Urban population, according to the United Nations, represents about 55% of world population and this proportion is expected to increase to 68% by 2050 (2018). In Canada in 2015, the building sector, accounting for residential, commercial and institutional buildings, was responsible for 28.3% of all energy used and 22.6% of greenhouse gas (GHG) emissions (NRCan 2016). Worldwide, 40% of building energy requirement is due to cooling and heating demands (IEA 2016). According to the Future of Cooling Report of the International Energy Agency (IEA), needs for space cooling will grow in the future worldwide, and the energy requirements for air conditioning are expected to triple by 2050 (IEA 2021). The energy consumption of buildings is not only influenced by the characteristics of the building envelope and the equipment (Santamouris et al. 2001; Ihara et al. 2008), but also by the environmental conditions. Because city-built areas have substantially different characteristics than natural surfaces, urban climates may differ significantly from original natural environment (Sun and Augenbroe 2014). The urban heat island (UHI) is characterized by higher temperatures in urban areas compared to surrounding rural areas (Santamouris 2007; Chapman et al. 2017), mainly due to heat storage in heavy constructions, lack of vegetation, reduced heat removal by ventilation and nighttime longwave radiation, anthropogenic impacts (particularly, waste heat released from heating, ventilation, and air-conditioning systems) (Magli et al. 2015). Numerous measures have been proposed to mitigate the negative impacts of UHI on the urban microclimate, particularly during heatwave periods (Jandaghian and Berardi 2020). One main strategy to alleviate urban thermal stress as well as to improve the human thermal comfort and decrease building energy demand, is to increase vegetation cover via green infrastructure such as forests, parks, and farmlands in urban areas (Wang et al. 2016; Kong et al. 2017; Arghavani et al. 2020). Several studies show that changes in ambient outdoor conditions caused by climate change (Pérez-Andreu et al. 2018; Ascione et al. 2021) and by UHI negatively impact the building energy demand.
Urban heat island effect and heatwave periods in cities have been studied for several types of climates. Building energy simulations (BES) (Eames et al. 2012; Fumo 2014) are influenced by weather data (climatic data), as shown e.g. in Toronto, demonstrating the effect of climate change on heating and cooling energy requirements for reference buildings (Berardi and Jafarpur 2020). In some cases, it is also shown that, in a cooling dominated climate (Ciancio et al. 2018; De Masi et al. 2021), not considering UHI leads to an underestimation of cooling energy consumption of 35–50%. The accurate prediction of the cooling energy demand (CED) of buildings during the summer period is not straightforward since input data for BES are numerous and uncertain and may change from year to year. For this reason, the Canadian National Energy Code uses the TMY algorithm (Typical Meteorological Year data) and software to develop Canadian Weather Year for Energy Calculation (CWEC) files for several locations in Canada (ASHRAE 2021). It is more and more accepted that weather data that reflect the local urban microclimate, heatwaves and potential future climate conditions has to be used for BES (Villa 2021). Mesoscale models are numerical meteorological models that have been used to simulate, analyze and predict urban weather conditions. Among the models that can be used, the Weather Research and Forecasting (WRF) model presents a high temporal resolution and flexible adaptability (Skamarock, W. C., Klemp, J. B., Dudhia, J., Gill, D. O., Liu, Z., Berner, J., … Huang 2019). The WRF model approach has shown promising results for studying the urban heat island effects (Bahi et al. 2020) and offers the possibility to overcome the difficulties and limitations of in situ measurement and assess potential future impacts (Kong et al. 2021). For example, WRF model was used to simulate waste heat release in cities at the mesoscale with high temporal resolution (Chen et al. 2014; Zhang et al. 2016). Krayenhoff et al. (Krayenhoff et al. 2021) conducted a systematic and critical review of the literature on numerical modeling of urban air temperature reduction resulting from green-blue infrastructure and reflective materials. When it comes to study the impact of vegetation in cities, several parametrization are available for WRF (Loughner et al. 2012). WRF with the urban canopy model (WRF-UCM) allowed estimating that covering urban streets with 50% tree coverage and reducing the quantity of asphalt on the roadway by 10% might lower daily urban canyon air temperatures by 4.1°C (Christopher et al., 2012) (Loughner et al. 2012).
Prior studies have estimated that increased urban albedo is an effective way of lowering ambient temperatures. Using also WRF with UCM and a dynamic downscaling approach, Vinayak et al. (2022) proposed and evaluated a mitigation strategy (urban albedo enhancement) to counter the unfavorable effects of future urbanization on the local climate over the Mumbai in India. Simulation results show that the average maximum surface air temperature of the Mumbai Metropolitan region would increase by 1.4°C due to future urbanization by 2050, whereas when adopting the proposed mitigation strategy, this increase would be restricted to 0.7°C (Vinayak et al. 2022).
Jandaghian & Akbari (2021) estimated in their study in Toronto and Montreal, Canada that an increase in the albedo could lead to 1–2°C reduction in ambient temperature (Jandaghian and Akbari 2021). More recently, Hosseini et al. (2022) in their study applied the degree-day method to assess building energy for the projected future climate changes (Hosseini et al. 2022). From WRF model results at a spatial resolution of 250 m, more local weather data can be determined and used to perform building energy simulations (BES) (Salata et al. 2022). BES are widely performed to predict the space heating and cooling energy demand of buildings (Allegrini et al. 2012; Zhou et al. 2020; Jafarpur and Berardi 2021).
As previously stated, there has been a significant amount of research devoted to the assessment of energy demand for buildings in a changing climate and/or taking into account the UHI. However, Canadian regulations as well as the majority of past research are highly based on a daily energy index (i.e. cooling degree-days) to correlate energy demand in scenarios' changing climate and UHI (Sun and Augenbroe 2014; De Rosa et al. 2014; De Masi et al. 2021; Hosseini et al. 2022). Although the CDD is an important index reflecting the climate and the energy demand of buildings, this index is based on the average daily temperature and does not take into account the variation in the hourly temperature. Contrary to that, applying cooling degree-hours (CDH) in changing climate scenarios, as an index linked to the hourly outdoor air temperature, can be represented as a term included in the computation of the cooling energy demand of a typical building. Indeed, this index makes it possible to evaluate the potential of natural ventilation and to analyze heat wave periods. This quantity index is proportional to the cooling energy demand.
The present study investigates the impact of land use of three scenarios (urbanization, river, and vegetation) to examine to which extent introducing large areas of vegetation and water (river) can dampen the effect of the urban heat island (UHI), especially during heatwave periods and their effect on the thermal comfort of building occupants and the cooling energy demand (latent cooling energy) of residential buildings in a large city. For this study, we select a large city surrounded by water, Montreal, a metropolitan region of Montreal, where the urban heat island becomes more and more prominent and in strong progression in certain of its districts (Cavayas and Baudouin 2008). The WRF ARW (Advanced Research and Weather Forecast) model is used to determine the local microclimate where the UHI phenomenon is taken into account. In addition, thanks to WRF's temporal and spatial high-resolution methods, cooling degree-hours are calculated taking into account land use properties covering two heat wave periods in the Montreal region.
We compare the actual scenario the river surrounding the island of Montreal to two fictitious (extreme) scenarios: in a first scenario the river is replaced by urbanization, and in a second by vegetation. Our investigation is conducted for late spring and early summer season, from May 15th to June 29th, 2020, covering two heatwaves and focusing on the effect of heatwaves and the urban heat island phenomenon at seven representative locations in Montreal. The three scenarios are compared in terms of indoor thermal comfort and building energy demand.