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Cool roofs represent an acknowledged, relatively simple, and low-cost strategy to reduce cooling energy demand of buildings and mitigate urban heat island phenomena. The purpose of this paper is to study the coupled passive-active effect produced by such a technology, where the active effect consists of the cool roof capability to decrease the suction air temperature of heat pump external units, when these units are located over the roof. This \"cooling\" benefit produces an extra increase of the energy performance of the heat pump in cooling mode, given that it produces the decrease of the temperature lift between the source and the output. In order to study this twofold effect, an industrial building with an office area located in Rome, Italy, was continuously monitored in summer 2012. The main results showed that the cool roof allows to decrease the roof overheating up to 20°C. The office indoor air temperature also decreased, even if the same set-point temperature was kept constant during the whole campaign. The energy requirement for cooling decreased by about 34%. In order to investigate the \"active\" contribution, suction air temperature was monitored and a new simple analytical model is proposed in order to estimate the cool roof active effect.

The urban heat island impacts the thermal comfort of pedestrians in cities. In this paper, the effects of four heat mitigation strategies on micrometeorology and the thermal comfort of pedestrians were simulated for a neighborhood in eastern Los Angeles County. The strategies investigated include solar reflective 'cool roofs', vegetative 'green roofs', solar reflective 'cool pavements', and increased street-level trees. A series of micrometeorological simulations for an extreme heat day were carried out assuming widespread adoption of each mitigation strategy. Comparing each simulation to the control simulation assuming current land cover for the neighborhood showed that additional street-trees and cool pavements reduced 1.5 m air temperature, while cool and green roofs mostly provided cooling at heights above pedestrian level. However, cool pavements increased reflected sunlight from the ground to pedestrians at a set of unshaded receptor locations. This reflected radiation intensified the mean radiant temperature and consequently increased physiological equivalent temperature (PET) by 2.2 °C during the day, reducing the thermal comfort of pedestrians. At another set of receptor locations that were on average 5 m from roadways and underneath preexisting tree cover, cool pavements caused significant reductions in surface air temperatures and small changes in mean radiant temperature during the day, leading to decreases in PET of 1.1 °C, and consequent improvements in thermal comfort. For improving thermal comfort of pedestrians during the afternoon in unshaded locations, adding street trees was found to be the most effective strategy. However, afternoon thermal comfort improvements in already shaded locations adjacent to streets were most significant for cool pavements. Green and cool roofs showed the lowest impact on the thermal comfort of pedestrians since they modify the energy balance at roof level, above the height of pedestrians.

The effects of urban heat islands (UHIs) have a substantial bearing on the sustainability of cities and environs. This paper examines the efficacy of green and cool roofs as potential UHI mitigation strategies to make cities more resilient against UHI. We have employed the urbanized version of the Weather Research and Forecasting (uWRF) model at high (1 km) resolution with physically-based rooftop parameterization schemes (conventional, green and cool), a first-time application to the Chicago metropolitan area. We simulated a hot summer period (16-18 August 2013) and assessed (i) UHI reductions for different urban landuse with green/cool roofs, (ii) the interaction of lake breeze and UHI, and (iii) diurnal boundary layer dynamics. The performance of uWRF was evaluated using sensible heat flux and air temperature measurements from an urban mini-field campaign. The simulated roof surface energy balance captured the energy distribution with respective rooftop algorithms. Results showed that daytime roof temperature reduced and varied linearly with increasing green roof fractions, from less than 1 °C for the case of 25% green roof to ∼3 °C during peak daytime for 100% green roof. Diurnal transitions from land to lake breeze and vice versa had a substantial impact on the daytime cycle of roof surface UHI, which had a 3-4 hour lag in comparison to 2 m UHI. Green and cool roofs reduced horizontal and vertical wind speeds and affected lower atmosphere dynamics, including reduced vertical mixing, lower boundary layer depth, and weaker convective rolls. The lowered wind speeds and vertical mixing during daytime led to stagnation of air near the surface, potentially causing air quality issues. The selection of green and cool roofs for UHI mitigation should therefore carefully consider the competing feedbacks. The new results for regional land-lake circulations and boundary layer dynamics from this study may be extended to other urbanized areas, particularly to coastal areas.

