The Influence of Planting Arrangement on Outdoor Thermal Comfort

A simulation study in a tropical urban public square

Abstract

Vegetation provides significant benefits for urban thermal environments attracting people to open spaces. Especially in tropical climates, squares are more vulnerable to thermal discomfort due to surrounding buildings and increased hard surface coverage compared to urban parks. Since the space for planting is limited, a proper arrangement is essential to enhance the outdoor thermal comfort (OTC). Nevertheless, the impact of different planting arrangements on OTC has not yet been adequately discovered in tropics. This research conducts numerical simulations using ENVI-met software employing physiological equivalent temperature (PET) to understand how planting patterns affect microclimate and OTC at pedestrian level (1.5m) under clustered, scattered, and equally distributed planting arrangements. The methodology includes three stages; onsite measurements, assessment of existing OTC level, and comparison of the impact of planting arrangements on microclimate and OTC. The changes in the planting arrangements considerably affect OTC in urban outdoors altering the air temperature (Ta), wind speed (WS), mean radiant temperature (MRT), and relative humidity (RH). MRT and the WS are found as the most important parameters influencing thermal comfort, thus collaborating with previous studies. Equally distributed planting arrangement has a significant impact providing the highest comfort improvement (9.8 ℃ PET reduction) at the pedestrian level. Reduced canopy overlaps and planting densities, and improved homogeneous shade coverage and ventilation are suggested to overcome thermal discomfort. The cooling effect of native tree species should be assessed in future research. These results guide urban designers and landscape architects in improving microclimate and OTC in warm-humid cities.

Introduction

Urban Climate change and increase in ambient temperature are extremely urgent issues to be addressed by urban policy-makers (Chatzidimitriou and Yannas, 2017; Lamarca, Qüense et al., 2018; Taleghani, Kleerekoper et al., 2015). Urban microclimate is affected by increased population and urbanization, which alter the metabolism of urban environments. This directly affects the outdoor thermal comfort (OTC) in public spaces and urbanites, creating several health issues. However, research conducted in warm-humid cities is few and inadequate, though the vulnerability to heat stress is high in particular regions. Urban heat island (UHI) effect is one of the most crucial phenomena caused by the alteration of energy balance and thermal properties of the built environment in urban cores compared to rural areas (Marando, Salvatori et al., 2019). This effect calls for designing and maintenance of thermally comfortable outdoor urban environments with alteration of urban morphology and inclusion of greenery (Akbari, Bretz et al., 1997; Norton, Coutts et al., 2015). Previous studies have highlighted the impact of urban design parameters such as morphology, geometry, density, greenery and configuration in hot-humid climates (Chow, Akbar et al., 2016; Johansson and Emmanuel, 2006; Johansson, Yahia et al., 2018; Nassima, Noureddine et al.; Sharmin, Steemers et al., 2015; Yahia, Johansson et al., 2018).

Three paramount factors affect the thermal comfort level: urban space morphology, orientation of elements and spaces, and vegetation (Mouada, Zemmouri et al., 2019; Yahia and Johansson, 2014) which should be taken into consideration by urban designers and planners. Urban greenery has been considered as one of the most important aspects of reducing the negative effect of the urban microclimate (Huang and Lin, 2013). Besides providing visual aesthetics for pedestrians, urban greenery accomplishes beneficial microclimatic effects, including air temperature reduction, which cures the UHI effect, provides shading, improves air quality, and reduces noise levels (Dimoudi and Nikolopoulou, 2003). Pedestrian level thermal comfort can be enhanced by vegetation as a better choice for tropical countries where solar radiation is intense (Taleghani, 2018). Trees are the most influential factor in reducing long-wave radiation exchange by blocking short-wave radiation penetration to the surface when compared to ground covers since turf lawns and shrubs only provide surface shading (Shashua‐Bar, Pearlmutter et al., 2011).

