Review on the impact of urban geometry and pedestrian level greening on outdoor thermal comfort

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Abstract

The number of hot days is increasing in many parts of the world because of the heat island phenomenon and global climate change. High air temperature greatly affects human thermal comfort and public health, particularly in urban areas. Therefore, the challenging task of, urban designers and urban planners in accommodating the increasing population is to make cities with the least level of vulnerability to future climate change. Interest in transferring urban climatic knowledge into urban planning practices, and developing mitigation strategies to adapt to climate change, has been increased in recent years. The use of vegetation and appropriate urban geometry are shown very promising in mitigating the adverse effects of heat island and providing a better pedestrian thermal comfort. This article reviews studies on pedestrian level urban greening and geometry in improving thermal comfort in cities. Such strategies can be applied at the preliminary stages of urban planning and thus directly affect the microclimate. The analyzed data include simulation and field measurement studies. The discussion of this research clearly reflects how urban design guidelines can be applied to enhance outdoor thermal comfort and minimize the heat island effect. This study is helpful in controlling the consequences of city design from the early design stage.

Introduction

Rapidly increasing urbanization takes place as a large portion of the world population migrates from rural to urban areas. Urban development and population growth have caused city-induced climate change, provoked serious consequences on public health, and stimulated a rising interest among researchers to overcome the adverse effects of urbanization on thermal condition of cities [1]. “Urban heat island” (UHI) phenomenon is the most obvious among the climatic manifestations of urbanization. Tracing its roots in the positive urban thermal balance, it contributes to higher air and surface temperatures in cities compared to the surrounding rural areas [2], [3], [4]. Extensive research has been conducted to understand the characteristics, significance, causes, and effects of UHI and to document the intensity of the phenomenon in various geographical areas of the planet [5], [6], [7]. The intensity of heat island exceeds 10 K in many cities, depending on the urban properties and the local climatic conditions [8], [9], and its temporal and regional variability is observed worldwide [10]. The increased urban air temperature seriously affects the energy consumption of building for cooling purposes [11], [12], [13], [14], intensifies pollutant concentration, and reduces the thermal comfort level of city inhabitants [15], [16]. The summer time cooling demand in a typical office building in Hangzhou metropolitan area of China increases by 10.8% for every 0.5 °C rise in the ambient temperature [17]. In Paris, research on the effect of UHI on regional atmospheric pollution has shown that the spatial distribution of pollutants is significantly affected by the intensity of UHI in each area [18].

Pedestrian summer time thermal comfort and mortality rate are also strongly compromised because of global climate change and heat island effect [18], [19], [20], [21], [22], [23], [24], [25], [26]. The recent increasing research attention on thermal comfort in outdoor environments is due to the high level of health-related risks caused by climate change and global temperature increase [27], [28]. For instance, the four-day heat wave in Melbourne in January 2009 resulted in 374 excess deaths, mainly among the elderly and people with cardio-vascular problems [29]. Similarly, the European heat wave during the summer of 2003 resulted in nearly 15,000 deaths in France alone [30], [31] and between 25,000 to 70,000 throughout Europe [32], [33]. Heat wave is an extreme weather phenomenon and practically a rare stochastic event. Studies have shown a sudden increase in mortality rate during summer months among people over 75 years old [34], but in case of long-term heat wave, the mortality rate extends to younger age groups [35], [36].

Thermal comfort in outdoor environment has been less under investigation due to the complexities that it entails. Moreover, due to a less time spent in outdoor environments, it is very difficult to conclude a thermal steady state and achieve a certain value for thermal comfort level [37], as people spend 10% of their summer time and 2–4% of their winter time in outdoors. Furthermore, the complex interaction between various variables in outdoor areas, where conditions are less controllable is the main challenge for scholars in scrutinizing the outdoor thermal condition. Thermal comfort, in its basic definition, is defined as the condition of the mind which expresses satisfaction with the thermal environment [38]. Thermal comfort can be defined from three different aspects, psychological aspect which is the mind׳s expression of satisfaction with the thermal condition of the outdoor environment. Thermo-physiological aspect which contributes to the biological reactions and thermal receptors on the skin towards the external environment. Finally, the energetic aspect which relates to the heat flow from and to the human body [39].

