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Promoting Citizens’ Quality of Life Through Green Urban Planning

  • Teresa SantosEmail author
  • Caio Silva
  • José António Tenedório
Conference paper
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Part of the Communications in Computer and Information Science book series (CCIS, volume 936)

Abstract

In dense urban areas, the pursuit of outdoor thermal comfort is a development goal included in the city’s sustainable plan. The aim of this study is to evaluate the effect of promoting new green areas, at ground and at rooftop levels, in the thermal comfort of the surrounding urban area. The simulation was made based on a recently concluded requalification project in a Lisbon neighborhood. This project was used as a case study to evaluate the effects of the new vegetation areas at ground level on microclimate and urban comfort [43], while the work of [44] was used as a case study to investigate the effect of green roofs.

The ENVI-met software is used to model the past and present (after requalification) scenarios, and a new scenario with green roofs. The simulation results indicate that the presence of new trees and shrubs results in: (i) increased urban comfort in the morning and in the afternoon resulting from the decrease in temperature; (ii) a reduction of up to 3° in the morning (9 h) and up to 3° in the afternoon (15 h); (iii) an increment of 10% in the relative humidity of the air, and (iv) a slight reduction in natural ventilation in both the morning and afternoon periods.

The microclimate simulation results confirm that vegetation is a key element when planning for comfortable public spaces.

Keywords

Thermal comfort Urban planning Green areas Green roofs 
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1 Introduction

1.1 Sustainable Urbanism

In 2016, the Lisbon City Hall has initiated the requalification of several public spaces within the city. One of these spaces is located in the central business district and foresees the substitution of traffic lanes by new pedestrian zones and green areas. The goal is to provide the citizens with less noise, more pedestrian spaces, comfortable sidewalks and lowered crosswalks for people with reduced mobility, more green areas, places for esplanades and bike lanes.

The role of green areas in urban planning is recognized as a key factor towards urban sustainable development. Green infrastructures promote healthier and safer environments by regulating the urban temperature, controlling environmental extreme events like floods caused by intense rainwater run-off, regulating air quality or wind speed, reducing noise, or promoting biodiversity within the city. Furthermore, the visual aesthetical quality provided by green landscapes also contributes to the communities’ well-being and quality of life. In this context, urban planning constitutes an efficient management tool to promote multi-scale sustainable green development. Local planning may be used to promote street tree plantation, green roofs, green pavements, or to preserve open spaces, natural corridors and parks in the public space. Regional planning, on the other hand, may address the preservation of forest or agriculture areas in the peri-urban fringe through zoning. In this sense, the concept of green infrastructure is generally associated with the concept of ecosystem services [22], i.e. the benefits that humans can obtain from ecosystem functions [14].

The Urban Heat Island (UHI) effect is the characteristic warmth of a settlement compared to its surroundings, and is the best-known climatic response to disruptions caused by urban development [6, 25, 35]. The temperature differential is generally larger at night than during daytime and larger in the winter than in summertime. Many factors contribute to UHI, like the presence of dark sealed surfaces (e.g. asphalt), building material with different thermal properties and albedo (e.g., concrete), or the lack of open green spaces, among others. The urban geometry also contributes to the UHI. Taller buildings implicate higher areas for sunlight reflection and absorption, thus promoting heating. Another urban geometry that affects local temperature is the effect created by having buildings on both sides of the street, also known as the urban canyon effect. The higher the aspect ratio (canyon height/canyon width) the higher is the impact of urban geometry in temperature, wind and air quality of the local. Other sources such as air conditioning, road traffic, or industry, also contribute to the UHI. There are several mitigation strategies like increasing the albedo of buildings and rooftops. Additionally, enhancing ecosystem service provided by the green infrastructures is another strategy to mitigate the heat island effect. In fact, shading and evapotranspiration provided by trees constitute an efficient way of regulating urban temperature [39], also the presence of open green spaces within the city’s quartiers helps controlling temperature, relative humidity or wind speed, all key factors contributing to the UHI. Assessing the fine-scale thermal variation due to such changes in land cover within urban environment is a way of quantifying the mitigation impact of green-space planning.

