Relating microclimate, human thermal comfort and health during heat waves: An analysis of heat island mitigation strategies through a case study in an urban outdoor environment
Introduction
Over the past ten years there has been a series of climatic changes in the world and an increase in the number of the global population concentrated in high-density and anthropized urban areas (World Urbanization Prospects, 2014). In particular, the developed countries also present other phenomena determined by historical and cultural factors which caused an increase in the average age of the population and those who are more exposed to thermal stress. Moreover, due to a scant redevelopment of the building heritage of the city, the materials used for the public spaces present thermophysical properties which amplify extreme heat phenomena (Grimm et al., 2008). In this type of scenario, countries characterized by hot climates during summer are exposed to a dangerous mix of events which is determining an increase in the global average overheating on a small scale related to the Urban Heat Island effect and intensified by heat waves (Michelozzi et al., 2005). Therefore in highly anthropized cities with a high number of old people this can lead to microclimatic conditions characterized by thermal discomfort, even for long time intervals, causing health problems. Even if some studies tried to solve this problem through solution as the natural ventilation in buildings (Coppi, Quintino, & Salata, 2013; Salata, Alippi, Tarsitano, Golasi, & Coppi, 2015a;Coppi, Quintino, & Salata, 2011), it is clear that in the future, if the climatic trend is this increase in the global average temperature, some characteristics of the urban centers that can be easy to redevelop must be changed. The finishing materials of the surfaces will have to be chosen in a conscious way while taking into consideration their thermophysical characteristics (Salata, Golasi, de Lieto Vollaro, & de Lieto Vollaro, 2015b), to control the Urban Heat Island effect and at the same time the buildings’ energy consumptions (Rosso, Pisello, Cotana, & Ferrero, 2014; Evangelisti, Battista, Guattari, Basilicata, & de Lieto Vollaro, 2014; Pisello et al., 2016a). For these reasons, urban materials will be chosen according to: (i) the radiative heat transfer coefficient with respect to the wavelength (optimizing radiative exchanges in terms of solar radiations and the surrounding environment); (ii) the heat capacity (to control the capacity to store and release solar thermal energy with a certain temporal shift). This has consequences on thermal comfort of outdoor spaces [11] in the cities which can be controlled to avoid high thermal stress situations affecting population and its health (Kovats & Ebi, 2016; D'Ippoliti et al., 2010).
From this point of view, a study (D'Ippoliti et al., 2010) focusing on the death rate of over 60-year-old people and intense climatic events was carried out in Rome and examined the data between 1992 and 2004 (where 2003 was historically one of the hottest years). To relate climatic data to thermal perception of people, this study used the Apparent Temperature (Steadman, 1979; Steadman, 1984), defined as reported in Eq. (1):where Tair is the air temperature [°C] and Tdewpt is the dew point temperature [°C]. It revealed how during heat wave days minimum values of the Apparent Temperature higher than 21 °C and maximum values higher than 35 °C were reported with reference to 90th percentile. These values led to an increase of 28.8% in the percentage of natural deaths, of 66.9% in the respiratory deaths, of 37.8% in the cardiovascular deaths and of 48.0% in the cerebrovascular deaths.
Given the negative consequences caused by a worsening of the microclimate, different solutions, able to improve outdoor thermal comfort, have been examined. Table 1 shows some scientific studies carried out in several cities over the past ten years analyzing the effect of various mitigation strategies.
Among the most analyzed and interesting solutions, the exertion of plants and green areas provides good results in terms of decrease in mean radiant temperature and air temperature. In those cases examined, many authors, besides changing the morphology of the streets and squares on the pedestrians level by increasing the number of trees (useful to limit direct solar radiation) or by installing some grass on the horizontal surfaces, highlighted the benefits determined by the presence of green roofs and walls to mitigate the Urban Heat Island effect. Even the presence of water surfaces has positive effects leading to a decrease in the Bowen ratio, defined as the ratio between the sensible heat flux and the latent heat flux (Oke, 1978). Other authors focused on the materials characterizing the surfaces of the buildings, both horizontal and vertical. This is why such kind of study concerned the thermophysical properties of the finishing materials of the buildings (examining, at different wavelengths, the reflection and absorption features of the solar energy or the infrared emission) and suggesting the exertion of high albedo materials on the boundary surfaces and cool roofs or cool pavements. Then many papers analyzed mixed solutions with the goal to increase as much as possible the mitigation of the microclimatic parameters characterizing outdoor urban environment.
