City-scale morphological influence on diurnal urban air temperature

Abstract

Urbanization patterns affect urban thermal environments. Studies have characterized the asymmetry of daily urban air temperature based on the increase in mean temperature, decrease in amplitude, and significant phase delay in the highly compact high-rise city of Hong Kong. In this study, a simple physical-based energy balance strategy was developed to investigate urban morphological impacts on city thermal climate. This energy balance model was first evaluated by using a meso-scale model, i.e. a weather research and forecasting model coupled to the BEP-BEM urban canopy model, and then applied to investigate urban daily air temperature profiles in a high-rise compact city. The modelling results show that urban thermal storage has no direct impact on the mean urban air temperature, but does affect amplitude and phase. The results reveal that different city morphologies lead to different urban air temperature profiles. Two important parameters for engineers and urban planners are introduced: a relative convective heat transfer number λ, and a time constant, τ, which respectively describe the relationships between convective heat transfer and urban thermal storage and ventilation rate in urban areas. These two parameters modulate the mean, amplitude term, and phase term of the daily urban air temperature cycles. The results have significant implications for design of city morphologies and urban planning of climate-resilient cities. The new results from this study and the proposed simple model should facilitate the rapid development and testing of potential urban heat island mitigation strategies.