Riyadh, the capital of Saudi Arabia, has a hot desert climate, ‘Bwh’, based on the Köppen-Geiger climate classification system [26]. Ambient temperature during the summer period may reach 45°C, skyrocketing the cooling energy consumption of buildings.
2.1 Current Climatic Conditions
The magnitude of the UHI in Riyadh is persistent and well-captured by the network of stations. The temperature distribution is rather regular, with almost no outliers and high daily average temperatures exceeding 40.0°C in the city. The differences between urban and reference contexts are systematic and stable, with frequent peaks exceeding 4.0°C and differences nearing 1.8°C for 75% of the examined period (3rd quartile) and a median of 1.2°C (Figure S6 (b)). Negative values, namely when the city is cooler than the surroundings, are rarely computed (Figure S6 (a)). Additional information on the climatic analysis is given in Supplemental Information 1.
The cooling degree days in Riyadh are quite consistent over the observed period, with very high values, exceeding 2000 at all locations (Figure S7) excluding stations 1 and 4, Table S2, where the four-year average CDDs exceed 1850 and 1800, respectively. The CDDs at urban locations are approximately 280 higher than those at reference locations (computed as the difference between urban and reference averages, considering the five-year average). The five-year average of the CDDs at reference (background non-urban) locations is equal to 1960, while it is 2236 at urban locations. This 14% increase in CDDs in the city with respect to the reference locations is in line with the literature [6], which states an average increase of 13%. However, even within the city, there is a difference of 160 CDDs between the hottest (Al Amal, 2291 CDDs) and the coolest urban area. These important intra-urban and urban-reference differences point out the influence of local factors (such as land cover and wind patterns), with the potential to exert a strong influence on the cooling energy performance of buildings.
The analysis as performed on the integrated data set (ground and simulated data) has resulted in the following results/conclusions.
- During the hottest conditions and considering all urban cells/data points, more than 50% of air temperature data exceeds 40.0°C, and 10% of the urban area has air temperature higher than 45.0°C. In contrast, on an average summer day, only 23% of the urban cells/data points have air temperatures exceeding 40.0°C, whereas air temperature does not exceed 45.0°C. This result shows that in the hottest climatic conditions, hot spots are not limited, and a considerable number of urban areas experience high ambient temperatures.
- The average UHI intensity of the entire urban area during the summer period, as predicted by the mesoscale model, is 1.5°C, Figure S12. The southern and eastern parts have the highest UHII with an average intensity of more than 2.0°C and the highest average value of 2.5°C. The mean UHI intensity increases with the increase of urban density. The mean UHI intensity corresponding to low-density urban cells exceeds 2.0°C for 6% of the time. In medium-density and high-density areas, the corresponding percentages of time when UHI intensity exceeds 2.0°C are 24.6% and 36.6%, respectively. This pattern of UHI with respect to the urban type is similar to the pattern of air temperature in relation to the urban type. These figures are consistent with the observations from the network of weather stations.
- The maximum calculated daytime UHI intensity in the whole city was close to 8.5°C and appeared at 14:00, Figure S13. The southeastern part of the city experienced high UHI intensity with a magnitude of above 8.0°C, Figure S13. It is observed that the UHI intensity and air temperature are closely related between 8:00 am and 8:00 pm. Outside this time range, the UHI intensity has a stronger relationship with the wind direction. The maximum UHI intensity was calculated to occur during northern winds, while the minimum intensity corresponded to southern and southwest winds.
- Land surface temperature (LST), in the greater Riyadh urban zone, presents a significant variability. Up to 5.0°C, higher average surface temperatures are observed in the south and southeastern parts of the city, Figure S10. LST in Riyadh obtains values higher than 50.0°C during summer months in all its districts, whereas districts in the northeast and the southeast of the city exhibit LSTs even higher than 58.0°C. This is an important finding as land surface temperature a) drives the transfer of heat from the ground to the overlying air and thus contributes to higher air temperatures, at least at levels close to the ground, and (b) reduces soil humidity. The land surface temperature at 14:00 ranges between 46.1°C and 53.3°C. Additional data about the distribution of the LST in the city is obtained through the Landsat 8 satellite Observations at 10:30 am local time per district was used to reveal using the QGIS software, the most thermally stressed districts of the city. Figure S14 shows the distribution of the mean daily land surface temperature per district during a hot day as calculated by the mesoscale simulations.