1.2 Planning New Green Urban Areas for Lisbon

The Praça Duque de Saldanha and Avenida da República Requalification Project.

One approach to deal with local urban climate challenges is to requalify typical neighborhoods and squares. Based on this premise, and from a total of 150 squares, 30 were identified by the Lisbon City Hall as priority site intervention. The Praça Duque de Saldanha and the Avenida da República constitute two of these intervention areas.

Several issues were identified in Praça Duque de Saldanha and the Avenida da República, which motivated the urban intervention, namely: (1) the discontinuity of tree alignments; (2) the use of the avenue as an express road generating speed and insecurity in zebra crossings, sometimes resulting in fatal hitches; (3) the avenue’s road channel space, with 60 m wide, presented an irregular profile along its axis, mainly as a result of the successive interventions it was subjected to from its initial conception to the present. Furthermore, the road axis presented problems and inadequacies concerning the infrastructures for soft mobility. In fact, the sidewalks had variable widths, sometimes too narrow for walking, with physical obstacles and parking areas. The requalification of the public space was also motivated by insufficiency of cycling lanes and its disconnection with the network in operation and the projected one to the city.

Given these considerations, the requalification project was designed with the intention of enhancing the scenic effect of the square recovering the avenue’s initial concept of “boulevard”, by eliminating obstacles to pedestrian circulation, improving crossings, providing infrastructure to support cycling and an improved accessibility in individual transport.

Concerning the square’s requalification, the project proposed the following actions: (1) to reshape the traffic lanes in the roundabout; (2) to increase the pedestrian space close to the buildings (11.80 m), allowing areas for esplanades; (3) to make a wide green belt (12.50 m) around the roundabout, separating the two urban functions (road and pedestrian); and (4) to eliminate parking spaces and reallocate taxicab stops (Fig. 1).
Fig. 1.

Praça Duque de Saldanha project [11].

Regarding the avenue’s requalification, the project proposed the maintenance of the main road central axis. Before the intervention, there were four lanes in one way and three in the opposite direction. The project dictated that one of these central lanes was to be eliminated and a new green central separator, 4 m wide was to be planned for the local (Fig. 2). The project predicted the cut of 15 trees, the transplantation of 30, and the plantation of 741 new trees. The selected species include Fraxinus angustifolia, Tipuana tipu, Jacaranda mimosifolia and Plantanus, among others.
Fig. 2.

Avenida da República project - two first blocks [11].

Nevertheless, the project implementation did not occur without some resistance from residents and other people who use this area of the city. The reduction of parking spaces in an area with a high concentration of commercial and tertiary activities, the traffic restrictions as well as the duration of the intervention, raised some reservations. To respond to these concerns, the City Hall promoted five public sessions. The aim was to present and debate the proposals with the local population. Furthermore, citizens were invited to participate with ideas and suggestions through an on-line public participative platform. Consequently, the final proposal emerged with the help of the population and the parish council. To compensate for the loss of parking spaces, the City Hall offered new public parks in the periphery of the city.

Green Roofing in Lisbon.

Covering roofs with vegetation contributes to mitigate the urban heat island effect by modifying not only the buildings’ microclimate but also the local climate of the city [4, 30, 34, 37]. Other public benefits of vegetation on the ground and on roofs include decreasing the indoor cooling load demand and promoting biodiversity [8, 9, 38, 40, 45]. Furthermore, green roofs constitute a valid alternative for increasing green urban areas, when the available space on the ground is generally sparse [7, 18, 21]. Nevertheless, knowing what are the best locations in the city to receive such structures constitutes an essential tool when it comes to green planning.

