Numerical assessment of the urban green space scenarios on urban heat island and thermal comfort level in Tehran Metropolis

The growing rate of urbanization and alterations in natural land-use/land-cover properties cause many worldwide environmental issues such as Urban Heat Island (UHI), low Thermal Comfort Level (TCL) and air pollution in megacities. Development of urban green spaces is one of the most proposed measures for mitigation of UHI and improvement of thermal comfort satisfaction which causes lower cooling energy consumption and indirectly improves the urban air quality. Therefore, the efficiency of realistic and executable green scenarios in both surface and roof levels (surface vegetation, green roof, surface vegetation + green roof) on early summertime UHI and TCL are evaluated and compared in Tehran Metropolis, using coupled Weather Research and Forecasting model with Single Layer Urban Canopy Model (WRF/SLUCM). Model predefined urban parameters (the urban density and vegetation fraction) are modified before the main simulations by data of remote-sensing methods which let us define surface green spaces developmental scenarios without any change in the basic urban morphology. Results are presented for three areas with different urban morphologies (Low and High-Intensity Residential (LR and HR) and Commercial and Industrial (C/I)). Three indexes (THI, ETI and RSI) are calculated for assessing TCL variations. Generally, results show that environmental operations of selected scenarios are different over selected areas. In LR area, diurnal cooling (up to -0.86 degrees C in green roof approach) and improvement in thermal comfort condition are observed in all scenarios. On the other hand, in HR and C/I areas, the daytime cooling (nighttime warming) effect up to -0.85 degrees C (up to +0.63 degrees C) in the third scenario are simulated. TCL alterations show similar deteriorating nightly effects in all scenarios in selected regions. Averaged diurnal decrease in wind speed is another undesirable impact of green development scenarios due to the enhanced surface roughness that can reduce urban natural heat and pollution ventilations. Current results clarify that the adoption of efficient urban green spaces programs needs to consider different environmental concerns. By comparison between roof level and surface greenery, it can be concluded that the green roof approach with the least undesirable nightly effects is a more efficient approach than surface vegetation development in high-density Tehran megacity. (C) 2020 Elsevier Ltd. All rights reserved.