High temperatures in urban areas have a direct impact on human health and are associated with heat-related stress and excess summer deaths [27]. We assessed the distribution of the urban heat risk in the city based on the collected and calculated meteorological characteristics expressing the degree of heat exposure, namely the physical characteristics and in particular, the urban planning characteristics that indicate the way the city has been built and the way it operates (building materials, buildings age, number of public facilities, etc), and the social characteristics involving demographic characteristics expressing the degree of sensitivity to extreme hot weather conditions estimated by census data such as the percentage of population over 65 or under 14 years old. For example, districts exhibiting high temperature values, low quality building structures and are inhabited by a high percentage of elderly people are more vulnerable to extreme heat than districts characterized by lower temperature values, high quality buildings and their population consists of younger people.
To assess the heat risk of Riyadh, the parameters presented above were combined into a composite heat risk indicator. To achieve this, each parameter was reclassified into three categories using the quantile classification, namely a data classification method that distributes a set of values into groups that contain an equal number of values. Table S3 provides the range of values for the three risk categories per parameter [28, 29]. The resulting three categories are defined as: 1) Low heat risk, 2) Moderate heat risk, and 3) High heat risk. Since the relative importance of each parameter is unknown, all parameters contributed equally to the composite heat risk index. The sum of all parameters was then reclassified into three categories using the quantile classification method, resulting in the composite heat risk index (Figure 1). It yields that the northeast and southeast districts of the city (in red) have higher heat risk than those to the west of the city. Several districts in the centre of the city are also exhibiting high thermal risk.
2.3 Design of The Mitigation Strategy
The detailed analysis of the current climatic and heat risk conditions in Riyadh reveals that the main axes of the potential interventions to mitigate urban heat in Riyadh should aim to: a) decrease the surface, LST, and ambient temperature and reduce the heat advection from the surrounding desert, b) reduce the strength of the sensible heat in the city, c) surge the magnitude of latent heat, d) improve solar control in the city, and e) decrease the levels of the released anthropogenic heat as much as possible.
We designed and evaluated, in detail, eight mitigation scenarios focusing on the above objectives. A short description of the scenarios is given in Table 1. To decrease the surface temperature in the city and reduce the release of sensible heat, reflective materials as well as super cool materials consisting of passive daytime radiative cooling coatings are considered. Reflective materials are commercially available and present a high reflectance to solar radiation and a high broadband emittance and contribute to reducing the surface temperature up to 10.0°C [30]. Super cool materials, (SCM), or photonic daytime radiative cooling coatings, or are recently developed and gradually penetrating the market. Super Cool Coatings exhibit sub-ambient surface temperatures and depending on the local climatic conditions can reduce the surface temperature of cities up to 15.0°C [18]. By lowering the surface temperature in the city, the height of the planetary boundary layer may decrease, resulting in reduced heat advection from the desert [31].
An increase in the green infrastructure in cities provides solar control and helps to decrease the release of sensible heat, while it highly enhances the release of latent heat in the city through evapotranspiration processes [20]. The cooling efficacy of urban greenery under high ambient temperatures, like in Riyadh, depends highly on the proper provision of irrigation [32]. Above a threshold ambient temperature and under low watering conditions, the magnitude of evapotranspiration of greenery is reduced significantly, while the released Biogenic Volatile Compounds, BVOCs, may surge, resulting in serious air quality problems [33].
Combined scenarios considering the implementation of both reflective or super cool materials with additional irrigated or non-irrigated greenery are designed and evaluated through mesoscale climatic modelling. Table 2 provides the calculated cooling performance and the corresponding mitigation potential regarding the ambient and surface temperatures for the eight scenarios. In addition, Table S2-1 provides the calculated average values of the Mean, Maximum and Minimum values of the main climatic parameters for the reference and the 8 mitigation scenarios, while Table S2-2 presents the number of hours that the average temperature in Riyadh is above a threshold ambient temperature as well as the corresponding value of the Cooling Degree Hours for the reference case and the eight mitigation scenarios during the whole summer period.
Analysis of the performance of the eight mitigation scenarios leads to the following main findings:
- An almost linear association between the reference temperature (no mitigation) and the potential temperature decrease is observed for both day and night periods. In general, and for all the mitigation scenarios, the higher the background temperature, the higher the potential temperature decrease. Figure S2-1 demonstrates the relation of the background temperature with the temperature drop for both the day and night and for the scenario ‘Very Reflective Riyadh’.
- The implementation of the super cool materials on the roofs of the buildings, combined with well-irrigated additional greenery, offers a higher mitigation potential and contributes to the reduction of the average 24 h ambient temperature in the city between 1.3°C to 7.5°C with an average value close to 4.2°C. The corresponding decrease of the ambient temperature at 2:00 pm varies between 0.0°C to 3.0°C, with an average value close to 1.4°C, while at 6:00 am the change of ambient temperature varies between an increase of 2.8°C and a decrease of 8.6°C, with an average decrease close to 3°C. In parallel, the reduction of the surface temperature in the city at 14:00 pm varies between 3.5°C to 7.3°C with an average value close to 4°C. During the whole summer period, the decrease of the overheating hours above 35°C and 40°C is 23.1% and 29.4%, respectively, while the decrease of the Cooling Degree Hours, CDH, base 35°C and 40°C is 28.4% and 47.6% respectively.
