Urban Heat Island Mitigation Research Articles

Evaluation of Uhi Mitigation Strategies in a Heterogenous Street Canyon in Kuala Lumpur from an Outdoor Thermal Comfort Perspective

Evaluation of Uhi Mitigation Strategies in a Heterogenous Street Canyon in Kuala Lumpur from an Outdoor Thermal Comfort Perspective

Analysis on the Effect of abnormal climate on Willingness to Pay for Urban Heat Island Mitigation Focused on Street Tree Planting Policy in Seoul, Korea

The paper attempts to analyze the effect of climate change on individual’s willingness to pay for urban heat island mitigation. In this paper, climate change represents change between heatwave and rainy season. Utilizing the contingent valuation method, each willingness to pay for street tree planting policy under two climate conditions is estimated. In order to specify the effect of climate change on willingness to pay, a dummy variable for two climate conditions is included as an explanatory variable for willingness to pay. Then, the statistical difference between the average willingness to pay under each condition is examined. The paper shows that the rainy season had a statistically significant negative effect on the willingness to pay for urban heat island mitigation. Also, it reveals that there is a statistical difference in the willingness to pay between heatwave and rainy season conditions. Finally, this paper suggests a policy direction for urban heat island mitigation based on the results obtained.

Kinetic Solar Envelope: Performance Assessment of a Shape Memory Alloy-Based Autoreactive Façade System for Urban Heat Island Mitigation in Athens, Greece

The paper explores the potentials of shape memory alloys (SMAs) for the design of autoreactive façade systems without using additional external energy. The exploration is conducted and assessed through the design of a façade concept for the city of Athens in Greece, aiming to improve both the indoor and outdoor environment by means of a kinetic autoreactive system featuring a dual-seasonal function, with a focus on the building’s direct and indirect impact on the urban heat island (UHI) effect. The paper covers a demonstration of the methodology followed, using a feedback-loop logic informed by environmental and energy performance evaluation studies in Grasshopper to optimize the geometry and movement of the shading component. During the façade design process, a comprehensive and systematic computational toolset is being developed, targeted on the abovementioned performance evaluation studies. Through the development and assessment of the façade concept, the objective is to explore the potentials and limitations for the application of autoreactive envelopes in the façade design. At the same time, the aim is to exploit the possibilities and optimization potentials offered through the developed iterative computational workflows, by showcasing the methodology and interoperability logic of the digital tools used for the data interchange.

How do urban morphological blocks shape spatial patterns of land surface temperature over different seasons? A multifactorial driving analysis of Beijing, China

Globally, rapid urbanization accompanied by a great deal of anthropogenic heat emission, further led to severe urban heat island (UHI) effects, posing a significant risk to human and environmental health. Understanding the combined driving effects of intra-urban temperature variation is vital to UHI mitigation. Although previous studies have extensively revealed the essential contribution of two-dimensional (2D) and three-dimensional (3D) architectural morphology to the land surface temperature (LST), their combined effects and seasonality on shaping the LST is still inconclusive, especially at finer scales. Therefore, this study aimed at broadening insight for thermal environment optimization from a block perspective. For the first time, the urban morphological blocks (UMBs) were introduced as the basic units to characterize the spatial-temporal variability of LST under various architectural morphological conditions. Multifactorial LST drivers, comprising architectural morphology, land cover, land use, and functions, were rigorously examined by the geographical detector model. The results showed that the highest LST generally occurred in the UMBs with the lowest building height and highest building density. The contribution of building aggregation to hot spots was most significant in the low-rise UMBs, while the cooling effects of the shadows were more pronounced in middle-density and high-density UMBs. In terms of different seasons, architectural morphological characteristics significantly contributed to LST variations under cold temperature, and the factors characterizing impervious surface and greening were always the two most powerful drivers in warm seasons. More factors contributed to the spatial patterns of LST variation in winter, suggesting a more complex LST driving mechanism under lower temperatures. Hierarchically, architectural morphologies, surface biophysical parameters, and urban land cover largely affected LST; however, the functional property factors were comparatively weak driving forces. The interaction detector model identified bilinear-enhanced and nonlinear-enhanced interactions from pairs of factors affecting LST over different seasons.

