High-resolution urban heat stress modelling: event-based downscaling in Swedish cities

The increasing frequency of heat waves in high-latitude regions emphasizes the necessity for a deeper understanding of their impacts on urban areas and residents. This study, conducted within the BRIGHT project, presents an innovative methodology for assessing urban heat stress by integrating event-based downscaling using dynamical climate model, low-cost sensor deployment, and citizen sensing approaches.

This study explores results from a network of low-cost thermohygrometers (2022-2024) in three Swedish cities (Stockholm, Norrköping, and Linköping) and climate simulations for normal, hot, and extreme hot summers under specific warming levels (SWL) of +0.9°C (historical), +2°C, and +3°C. The methodology employs the downscaling modelling approach, utilizing the HCLIM43-AROME convection-permitting regional climate model up to 3km resolution, the SURFEX land surface model at 300m, and the SOLWEIG radiation model at 1m resolution for detailed local assessments. SOLWEIG calculates mean radiant temperature (MRT), a key indicator of human thermal comfort.

The results reveal that the shade of foliage and buildings has a clear cooling effect, with parks reducing the MRT by over 20°C. For the “hot” and “extreme hot” summers (historical climate), the number of hours with a 5% increase in mortality risk varies up to 200 hours in the parks under historical period, while central areas of the cities, with sparse tree canopy, face 250-400 hours of such risk. Under the +3°C SWL, these ranges increase by 50h. When comparing SWL +3°C to historical conditions, the relative differences in heat stress are more pronounced for "normal" and "hot" summers than for "extreme hot" summers, indicating a trend, where the existing vegetation coverage becomes ineffective at mitigating the heat

These insights underscore the importance of holistic, climate-informed urban planning strategies that prioritize green infrastructure, consider varying warming scenarios, and address both extreme and moderate heat conditions to more liveable cities in a climate changing context.