Modelling the Outdoor Thermal Benefit of Urban Trees: A Case Study in Lecce, Italy ^†

Abstract

Urban vegetation plays a key role in mitigating thermal stress in cities, particularly in Mediterranean climates increasingly affected by urban heat. This study evaluates the impact of vegetation on outdoor thermal comfort in Piazzetta San Michele Arcangelo, a square in Lecce (Southern Italy), using the ENVI-met microclimate model. Two scenarios were simulated: the current configuration and a hypothetical one without trees. Results show that vegetation reduces air temperature during the hottest hours (up to −0.52 °C on average) and improves thermal comfort, as indicated by the Universal Thermal Climate Index (UTCI), with reductions in “very strong heat stress” up to 43% at peak times. At night, tree canopies limit radiative cooling, leading to slight temperature increases. The findings confirm the crucial role of urban greening in enhancing outdoor thermal comfort and provide quantitative support for sustainable urban planning strategies in Mediterranean contexts.

1. Introduction

One of the major problems facing cities in recent decades is the rise in air temperature, which causes numerous human health issues, as body temperature is a key factor in determining thermal comfort [1]. Moreover, climate change is intensifying the urban overheating phenomenon [2,3], also as a result of rapid urban expansion driven by population growth and land consumption. These processes are contributing to the degradation of the environment and urban microclimate, as well as to increased thermal stress in cities [4]. Given the wide range of physical, environmental, economic and social benefits offered by outdoor environments, urban planning must ensure that open and green spaces provide citizens with better outdoor thermal comfort conditions [5,6].

Increasing vegetation cover is a promising strategy to reduce urban heat stress and enhance outdoor thermal comfort, as it influences the urban climate both during the day and at night [7]. Vegetation alters the urban surface energy and water balance by increasing latent heat fluxes through evapotranspiration, reducing net energy absorption via canopy shading and limiting heat storage through permeable soils [8].

This study investigates outdoor thermal comfort in a selected area of Lecce, a typical Mediterranean city, by adopting a modelling approach aimed at evaluating the microclimatic mitigation effects of urban greening strategies. The analysis is conducted using ENVI-met, a Computational Fluid Dynamics (CFD)-based microclimate model extensively used to analyze microclimate dynamics and human thermal comfort in urban areas [9,10,11,12]. This case study is distinctive because continuous measurements taken before and after the planned greening interventions, combined with modelling, make it possible to assess and validate the effectiveness of the measures directly in the real urban context. To assess the effectiveness of the interventions, the results are evaluated through the Universal Thermal Climate Index (UTCI), a widely recognized indicator for quantifying thermal comfort in outdoor spaces and a key metric for examining urban heat mitigation strategies [13,14,15]. A key novelty of this study is the evaluation of the main micrometeorological variables and the UTCI across the study area, allowing us to capture the spatial heterogeneity of vegetation and its impact on thermal comfort.

2. Methodology

2.1. Study Area

The selected study area is located in Lecce (southern Italy) (UTM coordinates: 40°21′7.24″ N, 18°10′8.9″ E), a city of approximately 94,000 inhabitants (https://demo.istat.it, accessed on 15 September 2025). It extends over a flat territory at an elevation of approximately 40–50 m above sea level (Figure 1a) and is situated 13 km from the Adriatic coast and 25 km from the Ionian coast [16]. The city has a warm Mediterranean climate, classified as Csa according to the Köppen–Geiger system. Previous studies on Lecce have analyzed various aspects of its urban microclimate and thermal comfort, by investigating the impact of urban greening scenarios, building layout and meteorological conditions in several neighbourhoods [17].

The study area is in the San Pio residential district and corresponds to “Piazzetta San Michele Arcangelo”, a square approximately 180 m long, 40 m wide at its broadest point, and 20 m at its narrowest. It is surrounded by residential buildings, with the tallest reaching 29 m on the western side, while the remaining buildings range between 3 and 4 m in height. A busy road encircles the square, which is mostly paved with impervious concrete. The square also includes flowerbeds with sandy-clay soil planted with trees, primarily Quercus ilex L. and Pinus pinea L., ranging from 6 to 10 m in height (Figure 1b).