In this context, [44] estimated the potential of the city of Lisbon to receive green roofs using 3D data obtained with a LiDAR sensor. The selection of the most suitable rooftops was based on several criteria: roof covering material (tile, non-tile), available area, compatible slope and sunlight for plant development. Using remote sensing data and planimetric information, the city built environment was modelled and several scenarios were considered. In the most conservative scenario – non-tiled roofs, with available area equal or larger than 100 m2, and slope less or equal to 11°, and more than 3 h of sunlight per day – 2890 rooftops where identified, representing an overall increase in the city’s green area of 6.7% (Fig. 3).
Fig. 3.

Current vegetation cover at ground level and potential vegetation at rooftops.

1.3 Measuring Thermal Comfort

Thermal comfort, according to the ISO 7730 standard (1994), is “the condition of mind which expresses satisfaction with the thermal environment”. So, currently to assess outdoor thermal comfort, the human-biometereological methods rely on rational indexes determined by solving the human energy balance equation [5, 31].

Fanger‘s comfort equation is one of the most used formulas when it comes to the evaluation of thermal comfort conditions. Its input parameters include personal factors such as the metabolic rate estimation and clothing levels, physical factors such as air temperature (Ta), men radiant temperature (MRT), air velocity and humidity. The Fanger’s equation is the basis for calculating the Predicted Mean Vote (PMV) model. The PMV is one of the most used models of thermal comfort and it is included in the ISO 7730 standard (1994). Its calculation includes meteorological variables and personal settings. Using a biometeorological reference height of 1.6 m, the required variables include Ta, MRT, vapor pressure and local wind speed. Personal settings include clothing insulation, mechanical energy production of the body and mechanical work factor. The PMV reference person is always 35-year-old, male, with a height of 1.75 m and a weight of 75 kg. It scales from −3 (cold) to +3 (hot), where 0 stands for thermal neutrally, i.e., comfort.

Note that PMV was developed to measure indoor conditions, and its use to outdoor conditions is theoretically possible. Nevertheless, outdoor climatic variables can be diverse from indoor settings. Based on this fact, and to better estimate outdoor conditions, the Physiologically Equivalent Temperature (PET) index was proposed by [31]. It is defined “as the air temperature at which, in a typical indoor setting (without wind and solar radiation), the energy budget of the human body is balanced with the same core and skin temperature as under the complex outdoor conditions to be assessed”. PET is based on the Munich energy balance model for individuals (MEMI). So, booth PMV and PET calculus are based on models of human energy balance equation but, while PMV gives a mean vote representative for a large group of individuals, PET can be calculated for specific subjects, which are defined by their personal data [31]. Nevertheless, in most cases using only one of these models is sufficient for characterizing thermal comfort [31].

The Universal Thermal Climate Index (UTCI) [24], the most recent developed index for measuring outdoor thermal conditions, is derived from the Fiala multi-node model of human heat balance. The UTCI it is then defined as the air temperature of the reference environment outdoors, which produces the same strain index value indoors. Its different values are classified in terms of thermal stress, and vary from extreme heat stress to extreme cold stress.

1.4 State of the Art

Green areas are known to produce climatic modifications at the micro-scale. This premise has been the focus of researchers, and innumerous works are published in the scientific literature. Many studies address the impact of city morphology (built density and road orientation) and different green scenarios towards outdoor thermal comfort [5, 10, 19, 36, 46, 47, 48], others test the impact of different urban green covering scenarios without changing built density [26, 41], while others consider distinct urban densities [15, 16, 17, 20, 23, 27, 28, 32, 33, 49].

[49] simulated the impact of urban albedo and urban green covering on the urban microclimate of a planned new residential area in Osaka, Japan. A total of six scenarios were tested and showing that the scenario with low albedo and moderate greenery was the one with the greatest potential for improving urban microclimate. Furthermore, increasing the proportion of greenery is more effective than strategies of increasing the urban albedo for building facades in terms of mitigating Urban Heat Island effect to improve urban microclimate. The cooling effects of greening have already been reported by several authors. [33] in a study conducted in Hong-Kong, concluded that in high-density cities, the amount of tree planting needed to lower pedestrians level air temperature by around 1 ℃ is approximately 33% of the urban area. [46] also concluded that increasing 10% of urban vegetation could reduce the average temperature (Ta) and the Mean Radiant Temperature (MRT) all through the day and night-time up to 0.8 ℃, and that the heat reduction by adding urban vegetation was most observed in the high-rise area. Among the techniques to mitigate Ta at midday investigated in this paper, adding urban vegetation showed the most significant results.

