Aerodynamic roughness parameters in cities: Inclusion of vegetation

- Macdonald et al. (1998) and Kanda et al. (2013) morphometric methods are developed to include vegetation.
- Development is assessed at five study sites in London, UK.
- Effect of the inclusion of vegetation on zd and z0 depends whether buildings or vegetation are dominant.
- When vegetation dominates, the impact on the logarithmic wind profile is the greatest.

A widely used morphometric method (Macdonald et al. 1998) to calculate the zero-plane displacement (zd) and aerodynamic roughness length (z0) for momentum is further developed to include vegetation. The adaptation also applies to the Kanda et al. (2013) morphometric method which considers roughness-element height variability. Roughness-element heights (mean, maximum and standard deviation) of both buildings and vegetation are combined with a porosity corrected plan area and drag formulation. The method captures the influence of vegetation (in addition to buildings), with the magnitude of the effect depending upon whether buildings or vegetation are dominant and the porosity of vegetation (e.g. leaf-on or leaf-off state). Application to five urban areas demonstrates that where vegetation is taller and has larger surface cover, its inclusion in the morphometric methods can be more important than the morphometric method used. Implications for modelling the logarithmic wind profile (to 100 m) are demonstrated. Where vegetation is taller and occupies a greater amount of space, wind speeds may be slowed by up to a factor of three.