Vegetation in urban areas significantly improves thermal comfort levels regardless of its type and provides shade, and influences the wind flow patterns (El-Bardisy, Fahmy et al., 2016). Vegetation should not be neglected in urban planning because vegetation improves the microclimate by reducing hot air flows, evapotranspiration and shading as the most efficient (Bartesaghi-Koc, Osmond et al., 2020) and commonly used method (Declet-Barreto, Brazel et al., 2013; Wei, Chen et al., 2021) in urban microclimatic improvement. The cooling performance of vegetation depends on several vegetation parameters. Foliage density is one of the main determinants of heat reduction efficiency, even though other morphological characteristics of trees such as tree height, trunk height, and crown diameter are determinants of a tree's heat reduction potential (Morakinyo, Lau et al., 2018). Nevertheless, heat reduction capacity can be restricted depending on the location (Morakinyo, Ouyang et al., 2020) and tree location and arrangement should be carefully considered to maximize the average temperature cooling benefits in urban spaces (Morakinyo and Lam, 2016). Moreover, a larger tree-covered area (TCA) ensures improved OTC; the magnitude varies with tree-planting pattern (Li, Y. and Song, 2019; Morakinyo and Lam, 2016). According to Atwa, Ibrahim et al. (2020), the effective management of trees and higher densities improve thermal comfort. Isolated trees provide better cooling than loosely clustered ones, containing open spaces in between (Raman, Kumar et al., 2021). Morakinyo and Lam (2016) have found that the OTC benefits of trees are also responsive to Leaf area density (LAD) distributed across the different heights of the tree, while the trunk height seems to be the least important factor. However, the trunk height has a stronger effect than foliage density and tree height, resulting improved ventilation (Morakinyo, Lau et al., 2018).

According to the literature, assessing the impact of vegetation arrangement on OTC is essential, but still, it has not been explored adequately due to some limitations (Zhao, Q. and Wentz, 2016). With the aid of high-resolution remote sensed images, it has been recently found that clustered tree arrangement is more effective than dispersed planting arrangement (Fan, Myint et al., 2015; Myint, Zheng et al., 2015). However, remote sensed thermal images can only derive the canopy-top surface temperature (Yi, Zhao et al., 2018). Applied methods to quantify the impacts of urban design components on thermal comfort have various approaches (Tukiran, Ariffin et al., 2017). Modelling techniques which have been improved over several years are recently introduced to analyze the metabolism of the urban environments, giving consideration to regional climate. This provides the opportunity to assess and compare microclimatic conditions (air temperature, humidity, wind speed, etc.) and thermal comfort levels under the tree canopy by field measurement and simulations.

There are number of simulation models developed to analyse and predict the microclimatic conditions in past decades. Weather Research and Forecasting (WRF) Model is a mesoscale numerical weather prediction (NWP) system designed to serve both atmospheric research and operational forecasting needs (Zhao, Y., Zhong et al., 2021). WRF can be coupled with the urban canopy models (UCM) to predict the heat and moisture fluxes from the canopies to the atmosphere (Jandaghian and Akbari, 2018). Moreover, the model called “RayMan” can estimate the radiation fluxes and the effects of clouds and solid obstacles on short wave radiation fluxes to estimate the urban microclimate and thermal comfort (Matzarakis, Rutz et al., 2007). Further, the “Ladybug software” is a free and open source environmental plugin for “Grasshopper3D”. It creates interactive 2D and 3D graphics for weather data visualization to support the decision-making process using energy and daylighting modelling (Roudsari and Pak, 2013). However, ENVI- met model can analyze mesoscale, microscale to building scale environmental parameters. This is a three-dimensional atmospheric model which simulates the urban microclimatic conditions with time and spatial resolution of 1-10 s and 0.5-10m respectively. It can be used to investigate the building and vegetation impacts on different microclimatic parameters and human bio-climate (Huttner and Bruse, 2009). This is a reasonably reliable tool for studying plant-surface-atmosphere interactions (Tukiran, Ariffin et al., 2017). Previous studies have validated the simulation output results by field measurements largely with strong correlation between simulated and measured typical hourly air temperature (Ta) and mean radiant temperature (MRT) with minimal error (Morakinyo, Lau et al., 2018). Therefore, ENVI-met is the mostly used and reliable tool for microclimate and thermal comfort predictions. Further, computational fluid dynamics (CFD) modelling techniques provide the benefit of simulating and testing different vegetation arrangement scenarios that are not practical in real case studies. However, the majority of the OTC assessments provide results only for specific meteorological conditions instead of being based on a long-term analysis (Acero, Koh et al., 2020).