Influences of wind direction on the cooling effects of mountain vegetation in urban area

The mountain or hill with lots of natural vegetation in a city has obvious cooling effects, so it can play a significant role in the regulation of urban microclimate and mitigation of urban heat island (UHI). In this paper, the influences of wind direction on the cooling effects of the Zijin Mountain in Nanjing were evaluated. Firstly, the land surface temperature (LST) was obtained based on 15 Landsat-8 images from 2013 to 2020, and the wind directions in 4 h before the satellite crossing time were calculated. Then, two remote sensing images with opposite wind directions were compared and analyzed to explore the influences of wind direction on the cooling effects of the Zijin Mountain. The results showed that both the cooling buffer zone method and the cooling area method proposed in this paper had proved that the wind direction had great influences on the cooling effects of the mountain vegetation in urban area. In addition, it was also found that the larger the wind speed differences of group images, the greater the influences of the wind direction on the cooling effects. This study will be helpful and meaningful for urban microclimate analyses and urban planning research.

Microstructural and Pore Skeleton Characteristics of Pervious Concrete Containing RAP Aggregates Using X-Ray Microcomputed Tomography and Scanning Electron Microscope

Pervious concrete is special class of concrete that has application in reducing stormwater runoff and mitigation of urban heat islands (UHIs). This paper studied the morphological and pore structure characterization of pervious concrete (PC) mixes when reclaimed asphalt pavement (RAP) aggregates are used as a replacement of natural coarse aggregates. Scanning electron microscope (SEM) and X-ray microcomputed tomography (XRT) were used to quantify the three-dimensional (3D) changes in the pore structure of PC mix when RAP aggregates are used. SEM investigation conducted on 100% RAP validated that the involvement of RAP aggregates on PC mixes causes asphalt cohesion failure, which will be predominant in reducing the structural and functional properties of PC mixes. The 3D pore structure showed the large number of spheroid-shaped pores when binary-graded PC mix was used. These new findings on the pore morphology will assist in optimizing the use as well as selection of RAP aggregates when used in PC.

Environmental Sustainability Approaches and Positive Energy Districts: A Literature Review

During the last decade, increasing attention has been paid to the emerging concept of Positive Energy Districts (PED) with the aim of pushing the transition to clean energy, but further research efforts are needed to identify design approaches optimized from the point of view of sustainable development. In this context, this literature review is placed, with a specific focus on environmental sustainability within innovative and eco-sustainable districts. The findings show that some sustainability aspects such as sustainable food, urban heat islands mitigation and co-impacts, e.g., green gentrification, are not adequately assessed, while fragmented thinking limits the potential of circularity. In this regard, targeted strategies should be developed. On the other hand, the Key Performance Indicators framework needs some integrations. In this direction, indicators were suggested, among those defined in the Sustainable Development Agenda, the main European standards and initiatives and the relevant literature experiences. Future outlooks should be directed towards: the harmonization of the Life Cycle Assessment in PEDs with reference to modeling assumptions and analysis of multiple impacts; the development of dynamic environmental analyses taking into account the long-term uncertainty due to climate change, data availability and energy decarbonization; the combination of Life Cycle Assessment and Key Performance Indicators based techniques, from a holistic thinking perspective, for a comprehensive design environment and the analysis of the contribution of energy flexibility approaches on the environmental impact of a project.

