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Effect of urban heat island mitigation strategies on precipitation and temperature in Montreal, Canada: Case studies [1]
['Audrey Lauer', 'Department Of Earth', 'Atmospheric Sciences', 'University Of Quebec In Montreal', 'Montreal', 'Quebec', 'Francesco S. R. Pausata', 'Sylvie Leroyer', 'Meteorological Research Division', 'Environment']
Date: 2023-07
High-resolution numerical weather prediction experiments using the Global Environmental Multiscale (GEM) model at a 250-m horizontal resolution are used to investigate the effect of the urban land-use on 2-m surface air temperature, thermal comfort, and rainfall over the Montreal (Canada) area. We focus on two different events of high temperatures lasting 2–3 days followed by intense rainfall: one is a large-scale synoptic system that crosses Montreal at night and the other is an afternoon squall line. Our model shows an overall good performance in adequately capturing the surface air temperature, dew-point temperature and rainfall during the events, although the precipitation pattern seems to be slightly blocked upwind of the city. Sensitivity experiments with different land use scenarios were conducted. Replacing all urban surfaces by low vegetation showed an increase of human comfort, lowering the heat index during the night between 2° and 6°C. Increasing the albedo of urban surfaces led to an improvement of comfort of up to 1°C during daytime, whereas adding street-level low vegetation had an improvement of comfort throughout the day of up to 0.5°C in the downtown area. With respect to precipitation, significant differences are only seen for the squall line event, for which removing the city modifies the precipitation pattern. For the large-scale synoptic system, the presence of the city does not seem to impact precipitation. These findings offer insight on the effects of urban morphology on the near-surface atmospheric conditions.
Funding: This work was supported by the MCIN/AEI (PID2019-105253RJ-I00 project direct costs financed by MCIN/AEI/10.13039/501100011033 to DA) The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. All the numerical simulations in this study were performed on the supercomputer facilities at Environment and Climate Change Canada.
1 Introduction
Cities occupy a small fraction of the Earth’s surface, yet over half of the world’s population lives in urban areas, a number that will significantly increase in the next decades [1]. Cities modify the local environment because they are built with materials and geometries that clearly differ from the natural landscape. Built structures have an impact on the local climate because they alter surface exchanges of heat, moisture, momentum, and radiation with the atmosphere. A complete understanding of these effects is crucial to identify and reduce the risks that urban dwellers are exposed to.
Initially observed and documented in the 1800s, urban areas are warmer relative to their rural surroundings [2]. This phenomenon is referred to as the canopy urban hat island (UHI) and processes explaining the unique local climate of cities have been well documented [3,4]. Materials used in cities have low reflectance, are good thermal conductors and have greater heat storage capacity, so they are more efficient than the natural materials at absorbing atmospheric radiation fluxes and heat, which is then released at night mainly through sensible heat flux. Urban surfaces are mostly impervious, which alters the water budget by reducing infiltration and evaporation, and by increasing surface runoff. As a result, there is little water available for evaporative cooling and most turbulent heat exchanges are channelled through sensible heat fluxes. In addition, city landscapes are often less vegetated than rural areas, reducing evapotranspiration from plants and its effect on temperature. Urban geometry accentuates these effects by trapping energy because solar radiation is reflected multiple times by urban surfaces and thus the probability for it to be absorbed by the city fabric is larger [5]. Urban areas reduce the wind, which enhances the heat trapping in the city [5,6]. Anthropogenic heat sources (i.e. road traffic, industry, heating and air-conditioning) and atmospheric pollution also contribute to increasing the intensity of the urban heat island [5].
