Evaluating the Influence of Urban Canyon Geometry on Air and Surface Temperatures inside Modern Residential Neighborhoods in Hot-arid Climates: A Case Study of Riyadh, Saudi Arabi

The characteristics of the urban form and its surfaces influence the urban microclimate, forming what is known as urban heat island phenomenon (UHI). The UHI contributes to increasing the ambient air temperature inside urban areas to be higher than their rural surroundings leading to outdoor and indoor thermal discomfort during the hot seasons, particularly in extreme hot-arid climatic zones. In Riyadh, Saudi Arabia, the current residential neighborhoods have been developed based on urban planning regulations that were imported from moderate climates disregarding the extreme local hot-arid climate and introducing dispersed urban structures with wide streets and detached low-density residential units. As a result, the majority of urban surfaces become vulnerable to extreme solar radiation creating an undesirable microclimate around buildings. To mitigate the UHI intensity and reduce the ambient air temperature inside urban canyons, it is essential to minimize the penetration of direct solar radiation from reaching the canyon's surface leading to the creation of more comfortable outdoor and indoor environments. Within the urban canyon, solar radiation densities are governed by three key descriptors: building height to street width ratio (H/W) or aspect ratio, sky view factor (SVF), and solar orientation.;Driven by the lack of investigation tackling the problem of existing conditions, this research aimed to evaluate the thermal performance of the current condition of urban canyon geometry on the ambient air and surface temperatures inside residential neighborhoods in Riyadh, utilizing a three-dimensional microclimate simulation model ENVI-met 4.0. Specifically, this study sought (1) to determine the environmental applicability of current urban planning and design regulations and their effect magnitude on outdoor thermal conditions, and (2) to identify potential reductions in the air and surface temperatures when optimizing the H/W ratio in accordance with the current regulations. The thermal performances of 18 existing urban canyons [Base case (BC)], with H/W ratios ranging between 0.56 and 0.23, were evaluated. These cases were equally oriented toward northeast-southwest (NE-SW) and northwest-southeast (NW-SE); and due to the complexity of the actual urban structures, slight variations within each orientation existed and were taken into consideration to ensure accurate investigation. The main parameters included in the analyses were the ambient air temperature (Ta), ground or street surface temperature (Ts), mean radiant temperature (T mrt), and wall temperature (Tw). The results of the BC scenario were compared to the performances of two proposed scenarios, namely: minimum setback (MSET) and no setback (NSET) scenarios, to evaluate the ultimate improvement in the thermal performance of each selected case when increasing its H/W ratio and to identify which orientation provides the best performance.;The findings revealed that reductions in all temperatures were achieved as the H/W ratio increases, and vice versa. Regarding the effect of orientation, NW-SE oriented cases showed better performances compared to those oriented NE-SW. In general, lower temperatures were observed and increasing the H/W ratios tended to be more effective in reducing the investigated temperatures as the canyon's long-axis becomes further oriented toward the N-S direction (deviate more from the E-W orientation). This research was an endeavor to highlight the significant role of the existing urban canyon geometry as an influential factor in UHI intensity and establish a clear understanding of how to quantify urban geometry's influence on the air and surface temperatures in Riyadh, Saudi Arabia.