[41] tested the impact of vegetation and materials with a high albedo in the outdoor thermal comfort, by taking into consideration the PMV model. It was found that the presence of lawn, trees and shrubs leads to an improvement of the outdoor thermal comfort if the facades do not change: in this case, the magnitude of the multiple reflections inside the structure decreases and the reduction of the mean PMV is about 0.5 units. [27] analyzed the micro-climate effect of urban vegetation structures in Dresden, Germany. The cooling effects in air temperature ranged from 1 k to 2.1 K. according to the greenery provision. [36] quantified the cooling effect of a small green space in the surrounding atmospheric environment of a densely urbanized area located in Lisbon, using local measures. The highest difference found was of 6.9 ℃ in relation to air temperature, between shaded areas inside the garden and sunny areas located on the nearby street.

A different conclusion was found by [23] when investigating the effects of future structural plans on pedestrian thermal comfort in city, by modelling the existing and future scenarios. The study concluded that increasing the tree canopy coverage caused 1–2 ℃ reduction on PET level and adding a green roof did not show any improvement on PET at pedestrian level. In fact, the success of the increased tree canopy and green roof scenarios in reducing the mean radiant temperature was not as significant as in the increased building height scenario, probably due to the level of shading provided in the last scenario. Also, [16] in a study located in São Paulo, Brazil, found that vegetation had a limited cooling effect on the air temperature, ranging from 0.3 to 1.5 ℃. However, when it comes to mean radiant temperature, the differences when comparing a base scenario (without vegetation) with one with street trees, the values are up to 19.5 ℃. Also surface temperature is highly affected by the presence of vegetation, which can reduce the temperature by up to 13 to 16 ℃.

The aim of this work is to evaluate the effect of promoting new green areas, at ground level and at rooftop level, in the thermal comfort of the surrounding urban area. The simulation is applied in a Lisbon neighborhood that recently went through an urban requalification process. ENVI-met model was used in this study to simulate urban microclimate under three situations: (1) scenario 1, the situation before site intervention; (2) scenario 2, the situation where green cover was increased through an urban requalification project, and (3) scenario 3, where green roofs are proposed.

2 Study Area

2.1 Urban Context

Lisbon is the capital of Portugal, with an area of 84 km2 and 547 733 inhabitants, according to the latest census (2011). The largest green urban structure is the Monsanto Forest Park (1000 ha), that is linked to other green spaces in the city towards a green corridor. The city has a great diversity of green spaces, from arborized open squares, to tree alignments, public parks, communal gardens and patios. The Lisbon City Council has included in the new Master Plan, through the Biodiversity Strategy, a series of sustainable measures aiming to increase the green structure rates available in the city [13]. The Green Structure Plan predicts an increase of 20% in current areas, achieving 23.6% of the total of Lisbon´s area; generally concentrating the new parks and green connections over proposed greenways [12].

The municipality, in its pursuit of a strategy of urban regeneration, has promoted the rehabilitation of vacant buildings and the qualitative improvement of public spaces, by enhancing green spaces and their connectivity. In order to test the implications of this strategy towards urban comfort, an area located in Avenidas Novas neighborhood was selected. The square area has already many trees, being Tipuana tipu the most representative species, while Platanus x hybrida is the most common tree at Avenida da República.

Two strategies for promoting new green areas in this area will be investigated, considering vegetation at ground level and in rooftops:

  • Replacing traffic lanes by green areas and increase tree coverage. This is evaluated based on a requalification project that took place in the study area - the project of Praça Duque de Saldanha and Avenida da República, following a methodology proposed by [43];

  • Promoting green roofs in the buildings with known potential. The most suitable buildings are available at [44].