- Moderate increase of the low-level non-irrigated greenery at the city scale has a limited capacity to cool the city during daytime, 2:00 pm, while it may even increase slightly the temperature. The top layer soil moisture decreases during the summer because of the lack of precipitation during the summer period. By increasing the vegetation cover, the moisture level continues to decrease because of the larger surface of evapotranspiration. Under such soil moisture conditions, low-level vegetation with shallow roots can no longer evaporate effectively and plants cannot release latent heat, resulting in a very limited decrease or even increase of the daytime ambient temperature. Similar results were reported in [34] for Los Angeles, where it was demonstrated that adding and replacing existing plants in the city with drought-tolerant plants without additional irrigation water prevents plants from effectively helping the city to relieve UHI and heat wave conditions. During the night, the cooling contribution of low-level non-irrigated greenery is more significant as plants reduce the upward heat flux from the ground, resulting in a cooler soil surface at night. The average nighttime temperature decreases as caused by low-level non-irrigated greenery may reach 3.0°C. This is in full agreement with many other similar studies reporting the cooling potential of additional greenery in cities [35].
- A significant increase in the cooling potential of additional urban greenery is observed when high-level irrigated trees are considered. The average ambient 24 h temperature is found to decrease between 0.4°C and 7.2°C with an average value close to 3.5°C. The corresponding average decrease of the ambient temperature at 2:00 pm and 6:00 am is 0.6°C and 2.1°C, respectively. In parallel, the overheating hours above 35.0°C and 40.0°C decrease by up to 19.1% and 14.1%, respectively, while the corresponding decrease of the cooling degree hours is 17.1% and 21.5%, respectively.
- Although non-irrigated vegetation shows a cooling effect at night, they are not effective in reducing CDHs during the daytime. Non-irrigated vegetation leads to slightly higher CDHs compared to the reference scenario when the base temperature exceeds 38.0°C, while irrigated vegetation reduces CDHs during the day. Shading and evapotranspiration contribute most to the cooling effect of vegetation under greenery scenarios. The denser the plant canopy, the higher the cooling potential, as long as plants are sufficiently supplied with water for transpiration. However, the cooling potential of trees and other vegetation is severely reduced under dry conditions when soil water is limited, which results in drought stress to the plants and lower evapotranspiration. The variation of the CDHs during the day and night-time for different base temperatures, as well as for all the considered scenarios, is shown in Figure S2-2.
- An increase of the urban albedo contributes to the decrease of the peak daytime ambient temperature between 0.2°C and 2.2°C with an average value close to 1.2°C, while the daily average decrease of the urban surface temperature is close to 4.8°C. Reflecting materials decrease the CDHs with base temperatures of 35.0°C and 40.0°C, by 19.5% and 47.6%, respectively, while the corresponding decrease of overheating hours is 7.0% and 24.1%.
- Implementation of super cool passive daytime radiative cooling materials on the roofs of urban buildings presents a very significant heat mitigation potential. They decrease the peak ambient temperature at 2:00 pm between 0.3°C to 2.3°C with an average value of 1.3°C, while the corresponding decrease of the urban surface temperature is 6.6°C. Super cool materials contribute to the decrease of the CDHs with the base temperatures of 35.0°C and 40.0°C, by 21.9 % and 50.8%, respectively, while the corresponding decrease of the overheating hours is 8% and 28.1%. Given the very high reflectance of the super cool materials, their use should be limited on the roof of buildings to avoid optical annoyance problems. The development of coloured daytime radiative cooling materials presents a much lower reflectance but a quite similar cooling potential because of the addition of fluorescent components, which will extend the applicability of SCM’s in building facades and pavements and will boost their cooling capacity in cities.
2.4 Impact of Heat Mitigation Technologies on the Cooling Energy Consumption of Buildings
Using the CityBES simulation platform, the cooling energy consumption of 3323 buildings located in the Al Masiaf precinct of Riyadh is evaluated for the whole summer period using weather files corresponding to the current climatic conditions as well as to the eight designed mitigation scenarios. The simulated buildings consist of residential and commercial buildings of 1 to 4 storeys.
