City-wide analysis of outdoor thermal comfort during heatwaves: influence of building geometry, vegetation and ground cover

Urban citizens are particularly exposed to heat stress during heatwaves due to the urban climate conditions. Introducing more trees and/or changing building density and ground cover materials are examples of planning measures that can be used to mitigate heat stress. One challenge as an urban planner is to have knowledge on which mitigation measure to implement to achieve the highest cooling effect. The aim of this high-resolution modelling of outdoor thermal comfort on city-wide domains is to examine how different real-world urban settings reduce or exacerbate heat stress related building density, tree cover and/or ground cover. Here, we exploit an open-source tool, the Urban Multi-scale Environmental Predictor (UMEP), to investigate the influence of real-world information on building density, tree and ground cover on thermal comfort for the three largest cities in Sweden (Stockholm, Göteborg and Malmö). Two thermal comfort indices are calculated and compared: Physiological Equivalent Temperature (PET) and Universal Thermal Comfort Index (UTCI). Automated chain processes using Python scripting is demonstrated making it possible to derive microscale outdoor thermal comfort information (2-meter resolution) using only a standard personal computer and open data sources. Preliminary results show that tree cover is the single most effective heat mitigation factor, especially in areas with low building density. Results also show that altering ground cover has a minor cooling effect. The output from this study will be used as input in practical guidelines to resilient urban planning strategies against heat stress.