New Green Areas at the Ground Level.

Although the project covers a larger area (i.e., it requalifies an artery of the city with a total extension of approximately 1.5 km), due to the software constraints, only part of the square and the two first blocks of the avenue will be evaluated, occupying an area of 11.4 ha. This area includes part of the Central Business District of Lisbon, thus concentrating commercial and tertiary activities (Fig. 4). The built environment includes a mix of apartment buildings and multi-storey commercial and mixed-use buildings.
Fig. 4.

Study area located in Lisbon, Portugal.

In the study area there are 146 buildings, with a mean of 6 floors. Higher buildings go up to 13 floors and include commercial areas located in the square. In the area, there are approximately 667 residents (value extrapolated from the 2011 Census), but since it is an area with many services and commerce, one can anticipate that the number of people who will potentially benefit from this intervention is much higher.

The project was concluded in early 2017. In Fig. 5 are shown the changes within the square area. On the right side of the picture there used to be a parking area and a taxicab stop. After the requalification, the parking area was eliminated and the taxicabs were reallocated. Furthermore, where there used to be two traffic lanes, nowadays there is only one. The changing effect is now visible in new green areas and cycling lanes, but also in increased pedestrian space close to the buildings, with the possibility of implementing esplanades and other rest zones.
Fig. 5.

Praça Duque de Saldanha after site requalification.

The median divider of Avenida da República suffered a rearrangement: in fact, before intervention this axis was very narrow. So, the median divider was materialized and is now wider, with an alignment of trees and shrub vegetation (Fig. 6). Furthermore, the parallel lanes were also replanted and redesigned, and the parking spaces were reduced. Another important change was to provide the entire extension of the axis with a unidirectional cycle path on each side of the avenue.
Fig. 6.

Avenida da República central separator after requalification.

This new road design is expected to discipline the negative impacts of high traffic flows on this axis of the city of Lisbon by reducing the excessive speed of the vehicles, thus decreasing the chances for accidents and, at the same time, improving the quality of the public space.

New Green Areas at the Rooftops.

[44] estimated the city potential to receive green roofs. Through a multi-criteria analysis based on geographic variables, 2890 buildings were identified in the city of Lisbon as suitable locations. From this set, 8 are in the study area, and include 6 offices and 2 residential buildings (Fig. 7). These buildings have from 7 up to 11 floors, and a total roof area of 3264 m2.
Fig. 7.

Rooftops with potential to receive green roofs.

Along with the new vegetation at the ground level, planted within the Praça Duque de Saldanha requalification framework, potential new green areas at these rooftops will also be evaluated towards their impact in the urban thermal comfort.

2.2 Geographic and Climatic Dataset

The microsimulation of the study area before and after site intervention requires modelling the land cover, elevation and climatology for both situations. The urban requalification project, available at the City Hall, allowed identifying the changes occurring in the area, namely the lanes to be eliminated and replaced by vegetation. Then, using a satellite imagery available at Google Maps, the buildings and street level land cover before and after intervention were modelled.

For modelling the buildings’ height, a normalized Digital Surface Model (nDSM), from 2006, with elevation information available with a resolution of 1 m, was selected [42].

To evaluate the cooling effects of the urban requalification project, a climatic characterization of Lisbon city was performed (Fig. 8). Lisbon has a Mediterranean climate, with mild winters and hot and dry summers, classified as Csa according to Köppen system. According to [29], the “strong” UHI (intensity > 4 ℃) occurred more often during the summer, with median values of 2 ℃ by night and 1.8 ℃ by day. The wind regime is predominantly from the north and northwest all through the year, although western and southwestern winds are also common during the cold season [2]. Based on the climatological normals for the period 1970–2000, August was the hottest month, with an average air temperature of 22.5 ℃. For this month, the parameters air temperature, wind speed and direction, and relative humidity 2 m above ground were retrieved from www.portalclima.pt. The value for the roughness length was obtained in [3]. Regarding the specific humidity at model top, the Wyoming University site was consulted (http://weather.uwyo.edu/upperair/sounding.html).
Fig. 8.