Urban environments and heat waves interact synergistically and aggravate the thermal environment through the urban heat island (UHI) effect. Of concern is the potential for a projected warmer future climate to further compound heat waves in urban environments. The present study investigates the interaction of a 2006 heat wave in North America with two urban environments (Phoenix and New York City (NYC)) in current climate and future climate simulations. The future climate conditions were generated using the pseudo-global warming methodology. Multiple high-resolution (3 km) simulations were performed using the weather research and forecasting (WRF) model coupled with the single-layer urban canopy model to improve representation of urban processes and we explore how irrigated green roofs and cool roofs can mitigate heat wave amplification by UHIs. To quantify heat wave intensity, an analytical model is applied to the WRF model output that considers the urban surface heat and water vapor exchanges with the atmosphere. A future, warmer climate is found to amplify the UHI intensity during heat waves in both Phoenix (21%) and NYC (48%), but the amplification is of great uncertainty as its magnitude is comparable to the temporal variability of temperatures. The increase in urban heat index can be almost completely offset by adopting irrigated green roofs in urban areas, and partially offset by adopting cool roofs.

Mitigation of the urban heat island (UHI) effect at the city-scale is investigated using the Weather Research and Forecasting (WRF) model in conjunction with the Princeton Urban Canopy Model (PUCM). Specifically, the cooling impacts of green roof and cool (white high-albedo) roof strategies over the Baltimore-Washington metropolitan area during a heat wave period (7 June-10 June 2008) are assessed using the optimal set-up of WRF-PUCM described in the companion paper by Li and Bou-Zeid (2014). Results indicate that the surface UHI effect (defined based on the urban-rural surface temperature difference) is reduced significantly more than the near-surface UHI effect (defined based on urban-rural 2 m air temperature difference) when these mitigation strategies are adopted. In addition, as the green and cool roof fractions increase, the surface and near-surface UHIs are reduced almost linearly. Green roofs with relatively abundant soil moisture have comparable effect in reducing the surface and near-surface UHIs to cool roofs with an albedo value of 0.7. Significant indirect effects are also observed for both green and cool roof strategies; mainly, the low-level advection of atmospheric moisture from rural areas into urban terrain is enhanced when the fraction of these roofs increases, thus increasing the humidity in urban areas. The additional benefits or penalties associated with modifications of the main physical determinants of green or cool roof performance are also investigated. For green roofs, when the soil moisture is increased by irrigation, additional cooling effect is obtained, especially when the 'unmanaged' soil moisture is low. The effects of changing the albedo of cool roofs are also substantial. These results also underline the capabilities of the WRF-PUCM framework to support detailed analysis and diagnosis of the UHI phenomenon, and of its different mitigation strategies.

Assessment of mitigation strategies that combat global warming, urban heat islands (UHIs), and urban energy demand can be crucial for urban planners and energy providers, especially for hot, semi-arid urban environments where summertime cooling demands are excessive. Within this context, summertime regional impacts of cool roof and rooftop solar photovoltaic deployment on near-surface air temperature and cooling energy demand are examined for the two major USA cities of Arizona: Phoenix and Tucson. A detailed physics-based parametrization of solar photovoltaic panels is developed and implemented in a multilayer building energy model that is fully coupled to the Weather Research and Forecasting mesoscale numerical model. We conduct a suite of sensitivity experiments (with different coverage rates of cool roof and rooftop solar photovoltaic deployment) for a 10-day clear-sky extreme heat period over the Phoenix and Tucson metropolitan areas at high spatial resolution (1-km horizontal grid spacing). Results show that deployment of cool roofs and rooftop solar photovoltaic panels reduce near-surface air temperature across the diurnal cycle and decrease daily citywide cooling energy demand. During the day, cool roofs are more effective at cooling than rooftop solar photovoltaic systems, but during the night, solar panels are more efficient at reducing the UHI effect. For the maximum coverage rate deployment, cool roofs reduced daily citywide cooling energy demand by 13–14 %, while rooftop solar photovoltaic panels by 8–11 % (without considering the additional savings derived from their electricity production). The results presented here demonstrate that deployment of both roofing technologies have multiple benefits for the urban environment, while solar photovoltaic panels add additional value because they reduce the dependence on fossil fuel consumption for electricity generation.

