Canada’s Changing Climate Report has announced a doubled rate of warming for Canada compared to the rest of the world [1
]. The Crowther Lab at ETH Zurich University has predicted that some Canadian cities will experience an average temperature increased by more than three degrees Celsius by 2050. It is estimated at 3.1 °C for Montreal [2
]. Figure 1
shows the predicted increase of stress days across Canadian cities for the projected years starting in 1961. The impact of heat stress is already being felt by increasing the number of hot days, more than 30 °C [3
]. The rising frequency of heatwaves in many Canadian cities resulted in the death of 70 people in 2018 in Quebec alone [4
]. Health Canada (2011) reported that when the daily average temperature goes higher than 20 °C in seven Canadian cities, the relative mortality rate rises by 2.3% for every degree increase. This means the intensity of 2–3 °C would lead to a 4–7% increase in the mortality rate [5
]. Environment Canada (Ontario Region) introduces the heatwave as a period in which, for three successive days, the maximum temperatures stay or go over 32 °C [6
]. However, the air temperature is not the only factor for heat. The daily thermal experience is quite complex; for the same day and air temperature, there is a different feeling on a different pathway.
Human thermal comfort in the urban space is associated with four environmental parameters: air temperature, relative humidity, air velocity, and thermal radiation, which is the most complicated parameter. The thermal radiation within an urban open space is usually described by mean radiant temperature (Tmrt), which is defined as a precise measurement of urban sites prone to heat [7
]. Thorsson et al. (2014) identified it as a better indicator of the harmful heat stress condition [8
]. Tmrt depends on the temperature difference and the radiant net exchange between the object and the surrounded environment [9
]. Thus, the potential of a surface to absorb or negate heat and the amount of object seen by surrounding radiation sources has a crucial influence on Tmrt variation. The local convection coefficient is another practical factor for increasing Tmrt in the dormant area. If the wind speed decreases, the portion of radiation in the global temperature increases; in turn, Tmrt grows more [10
In the urban area, the physical characteristics of the built environment affect the distribution of Tmrt by changing the exposure to sunlight and the material coverage of the surfaces. When the intra-urban temperature shows a slight variation during the day, the Tmrt represents a considerable variation over short distances resulting in a noticeable difference in thermal perception [11
]. The effect of urban form on thermal comfort focused on the Tmrt indicator has been the subject of much research for different climate zones using simulation method or real data observation analysis [11
]. Two principle geometry factors in evaluating the heat stress of urban spaces include the aspect ratio, the ratio of building height to the street width and, sky view factor, representing the surface exposure to the skydome, ranging from zero to one [12
]. The finding highlighted the open structure design with low-rise buildings, and the lower aspect ratio is prone to increase the number of extreme heat stress hours. In contrast, the street canyon with a smaller sky view factor, shaded by surrounding buildings, provides more comfortable with lower Tmrt during summer days [12
]. For canyons of the same aspect ratio, their orientation has an influential role in managing the Tmrt [13
]. However, it is recommended to consider the simultaneous impacts of all critical factors such as the aspect ratio, canyon orientation, level of urban density and, latitude of canyon location [12
The management of geometry parameters is costing financially and needs to be considered in the first place since they will not change over a long period of time. Although a dense structure maintains the extreme swings in Tmrt and daytime thermal stress during seasons [12
], without respecting the flexible mitigators such as vegetations or adequate ventilation, it could increase nighttime temperature due to the urban heat island effects during summer [12
]. The contribution of urban heat island on the local scale and the increase of temperature at the regional level intensify the heat stress in the outdoor spaces [12
]. Many studies showed the potential of Tmrt reduction through urban vegetation [11
]. Planting trees is an effective measure to improve the urban microclimate by reducing summer day temperature within the tree’s boundaries and beyond the leeward side [19
]. In addition to their ecological, aesthetic, health and, physiological benefits, the distribution and density of trees also allow flexible strategies to manage sun exposure in urban areas. Trees help reduce the transmission of solar radiation [15
] and enhance the evapotranspiration and improve the shading pattern on surfaces underlying heat stress conditions [21
]. A more extensive canopy means a more significant contribution in reducing temperature and decreasing Tmrt in both daytime and nighttime. The effect of clustered tree planting is more than the isolated placement of trees in Tmrt reduction at low density and low height structure [15
]. However, dense construction must prioritize tree placement in areas not currently shaded by other sources like buildings, particularly in areas with heavy pedestrian traffic [14
The effect of surrounded coverage is another concern of designers to manage Tmrt through the albedo of surface materials. Albedo is a physical property of the material to change the thermal behavior of the surrounded environment. Albedo refers to the fraction of solar radiation reflected by a projected surface [22
]. Materials with low albedo, dark-colored, tend to absorb radiation and contribute to the urban heat island, while surfaces with a high albedo, white-colored, are prone to reflect it. Although using high albedo material is recommended for buildings to save energy during summer time [20
], it is a less desirable solution on the surfaces near the ground where the reflected beams bounce back to urban spaces where human activities are present [23
]. Alchapar, E.N. Correa (2015) demonstrated albedo’s effect on the temperature variation of surrounded surfaces, particularly in the high-density area, higher aspect ratio and, larger wall surfaces. When albedo increases by 10%, the air temperature increases by 0.5 K to 0.7 K, whereas it does not significantly change the low-density area [24
]. It is essential to highlight that the impact of albedo is not constant during the day. Hui Li (2016) examined that the effect of albedo is at the highest level during the early morning and late afternoon. While during midday, it tends to a low and constant value, and cloudy days intensify it as well [25
]. The summation of shortwave and longwave fluxes outgoing from the surrounded surfaces is comparatively small to incoming fluxes originating from the cardinal points [14
]. Hence, the simulated albedo effect is expected to be small, given the different turbulence evened out in local air temperature.