Urban planners tend to adopt many different strategies to reduce the strength of the UHI and its potential effects on the increasing urban population. Common mitigation strategies are, for example, adding green infrastructures such as green roofs, parks and trees [7–10], and increasing the reflectivity of urban surfaces [10–14]. Replacing urban surfaces with vegetation lowers air temperature due to increased evapotranspiration and less surface warming during the day. Furthermore, low vegetation might enhance heat release at night since it often has a high sky-view factor. On the other hand, vegetation also adds water vapor to the air, potentially decreasing human comfort on local population. Studies show that in general heat stress is typically lowered when vegetation is added [8,15], which is beneficial to urban population. The type of vegetation (i.e. low or high vegetation) added and its placement inside the urban canyon can have a different effect on thermal comfort, for example, trees offer shade and interact with radiation and are more effective than grass in improving comfort [7]. Increasing urban surface albedo decreases daytime air temperature due to higher reflection of solar radiation that causes less surface warming. Nighttime impacts of albedo change seem instead to be negligible [10,11,14]. For this mitigation strategy, the impact on human comfort can vary depending on the way it is assessed. Recent studies have shown that increasing the ground-level albedo may well decrease pedestrian comfort due to increased reflection [14,16,17]. The effectiveness of these strategies is also greatly affected by the geographical location, size, and composition of the city.
In the last decade, it has also been shown that urban areas can have a sizeable impact on precipitation. Observational and modeling studies in mostly North American and Asian megacities reviewed by Liu & Niyogi show a rainfall enhancement of 16% over and 18% downwind of the city (20–50 km from the city center) [18]. Our understanding on the urban processes that modify rainfall is still evolving because precipitation is influenced by many factors from large-scale synoptic systems to local cloud microphysics. The main mechanisms through which urban areas can influence precipitation are the following, in no particular order of importance:
An increase in low-level convergence due to increased roughness of cities which impacts convection over the urban areas [19];
Higher temperatures over cities due to the UHI tend to destabilize atmosphere, therefore create UHI-generated convective clouds [5,20,21];
Enhanced concentration of atmospheric aerosols over cities due to pollution are sources of cloud condensation nuclei (CCN) and influence the radiative transfer between the cloud layer and the surface. These effects are summarized in [22];
Storms tend to either bifurcate around cities [19] or split into small convective cells upwind from the city [23].
These processes are not always represented correctly in numerical studies, thus could explain the differences with observational studies reported in [18]. Nevertheless, numerical experiments have become more and more important to understand interactions between the cities and the atmosphere as different urban processes can be isolated to disentangle their relative impact on local climate.
In this study, numerical weather prediction (NWP) case studies in the Montreal (Canada) region are explored. During summertime, important UHI both night and daytime can be observed in Montreal. While the impact of this city on temperature and heat stress has been previously investigated [24,25], few studies have hitherto explored the impact of Montreal UHI on summertime precipitation. Located in the Saint-Laurence River, Montreal has been affected by significant flooding events. For example, springtime flooding in the Great Montreal region is typically linked to rainfall associated with extended thaw periods, hence leading to rapid melting of winter snowpack [26]. In July 1987, a series of strong thunderstorms that crossed the island in the afternoon generated significant downpours, which paralyzed the city. This event followed a significant heat wave over the region, which likely intensified the storm. Since previous studies have shown an enhancement of rainfall over urban areas and given that urbanized areas are growing, flooding events are more likely to occur in the future [27]. Moreover, impervious surfaces in cities intensify surface runoff and reduces water infiltration, which increases the flooding frequency [28]. Additional factors beyond the urban environment may produce an intensification of extreme events, for instance higher temperatures due to climate change increases the atmosphere’s water-holding capacity [29]. Studies have indeed shown a higher number of flooding events due to increasing urbanization and climate change [28,30,31], which urges cities to adapt.
The main objectives of this paper are, to understand how the urban environment of Montreal influences local temperature and human comfort during heat waves and to evaluate the impact of the city on rainfall following these heat waves. To achieve this, two heat events immediately followed by intense precipitation are studied using a high-resolution numerical model. Furthermore, different mitigation scenarios replicating urban design strategies are investigated to assess their effectiveness on improving comfort. The manuscript is divided as follows: section 2 presents the models used and the experimental design; section 3 shows the results from two different case studies; section 4 summarizes and discusses the key findings of this study.
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