Global Urban Heat Island Mitigation

By Ansar Khan

Global Urban Heat Island Mitigation provides a comprehensive picture of global UHI micro-thermal interaction in different built environments. The book explains physical principles and how to moderate undesirable consequences of swift and haphazard urban development to create more sustainable and resilient cities. Sections provide extensive discussion on numerous UHI mitigation technologies and their effectiveness in cities around the globe. In addition, the book proposes novel UHI mitigation technologies and strategies while also assessing the effectiveness and suitability of UHI mitigation interventions in various climates and urban forms.

• Adopts a multidisciplinary approach, bridging theoretical and applied urban climatology with urban heat mitigation
• Compiles disparate urban climate research concepts and technologies into a coherent framework
• Includes contributions from leaders in fields from around the globe

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 15, 2022
• ISBN: 9780323897945

Book preview

Chapter 1: Mapping and management of urban shade assets

a novel approach for promoting climatic urban action

Or Aleksandrowicz     Faculty of Architecture and Town Planning, Technion – Israel Institute of Technology, Technion City, Haifa, Israel

Abstract

One of the key challenges in urban heat island mitigation is the efficient allocation of resources to the most effective measures and to locations where they are most needed. While our knowledge on the potential of certain heat mitigation measures to reduce urban heat is generally broad and scientifically sound, in many times we still lack effective tools that would promote their large-scale adoption according to evidence-based prioritization policies. To drastically change the way our cities are planned and managed vis-a-vis the current climatic crisis, we should therefore also advance action prioritization methods that could support efficient and effective implementation of the measures we consider to be the most effective.

Planting of shade trees is regarded as one of the most promising heat mitigation measures because it can be widely and easily promoted and controlled by municipal planners and decision-makers for mitigating the effects of climatic vulnerabilities caused by exposure to solar radiation. Taking in mind this exceptional significance of street trees, we developed a novel and comprehensive approach to a city-scale analysis of street-level outdoor shade conditions that enables a municipality to effectively direct its shade intensification efforts, mainly through tree planting, to shade-deficient locations and to quantify the planting potentials of street trees according to the physical properties of the cityscape. The method is based on the production of shade maps that reflect the spatial hierarchy of street-level shading conditions in the city and on the high-resolution and comprehensive mapping of street trees. The article exemplifies how the method could be implemented in urban planning, taking Tel Aviv-Yafo, a city of hot summer Mediterranean climate, as a case study.

Keywords

Evidence-based urban design; GIS; Shade maps; Tree maps; Urban forest; Urban green infrastructures; Urban shade

1. Urban planning and urban microclimate: a challenge of knowledge communication

The term Urban Heat Island (UHI) describes microclimatic conditions in cities that are warmer than in surrounding rural areas (Arnfield, 2003; Landsberg, 1981; Oke, 1982, 1987; Stewart, 2019; Voogt, 2002). UHIs form because of the physical configuration of cities (including their topography, spatial morphology, and building density), the materials used for the construction of buildings, pavements, and roads, the flow of air through the street network, and heat-producing human activities such as transportation and industry (Akbari et al., 2001; Erell et al., 2011; Gartland, 2008; Grimmond, 2007; Santamouris, 2001; Taha, 1997; Unger, 2004). The most common indicator of urban overheating is higher air temperatures, usually recorded at street level. An increase in air temperatures can have diverse negative effects, including exacerbated outdoor heat stress (during daytime and nighttime alike), deteriorated air quality, increased energy consumption on indoor cooling, and even increase in mortality rates (Arnfield, 2003; Johansson and Emmanuel, 2014; Kleerekoper et al., 2012; Nikolopoulou et al., 2001; Oke, 1981).

