Effect of street orientations and aspect ratio on urban microclimate: A case Study of twin cities (i.e. Islamabad and Rawalpindi) of Pakistan

Abstract

In recent decades, a discernible rise in the percentage of the world’s population residing in urban areas has been observed in recent decades. The urban centers are catalysts for social and economic development of a country, but they may also present a number of difficulties, such as environmental, social, and economic issues, especially when the urban growth is hasty and poorly planned. Urban regions frequently experience the urban heat island effect, a phenomenon where urban air temperature is much greater than that of neighbouring suburban areas. The primary cause of urban heat island is the dense urbanization in these areas, which restricts airflow and emits heat stored from solar energy. Street canyon characteristics including canyon width, length, height, orientation, and sky view factor, as well as vegetation, are some of the important factors that can impact urban microclimate and thermal comfort. However, there is a lack of understanding about the specific nature and extent of this impact in the context of Pakistan. Therefore, it is crucial to investigate the relationship between these design elements and their effect on the urban microclimate, as this can provide valuable insights for urban planners and architects to design sustainable and energy-efficient cities. The aim of current research is to determine the optimum orientation and aspect ratio that would ensure optimal microclimatic and thermal comfort conditions in the planned

and semi-planned urban areas of Islamabad and Rawalpindi, respectively. For this pur- pose, the already developed urban setup of twin cities is simulated using 3D microclimate

simulation software ENVI-met. The simulated results of air temperature and relative humidity at the three selected locations in twin cities are compared to the observed values for validation purposes. After the initial validation of ENVI-met results, the effect of street canyon aspect ratio, and orientation is first evaluated on the important microclimate indicators, i.e., global radiation flux, air temperature and mean radiant temperature, and later on outdoor

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thermal comfort. It has been observed that global radiation intensity drops with in- crease in aspect ratio. However, these differences are more significant in Rawalpindi

(semi-planned) than that in Islamabad (planned). The peak temperature value (around 46.35 ◦C) is the same for both Rawalpindi (height-to-width ratio=0.37) and Islamabad

(height-to-width ratio=0.17). Likewise, by increasing the height-to-width ratio, Islam- abad and Rawalpindi’s daytime peak mean radiant temperatures are reduced by 1.211

◦C and 0.149 ◦C, respectively. Moreover, the limiting exposure to the radiation due to

shading provided by buildings is one of the key factors explaining the decrease in physio- logical equivalent temperature value in canyons with the increase in the height-to-width

ratio ratio. In terms of street orientation, street canyons that were NW-SE oriented in both Rawalpindi and Islamabad performed better thermally than those featuring other orientations.