Quantifying the relationship between land parcel design attributes and intra-urban surface heat island effect via the estimated sensible heat flux

Quantifying the relationship between land parcel design attributes (LPDAs) and associated surface urban heat island (SUHI) effect within cities is vital for enhancing urban adaption to artificial climatic modification. Taking four representative urban function zones of downtown Shanghai, China, as an example, this study investigated LPDAs and their associations with the summertime intra-SUHI effect, which was indicated with the parcel-based net blackbody sensible heat flux (NetBBF). Overall, the results revealed that the variation of the parcel-based NetBBFs should be attributed to the land parcels characterized with differential land developmental intensity. The results further showed that stepwise regression (SR) models exhibited lower statistical power for interpreting the relationships between the LPDA metrics and parcel-based NetBBFs, explaining 52.50–54.90% of the variance of the parcel-based NetBBFs. In contrast, the partial least square regression (PLSR) models better interpreted such relationships, explaining 59.9–71.6% of the variance of the parcel-based NetBBFs. The methodology and findings of this study can be referenced to facilitate the official decision-making towards sustainable land development, UHI mitigation, and climatic adaption.

Quantifying the dualistic nature of urban heat Island effect (UHI) on building energy consumption

UHI has significant impact on building energy consumption and associated greenhouse gas emissions in urban areas globally. However, the nature and extent of these effects are governed by the year-round climatic conditions at specific locations. In a dualistic relationship, UHI tends to increase the annual energy consumption of buildings located in hot climates and decrease it in cold climates. In this study, we conducted building energy simulations for 43 locations around the United States (U.S.) in order to quantify the relationships among the energy impacts of UHI and climatic variables. The energy impacts of UHI were found to be strongly correlated with several variables including degree days, absolute humidity, and solar radiation, while independent of wind speed and precipitation. Consecutively, we developed a multiple regression model to predict the relative energy impacts of UHI based on climatic context and evaluated the effect of UHI intensity on energy impacts across all 15 U.S. climate zones. The results of this study quantitatively validate the dualistic nature of UHI and present a statistical method to estimate the climatic variation in the impacts of UHI on building energy consumption. This can have significant implications in prioritizing UHI mitigation policies based on location and climate.

Passive action strategies in schools: A scientific mapping towards eco-efficiency in educational buildings

The research field on passive intervention strategies in schools is broad, complex, and fragmented due to the great diversity of disciplines, climates, and approaches. This article applies the scientific mapping software SciMAT to analyse research trends and developments from 1982 to 2020 of 537 papers and identifies the best available 24 passive intervention strategies in schools in 42 countries. The results show that, in the early years, research focused on natural ventilation, especially in arid climates. From 2010 onwards, and coinciding with the rise of energy efficiency regulations, green roofs increased as an alternative to declining urban forests and as a solution for urban heat island mitigation. In recent years, growing concerns about climate change, sustainable development, and numerical measurement methods have driven work on occupant comfort and IAQ, while research on cost overruns and payback of passive versus active design. The need for passive, climate-resilient design techniques is highlighted, building on the progress already made. It identifies the most optimised measures to promote guidelines to serve for future regulations. This study is a valuable contribution because it provides a detailed understanding of the status quo for researchers, practitioners, and policymakers and predicts the dynamic directions of the field.

Identifying Surface Urban Heat Island Drivers and Their Spatial Heterogeneity in China’s 281 Cities: An Empirical Study Based on Multiscale Geographically Weighted Regression

The spatially heterogeneous nature and geographical scale of surface urban heat island (SUHI) driving mechanisms remain largely unknown, as most previous studies have focused solely on their global performance and impact strength. This paper analyzes diurnal and nocturnal SUHIs in China based on the multiscale geographically weighted regression (MGWR) model for 2005, 2010, 2015, and 2018. Compared to results obtained using the ordinary least square (OLS) model, the MGWR model has a lower corrected Akaike information criterion value and significantly improves the model’s coefficient of determination (OLS: 0.087–0.666, MGWR: 0.616–0.894). The normalized difference vegetation index (NDVI) and nighttime light (NTL) are the most critical drivers of daytime and nighttime SUHIs, respectively. In terms of model bandwidth, population and Δfine particulate matter are typically global variables, while ΔNDVI, intercept (i.e., spatial context), and NTL are local variables. The nighttime coefficient of ΔNDVI is significantly negative in the more economically developed southern coastal region, while it is significantly positive in northwestern China. Our study not only improves the understanding of the complex drivers of SUHIs from a multiscale perspective but also provides a basis for urban heat island mitigation by more precisely identifying the heterogeneity of drivers.