Despite the solid scientific understanding of the relation between the physical composition of cities and urban climates, climatic factors are rarely considered in the common practice of urban planners and designers and, therefore, still have a relatively small impact on cities (Erell, 2008; Hebbert and Mackillop, 2013; Mills, 2014; Parsaee et al., 2019). This may be partly attributed to knowledge gaps among planning professionals that limit their ability to collect, comprehend, and analyze quantitative microclimatic data. As a result, planners and designers are usually unable to project and simulate the effects of certain design strategies on micro-level and local-level climatic conditions in cities (Heaphy, 2017; Hebbert, 2014; Mills, 2006, 2014; Oke, 2006). Even in cities that already promote climatic policies (Brandenburg et al., 2018; Francis et al., 2014; Osmond and Sharifi, 2017; Ruefenacht and Acero, 2017; Shorris, 2017), actions mainly focus on setting generalized guidelines and not on detailed planning of their concrete implementation. Detailed and site-specific climatic planning is still required for enhancing outdoor conditions because of the local climatic variance resulting from the inherent morphological, physical, and land-use diversity of streets and neighborhoods in every city.

The persisting failures in integrating scientifically sound and detailed climatic considerations into urban planning and design are becoming a major impediment to genuinely addressing the challenges of climate change, and, therefore, urgently call for new methods of translating scientific knowledge into terminologies, methodologies, and toolkits commonly understood and used by planners (Heaphy, 2017; Mauser et al., 2013; Mills et al., 2010). The main point of concern in this respect may lie in the inherently complex nature of urban climates and the diversity of factors affecting it, including air temperature, relative humidity, wind speed, incoming solar radiation, surface albedo and thermal absorption of materials, heat-generating human activities, air pollution levels, and the climatic performance of trees and vegetation (Erell et al., 2011; Mills, 2014). The main challenge is, therefore, to find ways for simplifying the transfer of climatic knowledge to planning professionals without compromising the quality, accuracy, and effectiveness of the decision-making process.

This chapter outlines a novel, quantitative approach to the evaluation of certain climatic properties of urban environments that could more easily be adopted in urban planning and design processes. It first examines the problematic nature of our current ways of evaluating urban overheating and the challenges of elucidating the outcomes of evaluation in directing concrete and effective planning and design actions. It then considers the applicability of a variety of common heat mitigation tools in real-life situations, acknowledging the limitations of municipal action. Based on this analysis, we then argue for a greater emphasis on outdoor shade provision in the adoption of climate-responsive urban policies and suggest a set of quantitative indicators that can support such policies. The chapter concludes with a demonstration of the applicability of the suggested indicators in actual situations, using Tel Aviv-Yafo, a city of hot summer Mediterranean climate, as a case study.

2. Correlating the language of urban morphology with its climatic outcomes

Until recent years, the quantification of urban overheating was usually done using the simple indicator of urban heat island intensity (Oke, 1987, p. 289), reflecting the maximum daily air temperature difference between air temperatures measured at a representative urban location and in a city's rural periphery. Evidently, this single indicator cannot capture the intricate variance in microclimatic conditions between different city parts, neighborhoods, and streets. It also ignores the effect of radiation fluxes on outdoor thermal comfort (Erell, 2017; Martilli et al., 2020). Therefore, additional and more precise indicators for describing urban climate should be developed and adopted for climatic urban planning and design.

One of the most promising approaches for enhancing the integration of scientific understanding of microclimatic effects in cities into urban planning and design is through the identification of statistically significant relations between certain morphological aspects of urban environments and the climatic properties of the same areas. Since planners regularly engage themselves in morphological questions by determining the spatial configuration and material properties of buildings, streets, neighborhoods, and cities, the language and terminologies of urban morphology are well understood by them. Thus, the identification of strong correlations between specific urban morphological properties and climatic conditions in cities can greatly enhance the capacity of planners to direct design decisions and actions in a way that will consciously and intentionally induce specific microclimatic conditions.

A recent comprehensive attempt to relate urban morphologies to climatic conditions was the development of the Local Climate Zone (LCZ) classification scheme by Stewart and Oke (2012). The system was initially conceived to address the ill-defined urban–rural dichotomy prevalent in many UHI studies, which is open to a variety of interpretations and does not describe well major differences in basic physical characteristics of cities and their environs (including building heights and densities, surface materials, and patterns of vegetative infrastructure). To resolve this problem, Stewart and Oke proposed a classification scheme that could be used to objectively define an environment based on several quantifiable morphological properties, as well as on its material characteristics.