Although previous studies have investigated both macro and micro scale climatic conditions, municipal level land use planning is critical in improving context-specific environmental quality (Kyttä, Broberg et al., 2016). Perera, N. and Emmanuel (2018) have highlighted the importance of integrating the local level planning process in climatic-responsive urban design to control the increasing trend of urban heat in tropical cities. Sri Lanka is a warm-humid tropical country where increasing heat stress in Temperature Humidity Index (THI) (from 1931-1961 to 2006-2016) has been recorded, resulting in several complications of health of the people in cities (Nanayakkara and Nianthi, 2018). Colombo is the capital city where vulnerability is high due to fast urbanization. Although the annual temperature humidity index and the bioclimatic map of Sri Lanka shows Colombo area as “moderately comfort” area as a regional representation, micro-level climatic conditions have been altered due to building density, surface coverage and lack of effective vegetation. Compared to urban parks, squares are more vulnerable to thermal discomfort due to surrounding buildings and the high percentage of hard surface coverage. Therefore, this study investigates how planting arrangement affects the microclimate and outdoor thermal comfort (OTC) in a tropical urban public square and finds ways to improve the microclimate and OTC.

Colombo is a warm-humid city because of high temperature and high precipitation levels throughout the year. The rainfall in Colombo is significant, with precipitation even during the driest month. According to Köppen and Geiger climate classification, this climate is classified as tropical rainforest (Af) climate (Peel, Finlayson et al., 2007). The average annual temperature is 26.5 °C (79.7 °F) and annual rainfall is 2387 mm (94.0 inch.). The driest month is February, with 61 mm of rain. In October, the precipitation reaches its peak, with an average of 350 mm. March is the warmest month of the year with 27.4 °C averages temperature, while December is the coldest month with 25.7 °C average temperature. June is the month with the highest relative humidity (86.80 %) and the lowest relative humidity is available in February (73.04 %). Figure 1 shows how the average annual mean temperature has increased from the year 1901 to 2020 in Sri Lanka.

Figure 1. Observed average annual mean-temperature of Sri Lanka for 1901-2020

Source: https://climateknowledgeportal.worldbank.org/country/sri-lanka/climate-data-historical

As shown in Figure 2, the selected site is Independence Arcade in Colombo (6.9027° N, 79.8688° E). Colombo is basically residential, and according to the initial local climate zoning (LCZ), 48.1% of the total land of Colombo municipal council area belongs to compact low-rise category, and 23.7% belongs to large low-rise LCZ category (Perera, N. G. R., 2016). However, there are ten build and seven land cover types in LCZ classification (Stewart and Oke, 2012). The rest of the CMC land cover area (18%) which have not specifically been mentioned in table 1 are belongs to other five built types (open mid-rise, open law-rise, lightweight low-rise, sparsely built and heavy industry) and the seven land cover types (dense trees, scattered trees, bush or scrub, low plants, bare rock or paved. Bare soil or sand and water). These areas are less vulnerable for urban heat island effect and thermal discomfort.

Figure 2. Location of the study area

The study area is categorized as large low-rise, and the urban/rural temperature difference (UHI effect) is 1.1°C. The size of the total site area is around 19500 m², with front and back yards are separated by a white-painted two-story (12m) building.