The quality of outdoor thermal environment and comfort level of individuals can be modified by small design details, such as surface materials, application of water bodies and adding shading devices to the buildings. However, it is more important to forecast the thermal consequences at the early stages of the design process [40]. Therefore, the arrangement of the buildings in urban canyons which influence the solar access and wind orientation, configuration and form of the urban areas and landscaping are some of the major considerations when it comes to modifying and improving the pedestrian thermal comfort [41].

According to the concept of urban atmosphere developed by Oke [42], the urban canopy layer is “a space bounded by the urban buildings up to their roofs.” Oke states that the urban canopy layer is a micro-scale concept, and its specific local characteristics at any given point are strongly affected by the nature of its surroundings. Thus, modifying the physical features of the city in relation to solar access and wind orientation would alter the above-mentioned variables which determine the human thermal comfort. Such an effect emphasizes the important role of urban planners and urban designers in creating a favorable urban micro-climate so that the potential consequences of city design would be analyzed from the early design stage. Integrating the climatic considerations into urban planning and design would contribute to the sustainable urban development and mitigate the adverse effects of increased urban air temperature.

The relationship between urban design parameters and pedestrian thermal comfort has been the subject of many studies [43], [44]. Several studies have examined the influence of city design on wind flow and reported the effective role of ventilation in mitigating the high urban air temperatures and improving thermal comfort [6], [45]. Other studies have focused on the effect of urban design on the magnitude of received solar radiation and its outcome on pedestrian thermal comfort [46]. Recent research has initiated the development of technological measures to create a thermally comfortable urban area [20], [47], [48], [49], [50], [51], [52], [53], [54]. Among the measures, urban area geometry and street-level urban greening appear to be very promising in ameliorating pedestrian thermal comfort. Therefore, this paper reviews the studies that investigated the effect of pedestrian-level mitigation techniques and presents the influence of geometry manipulation (through altering the aspect ratio (H/W), sky view factor (SVF), street orientation, and neighborhood configuration) and street-level urban greening (urban trees and urban parks) on outdoor human thermal comfort.

Section snippets

Research methodology

This paper systematically reviews recent research on the effects of urban geometry and street-level greening on pedestrian thermal comfort. As stated in the introduction, outdoor pedestrian thermal comfort is determined by both meteorological (air temperature, relative humidity, wind speed, and mean radiant temperature) and personal factors (clothing type and activity level). This review reports the results of research on the meteorological factors influencing outdoor thermal comfort. Studies

Urban geometry

In the relevant literature, streets with special geometric characteristics are known as “street canyons” owing to the formed geometries similar to natural canyons [55]. Oke defines an “urban street canyon” as the “basic geometric unit which can be reasonably approximated by two-dimensional cross-sections, neglecting street junctions, and assumes that buildings along the canyon axis are semi-infinite in length.” Urban canyons occupy two-thirds of city spaces and therefore play an important role

Urban greening

Cities are considered hostile environments for greenery because of the high level of impervious surfaces, reduced level of soil moisture, lack of nutrient and rooting volume, and presence of air/water pollutants [139], [140], [141]. Relevant literature indicated that application of vegetation in urban areas would alter the microclimate parameters such as air temperature, relative humidity, wind pattern, and precipitation [142]. Therefore, urban greening has been always recommended as an

Summary and discussion

In this review paper urban geometry and urban greening were studied as two promising strategies in ameliorating the pedestrian comfort. One may raise the question that how about the effect of other parameters such as anthropogenic heat, surface and pavement materials and green roofs on the energy budget of urban canyons.

According to the literature, the surface energy balance equation is influenced by the surface properties and anthropogenic heat [216]. However, there are contrasting estimations

Conclusion

Increased urban air temperature and rising risks of heat wave events are serious public health concerns. We summarized and reviewed the latest studies on the effect of the most promising strategies, (urban geometry and urban greening) on pedestrian level thermal comfort. The importance of each strategy to improve the microclimate, heat island effect, and thermal comfort has been discussed.

The first part of the study showed that the distribution and arrangement of the buildings in a city affect

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