Impacts of cool roofs on urban heat island and air quality in Dhaka, Bangladesh: A case modeling study during a heat wave

This study investigates the impacts of cool roofs on the urban heat island (UHI) and air quality in Dhaka, Bangladesh during an extreme heat wave event occurring in April 2021. A simulation with conventional roofs having an albedo of 0.2 and a simulation with cool roofs having an albedo of 0.8 are conducted using the Weather Research and Forecasting (WRF) model. Cool roofs reduce the 2-m temperature by 0.57 °C in the afternoon (1200–1700 LST) and cause the urban cool island in the daytime. In the afternoon, cool roofs reduce the planetary boundary layer height by 265 m and greatly suppress the urban breeze, reducing the 10-m wind speed by 0.8 m s⁻¹. As a result, the near-surface passive tracer (carbon monoxide) concentration increases by 45 ppb (52 %) in the afternoon. Cool-roof effects on the UHI and air quality are overall more pronounced in hotter areas. Cool roofs lead to statistically significant decreases in Humidex (−0.19), discomfort index (0.22), and heat index (−0.36 °C) in the afternoon, but all indices remain within the same stress levels. This suggests that additional measures such as urban greenery and other climate-sensitive urban designs are required along with cool roofs for an effective mitigation of urban extreme heat in Dhaka.