Stewart and Oke's underlying assumption was that each LCZ, which can be defined as a region of uniform surface cover, structure, material, and human activity that span[s] hundreds of meters to several kilometers in horizontal scale (Stewart and Oke, 2012, p. 1884), produces distinctively different patterns of air temperature fluctuations at screen height (usually 1.2m above ground) precisely because of the uniformity of its main physical features. While several comprehensive studies during the last decade have supported this assumption (Beck et al., 2018; Fenner et al., 2017; Kotharkar and Bagade, 2018; Leconte et al., 2015, 2018, 2021; Mandelmilch et al., 2020; Middel et al., 2014; Ren et al., 2019; Shi et al., 2018; Skarbit et al., 2017; Stewart and Oke, 2010), the climatic footprint of an LCZ changes from one city to another because of differences induced by their geographic location and overall composition and should, therefore, be mapped in each city separately (Stewart and Oke, 2015).

The LCZ classification scheme consists of 17 morphologically homogeneous urban formations, divided into 10 built types (compact high-rise, compact mid-rise, compact low-rise, open high-rise, open mid-rise, open low-rise, lightweight low-rise, large low-rise, sparsely built, and heavy industry) and seven land cover (or unbuilt) types (dense trees, scattered trees, bush/scrub, low plants, bare rock or paved, bare soil or sand, and water) (Stewart and Oke, 2012, 2015). Differentiation between the built types is almost entirely based on building morphology: three levels of average building height (above 25m, 10–25m, and 3–10m) and several levels of building surface fraction (ratio of building plan area to total plan area). By definition, the open built types, as well as the sparsely built type, consist of substantial quantities of trees and vegetation interwoven into the built fabric (Stewart and Oke, 2012). It is true that other combinations between building heights, built densities, surface cover, and vegetation types may exist in the real world, but the LZC classification scheme is intentionally meant to cover only the most common and familiar situations. This is done to prevent overclassification that would reduce the system's capability to standardize communication and reporting on urban climatic conditions, including intercity comparisons (Stewart and Oke, 2012, 2015).

While the definition of each of the LCZs is primarily given in verbally descriptive terms, the classification scheme also relates LCZ types to value ranges of several morphological properties of the urban environment, thus ensuring that the classification process is based not strictly on subjective impressions but also on objective and quantifiable factors. These morphological properties include sky view factor (SVF, the ratio of the part of sky hemisphere visible from ground level to that of an unobstructed hemisphere), street aspect ratio (mean height-to-width ratio of street canyons), building surface fraction, impervious and pervious surface fraction, and height of roughness elements (mainly buildings) (Stewart and Oke, 2012). While the urban environment can be described by using other morphological metrics and definitions (Dibble et al., 2019; Serra et al., 2018; Taleghani et al., 2015; Venerandi et al., 2017), it seems that Stewart and Oke chose the main properties that they perceived as contributing the most to the creation of local climatic conditions in cities.

According to Stewart and Oke, the way the classification scheme correlates urban morphology and urban climate offers a basic package of urban climate principles for architects, planners, ecologists, and engineers (Stewart and Oke, 2012, p. 1894) and can, therefore, be helpful in integrating urban climatic knowledge into city planning (Perera and Emmanuel, 2018). While the question whether these expectations for integration has been met is still open, it can be argued that the LCZ system already created a promising path for such an integration by correlating urban morphology with at least one important climatic factor (air temperature distribution across urban environments).

Notwithstanding its apparent merits, the LCZ system still suffers from two major drawbacks that limit its application in urban planning and design. The first is that of scale: LCZs are inherently defined as representing morphological homogeneity at the local level (from hundreds of meters to several kilometers) and are, therefore, not intended to capture micro-scale climatic differences at street level (Stewart and Oke, 2015). The second drawback of the LCZ system is its usability for outdoor thermal comfort evaluation; the system was originally developed to reflect intraurban differences in air temperatures and not in thermal comfort conditions. Although several studies have recently attempted to correlate LCZs to outdoor thermal comfort according to common comfort indices (Lau et al., 2019; Unger et al., 2018; Verdonck et al., 2018), they are, in a way, ignoring the fact that outdoor thermal comfort levels can show significant variance at street level (Quanz et al., 2018) because of micro-scale physical differences (such as street orientation, tree canopy cover, etc.) that are not considered in the LCZ classification scheme.