Meteorological input parameters for summer time of Lisbon, Portugal.

3 Methodology

3.1 Green Urban Scenarios

Three green scenarios were tested to assess vegetation impact towards thermal comfort:

  • Scenario 1 describes the situation, before the site’s requalification, with less vegetation;

  • Scenario 2 describes the present-day situation, after implementing the requalification project. This scenario represents an increased area of vegetation at ground level, resulting from replacing traffic lanes by shrubs, herbaceous and new tree alignments;

  • Scenario 3 foresees the increase in vegetation area though green roofing. In this situation, new green areas are obtained not just by considering the space available at ground level but also the roof areas that have geographic potential to receive green structures.

3.2 Microclimatic Simulation

To evaluate the impact of the new green areas implemented within the requalification project as well as new potential ones at rooftops, the ENVI-met software, v3.1 (www.envi-met.com) was selected. The ENVI-met is a 3D microclimate model designed to simulate the surface-plant-air interactions in urban environment, at a microscale level (0.5–10 m in space, and 10 s in time). The main prognostic variables of the software are wind speed and direction, air temperature and humidity, turbulence, radiative fluxes, bioclimatology and gas and particle dispersion. The software considers in the calculations, the radiation flux of short and long waves, and the latent heat of vegetation and water elements. Further details about the equations and architecture of ENVI-met model are presented in [1].

The ENVI-met was chosen due to the simplicity of the modeling process. In addition, the program allows the generation of numerous types of scenarios and the generation of spatialized results. This is a key factor when evaluating the impact of vegetation scenarios towards the pedestrian level of comfort, i.e., for local scale analysis.

Among others, the ENVI-met model includes the calculation of biometrical indices like PMV (Predicted Mean Vote) that are used to measure and compare human thermal comfort in different environments. Another well used index - PET - will not be considered in this study, since the free ENVI-met v.3.1 does not deliver this index.

The ENVI-met model is designed in a 3D rectangular grid. To run, an area input file and a configuration file are required. The area input file includes information about the environment morphology, such as position and buildings’ height, plant types’ distribution, surface materials and soil types. The configuration file, on the other hand, includes simulation date and duration, as well as basic meteorological data.

The ENVI-met adopts a numerical model. Therefore, the study area is reduced to grid cell and the user must manually introduce each element in the area. To visually aid this stage, a satellite image from the study area, available at Google Maps was used. The first elements to model are the buildings. For each element in this class, information about the location and the height must be introduced. In this task, the satellite image allowed locating the buildings, while the nDSM indicated the respective height.

After introducing information regarding the built environment, the next element to be modelled is the vegetation cover. ENVI-met has a plant database with several species characterized according to the CO2 fixation, plant type (deciduous, conifers or grass), short wave albedo, plant height, leaf area density, among others.

For Scenario 1 – the past situation – the tree cover was modelled based on the visual interpretation of the image. For Scenario 2 – the present-day situation, after site requalification – the urban requalification map was used to locate the new sidewalks, and new green areas, including trees, and hedges. The species available at ENVI-met plant database that were selected for both scenarios included: (1) trees with 20 m, dense and with a distinct crown layer, (2) trees with 15 m, less dense, (3) grass with 50 cm, and (4) dense hedges. For Scenario3 – considering also green roofs – the layer created by [44] was used to identify the suitable rooftops. The plant species selected from the database for being the one that most resembles those used in Portugal in this type of green infrastructure was Luzerne with 18 cm (Fig. 9).
Fig. 9.

Green Roof modelling in ENVI-met.

The last elements to be modeled are the soil and surface. In the study area, concrete and asphalt are the most common surfaces. This information is available at the satellite image and in the requalification map.

