Green and Cool Roofs to Mitigate Urban Heating: An Analysis with a Regional Climate Model

Escalating urban population growth has put a burden on the local environments and climates of urban areas such as Chicago. As urban dwellers continue to rise to a predicted 70% of world population by 2050 [UNPD 2008], there is a greater impetus to adopt policies to make cities more sustainable and more resilient against natural and man-made hazards, like heat waves. This paper addresses one related physical phenomenon threatening urban sustainability called urban heat island (UHI) – elevated urban temperatures in comparison to nearby rural and non-urban areas – and potential UHI mitigation strategies with a regional climate model that incorporates green and cool roofs. Primarily UHI exists due to human-induced modifications of the atmospheric and surface properties of urban regions, in particular, reduced vegetation, high thermal capacity building materials, anthropogenic heat emissions, trapping of outgoing radiative heat flux in urban canyons, and increased surface roughness. When heat waves envelop cities and combine with UHI, they have disproportionate impacts on human mortality, economies, and local ecosystems within cities. UHI also affects the lowermost layer of atmosphere, the boundary layer, where surface effects of mass, momentum and heat transport are critical to human infrastructure, modifying the heat and moisture exchange between land surface and atmosphere. Over Chicago, complex urbanization with varying building heights and surface types, lake breeze, and UHI lead to a complex boundary layer structure, which is studied in our case with a regional climate model as high-density field measurements are wanting. Although Chicago benefits from its proximity to the Lake Michigan and resultant lake breeze that acts as a natural UHI mitigation mechanism, there remains a need for man-made UHI solutions. In this study, we modeled the Chicago UHI with physically based schemes for varying green roof fractions and cool roofs during a hot summer period of August 2013 with an urban-WRF (uWRF) model. The green roof model has a 4-layer soil structure with grassland and loam as vegetation and soil type, including a dynamic irrigation algorithm to enhance soil moisture over urban ground vegetation and green roofs. This green roof model generates a cooling effect as the net radiation is conserved and sensible heat is reduced to compensate for the increased latent heat due to evapotranspiration from vegetation and soil moisture. The cool roof model reduces overall net radiation available at the roof by reflecting incoming shortwave radiation with a high-albedo surface material like glass/solar panels or white paint. For our simulations, we modified albedo for white paints (0.85) to simulate the impact of cool roofs on UHI. The performance of uWRF with green and cool roof algorithms was verified and validated using surface sensible heat flux and air temperature measurements from an urban field campaign in a Chicago neighborhood. The uWRF model's predicted diurnal cycle of sensible heat fluxes and 2-m air temperature compared well with real-world conditions for conventional and green rooftops. Green roof results showed that daytime change in UHI reduced linearly with increasing green roof fractions, from less than 1 °C to as much as 3 °C during peak daytime for 25% green roof case and 100% case, respectively. Cool roofs were evaluated for only one test case of 100% cool roof, and results showed that daytime surface temperatures for core high intensity urban areas were reduced 7-8 °C, a ~1 °C greater reduction than 100% green roof case. An analysis of surface energy balance for both green and cool roof verified our hypothesis for energy distribution with respective algorithms. The lower atmosphere's temperature, winds and relative humidity also changed due to reduction in UHI, as the boundary layer depth decreased with green and cool roofs during daytime convective period.