Homogeneity and heterogeneity of diurnal and nocturnal hotspots and the implications for synergetic mitigation in heat-resilient urban planning

Many cities are under intense heat challenges with severe environmental, social, and economic consequences, sparking great concern on heat-resilient urban planning, yet normally with biased focus on limited (e.g., diurnal) mitigation needs. Particularly, the recognition of urban thermal hotspots is crucial for adding effective cooling interventions for mitigation and avoiding overheating in newly built areas. However, the hotspots and associated drivers vary across time and space, bringing challenges to urban planners to make win-win decisions to synchronously address diurnal and nocturnal heat stresses through an integrated set of cooling strategies. This study aims to recognize the homogeneity and heterogeneity of diurnal and nocturnal hotspots and interpret principal and synergetic drivers behind them by developing a robust methodological scheme in addressing uncertainties associated with temperature data and analytical models. It explicitly 1) identified summer diurnal and nocturnal hotspots using rigorously screened satellite data; 2) recognized the typical typologies of hotspot-prone urban landscape according to urban composition, morphology, and function; 3) explored the day-night similarities and disparities in major urban factors and their robust effective ranges for synergetic mitigation through multi-model non-linear analysis with diverse machine learning techniques covering random forest, gradient boosting machines, and boosted regression trees. Results revealed that the specific locations and typical urban landscape features varied between diurnal and nocturnal hotspots. Among the six typologies recognized, industrial-dominated ones were more inclined to emerge as diurnal hotspots, while mid- to high-rise and mid-density blocks, with diversified land uses (mostly residential-dominated), tended to become diurnal, and more likely, nocturnal hotspots. All three models reached robust conclusion that urban morphology exhibited significant influence on both diurnal and nocturnal hotspot formation. Although trade-offs remained unavoidable in many cases, synergetic mitigation could be achieved through optimizing area averaged building height below 15 m or above 25 m, and building volume density under 2 % for Wuhan, China. Overall, this study responds to the emerging multidimensional urban science and praxis and extends the conventional one-dimensional planning against urban heat to win-win decisions over both diurnal and nocturnal hotspots. The empirical findings can benefit the development of complete, unbiased, and implementable actions for enhanced climate-resilience.