While the morphological approach to climatic urban design can become effective at certain scales, there is still a need for additional tools that would help planners and designers to effectively control the negative microclimatic effects of urban environments, focusing not only on air temperature levels but also on outdoor thermal comfort. Yet, such a morphological approach also lacks in a more fundamental sense, namely its applicability to mitigating heat stress in older city parts. While new developments can easily follow certain morphological guidelines to secure better climatic performance, a comprehensive morphological transformation of existing neighborhoods and city parts is in many times impractical. These older areas, typically located at a city's core, are also the same areas in which urban overheating is usually more profound. For these situations, evaluation of the applicability and effectiveness of available heat mitigation measures is, therefore, required.

3. Urban heat island mitigation measures: the limits of intervention

The concept of UHI mitigation describes strategies, measures, or actions that are meant to ameliorate the negative effects of urban overheating through modifications of the physical environments of cities (Aleksandrowicz et al., 2017; Gago et al., 2013; Solecki et al., 2005). Its origins probably date back to the pioneering work of Hashem Akbari, Arthur Rosenfeld, and Haider Taha at the Lawrence Berkeley National Laboratory. In 1985, they founded the Heat Island Research Project and in 1989, organized a first workshop that focused not only on the phenomenon of UHI itself, but also on the ways to mitigate it (Huang et al., 1990; Rosenfeld, 1999). Since then, research on the possible strategies to mitigate UHI intensity has significantly expanded and developed, creating a solid and rigorous body of knowledge that could potentially benefit urban planners and designers.

Contemporary scientific literature documents multiple mitigation measures that may be applied to combat urban overheating, with diverse perspectives or classification systems suggested by different authors (Gago et al., 2013; Gartland, 2008; Giguère, 2009; Jamei et al., 2016; Kleerekoper et al., 2012; Nuruzzaman, 2015; Wong and Jusuf, 2013). Following a previous work by Aleksandrowicz et al. (2017), this section classifies the central mitigation measures according to the physical domain of intervention: building envelopes; pavements and roads; urban landscape elements; and the geometric and morphological aspects of streets. This type of categorization reflects a practical approach toward UHI mitigation, focusing on concrete actions that can be realized as part of climatic urban policy.

3.1. Building envelopes

Cool roofs: The term cool roof describes roofs whose finishing material is relatively reflective. The high reflectance of the roof surface results in lower absorption of heat in the roof surface itself and in the building's envelope, as long as the solar radiation is reflected to the sky and not toward adjacent building parts. The application of cool surface finishes (for example, white paint) can thus reduce the heat emitted by the building envelope at night, as well as the heat emitted by air conditioning units because of indoor overheating.

Cool walls: Cool (i.e., relatively reflective) surface finishes can also be applied to exterior walls. The result of using cool finishing materials for exterior walls is similar to applying the same materials to roofs, although the degree of relative improvement expected from their application is smaller compared to roofs, due to the significant difference in sun incidence angle (walls absorb less solar radiation compared to roofs, assuming the application of similar finishing materials). In addition, the efficacy of solar reflectance from walls in terms of urban overheating reduction can be significantly reduced in densely built areas, since the solar radiation reflected from the walls is usually absorbed by adjacent surfaces of neighboring buildings.

Green roofs: Green roofs are roofs covered with vegetation. Green roofs are expected to significantly reduce the heat load on indoor spaces below the roof, due to their thermal insulation properties and the absorption of solar radiation in the vegetated layer on the roof. They can also locally cool the air around them because of evapotranspiration from the vegetative layer. Compared to cool roofs, the main downside of green roofs is their additional maintenance and irrigation requirements.

Green walls: Green walls are vertical building parts whose external surface is covered with an expansive layer of vegetation. Like green roofs, green walls block the incidence of solar radiation on parts of the building envelope and can thus help in reducing the building's heat load and the heat a building emits to its surroundings. In addition, evapotranspiration from the vegetative vertical layer is expected to reduce air temperature in close vicinity to the green wall itself.

