Urban climate: Verfication of urban heat island mitigation strategies in the Swiss city Basel

Cities worldwide will be affected by anthropogenic climate change and additionally cope with additional heat due to greater heat storage capacity of artificial surfaces, less ventilation, and a higher risk of extreme floods due to sealed surfaces. Urban heat island mitigation strategies (such as rooftop greening, increasing surface albedo of the city and irrigation of green surfaces, which leads to significant evaporative cooling) are well known, but the magnitude in air temperature reduction is still not fully understood and significantly differs between cities.

A small area (“Triangel area”) was desealed and planted with 18 young trees at a 1 ha area in the center of the Swiss City Basel in March 2021. This urban heat island mitigation strategy was validated with a high dense low-cost IoT measurement network, which was installed in Basel in 2020 to detect urban heat islands. To validate the mitigation strategy properly, 3 air temperature measurements were installed in the Triangel area in 2020 and compared with more than 20 air temperature measurements in the reference area outside the mitigated area.

The measurements showed that the Triangel area is in general around 0.2 K cooler than the reference area. Furthermore, the differences in air temperatures between Triangel and reference area were calculated before and after the mitigation action to test the effectiveness of the method. An air temperature reduction of the Triangel area of 0.4 K after the mitigation action (in comparison with the reference area) was observed by the measurements.

The results from the measurement campaign were compared with model results using the surface energy budget model SUEWS. The SUEWS model results confirm an air temperature decrease of 0.4 K for the chosen urban heat island mitigation strategy and suggest other mitigation strategies (such as rooftop greening, watering and more). This approach allows to estimate the best possible mitigation strategy for the Triangel area with the largest reduction of surface and air temperatures. In summary, the approach helps city councils taking the right decision by choosing the optimal cost-value ratio in urban heat island mitigation strategies and prevents costly (and non-climate effective) strategies.