After modelling the land cover of the study area, the basic meteorological framework for simulation must be defined (Fig. 8). The last step is to run the ENVI-met model. The ENVI-met simulations typically cover 24 to 48 h ahead and result in atmospheric outputs for each grid cell in the 3D raster as well as surface and soil variables for the simulated environment. For calculating outdoor thermal comfort and compare it among the three scenarios, three main parameters were selected: air temperature, wind speed, and relative humidity. Additionally, the human thermal comfort index PMV, which is derived from these variables, was also selected. The steps for simulating the human thermal comfort in the study area, considering three distinct green scenarios are present in Fig. 10.
Fig. 10.

Methodological steps for climate modelling of the study area under three scenarios using ENVI-met.

4 Results and Discussion

Three contrasting urban scenarios were modelled to assess the impact of promoting new green areas at ground level and rooftops in the urban thermal comfort. The first scenario (1) represents the situation before requalification, where there was already some vegetation. The second scenario (2) is the present situation, where green areas at ground level where increased through the requalification plan. The third scenario (3) is a theoretical proposal for installing green roofs, based on the findings of [44] (Fig. 11). The three scenarios have different proportions of pervious (vegetation on the ground), semi-pervious (sidewalks – cobblestone) and impervious elements (buildings and asphalt) (Fig. 12).
Fig. 11.

Scenarios 1, 2 and 3 simulated in ENVI-met.

Fig. 12.

Percentage of each land cover element in the three scenarios.

In every scenario, the same conditions were applied: (1) the area input data was a 135 × 135 × 30 grid, (2) the grid cell size was dx = 2.5, dy = 2.5 and dz = 2 m, and (3) Lisbon geographical position. The simulation duration was 24 h. Each scenario simulation required a total time of 30 h. Table 1 shows the results from the simulation of each scenario.
Table 1.

Simulation results for scenarios 1 (sc1), 2 (sc2) and 3 (sc3), in Lisbon, Portugal, during summer time.

Parameter

9 a.m.

3 p.m.

sc1

sc2

sc3

sc1

sc2

sc3

Air temperature

21°C

18°C

18°C

25°C

22°C

22°C

Relative humidity

50%

60%

60%

35%

45%

45%

Wind speed

3.0 m/s

2.5 m/s

2.5 m/s

3.0 m/s

2.5 m/s

2.5 m/s

PMV

+1 (slightly warm)

−1.5 (slightly cold)

−1.0 (slightly cold)

+1.5 (slightly warm)

−1 (slightly cold)

−0.5 (slightly cold)

The simulation process reveals an improvement in the comfort indexes for the morning and afternoon periods. The PMV summarizes the effects of air temperature, radiation, humidity and wind, on the persons’ energy balance in one value weighed with level of influence. In the morning period, the PMV simulated for scenario 1 indicates a slightly warm environment while for the greener scenarios 2 and 3 it indicates a slightly cold environment. In the afternoon period, an inversion of the comfort assessments occurs, going from slightly warm to a slightly cold environment (Fig. 13). The green roof scenario produced a PMV value closer to 0 (comfort), meaning that this type of greening preserves the comfort in both periods of analysis.
Fig. 13.

PMV (predicted medium vote) in the three scenarios simulated in ENVI-met at 9a.m and 3p.m.

Considering the different meteorological parameters individually, the comparative results between scenario 1 and scenarios 2 and 3 show a great improvement in the comfort of the area. Observing the air temperature simulation, the results indicate a potential reduction of up to 3° in the morning (9a.m) and in the afternoon (3p.m) (Fig. 14). This condition can be explained by the presence of trees. These contribute to reduce the incoming solar radiation, which consequently reduces the mean radiant temperature. Nevertheless, this effect is more visible on the avenue, rather than on the square. This is due to the fact that, in the current scenario, trees are already present in the square area.
Fig. 14.

Air temperature in the three scenarios simulated in ENVI-met at 9a.m. and 3p.m.