The complexity of outdoor thermal conditions in response to Tmrt, as a significant indicator of thermal comfort, demands the combination of 3D city modeling and climate assessment program to support climate sensitive spatial design. Traditional urban design methods were mostly based on 2D spatial design. However, with the continuous evolution of geospatial technologies and the added benefit of analyzing and virtually visualizing our world in three dimensions, it is possible to model and evaluate the reflection of the urban decisions before implementation and reduce the cost of trial errors. Thus, this research aims first to make use of 3D city modeling tools to design and create the study area and then uses these 3D city models to calculate the mean radiant temperature (Tmrt) in terms of assessing the spatiotemporal distribution of the heat stress for an upcoming transformed district in the city of Montreal. Alongside that, the study introduces a systematic workflow to evaluate and improve outdoor thermal comfort through the accurate placement of vegetation (trees), convenient in city scale.
This research identifies a systematic workflow to evaluate and upgrade the outdoor thermal comfort relying on the contribution of 3D city modeling and climate assessment application for various time scales. A case study district under development in Montreal was used to analyze 3D geometry’s impact on thermal comfort. Geometries were generated using the ArcGIS CityEngine. The generated spatial design was rasterized to serve the SOLWEIG program to model the spatiotemporal distribution of mean radiant temperature (Tmrt) as an outdoor thermal comfort indicator. Analyzing the outcome of SOLWEIG reveals the duration and pattern of the area under heat stress. The spatial variation of Tmrt demonstrates almost 25 K difference over the spaces investigated. One of the critical reasons was the variation of the average sky view factor resulting from surrounding buildings, with a non-uniform distribution of geometry in three dimensions. Moreover, the North-South dominant direction for some urban spaces resulted in a durable shadow for the area adjacent to the west-facing walls. Whereas the areas in the proximity of east-facing walls in the wide canyons suffered from being long-time exposed to solar irradiance during the hottest period of the day. Moreover, varying the albedo value on the wall surfaces showed an almost 0.5 K decrease per twenty percent reduction of albedo. Focusing on the albedo value, deep canyons with low sun accessibility showed more sensitivity to albedo effects, and their average mean daytime Tmrt changes more than wider canyons.
Analyzing and classifying the results with the physiological equivalent temperature (PET) index indicated the potential regions under extreme heat stress. By overlaying the classified map and the human walkway’s principal structure, the priority of regions subject to heat mitigation action were clustered as a hot-spots map. The hot-spots map converted to polygon shapefile was used for automated tree generation in the CityEngine and converted to vegetation DSM for reassessment in the SOLWEIG program. The workflow was iterated two times. In the first round, the heat mitigation action’s target area was the improvement of human walkways. The response strategy results in proposing 264 number of trees with a 19% density of canopy coverage over the site (30% of the landscape). It led to improving the mean Tmrt during the hottest period of the day by 7.5 K in the entire study area, 8.5 K on the footpaths, and 5 K in the urban spaces. In the second try, the focus was on the urban space enhancement regarding their level of activity. In this case, the local community spaces and the park were in priority. The second intervention’s outcome added 58 more trees and increased tree canopy density by 4% (6% of the landscape). At this level, the mean Tmrt of the site was reduced by 1.3 K and determined urban spaces by 3.5 K during the hottest period of the day.
The designed workflow is scalable and allows for the outdoor thermal evaluation in multiple urban levels. The simultaneous designing, visualizing, and assessment supports architects and urban designers for a collaborative practice toward climate-responsive decision-making to reduce the cost of human errors, as today’s design decision has implications for the next decades. However, using such a workflow, as demonstrated in the present work, reiterates the need for urban designers, geoinformatics, and sustainability experts to work hand-in-hand. That is because executing such a 3D city modeling driven workflows needs an acceptable level of experience and familiarity with the geographic information system (GIS), 3D data modeling methods, and basic computer programing ability in the case of using ArcGIS CityEngine. From the technical side in the SOLWEIG model, the hourly Tmrt on the spots near the buildings, in a nonuniformed surrounding, is prone to overestimating around the peak hour when the albedo value increases significantly from 0.15 to 0.75. Whereas in the central points with uniform surroundings and lower albedo value, the estimation of hourly Tmrt is close to the expected influence of albedo effect, namely lower than 2 K.
The next issue is relevant to converting 3D building geometry with open space between the bottom surface of a bridge-type building and the ground surface (Figure 6
) provided complication and unreal data when converting to DSM. This problem resulted in missing correct data under the bridge building section and, consequently, no tree plantation suggestion could be made through the workflow. Such an issue can result in a wrong heat stress calculation from models such as SOLWEIG for which primary inputs are raster datasets. Since such complexity in the arrangement of building blocks is also available in the real-world, development of SOLWEIG type models which can directly accept 3D city models as inputs are suggested. It reduces the information loss, which happens while converting 3D city model geometries to raster datasets. Finally, this study clearly demonstrates the workflow’s capacity to provide an effective collaboration between 3D city modeling and climate assessment application to mitigate outdoor thermal stress through objective and systematic intervention (tree placement), which is currently a challenge for urban planners due to the lack of easy-to-use tools.