Influence of the underground urban morphology on subsurface heat islands

The subsurface of cities is warming up-undergoing an underground climate change caused by subsurface urban heat islands (SUHIs). This phenomenon poses hazards while offering opportunities for sustainable urban heating. Although the morphology of urban areas above the ground is renowned for markedly influencing surface heat islands, the impact of the underground urban morphology on SUHIs remains largely unexplored. This study aims to (i) extend the definition of quantitative variables for analysing the urban morphology from the surface to the subsurface of cities and (ii) systematically examine how the underground urban morphology affects the intensity of SUHIs. Using three-dimensional (3D), time-dependent finite element simulations, we assess the role of different underground morphological features of cities in the development and intensity of SUHIs, such as heat source dimensions and distribution, green-space density and ground properties. Results indicate that the size and density of underground heat sources primarily drive the overall intensity of SUHIs, strongly depending on the presence of groundwater flow and only secondarily on the ground thermo-physical properties and the presence of green areas at the surface, whose influence substantially vary with depth. These findings enhance the understanding of the mechanisms governing SUHIs and provide insights to mitigate them globally.This article is part of the theme issue 'Urban heat spreading above and below ground'.