Exploring the thermal effects and influencing factors of urban parks during diurnal variations

Urban parks are crucial in mitigating the urban heat island (UHI) effect, particularly during the daytime. However, warming effects associated with the diurnal cycle pose environmental and health risks that remain underexplored in existing studies and planning strategies. This study examines urban parks in Henan Province as a case study, analyzing park geometry, landscape metrics, biophysical parameters, surface albedo, and anthropogenic factors using machine learning algorithms. The aim is to examine the different effects of urban characteristics in the park's internal and external environments on thermal intensity (TI) across diurnal variations in different seasons and within the diurnal cycle during the hot weather. The results reveal that (1) Daytime cooling is most prominent in summer, while nighttime warming is strongest in winter. During the hot weather, over 70 % of parks exhibit a cooling effect during the day, while about 60 % show a warming effect at night. (2) The total landscape area of the park shows biphasic modulation, with enhanced cooling during the day and increased warming at night within specific threshold ranges. Park albedo, except during the summer daytime, exceeded 0.5-1.0, which reduces daytime cooling and diminishes nighttime warming. A larger total park landscape area, combined with a lower park albedo, amplifies both daytime cooling and nighttime warming. (3) When the builtup area outside the park is more than 10 %, daytime cooling is more effective in summer. Additionally, a larger total park landscape area combined with a higher percentage of park outbuildings also contributes to significant cooling effects. (4) During the daytime of the hot-weather diurnal cycle, the cooling effect significantly diminishes when the percentage of bare land area within the park is below 40 %. The interaction between a park's total landscape area and the percentage of bare land within it has a considerable impact on this cooling effect. This study underscores the complexity of diurnal thermal effects in urban parks, providing a theoretical foundation for optimizing park design and cooling strategies, thereby enhancing urban thermal management.