Assessing the Effect of Green Roofs on Urban Microclimate, Case study: the Low-Rise Context in Hot-Dry Climate of Shiraz

In recent years, various solutions have been proposed to reduce urban heat islands. The most common of these strategies is the use of cool surfaces (on roofs and sidewalks and vegetation surfaces) and increasing green space. However, the shape of urban blocks, the duration of direct sunlight, and the average radiant temperature play an important role in the thermal comfort of the outer space of cities. The microclimatic temperature conditions of the city affect the thermal behavior of buildings as well as the thermal comfort of pedestrians. Since the simplest way to change the microclimate and reduce the urban heat island is to use cool surfaces and increase green space, in recent decades, the use of green roofs in different climatic regions has expanded. The application of green roofs is one of the common strategies to reduce the thermal load of buildings. On the other hand, it is recommended to use cool surfaces and greenery to change the microclimate and mitigate urban heat islands. This research aims to investigate the effect of green roofs on the urban microclimate in the hot and dry climate.

This study aims to determine the effect of green roofs on local microclimate and urban heat island in the low-rise contexts in the hot and dry climate of Shiraz. The research method used in this research is a computer simulation method, carried out using Envi-met software. Envi-met is a 3D simulation and measurement software for microclimatic conditions of urban climate and is used to analyze the open spaces of urban environments. This software has been selected due to its numerous capabilities and different validated results. The case study is a part of a low-rise urban context in Shiraz, and the research was carried out using Envi-met software for thermal simulation of the outside environment on one of the hottest days of the year. Thermal effects of three roof types on the surrounding environment have been evaluated: a common roof with reflecting isolation layer, a green roof with grass, and a green roof with trees. The simulation was done for the first day of July as one of the hottest days of the year. Data output is taken at the height of 0.5 m above the roof surface (6.5 m above the ground).

The results show that the application of green roofs with trees or grass would slightly improve thermal comfort in the neighborhood. The maximum reduction in air temperature is between 0.27 to 0.74 degrees in the receptors, and 5.05 degrees in mean radiant temperature. In the case of relative humidity, the maximum changes are an increase of 0.36% or a decrease of 0.48%, while the maximum fluctuation of wind speed is an increase by 0.68 m/s. The analysis of the correlation coefficients between the variables and the thermal comfort index of PMV shows a significant correlation. In verifying the results, they are represented as descriptive statistics, including the changes in the variables of air temperature, radiant temperature, wind speed, humidity and thermal comfort. In addition, the relationship between radiant temperature, air temperature, relative humidity and wind speed and the thermal comfort has been investigated using statistical analysis and the coefficient of correlation between them has been calculated in three modes: green roof with grass and green roof with tree at 9 am and 5 pm. The correlation coefficients in most cases are more than 0.7 and indicate a strong correlation. In this way, the obtained correlation coefficients show a strong significant direct relationship.

Providing A green roof with trees is more efficient than a green roof with grass providing better thermal comfort conditions in urban microclimates. The green roof area is more important than the green roof itself. The application of green roofs in small areas (urban blocks and neighboring) has little effect on the thermal comfort of the microclimate. It would effectively reduce air temperature and cooling effect only if it is used in a vast area. The correlation between the variables and the thermal comfort created in different conditions shows a strong correlation. Based on the results, the extent and the size of the green roof have a more important effect on the region’s microclimate. These results are consistent with previous studies (Battista et al., 2016: 1058) and (Peng et al., 2013: 598) and (Alcazar et al., 2016: 304) and indicate that considering green roofs in small extents such as an urban block or a neighborhood unit has little effect on the microclimatic conditions of the region. In order to benefit from the cooling effects of green roofs and reduce the heat on an urban microclimate scale, it is necessary to use green roofs in a wider area. In summary, the following can be extracted from the results of the present study:- Using a green roof can slightly reduce the outdoor temperature and improve thermal comfort conditions on a neighborhood scale.- A green roof with trees is more effective than a green roof with grass, and from this point of view, it is more preferred.- If the use of green roofs is considered as a solution to reduce air temperature and improve thermal comfort conditions, it should be considered in large urban areas (several urban blocks or several neighborhood units).