While examining the studies reported in Table 1, it can be noticed how the use of software to perform studies concerning outdoor spaces (Nardecchia, Gugliermetti, & Bisegna, 2016;de Lieto Vollaro, Galli, & Vallati, 2016; Nardecchia, Gugliermetti, & Bisegna, 2017) and the influence of environmental variables on several aspects related to buildings (Salata, Golasi, di Salvatore, & de Lieto Vollaro, 2016b) has gradually increased. Performing a series of experimental measurements on the field is often too expensive in terms of both time and resources. This is why 3D numerical simulations could be considered most appropriate if the goal is to obtain the prediction of the environmental performance. In particular, the situation becomes even more difficult when there is the necessity to model urban microclimate due to the high number of variables involved.
From this point of view, several studies used the CFD solver FLUENT-ANSYS (Anon, 2017a). Li et al. (Li, Pan, Zheng, & Shao, 2016) examined the microclimate in Xianghe Segment of China’s Grand Canal (XSCGC) suggesting the realization of different functional areas to improve its livability. Kakoniti et al. (Kakoniti, Georgiou, Marakkos, Kumar, & Neophytou, 2016) analyzed the influence of the materials on the microclimate coming to the conclusion that if chosen meticulously they can affect more the micrometeorological variables than an optimization of the built packing density. Then Liu and Niu (Liu & Niu, 2016) investigated different turbulence models and compared the performances of the Steady Reynolds Averaged Navier–Stokes (SRANS) RNG k-ε, Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) examining the wind flow around an isolated building. Allegrini et al. (Allegrini and Dorer, 2015) studied the microclimate coupling building energy simulation (BES) with CFD simulation. To be more specific, the influence of the building surface temperatures and the outdoor air temperatures on the thermal performance of buildings and thermal perception of pedestrians was examined. The studies carried out by Tominaga (Tominaga, 2012) and Tominaga et al. (Tominaga, Sato, & Sadohara, 2015) are also interesting. The first study (Tominaga, 2012) analyzes different urban layouts with buildings characterized by a squared map and it was possible to evaluate how a different height affects the air temperature and wind speed. The second study (Tominaga et al., 2015) investigates the effect of the evaporative cooling related to water surfaces on the microclimate together with the decrease in the air temperature at the pedestrian level which was estimated to be of 2 °C. Yumino et al. (Yumino, Uchida, Sasaki, Kobayashi, & Mochida, 2015) also had their focus on the mitigation strategies of the microclimate: they examined the influence of highly reflective materials and greening on the vertical boundaries of the buildings, while evaluating at the same time radiation, conduction and convection. A different approach was adopted by Mirzaei and Haghighat (Mirzaei & Haghighat, 2010b; Mirzaei & Haghighat, 2012), who analyzed even the pedestrians’ health. This is why they developed mitigation techniques while evaluating the Urban Heat Island (UHI) and Urban Pollution Island (UPI) (Mirzaei & Haghighat, 2012) and they studied the pedestrian ventilation system (PVS) to ventilate building canopy (Mirzaei & Haghighat, 2010b).
In other studies MITRAS (Schlünzen et al., 2003) was used, a model able to predict temperature, wind, humidity, tracer concentrations and solve equations for chemical reactions, cloud- and rainwater. Bohnenstengel et al. (Bohnenstengel, Schlünzen, & Grawe, 2004) reported an increase in the circulation in a street canyon in presence of convective stratification whereas Schlünzen et al. (Schlünzen, Grawe, Bohnenstengel, & Schlüter, 2011) evaluated large-scale phenomena, showing how MITRAS can be combined with mesoscale models and examining the influence of possible obstacles.
