Quantifying the effects of microscale heterogeneity in urban surface structure on the urban heat and park cool islands in a mid-sized city in Central Europe

As the intensity and duration of heat waves increase with climate change, the importance of the cooling effect of cold air drainages from surrounding elevated terrain on the urban heat island increases. In the urban boundary layer, air temperatures throughout the diel cycle are commonly higher than in rural areas due to modifications in the radiation and energy transfers which depend on the microscale urban surface structure. This well-known urban heat island phenomenon also applies to mid-sized (< 100 000 citizens) cities, but has received less attention. Spatial variability of nighttime air temperatures showed that flow obstacles including dams can impede the nocturnal cold air drainage in urban areas. During the daylight period, parks can form cool islands in the heated city body because of evapotranspirational cooling from leaf surfaces and interception of a significant fraction of the sunlight leading to lower surface air temperatures. The aim of this study was to quantify the impacts of flow obstacles on nocturnal cold air drainages and of heterogeneity in the urban surface structure including city parks on the daytime thermal variability in a mid-sized city (Bayreuth, Germany, about 75 000 inhabitants) in Southern Germany, Europe. Observations were collected using a city-wide meteorological microclimate station network and mobile fast-response air temperature measurements by bicycle during 6 cloud-free nights and days in August 2023. The results showed that an 8 to 10 m high dam blocked the cold air drainage originating from surrounding topography elevated by about 150 m approaching the obstacle at an average wind gust speed of 0.9 m s^-1 and depth of several meters. The blocking resulted in the formation of a cold air pool on the upwind side and a distinct microclimatic difference on both sides. While the direct cold-air drainage was blocked, some cold-air drainage was able to circumflow the dam mostly alleviating the air temperature difference between both sides. On average, the nocturnal cold air drainages reduce the ground-level air temperatures on the urban surrounding of a medium-sized city on average by 1.5 K, while instantaneous cooling effects were quantified up to 3.2 K. Additionally, areas with a high proportion of vegetation had a cooling effect on the surroundings in the afternoon, but also at night, as less energy was stored in the ground. The research highlights the importance of considering cold air flow drainage paths in urban planning. The reconstruction of barriers can contribute to the reduction of urban heat island at night.