The impact of an urban canopy and anthropogenic heat fluxes on Sydney’s climate.

We use the Weather Research and Forecast model and estimate anthropogenic heat (AH) fluxes based on fine-scale energy consumption data for Sydney, Australia, to investigate the effects of urbanization on temperature. We examine both the impact of urban canopy effects (UCE) and AH which in combination causes the urban heat island effect. Sydney’s urban heat island (UHI) varies from −1 to >3.4 ∘ C between day and night and between seasons. UHI intensity is highest at night and an urban cool island is often experienced during the day. UCE contributes 80% of the UHI during summer nights because of the release of stored heat from urban infrastructure that has been absorbed during the day. During the day for UCE, the reduced net radiation and greater heat storage by urban infrastructure combine to slightly cool. In contrast, AH contributes 90% of the UHI during winter nights because it does not dissipate into the higher levels of the boundary layer efficiently. The opposite applies during summer nights and during daytime in both summer and winter where heat mixes effectively into the atmosphere. Our results show contrasting impacts of UCE and AH by time of day and time of year and point to major simulation biases if only one of these phenomena is represented, or if their seasonal contributions are not accounted for separately.

Figure 3. Impact of the urban canopy (a–d) and AH (e–h) on temperature at 2 m as well as UHI intensity (i–l). UCE expressed as the difference of urban land (URB) and natural land (NAT) experiment. AH is the difference of urban land plus AH (URB + AH) experiment and urban land (URB) experiment. UHI is expressed as the difference of urban land including AH (URB + AH) and natural land (NAT) experiment.