Nature-Based Solutions for Urban Thermal Resilience: Modeling the Influence of Vegetation on Urban Heat Island Intensity During Heatwaves in Turku, Finland.

Urbanization and climate change-induced extreme weather conditions are intensifying the magnitude and duration of the Urban Heat Island (UHI), which is characterized by higher temperatures in urban areas compared to their rural surroundings. Heatwaves are expected to be more frequent in the future, which may worsen the UHI-related problems by exacerbating thermal stress in cities, resulting in heightened health risks and higher energy demands. A heatwave was classified as an event occurring when the daily mean temperature remained above a predefined threshold for no fewer than three consecutive days.  Threshold temperatures in this study range from 20 to 26°C.  Temporal and spatial dynamics of heatwaves, UHI, their relationship, and how vegetation can mitigate their effects in urban areas were examined in this paper. In this study, we examined half-hourly air temperature (3m elevation) measurements over 22 years (2002–2023) recorded at 20 stations over the city of Turku, located on the southwest coast of Finland in a humid continental climate zone. The temperature records were supplemented with 2m resolution land use land cover (LULC) data along with vegetation height (from grass to vegetation with height more than 20m), a high-resolution digital elevation model of the case area, and national urban-rural classification for Finland. Statistical analyses were used to quantify heatwave intensity, UHI, and the relation between heatwaves and UHI, and Gradient Boosting (GB) models were used to assess the LULC impact on UHI and to search for the most suitable vegetation characteristic to mitigate UHI. Results intensity was strongest t during nighttime. High trees (height more than 15m) are most prominent in mitigating the UHI during no heatwave or low heatwave conditions, while grassland played a vital role during intense heatwave conditions. These findings highlight the importance of urban greenery, especially high urban greenery, to enhance thermal resilience and may guide sustainable urban planning to meet the challenge of climate change-induced heat stress through nature-based solutions.