3.2. Pavements and roads

Cool pavements: Like cool roofs, the application of reflective materials to sidewalks and roads is expected to reduce their heat absorption and their resulting heat release. However, and unlike the application of cool materials to roofs, the use of highly reflective materials at street level may cause undesirable levels of glare and heat stress through the increased exposure of road users to reflected shortwave radiation. This may limit the use of highly reflective paving materials and, as a result, the practical cooling capacity cool pavements.

Water-retentive pavements: In climates where the hot season is also characterized by frequent rainfall, pervious paving materials that are applied above a water-retentive sublayer can help in reducing air temperatures in their vicinity through evaporation. The pavement's exposure to direct solar radiation causes its upper layer to heat during daytime, which, in turn, leads to the evaporation of the water stored in the sublayer and the cooling of the upper layer. The cooler the pavement, the cooler is the ambient air near it.

3.3. Urban landscaping elements

Shade trees: Shade-giving trees (as opposed to trees with underdeveloped tree canopies) can play an important role in reducing urban heat, especially by shading streets in a way that absorbs solar radiation before it affects sidewalks, roads, and building facades. Shading can also play an important role in relieving heat stress in urban spaces during the hot season since they block solar radiation that affects the human body. In addition, evapotranspiration from trees exposed to solar radiation can help in reducing air temperatures in their immediate vicinity.

Ground cover through vegetation: Unlike synthetic flooring materials, vegetation (grass, shrubs) absorbs solar radiation in a way that does not release heat into the air but rather cools the air around it through evapotranspiration. Nevertheless, this immediate cooling effect depends on the water content of the soil and, therefore, may be less effective in plants that require small amounts of irrigation.

Lakes, streams, and ponds: Water sources may lower air temperature mainly through water evaporation. Streams or canals that pass through the heart of a city may also help in transporting heat away from within the city center due to water movement, while large water reservoirs may help in reducing air temperature in their vicinity because of the heat capacity of the water.

3.4. Street geometry

Sky view factor: The geometric proportion between the height of buildings along a street and its width (the height-to-width ratio) can have the positive effect of reducing air temperatures at street level depending on a street's degree of openness to the sky or its sky view factor. Streets of lower sky view factor, in which the street section is deep and narrow, may become cooler during the day, because they are less exposed to incoming solar radiation. However, extremely deep and narrow street sections may become too dark for long periods, creating gloomy indoors (infringing the residents' solar rights) and undesirably dark outdoors during the cold season. Another possible downside of deep and narrow street sections is the lower heat release capacity of street surfaces to the sky precisely because of relatively low proportion of their exposure to the sky.

Wind corridors: Winds entering the urban area from contiguous open spaces tend to carry cooler air with them, and winds passing through the city may thus remove hot air from it. To secure effective passage of wind through a city, care must be taken to the way the street network is laid out. Main streets should, therefore, be oriented toward the prevailing wind directions, in a way that maintains effective corridors through which wind can penetrate the city core.

Solar orientation of streets: In subtropical latitudes, streets oriented along a north-south axis will usually provide significant shading from buildings at street level during the hot season, thus reducing street-level air temperature and pedestrian thermal discomfort during daytime. On the other hand, streets oriented along an east-west axis will be almost fully unshaded without the use of additional shading elements (trees, arcades, pergolas). Therefore, clever design of a street network that is based on street-level shading through the orientation and massing of the built volumes can generate cooler urban environments with lesser reliance on additional mitigation measures.

The broad range of tools for mitigating the negative effects of urban heat may create the false impression that an urban planner enjoys great flexibility in formulating an effective climatic action strategy. Yet, the applicability of each of the measures described above varies from one location to another in a way that may leave a planner with less than a handful of tools for facing the challenge of overheating urban environments. As the above description may suggest, the four different domains of intervention call for different scales of application and direction, as follows:

• Actions directed to building envelopes can bring about genuine urban change only when applied on a large scale and, therefore, seem to be less effective without the cooperation of numerous property owners. In real-life situations, while municipal planners can regulate and direct the application of cool or green materials to building envelopes, they may not have the means for supporting their comprehensive application by the private sector.

