Urban boundary layers over dense and tall canopies

TL;DR

This study investigates how urban canopy height heterogeneity influences boundary layer flow, revealing that heterogeneity enhances mixing and deep flow penetration, contrasting with uniform canopies which exhibit shallow flow regimes.

Contribution

It provides new insights into the effects of height heterogeneity on boundary layer structure over dense urban canopies, challenging previous assumptions about inertial sublayer presence.

Findings

Heterogeneous canopies promote deeper flow penetration.

Uniform canopies exhibit shallow roughness sublayers.

Inertial sublayer exists despite high surface roughness.

Abstract

Wind tunnel experiments were carried out on four urban morphologies: two tall canopies with uniform-height and two super-tall canopies with a large variation in element heights (where the maximum element height is more than double the average canopy height, $h_{max}$=2.5 $h_{avg}$). {The average canopy height and packing density were fixed across the surfaces to $h_{avg} = 80$ mm, and $\lambda_{p} = 0.44$, respectively.} A combination of laser doppler anemometry and direct drag measurements were used to calculate and scale the mean velocity profiles {within the boundary layer depth, $\delta$}. In the uniform-height experiment, the high packing density resulted in a `skimming flow' regime with very little flow penetration into the canopy. This led to a surprisingly shallow roughness sublayer ($z\approx1.15h_{avg}$), and a well-defined inertial sublayer above it. {In the heterogeneous-height canopies, despite the same packing density and average height, the flow features were significantly different.} {The height heterogeneity enhanced mixing thus encouraging deep flow penetration into the canopy. A deeper roughness sublayer was found to exist and extend up to just above the tallest element height (corresponding to $z/h_{avg} = 2.85$)}, which was found to be the dominant lengthscale controlling the flow behaviour. {Results points toward the existence of an inertial sublayer for all surfaces considered herein despite the severity of the surface roughness ($\delta/h_{avg} = 3 - 6.25$)}. This contrasts with previous literature.