To simulate realistic impact of rooftop-based heat mitigation strategies (HMS), we introduce a new capability in the Weather Research and Forecasting model coupled with the building effect parameterization urban physics module. This enhancement allows the model to ingest two dimensional fields of grid aggregated roof albedo and fractional rooftop area availability for HMS implementation. By incorporating these spatially varying inputs, the model overcomes previous limitations that relied on idealized scenarios. Using Austin, Texas, as a testbed metropolitan area, we assessed the realistic potential for cooling the city by reducing the 2 m air temperature (T2M) and universal thermal climate index (UTCI) through various HMS. Specifically, we evaluated cool roofs, green roofs, and solar photovoltaics under two scenarios: (i) complete roof area coverage and (ii) realistic implementation based on the available flat (non-pitched) roof area. Additionally, we implemented urban gardens and street trees and assessed their effectiveness in reducing T2M and UTCI. All experiments, including a control scenario without any HMS, were conducted during clear-sky days in August 2020, which was one of the hottest months recorded in Austin. The results indicate that while cool and green roofs are effective, their effectiveness is obfuscated in the city-wide mean T2M and UTCI reduction. Further, the realistic implementation of HMS had a negligible impact on city-wide mean T2M and UTCI. Nevertheless, these realistic rooftop simulations demonstrated potential for localized cooling in some areas of the city. Combining realistic rooftop-based HMS with urban gardens and street trees similarly resulted in limited city-wide cooling while yielding noticeable cooling in some grid cells. Street trees showed a large potential for locally reducing UTCI. These findings have direct implications for urban planning and heat mitigation approaches, as they highlight the need for evaluating HMS based on local (e.g. urban-block-scale) impacts using realistic implementation constraints, rather than emphasizing city-wide reductions in T2M or UTCI.

A novel material open to warm air stays below ambient temperature under maximum solar intensities of mid‐summer. It is found to be 11 °C cooler than a commercial white cool roof nearby. A combination of specially chosen polymers and a silver thin film yields values near 100% for both solar reflectance, and thermal emittance at infrared wavelengths from 7.9 to 13 μm.

The air temperature cooling impacts of infrastructure-based adaptation measures in expanding urban areas and under changing climatic conditions are not well understood. We present simulations conducted with the Weather Research and Forecasting (WRF) model, coupled to a multi-layer urban model that explicitly resolves pedestrian-level conditions. Our simulations dynamically downscale global climate projections, account for projected urban growth, and examine cooling impacts of extensive cool roof deployment in Atlanta, Detroit, and Phoenix (USA). The simulations focus on heatwave events that are representative of start-, middle-, and end-of-century climatic conditions. Extensive cool roof implementation is projected to cause a maximum city-averaged daytime air temperature cooling of 0.38 °C in Atlanta; 0.42 °C in Detroit; and 0.66 °C in Phoenix. We propose a means for practitioners to estimate the impact of cool roof treatments on pedestrian-level air temperature, for a chosen roof reflectivity, with a new metric called the Albedo Cooling Effectiveness (ACE). The ACE metric reveals that, on average, cool roofs in Phoenix are 11% more effective at lowering pedestrian-level air temperature than in Atlanta, and 30% more effective than in Detroit. Cool roofs remain similarly effective under future heatwaves relative to contemporary heatwaves for Atlanta and Detroit, with some indication of increased effectiveness under future heatwaves for Phoenix. By highlighting the underlying factors that drive cooling effectiveness in a trio of cities located in different climatic regions, we demonstrate a robust framework for estimating the pedestrian-level cooling impacts associated with reflective roofs without the need for computationally demanding simulations.

This paper investigates the thermal performance of green roofs, cool roofs, and their combined effects in tropical climates. Although each technology has been studied independently for its potential to reduce cooling energy consumption in buildings, their combined effects have not been thoroughly examined in tropical climates. The study employed experimental and numerical methods, demonstrating that combining green and cool roofs can lead to even greater cooling energy savings. The research involved fabricating four identical cubicles made of mild steel sheets and placing them in an open space for testing under two operational conditions: closed window and open window/door. The combined green and cool roof technology achieved a temperature difference reduction of 4.14 °C compared to the original roof under the closed window and door state, with green and cool roofs achieving 0.72 °C and 0.79 °C, respectively. Combining green and cool roofs led to even more significant cooling energy savings, with 53.57 kWh energy savings compared to 20.1 kWh and 3.68 kWh for combined, green, and cool cubicles, respectively. The study found that combining green and cool roofs led to even more significant cooling energy savings, with 53.57 kWh energy savings compared to 20.1 kWh and 3.68 kWh for combined, green, and cool cubicles, respectively. The research suggests that combining these technologies can lead to greater cooling energy savings and highlights the potential benefits of green and cool roofs for tropical climates.