The Effect of Plants on Extensive Green Roofs in Urban Heat Island Mitigation Efforts in Humid Tropical Cities

One of the urban heat island mitigation strategies in reducing urban temperatures in tropical cities is the application of a green roof system. This study compares the reduction in temperature and heat flow rate provided by three types of plants on extensive green roofs (EGR). We demonstrated that a EGR constructed with three types of plants (ground cover, and shrubs) could result in a decrease in temperature relative to the normal roof (NR). The results showed that the base temperature of the EGR of the bush and ground cover was lower than the base temperature of the NR which was 10.2ºC on indoor air, 17.8ºC on the inside and 19.1ºC on the outside. The peak indoor temperature was over 50ºC for the NR prototype. In the model with pennisetum purpureum schamach as the EGR, the maximum temperature was 40.1ºC, while for portulaca grandiflora and tradescantia spathacea the peaks were 37.6ºC and 37.5ºC, respectively. This shows that plants with large leaf widths are able to reduce heat greater than plants with small leaf widths.

Mitigation measures for urban heat island and their impact on pedestrian thermal comfort

A multiscale coupled model is presented that allows for the detailed analysis of the local impact of urban heat island mitigation measures. The model uses coupled computational fluid dynamics (CFD) simulations with unsteady heat and moisture transport (HAM) in porous urban materials in order to take into account the dynamic heat and moisture storage in the built environment. A realistic case study is performed for a public urban square in the City of Zurich during heat wave conditions. The impacts of two different mitigation strategies, i.e. adding artificial wetting of pavements and adding vegetation, on pedestrian thermal comfort are evaluated and compared with the existing situation. The results show an improvement in thermal comfort in both conditions. The improvement resulting from the addition of trees is larger and lasts longer due to shadowing effects, even though a reduced ventilation and an increased relative humidity by trees have an adverse effect on the thermal comfort.

Solar reflective ink-jet printed porcelain stoneware tiles as an alternative for Urban Heat Island mitigation

Digital ink-jet printing is a decoration system for ceramic tiles that requires the application of engobes with high whiteness and opacity, characteristics that can be used to design solar reflective ink-jet decorated tiles. These materials can help to mitigate Urban Heat Island (UHI) phenomenon. Here, the fabrication of ink-jet printed solar reflective tiles using an engobe with high solar reflectance was investigated. Two printing modalities (binary (BIN) and grayscale (GS)), five printing intensities, and four colours were tested. It was found that some of the prepared tiles can be used for mitigating the UHI phenomenon. The solar reflective properties were mainly derived from the reflective engobe, and in some grade, from the mineral composition of the inks. The colour and roughness measurements revealed that the GS modality could prepare ink-jet decorated tiles with good solar reflective properties with a smaller quantity of inks and good soiling resistance.

Nature-Based Solutions: Thermal Comfort Improvement and Psychological Wellbeing, a Case Study in Genoa, Italy

The urban heat island (UHI) effect is among the most critical issues caused by human activities and high building density. UHI has severe impacts on the urban and natural environment as well as on human health and wellbeing. The research presented here aims at evaluating the effects of nature-based solutions (NBS) in improving the livability of a district in the city of Genoa, which is heavily cemented and a major example of the heat island phenomenon. This study focuses on the microclimatic benefits of urban heat island mitigation as well as on psychological and perceptual aspects. A preliminary analysis of the district through CFD simulations using Envi-met software allowed for selection of the most suitable areas for a system of punctual interventions in urban regeneration using nature-based solutions. For each area identified, we simulated the effects of different design scenarios on microclimate mitigation and thermal comfort improvement. In addition, to evaluate the perceptual benefits of the most well-performing design scenarios, we set up a web-based survey that was administered to a convenience sample of Genoa residents. In terms of aesthetic satisfaction and perception of improved conditions of physical and psychological well-being, the preferred design outcomes were those which emphasized a freer and more natural environment. This study shows that nature-based solutions can improve the overall conditions of dense urban areas; microclimate performance and psychological effects should be both considered in the design process in order to improve the wellbeing of urban citizens.