• Applying reflective materials to pavements and roads can be effectively directed as a top-down solution but may introduce only local and minor reduction in air temperatures since the reflectivity of the applied materials should be kept low to prevent glare. The reflectivity of pavements can also cause undesirable heat stress for pedestrians since it may significantly increase the overall radiation flux on the human body during daytime. Water-retentive pavements, on the other hand, can be effective only in nonarid locations, where frequent rainfall is typical to the hot season.

• Using landscaping elements for UHI mitigation is probably the most effective mitigation strategy in many urban locations. Landscaping elements can be comprehensively introduced into public spaces and can induce change at different scales, from the very local to the urban. This is especially true in respect to trees, which can be planted in a variety of locations, including streets, plazas, gardens, and parks. While trees may have a negative effect on the cooling capacity of streets at night because their canopies may reduce the flow of wind and heat released to the sky from street surfaces, it seems that their overall positive effect of daytime shading and evapotranspiration is in many times much more significant. Proper pruning of tree canopies may improve wind flow, as long as the trees are allowed to grow their canopies high enough. Additionally, desealing of paved surfaces and their replacement with vegetated landcover are another action that can be easily applied to existing cities in a comprehensive way and as a top-down policy directed by municipalities.

• As for street geometry, its modification to improve climatic conditions is almost impossible to realize in older city parts. While municipal planners have a decisive control over the regulation of street geometry, in cities where the rate of urban transformation and rebuilding is low or where new construction concentrates in the outskirts of cities, this control cannot be translated into effective mitigation of urban overheating.

It can, therefore, be concluded that the most climatically effective domain of intervention for urban planners and designers attempting to combat the climatic challenge of urban overheating is that of urban landscaping. More specifically, the green infrastructure of a city, and especially the clever use of trees, is essential to the success of an urban overheating mitigation strategy (Bowler et al., 2010; de la Barrera and Reyes-Paecke, 2021; Hiemstra et al., 2017; Saaroni et al., 2018). Yet trees can be regarded as an almost ideal design element for urban cooling not only because of their effect on air temperatures. While air temperatures are commonly used as an indicator of urban overheating intensity, in many cases the effect of a slight increase of air temperature is not the decisive factor generating outdoor thermal stress, especially during daytime. In that respect, the benefits of trees as shade providers at street level may be more important for improving the climatic conditions in cities than their direct air cooling effects (Jamei et al., 2016; Rahman et al., 2020; Sanusi et al., 2017; Taleghani, 2018).

4. Shade's pivotal effect on outdoor thermal comfort and its urban-scale mapping

Outdoor thermal comfort can be evaluated using comfort indices that correlate objectively quantifiable climatic variables with thermal stress. Two of the most prevalent indices in current scientific literature are the Physiologically Equivalent Temperature (PET) and the Universal Thermal Climate Index (UTCI), which follow an almost similar approach, calculating an equivalent temperature based on climatic variables and providing an estimated thermal comfort or discomfort levels for a range of equivalent temperatures. Both indices use air temperature, mean radiant temperature (MRT), relative humidity, wind speed, clothing level, and metabolic rate for calculating the equivalent temperature. UTCI index is simpler to calculate since it presupposes clothing and metabolic rate values (Bröde et al., 2013; Höppe, 1999; Matzarakis et al., 1999, 2015).

Recent studies have consistently demonstrated that in multiple geographic locations, during the hot season, direct and diffuse solar radiation largely contributes to excessive daytime heat stress, making shade provision a viable tool for heat stress mitigation (Balslev et al., 2015; Chen and Ng, 2012; Colter et al., 2019; Coutts et al., 2015; de Abreu-Harbich et al., 2015; Du et al., 2020; Hiemstra et al., 2017; Huang et al., 2020; Lee et al., 2013, 2018; Middel et al., 2021, 2016; Shashua-Bar et al., 2010, 2011). Shade has also the potential to reduce street-level air temperatures and cooling loads in buildings because it decreases irradiation of surfaces in cities (roads, pavements, building facades), thereby reducing the amount of heat absorbed and released by man-made elements in the built environment (Erell et al., 2011, pp.