Outdoor thermal comfort in Drottninghög, Helsingborg: a study on the effects of urban densification in a warmer climate

Abstract: Extreme heat can lead to a significant increase in mortality, especially for urban citizens. Densification of urban areas can dramatically affect the local climate of the city by replacing green areas with impermeable materials and changing both radiative and turbulent fluxes. The assessment of the thermal comfort in urban areas is an important consideration for local municipalities to mitigate zones of extreme heat and to use in climate adaptation strategies. In Drottninghög, a district in Helsingborg, the urban thermal comfort can change dramatically as the area will be densified with over 1100 apartments through the construction of multi-story buildings. The aim of this study is therefore to study the consequences of the future building situation in Drottninghög for a normal summer and an extremely warm summer, using the model SOLWEIG, which computes the mean radiant temperature (Tmrt), a variable commonly used to assess the thermal comfort of humans. Future scenarios are considered using two different future scenarios with a 2°C and 3.5°C summer temperature increase by the end of the century. The increased mortality risks are assessed by computing the number of hours with an increased risk. The effect of increased tree cover is also evaluated as a potential mitigation strategy. The results indicate that open areas are more susceptible to high values of Tmrt with a paved schoolyard and a market square experiencing the most hours with an increased health risk. The future buildings situation leads to some of the parking lots being replaced, causing the Tmrt to decrease. The multi-story buildings create significant shade leading to lower Tmrt as a result of less incoming radiation. An extreme summer has a Tmrt daytime average of 6°C warmer than a normal summer for the months June to August. The future scenarios lead to an increase in Tmrt similar to the increase air temperature. The addition of trees causes a decrease of Tmrt by 10°C for open areas and can be regarded as an appropriate mitigation strategy. The findings of this study assess the thermal environment in the study area in regard to radiation patterns and can aid the decision makers in future plans for the district. However, future studies should include wind patterns as it would further improve the evaluation of the influence of constructing tall and dense buildings in Drottninghög, as well as incorporating the cooling effects of wind in regard to outdoor thermal comfort.