Modeling the Urban Heat Islands Mitigation Using the Green Roof Approach: A Case Study of the 17 District of Tehran municipality

The urban heat island (UHI) effect is one of the main consequences of a changing nature in the cities. The heat island effect is the rise of the temperature in urban areas compared to the surrounding rural areas and today has become a very important issue in urban climatology. Rapid growth and insufficient attention to natural elements affecting the planning and implementation of urban plans has many environmental consequences, including the phenomenon of heat island that results from the concentration of human activities and land use/cover changes. Many studies established the correlation between expansion of urban green spaces and reduction in UHI intensity. However, higher economic values and lack of urban lands due to high density, population growth and etc. are the main limitation of green space expansion in horizontal scale in urban areas. In this regard, improvement in the surface cover of buildings and constructions that have been covered with cement and asphalt concrete is examined as one of the measures to mitigate the urban heat island phenomenon. Roofs constitute about 20–25% of the urban surface. Conversion of traditional roofs into planted roofs or green roofs can give rise to many benefits such as mitigation of UHI effect, increase air quality, urban flood management, energy consumption management and etc.Tehran, as the capital of Iran, has a relatively irregular and unplanned urban design pattern. On the other hand, the per capita green space in this city is less than standard at global scale. Furthermore, the distribution of green space across the city is heterogeneous. Lack of green space across the city led to formation of UHI with multiple cores. Hence, the main objective of the present research is to explain and evaluate the effect of development of green roofs as a way to reduce the intensity of Tehran Heat Islands and regarding to lack of space in horizontal scale in Tehran.

After reviewing library resources and previous studies, statistical combination method and numerical modeling of climate micro scale were used to study the effect of green roofs on the intensity adjustment of Heat islands of the city. In this study, the data obtained from field survey as well as the outputs of the Envi-met micro-scale model in the spring, summer and autumn periods, were used to extract the temperature difference between the two green roofs and the reference roof in 17 district of the Tehran’s municipality. To collect field data two air temperature and relative humidity were installed both on the studied roofs (one meter above the selected green roof and reference roof located in 17 district of the Tehran’s municipality). The time intervals for data recording was considered in 15 minutes. Then, the studied area was design using Envi-met micro-scale model to determine the ability of the model in simulation of the roof’s materials impact on urban microclimate. Finally, the correlation between actual and simulated data were determined.

Results demonstrated that, temperature difference between the hours of afternoon and night-time was greater than in the morning during the all studied seasons. Also, the maximum temperature difference between the two green roof and the reference one in this season was occurred at 00:00 and 06:00 A.M. while the maximum temperature differences was observed at 22:00 P.M. Furthermore, the comparison of temperature in both studied roofs during the four seasons of the year illustrated the maximum differences between seasons was occurred in the autumn in comparison with other seasons. By examining the outputs of the model in terms of the actual situation in the area, it became clear that the amount of difference between the two studied roofs decrease as we approach the early morning hours. But in the afternoon there was an increase in the range that continues until night, and with numerical and graphical data analysis, it is determined that the maximum temperature difference between the two green roofs and the reference occurred at night. That is exactly when the heat island reaches its peak. Therefore, green roofs are one of the most important ways to adjust the intensity of the heat islands. On the other hand, to evaluate the accuracy of the Envi-met model for simulation of the reality, the simulated data were correlated with actual and measure data. Based on the results, simulated and actual data in spring and summer are moderately correlated (R=0.53). While the correlation between simulated and actual data in autumn is low.

Increasing the proportion of green spaces in urban areas has been recommended as one of the key UHI mitigation strategies in several research studies. Green infrastructure, including green roofs and green walls, is important for future urban forms. These infrastructure components have the potential to reduce temperatures in urban areas, particularly in the summer time and on individual hot days. in this regard, a methodology to test the resilience of the most commonly used UHI mitigation strategies in city of Tehran, Iran, was presented with assistance of simulation results using a prognostic three-dimensional microclimate model simulation (Envi-met). In addition, this research highlights the positive effects that vegetation has on the UHI mitigation, and at building scale. Indeed, green roofs are capable of decreasing the use of energy for cooling and heating and as a consequence, the peaks of energy use. Based on the results, the urban-wide conversion of the black roofs into white or green roofs can have positive effects not only on micro-scale, but also on urban scale. In conclusion, the microclimate study proved that a green roof retrofit will have significant impact on the rooftop microclimate. However, the expansion of vertical green spaces in the city of Tehran needs special incentive policies. Also, the function or the performance of a green roof needs to continue over time in order for the benefits to be realized. Therefore, the green roof must be properly maintained and reorganized at least each 2 years.