Simulation of the heat mitigation potential of unsealing measures in cities by parameterizing grass grid pavers for urban microclimate modelling with ENVI-met (V5)

Abstract. Many urban areas are characterized by both a growing population and an intensification of summer heat events in the context of climate change. Thus, more and more people are exposed to heat stress and corresponding health consequences. Measures for climate change adaptation such as unsealing strategies are needed in the existing urban fabric to reduce sensible heat flux by increasing latent heat flux to cool down the urban environment without requiring additional space or changing the basic function of the area. Unsealing measures like grass grid pavers (GGPs) can also help to reduce flooding risks due to increased infiltration and water storage capacities. Up to now, a parameterization of GGPs for microclimatic simulations is not available. To fill this research gap, we here present a new GGP model parameterization developed for the fluid dynamics microclimate ENVI-met model based on field measurements with double-ring infiltrometers etc. To analyse the microclimatic effects and the cooling potential of this GGP parameterization, scenario analyses were performed using a validated ENVI-met model setup for an urban high-density study area in Cologne/Germany. An extreme scenario was designed to address the maximum cooling potential of the GGPs in comparison to the dominant sealed asphalt surfaces in the study area, and a more realistic scenario with a usage-compatible installation of GGPs in the model domain only in side streets and inner courtyards while main streets remain sealed We found a maximum cooling potential of up to -20.1 K for ground surface temperature and up to -7.1 K for air temperature in 1 m above ground level for a simulated 3-day heat wave in summer 2022 which represents a 20-year heat event in Cologne. On spatial average, a decrease of up to -11.1 K for surface temperature and up to -2.9 K for air temperature was determined. On temporal average for the 3-day heat event, statistically significant mean temperature differences of -5.8 K for surface temperature and -1.1 K for air temperature were simulated. Cooling effects are more pronounced during daytime for surface temperature especially on unshaded areas, while cooling effects for air temperature are strongest during nighttime. Model results also show that the entire air volume in the study area is cooled down due to this adaptation measure, even in areas of the domain where no surfaces have been unsealed in the scenario design. The more realistic GGP scenario shows cooling effects of a comparable magnitude as the extreme GGP scenario. Thus, even partial GGP unsealing is an effective adaptation measure for reducing extreme temperatures in cities if water availability is not limited.