A thermo-hygrometer has been installed on the balcony of a southeast-facing building around the square, approximately 4 m above ground level, to monitor temperature and humidity. The instrument is a waterproof data logger (Tinytag Plus 2-TGP-4505, Gemini Data Loggers Ltd., Chichester, West Sussex, UK), equipped with air temperature and relative humidity probes, with an accuracy of ±0.3 °C and ±3%, respectively, at 25 °C.

2.2. Meteorological Data and Modelling Setup in ENVI-Met

Meteorological data were collected from two stations in the Lecce city centre. The Ateneo station (40°21′22.22″ N, 18°10′4.59″ E), managed by the OMD Foundation (Osservatorio Meteo Milano Duomo; https://www.fondazioneomd.it, accessed on 24 July 2025) and positioned 11 m above ground, recorded temperature, relative humidity, wind speed and direction. The ARPA station (40°20′44″ N, 18°10′37″ E), managed by the Environmental Protection Regional Agency (https://www.arpa.puglia.it; accessed on 24 July 2025) and 13 m above ground, measured solar radiation. These stations are located 1.1 km to the east and 2.2 km to the southeast, respectively, from the study area.

Measurements from 15 July to 15 August 2024, were processed to derive the mean daily cycle of a typical summer day in Lecce for each meteorological variable, with a 30 min interval. These processed measurements served as meteorological input for ENVI-met 5.1.0 (https://envi-met.com; accessed on 24 July 2025), with August 1st selected as the representative day. The wind speed ranged between 1.2 and 2.8 m/s, and the maximum temperature was 33 °C at 14:00; a northwesterly wind direction and no precipitation were set. The initial 6 h were designated as the initialization (spin-up) period.

A 400 × 400 m geometric model of the study area was created in QGIS (qgis.org; accessed on 24 July 2025) and imported into ENVI-met. The grid was set to 225 × 225 × 35 cells, with a resolution of 2 m and 5 nesting grids. A 10% telescoping factor was applied from ground level, extending the model domain to a maximum height of 160 m. This ensured sufficient vertical space (130 m) above the tallest building (30 m) to the model’s upper boundary.

Two simulations were performed in ENVI-met to compare different scenarios: (1) the current urban configuration (Figure 2a) and (2) a hypothetical scenario identical to the current configuration but without vegetation in the square (Figure 2b).

Piazzetta San Michele Arcangelo is mainly composed of evergreen trees in its current configuration. The SE area is predominantly planted with Quercus ilex L. (approximately 10 m tall), with a few specimens of Melia azedarach L. (approximately 10 m tall) and Robinia pseudoacacia L. ‘Umbraculifera’ (approximately 4 m tall). The centre of the square is bare, while the NW area is predominantly planted with Pinus pinea L. (approximately 6 m tall), arranged in two rows separated by an internal row of Quercus ilex L.

3. Results

3.1. Model Validation

Figure 3 presents the temperature and relative humidity outputs from the simulation of the current scenario, compared with observations acquired by the thermo-hygrometer, which were used to validate the results. All results are expressed in local time. The rapid temperature increase observed during the early morning hours is likely attributable to the thermo-hygrometer’s placement on the south-facing wall of the building. The simulated temperature and relative humidity values correspond to the thermo-hygrometer’s installation point, approximately 4 m above ground. Validation against observed data yielded a Root Mean Square Error (RMSE) of 1.04 °C for temperature and 6.4% for relative humidity. For temperature, the simulation showed a slight underestimation with a bias of −0.24 °C, while for relative humidity, there was an overestimation with a bias of 5.5%.

3.2. Analysis of Temperature Differences

Figure 4 reports the Absolute Temperature Difference (ATD) values between the current scenario and the hypothetical scenario without trees, specifically for Piazzetta San Michele Arcangelo. The boxplots show more pronounced negative mean values from 10:00 to 16:00 (mean of −0.52 °C) and higher positive values during the night from 00:00 to 05:00 (mean of 0.26 °C) and during the evening from 19:00 to 23:00 (mean of 0.17 °C).

