Cities are often much hotter than their surrounding rural areas. This is a result of darker materials such as asphalt that absorb more solar energy, denser materials such as concrete that store more heat, lack of vegetation that reduces cooling by evaporation, and the various sources of heat in the city such as cars or building heating. The resulting Urban Heat Island (UHI) poses a threat to humans, especially during the summer and during heatwaves. Mitigation of this hazards requires accurate tools to predict the UHI and to examine how it can be reduced. This project focused on (i) detailed physical modeling of the UHI phenomenon, (ii) fast probabilistic modeling of the UHI, and (iii) reliable assessment of heat mitigation strategies.

Intellectual Merit: On the physical modeling side, we clarified how air flows in cities at the street scale and the whole city scale, and what is the impact on temperature that pedestrians experience. We demonstrated that roof shape has a significant impact on air and heat flow, and that a strong UHI can create a bubble that recirculates air into the city, preventing good ventilation (Fig. 1). We illustrated and quantified how rainfall can quench that heat and cool the city. We developed approaches for emulating the impact of cities in coarse climate models where the urban physics are not sufficiently captured. On the probabilistic modeling side, we collaborated with the lead institution on the grant (Carnegie Mellon University) to develop two different approaches for rapidly modeling the UHI. These would allow city managers to quickly update their extreme heat forecasts to have better, faster information when dealing with heat extremes and their hazards. Both the physical and probabilistic models were then used to develop strategies for monitoring urban heat extremes more reliably and with fewer sensors.

Broader Impact: For mitigation, we explored novel material that change color with temperature to reflect more solar energy in the summer but absorb more in the winter. We examined a large range of worldwide cities to infer how mitigation measure effectiveness depend on background climate and city size, finding that urban green infrastructure is most effective in arid regions but it would require irrigation. We modeled a human being in the city to examine thermal comfort directly rather than inferring it from temperature and radiation. We showed that cities interested in mitigating their UHI should carefully consider the scale of implementation and how the benefit depends on the location of the urban core relative to the rest of the city. This scale of implementation could benefit from a new conceptual framework of environmental neighborhoods that we introduced, defined as the surrounding area influencing the environmental quality at a given point in a city.

Extreme heat is one of the most lethal natural risks humanity faces (estimates of heat fatalities in the US range between 500 and 1000 per year, more than any other natural disaster). Our findings point to the fact that a single silver bullet (green roofs or cool roofs for example) will not solve the Urban Heat problem and a portfolio of interventions is needed. Globally, this project significantly advanced the science needed by city managers to make better designs and decisions for combating heat. We are now developing syntheses specifically targeted for city decision makers.

Last Modified: 10/08/2021
Modified by: Elie R Bou-Zeid