Impacts of urban densification and vertical growth on urban heat environment: A case study in the 4th Ring Road Area, Zhengzhou, China

Rapid urbanisation has significant impacts on the surface morphology, local regional climate, and thermal environment in urban areas. Previous studies have focused on horizontal urban expansion. Here we analyse both the impacts of horizontal sprawl and vertical changes on the urban thermal environment. We develop a method to understand the relationship between urban morphology and urban heat. First, we classified urban land-use and stereo image pairs to extract building heights. We then determined the links between the land surface temperature (LST) characteristics and the variations in building density (BD) and building height (BH). The method is applied to the 4th Ring Road Area, Zhengzhou City, China, that is shown to have experienced significant 3D expansion between 2013 and 2017, with a widespread shift from low to high BD, a marked decrease in single-storey buildings, and a sharp increase in high rise (>= 50 m) buildings. However, the relationship between BH, BD and LST differs between summer and winter, is non-monotonic and switches directionality at BD = 0.2 and BH = 24 m and 100 m. The exact nature of these relationships, and in particular the inflection points that are critical to building density and height decisions, will vary between cities. However, the method provides critical information for decision-makers seeking to address the challenge of climate change within the spatial planning process.