Further, the Percentage Temperature Variation (PTV) was calculated for both scenarios, as follows:

$$PTV \left(\%\right)= {\displaystyle \frac{Ttrees-Tnotrees}{Tnotrees}}·100$$

(1)

where T_trees = temperature obtained in the current scenario with trees (°C); T_notrees = temperature obtained in the hypothetical scenario without trees (°C). Figure 5 illustrates the PTV values based on the two simulated scenarios for Piazzetta San Michele Arcangelo, highlighted with a black line, at 09:00, 12:00, 16:00, and 21:00. These time-steps allow us to evaluate the spatial pattern of temperature differences, highlighting the cooling effect of vegetation shading during daytime and the slight nighttime increase due to limited radiative cooling. At 09:00, values primarily range between −1.5 and −0.5. By 12:00, even more negative values (from below −3 to −2) extend over a larger area, particularly where groups of trees are present. At 16:00, positive values, just above 0, begin to extend over a larger area towards the centre of Piazzetta San Michele Arcangelo, where trees are absent. At 21:00, the square exhibits positive values. The southern section shows values from approximately 0.6 to 1.00, the central section shows values from about 0 to just above 0.2, while the northern section displays the highest values, ranging from 1.2 to 1.60. This spatial difference is attributed to the presence of trees in the southern and northern sections and their absence in the central section.

3.3. Outdoor Thermal Comfort Analysis

Outdoor thermal comfort was analyzed for Piazzetta San Michele Arcangelo, with results provided in terms of the Universal Thermal Climate Index (UTCI), extracted at a height of approximately 2 m above the ground.

Table 1. Classification of UTCI based on thermal sensation levels [7].

UTCI Range (°C)	Thermal Sensation
>46	Extreme heat stress
38 to 46	Very strong heat stress
32 to 38	Strong heat stress
26 to 32	Moderate heat stress
9 to 26	No heat stress
9 to 0	Slight cold stress
0 to −13	Moderate cold stress
−13 to −27	Strong cold stress
−27 to −40	Very strong cold stress
<−40	Extreme cold stress

provides the UTCI thermal sensation scale, classifying different levels of heat and cold stress.

Figure 6a shows the hourly percentage frequency of UTCI classes in the study area for the scenario with trees (1), while Figure 6b shows it for the hypothetical scenario without trees (2). Comparing the two graphs, we observe an improvement in outdoor thermal comfort in scenario 1 compared to scenario 2, with a reduction in “very strong heat stress” of 38% at 09:00, 43% at 10:00, and 24% at 11:00. During the afternoon, the reduction is 32% at 15:00, 43% at 16:00, and 34% at 17:00. No significant improvement is observed for the central hours of 12:00, 13:00, and 14:00 (a reduction of 2 to 4%). During nighttime hours (00:00–05:00), two distinct situations are observed. In the current scenario, the presence of trees causes moderate thermal stress during the night, partly due to tree canopies limiting radiative cooling. In the treeless scenario, no heat stress is observed during the same hours, leading to an average reduction in moderate heat stress of approximately 49%. No changes in heat stress are observed from 18:00 to 23:00. The presence of trees reduces the frequency of very strong heat stress during the day by 25%, thus demonstrating a significant benefit for thermal comfort, especially during extreme heat conditions.

4. Conclusions

This work involved a microclimatic study of an area in Lecce, a typical Mediterranean city, focusing on the mitigating role of vegetation. This study utilized a modelling approach to provide microclimatic simulation results and analyze outdoor thermal comfort.

UTCI results demonstrate that the presence of trees in Piazzetta San Michele Arcangelo improves outdoor thermal comfort compared to a hypothetical scenario without trees, leading to a reduction in “very strong heat stress” of up to 43% at 10:00 and 16:00. The calculated PTVs also indicate that at 12:00, the presence of trees in Piazzetta San Michele Arcangelo results in more negative values than in treeless areas, which show values close to zero. This is a direct consequence of the air temperature lowering caused by trees. Conversely, at 21:00, higher positive values are observed because the trees impede the release of heat absorbed by the pavement during the day.

This study confirms that vegetation is a key element in mitigating the microclimate by improving outdoor thermal comfort and should be integrated into urban planning from a sustainable perspective to enhance residents’ daily lives. Future studies aim to apply the same modelling approach to investigate the microclimate in the same area by modifying the distribution, quantity, and type of vegetation, as well as replacing impermeable pavements with permeable ones, as part of an ongoing urban redevelopment project in Piazzetta San Michele Arcangelo.