Assessing the relationship between Urban Morphology and Mean Radiant Temperature - A study over 5 cities in Sweden

Abstract

Thermal comfort has become an increasingly important topic in urban areas due to climate change and rapid urbanization. The most critical meteorological variable influencing experienced heat and thermal comfort during warm and clear weather is the mean radiant temperature (Tmrt). This study examines how the thermal environment is influenced by variations in urban morphology. Modeling of Tmrt using SOLWEIG was performed for five cities in Sweden: Malmö, Gothenburg, Stockholm, Östersund, and Luleå. For each city, simulations were executed twice - once including vegetation and ground cover, and once without. This approach allowed for a clearer assessment of the influence of vegetation and urban geometry on the thermal environment. The results for each city were compared, identifying the most influential variables affecting Tmrt. The findings indicate that trees have the highest influence and correlation with lower Tmrt in urban areas, primarily due to the shade they provide. The impact of trees varies depending on the surrounding urban geometry, with their cooling effect diminishing in denser areas, especially those with tall buildings. The relationship between urban geometry and the thermal environment is complex, with many variations on a microscale. On average, higher density leads to lower Tmrt over an entire 500 x 500mcell for an entire day. Due to morphological differences among the five cities, Malmö emerged as the warmest city thermally, mainly due to its low number of trees. Conversely, Luleå was the least thermally warm city, showing the highest decrease in Tmrt for both geometry and vegetation. Despite having the fourth-lowest density and tree fraction, Luleå's results suggest that solar altitude significantly influences Tmrt, indicating that the geographical location of cities affects the spatial variations of Tmrt.