The majority of the world’s population already lives in cities and with a continuing trend of urbanisation cities are under increasing pressure from resource scarcity, air pollution and climate change. In order to develop effective adaptation strategies, it is important to understand the local climate effects of urban environments. This project investigates the role of urban heterogeneity in the urban microclimate: densely mixed building units of various sizes and shapes, complex road networks, paved surfaces, water and urban vegetation all interact with each other and the atmosphere above.
Using Large-Eddy Simulations with several setups of idealised urban areas, we analyse mean air flow and momentum transport within the urban boundary layer. We compare simulations with similar building density and frontal aspect ratio, but varying levels of complexity of buildings and street geometry. Increasing the heterogeneity of surface layout increases average building-induced drag and turbulence-related momentum fluxes, and thus increases overall momentum sinks. Heterogeneity parameters such as maximum building height, average building height weighted by frontal area, and weighted building height deviation are identified as predictors for an increased rate of momentum loss within heterogeneous urban canopies.