Greenery systems for the improvement of outdoor comfort in urban built environment with artificial ground depression. A case study in Sicily

Introduction Rising temperatures caused by climate change have pushed countries around the world to establish agreements aimed at reducing greenhouse gas emissions, encouraging sustainable forest management, and increasing the use of energy from renewable sources. To keep global warming below 1.5°C above pre-industrial levels, the Paris Agreement mandates that participating nations to review their national energy, and climate plans every five years. Recently, the United States of America officially withdrew from the Paris Agreement for the second time, a position that could alter the global political direction on efforts of reduce effects of climate change. To reduce greenhouse gas emissions and mitigate the effects of Urban Heat Islands, greenery systems can be implemented in urban contexts [1]. Greenery systems can improve both outdoor and indoor comfort within the built environment. Actually, he positive effects of greenery systems - such as trees, green corridors, green walls, and green roofs - have already been confirmed by several studies carried out in different climatic zones [1–5]. Methodology A multi-scale methodology -already tested in another works - has been used to analyse a particular area located in the campus of the University of Palermo. To evaluate the thermofluidic process and the effects of greenery systems a well know CFD software (ENVI-met ®) has been used to analyse both microclimate urban scale and building scale [1], considering the specific climatic conditions (Csa a hot-summer Mediterranean climate according to the Köppen-Geiger classification). The analyses have been focused on the area of the Department of Architecture that consists in a main building located within an artificial ground depression at the lower point of 39 m above sea level (m.a.s.l.). It has four floors and totally its height reaches approximately 22 m. A secondary lower building is positioned along a side of the ground depression. It has two floors and totally its height reaches approximately 7 m. The streets outside the ground depression are between 47 and 51 m.a.s.l. The weather data was obtained from the weather station of the University of Palermo located near the analysed area, inside the campus [6]. Based on the increasing trend of temperatures registered and published by Copernicus [7] this work used the available weather data of the 2024, instead of using an average weather data of a wider range of years than can usually goes from 5 to 10 years (as it can be found in literature). Simulations have been carried out considering 24 hours, by selecting the hottest and the coldest day of the year, which corresponded to 16 August 2024 and 24 December 2024. These weather data have been used as input for the urban scale simulations. Urban scale simulations aimed to evaluate the effects of urban morphology by considering building surfaces, existing vegetation, and thermal and wind flows. The analysed area measures 500x500 m and it was modelled using a grid of 100x100x30 cells, with each cubic element measuring 10x10x5 m. The modelling process was carried out using the QGIS software, where it is possible to set up the area, the surfaces, the buildings, and the vegetation; the model has been exported by using ENVI-met plugin®. The terrain was modelled using the Digital Terrain Model (DTM) of the Sicily Region, which has a resolution of 1 m and derived from LIDAR scanning conducted in 2008 [8]. The buildings were modelled using open data shapefiles of the Municipality of Palermo to obtain building shapes and their heights [9]. Adjustments have been carried out to overcome inaccuracies found in the shapefiles The results obtained from these simulations - regarding air temperature, relative humidity, wind speed, and wind direction - were subsequently used as input data for the building scale simulations. Building scale modelling was carried out over an area measuring 200x250 m, using a grid of 100x125x40 cells, with each element measuring 2x2x2 m. In front of the main entrance of the building there are cylindrical deciduous trees (Populus nigra). Based on the results of the baseline scenario, critical zones were identified, and three scenarios were proposed and evaluated to improve outdoor thermal comfort. In the first scenario, vegetation was enhanced with additional shrubs and trees. In the second scenario, artificial ponds were added. The third scenario considered a green wall installed on the façade of the lower building. A fourth scenario evaluated the condition in which the trees did not exist, as was in the past. This scenario was assessed to determine whether the introduction of these threes had a significant effect on comfort. Results and discussion The results obtained from the building scale simulations of the baseline scenario confirmed that the artificial ground depression contains several critical thermal zones. The lowest wind speeds were observed along the borders inside the artificial ground depression, and it also corresponded to the highest temperatures recorded in the baseline scenario. The peak temperature was registered at 14:00 (16 August 2024) in all scenarios. Several points were selected – along the area – to quantify and to assess the effects of each proposed intervention compared to the baseline. Scenario 1, which involved enhancing vegetation, showed temperature reductions ranging from 0.05 °C to 0.4 °C, with the magnitude of reduction depending on the distance from the implemented vegetation. Scenario 2, which included artificial ponds, demonstrated temperature reductions between 0.1 °C and 0.6 C, also varying with distance from the ponds. As expected, Scenario 3, which implemented a green wall, registered the highest temperature reduction near the intervention area, although this reduction was not significant, ranging from 0.05 °C to 0.1 °C. Scenario 4 has confirmed that the introduction of threes reduced the temperature ranging from 0.05 °C to 0.3°C. The localized proposed interventions demonstrated that the beneficial effects on the improvement of the outdoor thermal comfort were limited. This is primarily due to the site's topography: the artificial ground depression negatively affects natural ventilation. In some zones of the area the wind speed is close to zero. Since the presence of vegetation usually reduces the wind speed, in the case of a ground depression area it can negatively affect zones located farther. Traditional greenery systems, such as those applied in this study, have not shown significant effects on improving outdoor comfort. Therefore, the sole traditional greenery interventions are not able to sufficiently improve outdoor comfort, and further strategies have to be considered in alternative or in a complementary way in order to enhance natural ventilation and mitigate the thermal conditions.