Even if there is a concern regarding its performance and resolution, over the past few years the use of the software ENVI-met (Anon, 2017b) has been growing. It was mainly used to study the impact of the urban vegetation on microclimate and outdoor thermal comfort (Lee, Mayer, & Chen, 2016;Duarte, Shinzato, dos Santos Gusson, & Abrahão Alves, 2015;Tsilini, Papantoniou, Kolokotsa, & Maria, 2014;Morakinyo & Lam, 2016;;Ketterer & Matzarakis, 2015; Wang & Zacharias, 2015). Morakinyo and Lam (Morakinyo & Lam, 2016) simulated in Hong Kong different scenarios, changing green coverage, Leaf Area Density (LAD) and Leaf Area Index (LAI). Wang and Zacharias (Wang & Zacharias, 2015) studied possible interventions of urban requalification in Beijing (China) estimating a decrease in the air temperature of about 0.5–1 °C due to the substitution of roads with urban greening and permeable soils. Other researches focused their attention on the morphological features (Ghaffarianhoseini, Berardi, & Ghaffarianhoseini, 2015;Ketterer & Matzarakis, 2014; Middel, Häb, Brazel, Martin, & Guhathakurta, 2014;Perini & Magliocco, 2014; Yahia & Johansson, 2014) and some interesting results were obtained by Ketterer and Matzarakis (Ketterer & Matzarakis, 2014). They discovered how thermal comfort can be improved in an urban canyon characterized by a Northwest-Southeast orientation and an aspect ratio of at least 1.5. Then other studies concerned the roofs’ level (Sodoudi, Shahmohamadi, Vollack, Cubasch, & Che-Ani, 2015;Wang and Akbari, 2014; Ambrosini, Galli, Mancini, Nardi, & Sfarra, 2014;Razzaghmanesh, Beecham, & Salemi, 2016;Lobaccaro & Acero, 2015). For example Wang and Akbari (Wang and Akbari, 2014) implemented different materials for ground, walls and roofs and to have a useful parameter for the microclimate analysis introduced a “Thermal Radiative Power” (TRP). Further studies were carried out to examine topographical aspects (Johansson, Spangenberg, Gouvêa, & Freitas, 2013; Taleb & Taleb, 2014; Qaid & Ossen, 2015;;Taleghani, Kleerekoper, Tenpierik, & van den Dobbelsteen, 2015). An example can be found in the study performed by Johansson et al. (Johansson et al., 2013) who, in São Paulo (Brazil) and during a summer day, evaluated the thermal environment in six different urban contexts or in the study of Taleghani et al. (Taleghani et al., 2015) who analyzed different urban forms in Holland.
Therefore, while taking into consideration what previously said, this study examines the microclimate in the campus of Sapienza University of Rome. This site is characterized by a wide variety in terms of topographical features and urban morphology and allowed to evaluate the influence of different mitigation strategies on the micrometeorological variables and outdoor thermal comfort. Hence different configurations for the site are here presented: one is characterized by an increase in the urban greening, a second configuration presents the implementation of a cool pavement in concrete, a third configuration presents cool roofs and another one a combination of the previous solutions. In order to perform an evaluation of their impact during a heat wave, they were examined with both the current site configuration and one characterized by asphalt only for the ground surface in total absence of vegetation. To carry out such analysis, numerical simulations with the software ENVI-met V 3.1 (Anon, 2017b) were performed and maps of the Mediterranean Outdoor Comfort Index (MOCI) (Salata, Golasi, de Lieto Vollaro, & de Lieto Vollaro, 2016a) were obtained. Moreover the hourly trend of the air temperature and mean radiant temperature was examined, the two variables which in the Mediterranean area affect the most thermal perception (Salata et al., 2016a).
Then combining the Mediterranean Outdoor Comfort Index (MOCI) (Salata et al., 2016a) with the Apparent Temperature (Steadman, 1979; Steadman, 1984) it was possible to calculate the value of the first index which once exceeded leads to an increase in the heat related casualties. Finally the influence of the mitigation strategies on the health risk was analyzed.
Section snippets
Case study
The site chosen to perform the study is the campus of the Sapienza University of Rome and it is located in the center of Rome. It is formed by 27 buildings hosting many of the Faculties of the University and covering an area of about 23 ha (Fig. 1).
Despite all the initiatives to redevelop and transform it, the campus still presents the historical and architectural features of the period in which it was realized. The buildings characterizing the site are made of different materials, as white
Results and discussion
Given the importance of the urban microclimate, it is essential trying to identify the possible actions that might mitigate the effects of the Urban Heat Island (UHI) phenomenon. As a matter of fact, different urban features can determine a high variability in the thermal conditions in the same city, and in order to verify their influence, the study was carried out without taking into consideration the anthropogenic heat flux. It is an extra heat source provided by human activities as traffic,
Conclusions
In this study different mitigation strategies of the urban microclimate were compared with respect to the campus of the Sapienza University of Rome.
Over the past few years there has been a progressive increase of conditions of intense heat and heat waves in the Mediterranean area with a resulting increase in people’s thermal stress. This determined, when temperature reports high values, an increase in the mortality rate among those exposed to these phenomena. The situation got worse with the
Acknowledgements
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. A special thanks to Mrs. Flavia Franco for the help she provided in the preparation of this paper. The authors also want to thank the reviewers for their suggestions.
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