Coupling Urban Microclimate and Heat and Mass Transfer Green Roof Models to Evaluate the Role of Green Roofs on Urban Heat Island Effect in a Semiarid Climate of Chile

The Urban Heat Island (UHI) phenomenon is a consequence of urbanization that intensifies city temperatures, adversely affecting public health, increasing cooling energy demand, and deteriorating air quality. Moreover, heat waves are more frequent and intense, causing thousands of fatalities. Green roofs have emerged as an effective strategy to mitigate these effects, potentially reducing urban temperatures up to 3°C, depending on the climate. However, most urban climate simulation tools do not adequately model the green roof evapotranspiration phenomena and consider green roof vegetation in very simplistic ways.

This research evaluates the effect of green roofs in several Santiago (Chile) neighborhoods on reducing urban air temperature. UWG, a microclimatic simulation tool in Matlab (Bueno et al., 2014), models vegetation in simple ways, which also happens in other urban climate tools. A heat and mass transfer green roofs (HMTGR) model developed and validated by Vera et al. (2019) in Matlab considers the complexities of biophysical properties of the plant-substrate system that affect sensible and latent heat and moisture transport. UWG and HMTGR tools were coupled at each time step to investigate the impact of vegetative roofs on reducing urban microclimate temperatures in Santiago, based on eighteen annual simulations varying the neighborhoods' density, the percentage of green areas at the pedestrian level,  and the green roof vegetation's cooling potential and coverage. The cooling potential of green roofs is given by choosing species with different stomatal resistance and leaf area index.

The simulation results were analyzed using normality tests (Shapiro-Wilk, Kolmogorov-Smirnov, and Jarque-Bera) and correlation tests (Pearson and Spearman). The statistical significance of the results was assessed using the non-parametric Wilcoxon signed-rank test. The main results evidence that green roofs significantly reduced urban temperatures. The most significant temperature reduction was 2°C for green roof species with high cooling potential.