High resolution modelling of Stockholḿs urban heat island and the impact of urbanization

Stockholm in Sweden is growing fast in response to an increasing need of housing and transportation. In parallel, recent heat waves pose new challenges to human health and well-being, especially the most vulnerable people. Understanding the effect of on-going urbanisation on the urban climate of Stockholm is therefore fundamental for the success of adaptation, in particular of those measures taking profit from the cooling potential of vegetation during heat stress events.

In this work we investigate the urban climate of Stockholm, with a focus on the spatial variation of air temperature. Dynamical downscaling at 1 km grid space is carried out with the Numerical Weather Prediction system HARMONIE-AROME, a technique validated over different European cities within the Copernicus Climate Change Service UrbanSIS.

Validation against observations taken during 5 years at 8 weather stations shows that the high resolution model provides the required performance skills. The summers of 2014 and 2018, two of the hottest in the last decades in Scandinavia, were then selected as baseline for the study. The high-resolution modelled data reveals the spatial and temporal dynamics of the urban heat island (UHI), with a number of inner-city temperature gradients resulting from vegetated areas and water bodies. This cooling effect is also analyzed in a field campaign with 10 low-cost air temperature data loggers placed in the Tantolunden park and nearby street-canyons.

Furthermore, the impacts of an urban and a regional development scenario, targeted at 2030 and 2050 respectively, are investigated. Results show that the average summer temperature increases in relation to 2014 by 0.29 °C in 2030 and 0.46 °C in 2050, up to a local maximum of 1.35 °C in the latter, with a stronger effect at nighttime. An increase in the number of hot days by up to 10 in 2050 is found, with locations closer to the sea being less prone to temperature maxima. The coverage of this warming effect is predominantly localized over the transformed/densified area.

Finally, this dataset will be available as a city demonstrator in the information system developed in the Horizon2020 project CLARITY. User-tailored downscaled urban climate data, in the example of Stockholm, provide new intelligence for urban planning that assimilates fit-to-purpose Nature-based Solutions.