Humans in the city: Representing outdoor thermal comfort in urban canopy models

The negative effects of urban heat islands (UHIs) on citizens' well-being and life quality are widely acknowledged, but they still represent critical challenges, particularly since urban population is predicted to rise to 60% of the world population by 2030. Computational models have become useful tools for addressing these challenges and investigating urban microclimate repercussions on citizens' comfort and urban liveability. Despite that, humans typically remain absent from such models. This work bridges this gap, moving beyond purely thermodynamic Urban Canopy Models (UCMs) to highlight the importance of integrating even simplified pedestrians' biophysics for comfort assessment. Human physiology parameterization is therefore introduced into the Princeton Urban Canopy Model (PUCM), which had been designed to investigate the effect of greenery and novel materials on the UHI. Human thermal comfort is assessed in terms of the skin temperature and then evaluated against the apparent temperature, a widely-used thermal comfort indicator. Different configurations of the same urban canyon are therefore tested to assess the effectiveness of cool materials and trees for human thermal comfort enhancement. Results show that cool skins in the canyon's built environment lead to an air temperature reduction up to 1.92 K, but slightly worsen human comfort in terms of a warmer computed skin temperature by 0.27 K. The indirect effect of trees, that exclude shading, are negligible for human thermal comfort. The new integrated human-centric model can help policymakers and urban planners to easily assess the potential benefits or threats to citizens' well-being associated with specific urban configurations.