertical Profile of Wind Speed and for Source Areas

Abstract

Nine methods to determine local-scale aerodynamic roughness length $\left(z_0\right)$ and zero-plane displacement $\left(z_d\right)$ are compared at three sites (within 60 m of each other) in London, UK. Methods include three anemometric (single-level high frequency observations), six morphometric (surface geometry) and one reference-based approach (look-up tables). A footprint model is used with the morphometric methods in an iterative procedure. The results are insensitive to the initial $z_d$ and $z_0$ estimates. Across the three sites, $z_d$ varies between 5 and 45 m depending upon the method used. Morphometric methods that incorporate roughness-element height variability agree better with anemometric methods, indicating $z_d$ is consistently greater than the local mean building height. Depending upon method and wind direction, $z_0$ varies between 0.1 and 5 m with morphometric $z_0$ consistently being 2-3 m larger than the anemometric $z_0$ . No morphometric method consistently resembles the anemometric methods. Wind-speed profiles observed with Doppler lidar provide additional data with which to assess the methods. Locally determined roughness parameters are used to extrapolate wind-speed profiles to a height roughly 200 m above the canopy. Wind-speed profiles extrapolated based on morphometric methods that account for roughness-element height variability are most similar to observations. The extent of the modelled source area for measurements varies by up to a factor of three, depending upon the morphometric method used to determine $z_d$ and $z_0$ .

Keywords: Aerodynamic roughness length; Anemometric methods; Logarithmic wind-speed profile; Morphometric methods; Source area; Zero-plane displacement.