Effect of grass grid pavers on thermal outdoor comfort in urban areas - A microclimate modelling study to analyse spatial variability in heat stress mitigation potential in the context of climate change adaptation

The urban heat island (UHI) effect leads to substantial health risks for urban dwellers. A significant increase and intensification in heat exposure is expected due to climate change. A key cause for the UHI is the large proportion of low albedo and impervious surface materials. To increase thermal comfort and reduce heat stress, nature-based solutions including green infrastructure are often proposed. Unsealing strategies such as grass grid pavers (GGPs) can support heat mitigation and contribute to reducing surface flow without requiring additional space. However, GGPs cannot be installed everywhere and require high installation and maintenance costs. Thus, our research aims to realistically assess the efficacy of GGPs for cooling urban areas, considering limitations like implementation suitability. In addition, factors such as ventilation and irradiation have a strong impact upon the cooling effect. We compared microclimatic effects and thermal comfort improvement of GGPs for shaded and unshaded areas in a densely developed neighbourhood in the city of Cologne/Germany during an extreme heat event by using high-resolution microclimate simulations (ENVI-met). A new parametrization of GGPs based on field measurements was developed. Model performance is assessed using data from a quality-controlled citizen science sensor network. Asphalt and concrete surfaces in the status-quo model were replaced by GGPs. Model outputs show a significant improvement of biometeorological comfort quantified by the Universal Thermal Comfort Index which indicates the change in human heat perception. Physiological Equivalent Temperature is reduced by up to -5.43 K. These findings can support decision-making and optimization of adaptation strategies aiming to improve heat resilience. Modelling the human perception of heat and the effects of adaptation measures has the potential to strengthen citizen participation in the needed transformation process towards climate change adaptation. Pathways for citizen participation are currently researched in the AKT@HoMe project at University of Cologne.