The average summertime cooling load of all the buildings, COP=1, is 104.6 kWh/m2. The total summertime cooling load of all buildings is 222. 3 GWh. As expected, the taller the building, the lower the cooling load. The calculated average load for the one, two, three and four-storey buildings was 122.1 kWh/m2, 103.3 kWh/m2, 103 kWh/m2 and 88.2 kWh/m2, respectively (Figure 2).
The calculated average summertime cooling load corresponding to the eight mitigation scenarios is given in Table 3. Calculations for all cases are performed using the same building characteristics considered in the reference scenario, Table S1, and the albedo of the buildings has not been modified. Thus, the reduction of the cooling load is due only to the decrease of the ambient temperature caused by the mitigation technologies and not to the lower absorption of solar radiation by the structure of the building.
As shown, the mitigation scenarios investigated here result in a decrease in the average summertime cooling, with the reduction ranging between 3.6% and 16%. The use of high albedo and supercool materials (Reflective and Very reflective scenarios) reduces the cooling load by 4.4% and 5.2% compared to the reference scenario, respectively. The Green and Dry and the Very Green and Dry scenarios lead to a decrease of 3.9% and 10.6% of the average summertime cooling loads compared to the reference condition, while this reduction is slightly higher once considering the irrigated vegetation (i.e., 5.4% and 13.4%, respectively). The combination of Very Reflective and Very Green with the non-irrigated vegetation scenario shows a 14.8% reduction in the average cooling loads compared to the reference scenario. The maximum reduction of 16.0% in the average summertime cooling load is achieved by the combination of Very Reflective and Very Green with the irrigated vegetation scenario. Therefore, the implementation of heat mitigation technologies in the considered urban area can provide a reduction of the cooling load up to 35.5 GWh during the summer period. Figure 3 demonstrates the distribution of the annual cooling load in the study area for the eight investigated mitigation scenarios.
The energy conservation potential of the mitigation technologies implemented on the façade or roof of the buildings is considerably increasing when the direct benefits arising from the reduction of the absorbed solar radiation are considered. Tables S2-3 present the potential cooling load reduction in buildings of one to four storeys under the reflective and very reflective mitigation scenarios, considering both the direct and indirect benefits arising from the implementation of the reflective and super cool materials on the roof of the buildings. Under the reflective scenario, when the decrease of the absorbed solar radiation is considered, the cooling load conservation increases on average from 4.4% to 5.6%. Under the very reflective scenario, the cooling load conservation increases from 5.2% to 6.9%. In absolute values, the total cooling load reduction in the urban area under the reflective scenario will rise from 9.8 GWh to 12.4 GWH, while for the very reflective scenario, the corresponding reductions are 11.6 GWh and 15.3 GWh. The calculated reductions indicate that for both scenarios, almost 75% to 78% of the potential conservation of the cooling load in the 3323 buildings is attributed to the decrease of the ambient temperature induced by the implementation of the mitigation technologies at the city scale, underlying the energy conservation impact and consequently the considerable decarbonisation potential of urban heat mitigation technologies.
Energy retrofitting of buildings is the most efficient way to decrease their energy demand. To evaluate the importance of the combined impact of energy retrofitting and heat mitigation technologies implemented at the building and city scale, respectively, we designed and simulated the energy impact of building retrofitting measures for all the 3323 buildings combined with heat mitigation technologies implemented at the urban scale. The energy retrofitting measures mainly included measures to improve the thermal quality of the envelope, namely better windows, better insulation, improved solar control, cool roofs, and improved air permeability. Measures related to the HVAC system are not considered. A full list of the selected energy retrofitting measures is given in Table S2-4.
The calculated reduction of the summer cooling load in the urban area, considering the combined implementation of the heat mitigation and energy retrofitting measures (Qcomb) as well as the corresponding cooling benefits (Qmit) when only mitigation measures are considered, are given in Table 4 for all the scenarios. Given the important thermal interaction between the energy retrofitting and the heat mitigation measures during the building operation, the difference between Qcomb-Qmit, does not represent the exact contribution of the retrofitting measures and is lower than when retrofitting measures are applied individually. Simulation of the energy impact of the retrofitting measures under non-mitigated climatic conditions is also performed for all the buildings, and the corresponding reduction of the cooling load of the reference building is calculated (Qretr). However, under the combined implementation of the mitigation and retrofitting measures, the real contribution of the heat mitigation and energy retrofitting measures are lower than Qmit and Qretr, respectively, because of the important thermal interaction between the considered measures. Nevertheless, a comparison between Qmit vs Qcomb and Qretr vs Qcomb can provide an approximate but quite realistic contribution of the mitigation and retrofitting technologies. As shown in Table 4, combined heat mitigation technologies can contribute up to 46% of the total cooling load conservation of urban buildings under the combined implementation of heat mitigation and energy retrofitting measures.