Urban boundary layers over dense and tall canopies

Wind-tunnel experiments were carried out on four urban morpholo-6 gies: two tall canopies with uniform height and two super-tall canopies with a 7 large variation in element heights (where the maximum element height is more 8 than double the average canopy height, h max =2.5h avg). The average canopy 9 height and packing density are fixed across the surfaces to h avg = 80 mm, 10 and λ p = 0.44, respectively. A combination of laser doppler anemometry and 11 direct-drag measurements are used to calculate and scale the mean velocity 12 profiles within the boundary-layer depth δ. In the uniform-height experiment, 13 the high packing density results in a 'skimming flow' regime with very little 14 flow penetration into the canopy. This leads to a surprisingly shallow rough-15 ness sublayer (z ≈ 1.15h avg), and a well-defined inertial sublayer above it. 16 In the heterogeneous-height canopies, despite the same packing density and 17 average height, the flow features are significantly different. The height het-18 erogeneity enhances mixing, thus encouraging deep flow penetration into the 19 canopy. A deeper roughness sublayer is found to exist extending up to just 20 above the tallest element height (corresponding to z/h avg = 2.85), which is 21 found to be the dominant length scale controlling the flow behaviour. Results 22 point toward the existence of a constant stress layer for all surfaces considered 23 herein despite the severity of the surface roughness (δ/h avg = 3 − 6.25). This 24 contrasts with previous literature. 25 Keywords Laser doppler anemometry · Turbulent boundary layers · Urban 26 roughness · Wind-tunnel experiment