When profiling the green roof influence in the avenue (Fig. 15) a slight modification can be observed near the buildings. In the morning, it is cooler than its surroundings, while in the afternoon, an inversion occurs.
Fig. 15.

3D profile of air temperature.

Regarding the relative humidity of the air, the presence of new trees and shrubs in scenario 2, contributes to an increment of 10%. This fact can be attributed to the evaporation of the trees and lower vegetation (Fig. 16). For the green roof scenario 3 the relative humidity is the same as in scenario 2, i.e., it is not modified. Regarding natural ventilation, as expected, there is a slight reduction in natural ventilation in both morning and afternoon periods. In fact, wind speed decreases from 3.0 to 2.5 m/s due to the barrier caused by the treetops, although ventilation is preserved always. Nevertheless, and as anticipated, the green roofs have no influence in this parameter (Fig. 17).
Fig. 16.

Relative Humidity in the three scenarios simulated in ENVI-met at 9a.m. and 3p.m.

Fig. 17.

Wind speed in the three scenarios simulated in ENVI-met at 9a.m and 3p.m.

5 Conclusions

The microclimate software ENVI-met allowed modelling the improvements towards outdoor thermal comfort of different greening strategies in a public space located in a Lisbon neighbourhood. Two strategies for increasing green areas were investigated: considering vegetation at ground level and in rooftops. The first strategy was based on the urban requalification project implemented in the study area in early 2017. The second strategy was based on the results of [44], where the rooftops with green potential were identified.

The situation before urban requalification included 12% of the area with green cover (trees and shrubs). The present-day situation, after site requalification, is characterized by an increase of 26% of vegetation at the ground level. When considering rooftop level, the proposed increase is 0.3%.

As already indicated in a previous study conducted in the same area [43], the microclimate simulation results confirm that vegetation is a key element when planning for comfort public spaces.

The methodology disregards the details of important urban structures as sidewalks, or urban design. This is due to the model’s pixel size. Nevertheless, it is clear that the new urban design, where the road includes more sidewalk area and less area of asphalt, promotes an increase in thermal comfort, and contributes to the reduction of heat island formation.

The role of green roofs in thermal comfort requires further investigation, namely considering the private and public benefits of such structures, based on a multi-temporal analysis, where winter and summer time periods should be investigated. In that context it is also relevant to understand the species of vegetation implanted in green roofs, going beyond the height of the substrates used, as this may interfere with the heat gains of the building.

The area under study is already heavily forested, and the large tree mass might have overshadowed the gains of the green roofs. Consequently, in addition to the climatic seasonality analysis, it is recommended to evaluate the green roofs as one of the mitigation strategies in areas with fewer trees.

A major constraint in ENVI-met is the specified values for vegetation functional types and their form (shape, structure and size). These are broad and generalized and not specific to individual species. A detailed description of the species used in the requalification of the study area would allow refining the results. Nevertheless, the contribution of vegetation as a mitigation factor should be quantified when planning for new urban areas and ENVI-met has demonstrated its efficiency towards that goal.

Notes

Acknowledgements

The authors would like to thank Logica the opportunity to use the LiDAR data set. This paper presents results partially supported by CICS.NOVA - Interdisciplinary Centre of Social Sciences of the Universidade Nova de Lisboa, UID/SOC/04647/2013, with the financial support of FCT/MCTES through National funds. The first author was funded by the Fundação para a Ciência e Tecnologia, under a post-doctoral grant (Grant SFRH/BPD/76893/2011). The second author was funded by Fundação de Apoio a Pesquisa do Distrito Federal do Brasil (Foundation for Research Support of DF).

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Teresa Santos
    • 1
    Email author
  • Caio Silva
    • 2
  • José António Tenedório
    • 1
  1. 1.Interdisciplinary Centre of Social Sciences (CICS.NOVA)Faculty of Social Sciences and Humanities (NOVA FCSH)LisbonPortugal
  2. 2.Faculdade de Arquitetura e UrbanismoUniversidade de BrasíliaBrasíliaBrazil

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