Basic Principles, Most Common Computational Tools, and Capabilities for Building Energy and Urban Microclimate Simulations

This paper presents basic principles of built-environment physics’ modelling, and it reviews common computational tools and capabilities in a scope of practical design approaches for retrofitting purposes. Well-established simulation models and methods, with applications found mainly in the international scientific literature, are described by means of strengths and weaknesses as regards related tools’ availability, easiness to use, and reliability towards the determination of the optimal blends of retrofit measures for building energy upgrading and Urban Heat Island (UHI) mitigation. The various characteristics of computational approaches are listed and collated by means of comparison among the principal modelling methods as well as among the respective computational tools that may be used for simulation and decision-making purposes. Insights of coupling between building energy and urban microclimate models are also presented. The main goal was to provide a comprehensive overview of available simulation methods that can be used at the early design stages for planning retrofitting strategies and guiding engineers and technical professionals through the simulation tools’ options oriented to the considered case study.

Procedure for the selection and evaluation of prefabricated housing buildings for the implementation of green roofs in the context of Urban Heat Island mitigation. The example of Wrocław, Poland.

The assessment of the suitability of existing buildings for implementation of green roofs is an important research issue, especially in the context of Urban Heat Island (UHI), the negative impacts of which are locally exacerbated by the global warming. The studies carried out so far have covered a variety of buildings and have taken into account a range of different conditions. Relatively little attention has been paid to the possibilities of greening the roofs of prefabricated apartment blocks from the second half of the 20th century in the context of the potential climate effect. Yet, these buildings are found in many cities around the world, and seem in fact attractive for greening. In view of the above, we proposed a three-stage investigatory procedure to: (I) identify and classify buildings based on the number of floors and the rooftop available area; (II) select buildings by designating priority areas depending on the highest UHI intensity and roof density; (III) analyse the roof load capacity to develop retrofit scenarios. The procedure was applied to prefabricated housing estates built in the 1970s and 1980s in Wrocław, Poland. The research shows that there are 1962 buildings of different heights and roof area of 722405 m2, of which 480 buildings with a roof area of 122749.1 m2 were selected for greening within priority areas. The structure of the studied roofs was not designed to carry additional loads, which requires the application of complementary solutions. Scenario 1 assumes extensive greening provided that the existing ventilated roof is strengthened, scenario 2 –semi-intensive greening, which however requires the conversion of the ventilated roof to a non-ventilated one. The presented procedure can be applied in any other city with prefabricated apartment blocks and available UHI data, and serve to support the decision to implement green roofs to mitigate UHI.

Modelling spatiotemporal variations of the canopy layer urban heat island in Beijing at the neighbourhood scale

Abstract. Information on the spatiotemporal characteristics of Beijing's urban–rural near-surface air temperature difference, known as the canopy layer urban heat island (UHI), is important for future urban climate management strategies. This paper investigates the variation of near-surface air temperatures within Beijing at a neighbourhood-scale resolution (∼ 100 m) during winter 2016 and summer 2017. We perform simulations using the urban climate component of the ADMS-Urban model with land surface parameters derived from both local climate zone classifications and OpenStreetMap land use information. Through sensitivity simulations, the relative impacts of surface properties and anthropogenic heat emissions on the temporal variation of Beijing's UHI are quantified. Measured UHI intensities between central Beijing (Institute of Atmospheric Physics) and a rural site (Pinggu) during the Atmospheric Pollution and Human Health in a Chinese Megacity (APHH-China) campaigns, peak during the evening at ∼ 4.5 ∘C in both seasons. In winter, the nocturnal UHI is dominated by anthropogenic heat emissions but is underestimated by the model. Higher-resolution anthropogenic heat emissions may capture the effects of local sources (e.g. residential buildings and adjacent major roads). In summer, evening UHI intensities are underestimated, especially during heatwaves. The inability to fully replicate the prolonged release of heat stored in the urban fabric may explain this. Observed negative daytime UHI intensities in summer are more successfully captured when surface moisture levels in central Beijing are increased. However, the spatial correlation between simulated air temperatures and satellite-derived land surface temperatures is stronger with a lower urban moisture scenario. This result suggests that near-surface air temperatures at the urban meteorological site are likely influenced by fine-scale green spaces that are unresolved by the available land cover data and demonstrates the expected differences between surface and air temperatures related to canopy layer advection. This study lays the foundations for future studies of heat-related health risks and UHI mitigation strategies across Beijing and other megacities.

