The Urban Surface and Heatwaves

This chapter considers the role of urban surfaces in exacerbating heatwave conditions in urban environments. The chapter opens by exploring the morphological complexity and climatic diversity of urban surfaces, highlighting how urban materials, along with the three-dimensional structure of cities, influence the exchange of heat, mass and momentum between the urban surface and the overlying atmosphere. The characteristics of the urban boundary layer (UBL) and how this is influenced by urban surface climate processes is also outlined. The concept of the urban energy balance is used to explain how the partitioning of available energy between sensible and latent heat flux, as influenced by moisture availability, determines the thermal response of cities during heatwaves. It also addresses the role of heat storage in urban materials and its impact on maintaining high nighttime temperatures. Anthropogenic heat flux as an additional heat source and its role in the exacerbation of urban heat during heatwaves is also discussed. Because debate persists on the synergistic effects of heatwaves and urban heat islands (UHI) the nature of the four types of UHI is described before exploring whether heatwaves amplify UHI intensity. The chapter concludes that heatwaves can enhance levels of urban heat by altering energy partitioning between the turbulent and conductive heat fluxes but cautions that to understand the interaction between heatwaves and urban heat island intensity, a full consideration of the simultaneous processes in adjacent urban and rural settings is required. The specific role of air conditioning in enhancing levels of urban heat is also touched upon.