Constructing a Multi-Scale Urban Cooling Island Ventilation Network to Mitigate the Urban Heat Island Effect: A Case Study of Changsha, China

         To address the intensifying urban heat island (UHI) effect driving by rapid urbanization, current research reveals a significant scale discontinuity between macro-level strategies, such as regional cooling network design, and micro-level studies that focus on localized cooling mechanisms of individual green patches. Macro-scale approaches often overlook small cooling islands embedded within dense urban fabrics, while micro-scale investigations lack systematic understanding of inter-patch connectivity. This study proposes a multi-scale cooling island ventilation network to synergistically mitigate UHI impacts across spatial hierarchies. Using the core area of Changsha City as a case study, the research introduces an innovative three-tier scale classification framework incorporating building density. By integrating relative land surface temperature and morphological spatial pattern analysis, the study identifies core cold island sources. Further, a cold island ventilation resistance surface is constructed using the CRITIC objective weighting method, enabling the identification of key nodes and corridors for establishing a comprehensive multi-scale ventilation network. Findings reveal that, amidst urban expansion and increasing building/road densities, landscape fragmentation has led to a “shrinking-in-size, growing-in-number” trend for both primary and secondary cold island sources. From 2009 to 2016, the total area of primary-scale cold sources declined sharply from 45 km² to 19.8 km², while their number rose from 130 to 151. The average patch size fell from 0.35 km² to 0.07 km², and the minimum temperature increased from 28.7 °C to 35.3 °C-signaling a depletion risk. Similarly, secondary cold sources shrank from 215.38 km² to 144.83 km², as their number increased from 123 to 169, with average patch size dropping from 1.75 km² to 0.86 km²-weakening their thermal buffering capacity. Despite this, ventilation corridors peaked in 2020, totaling 371 in number and 528.5 km in length, continuing to act as "relay stations" transmitting peripheral cooling effects to the urban core. Notably, tertiary cold sources rebounded after 2016 due to strengthened ecological conservation efforts, expanding by 237.5 km² by 2020. Their temperatures stabilized between 35–38 °C—significantly cooler than the urban core—demonstrating sustained cooling potential. Policy recommendations are proposed across three spatial scales: 1) primary scale, remove obstructions at cold source points to broaden cooling supply channels; 2) secondary scale: prioritize the protection of key corridors and junctions to preserve inter-patch connectivity and maintain dynamic cold air flow; 3) tertiary scale: safeguard and enhance core ecological areas to ensure stable and continuous cooling output. By identifying cold island sources and constructing a multi-scale ventilation network, this study offers a science-based framework for optimizing thermal environments in high-density urban areas.

Graphical Abstract