A multi-layer urban canopy model for neighbourhoods with trees

Over 50% of the world’s population lives in cities, many of which are hot, polluted, and expanding. The design of cities impacts local meteorology and climate, which affect building energy use and the comfort and health of urban residents. Numerical models that incorporate the relevant urban elements and physical processes can predict these effects and guide management strategies. Addition of vegetation is a key design strategy for moderation of local urban climate, and many cities already boast extensive vegetation. Relative to shorter vegetation, urban trees have unique effects on local climate and pollutant dispersion: they provide shade and shelter, interacting with buildings and streets to alter climate and wind flow. Urban canopy models (UCMs) predict neighbourhood-scale (10² – 10⁴ m) energy exchange and climate of atmospheric layers between and above the buildings. Few UCMs represent the urban canopy with multiple layers, which permit more flexible and process-based representation of canopy physics. Most UCMs neglect vegetation, or incorporate it with a separate model, neglecting interaction between vegetation and built elements. This dissertation develops BEP-Tree, the first multi-layer urban canopy model that explicitly includes trees and their interaction with buildings. It consists of an existing multi-layer UCM, a foliage energy balance model, and two major developments: firstly, a model that distributes solar and infrared radiation amongst tree foliage, road, roof, and wall elements at multiple heights, accounting for radiation ‘trapping’ and mutual shading; secondly, parameterization of building and tree foliage effects on flow, including generation and dissipation of turbulence, drag on the mean wind, and explicit consideration of sheltering. The combined model permits a range of building and tree configurations, and makes possible advanced assessment of impacts of trees on urban climate, air quality, human comfort and building energy loads. BEP-Tree is compared with measurements from the Sunset neighbourhood in Vancouver, Canada. Urban trees channel sensible heat into latent heat (evaporation), shift surface-atmosphere energy exchange upwards, slow canopy wind, and dissipate turbulence, especially if taller than nearby buildings. Effects of trees on canopy thermal climate depend on representation of neighbourhood-scale foliage clumping in radiation versus dynamical processes, and further theoretical advances are required.Arts, Faculty ofGeography, Department ofGraduat