QUANTIFYING THE IMPACT OF URBAN INFILL ON THE URBAN HEAT ISLAND EFFECT – A CASE STUDY FOR AN ALTERNATIVE MEDIUM DENSITY MODEL

Abstract. Urban Heat Islands (UHIs) impact the quality of life in many urban centres. Metropolitan areas of Australian cities and urbanised regional centres, in particular, show vulnerability towards UHIs due to challenging climatic conditions and the model of greater subdivision of established properties whereby backyards and mature trees are replaced with more residential dwellings and sealed areas. The measurements for the UHI mitigation, such as imposing reforestation, employing sustainable and medium density housing build form typology must be quantified. Simulation-based identification and mitigation of UHIs can be used for planning decisions. There are several advantages to use simulations. For instance, alternative subdivision design, building design and the placement of trees and other measures is only required in the digital twin. Experimenting with the digital twin saves resources and maximises the outcome by being able to quantify the heat reduction. The aim of this work is to quantify the impact of the current urban infill methods on UHI. We focus on an area that has gone through a process of re-subdivision that is bounded by roads within Perth, Western Australia. For the same area we propose an alternative design with an urban infill model based on a medium density housing guidelines with an improved green space allocation. While the current model shows significant effects of UHIs we could mitigate those effects in the alternative proposed model. In our simulation the hottest surfaces are roads with a temperature of approximately 45 °C at 2PM in the afternoon. In the current model close to 100% of the road surfaces reach this temperature. In contrast, in the alternative model only 45% of all road surfaces reach this temperature, significantly reducing the impact of UHI for pedestrian walking close by to those roads. At 2PM, the most frequent temperature of all surfaces for the current model is around 44 °C while the alternative’s model the most frequent temperature is approximately 28 °C – a difference of 16 °C.

Development of microclimate modeling for enhancing neighborhood thermal performance through urban greenery cover

Green spaces and vegetation cover offer various environmental benefits, including building energy-saving. Enriching the urban vegetation is fundamental in urban heat island (UHI) mitigation policies promoted over the last years in many large North American cities. For example, the Greater Toronto Area (GTA) has been promoting significant increases in urban greenery by intensifying the urban trees and green roofs. In this paper, an updated version of the open-source code of the Urban Weather Generator was developed and validated to assess the effect of the increased horizontal and vertical green infrastructure on the microclimatic thermal performance of three urban typologies within the GTA. The updates of the model efficiently estimate the cooling and warming effects of the urban greenery cover on the microclimate thermal environment. The proposed enhancements included intensifying the tree canopy and incorporating the vegetated façades and green roofs. The validation ensured the model’s ability to predict the cooling and warming effects of the green infrastructure. The results confirmed that tree canopy and green façades were the most effective mitigation strategies for reducing the canyon air temperature and building energy consumption, respectively. A parametric analysis utilizing a multi-objective algorithm was also designed to evaluate the effectiveness and affordability of the applied strategies. The results showed that the ambient peak air temperature, the outdoor heat stress index, and building energy savings were reduced by up to 4.6˚C, 5.4, and 42.4%, respectively. Considering reducing the green infrastructure cost by 50%, the parametric study resulted in a reduction of up to 3˚C, 2.9, and 35.6% in the ambient peak air temperature, the outdoor heat stress index, and building energy savings, respectively.