Table 1. The highest vulnerable zones referring to urban/ rural temperature difference and fraction of Colombo municipal area

LCZ classification	Urban/ rural Temperature difference (°C) – UHI effect	fraction of Colombo municipal council (CMC) land area
Compact high-rise	4.40	0.3%
Open high-rise	4.35	1%
Compact mid-rise	3.96	8.9%
Compact low-rise	3.19	48.1%
large low-rise	1.10	23.7%
Other LCZ types	-	18%

*Source: (Perera, N. G. R., 2016)

Methodology

This research is conducted over three stages; (1) Onsite measurements, (2) Assessment of base case OTC level and (3) impact of planting arrangements on OTC level. First the base case model area was configured using ENVI-met Software. A fieldwork measurement campaign was conducted to receive input climate data for numerical simulations. Micrometeorological fluid dynamics (CFD) modelling was used to investigate the effect of different planting arrangements on OTC in a tropical urban public square (Independence Arcade in Colombo). Outdoor microclimate and human thermal comfort conditions were simulated using ENVI-met software employing Physiological equivalent temperature (PET) to compare different planting arrangements. The Figure 3 shows the methodological framework of the study.

Figure 3. Methodological framework

Fieldwork Design

Conducting an onsite measurement campaign, Air temperature (Ta), relative humidity (RH), wind speed (WS), and wind direction (WD) were measured at 1.5m above ground level as the inputs for the ENVI-met 5.0 simulation model. The height of 1.5m is the level where the human thermal comfort is affected the most (Davtalab, Deyhimi et al., 2020; Li, J., Wang et al., 2020; Ouyang, Morakinyo et al., 2020; Yin, Lang et al., 2019). The field measurements were conducted in a clear sunny day without cloud cover. Measuring the real time onsite climatic data conducting field measurement campaigns have been widely used in many researches using weather station and other related equipment (Hsieh, Jan et al., 2016; Johansson and Emmanuel, 2006; Krüger, Minella et al., 2011; Li, Y. and Song, 2019; Morakinyo, Lau et al., 2018; Wai, Xiao et al., 2021). Therefore, a fixed weather station (PCE-FWS 20N) was used to measure the climatic parameters mentioned above. Site conditions/dimensions of vegetation, water bodies, surface conditions and materials and building morphology, were obtained with field observation and using related measuring equipment, detailed maps, field surveys, and Google Earth. Weather station was fixed in the morning from 11.00 am to 6.00 pm on 26^th October 2021. Figure 4 shows the type of the weather station used for onsite measurement campaign. Measurements were taken only in day time since the study investigates the outdoor thermal comfort at the peak hours in daytime temperature in Colombo (2.00 pm to 3.00 pm).

Figure 4. Weather station (PCE-FWS 20N) used for onsite measurements.

Source: https://www.pce-instruments.com/english/measuring-instruments/testmeters/weather-station-air-quality-station-pce-instruments-weather-station-pce-fws-20ndet_5932919.htm

Micro-meteorological simulations

Computational fluid dynamics (CFD) modelling can be used to assess generic urban environments (Huang and Lin, 2013; Ignatius, Wong et al., 2015; Wai, Xiao et al., 2021; Yahia and Johansson, 2014) and real urban contexts (Emmanuel, Rosenlund et al., 2007; Krüger, Minella et al., 2011; Raman, Kumar et al., 2021). In this study, a real urban context was assessed. The simulations were conducted using “ENVI-met 5.0 Bio met” software employing physiological equivalent temperature (PET) comfort index (Höppe, 1999; Matzarakis, Mayer et al., 1999). ENVI-met is a three-dimensional atmospheric model which simulates the urban microclimatic conditions with surface-plant-air interactions, and has been used to investigate the air flows in urban outdoors surrounded by buildings, vegetation impacts on ambient temperature, heat exchange processes at the building walls or ground surface, and bioclimatology and pollutant dispersion Huttner and Bruse (2009). Since there is a strong correlation between ENVI-met modelled and measured typical hourly Ta and MRT with minimal error according to previous studies, ENVI-met model is a reasonably reliable tool for studying plant-surface-atmosphere interaction validated by field measurements (Duarte, Shinzato et al., 2015; Tukiran, Ariffin et al., 2017). A relatively low root mean square error (RMSE) and mean absolute percentage error (MAPE) has been found either due to model limitations or the quality of input data (Morakinyo, Lau et al., 2018). Simulations were conducted using measured microclimatic parameters to the “ENVI-met 5.0” software package. The ENVI-met model area domain was 60 x 60 x 30 m with vertical and horizontal grid resolution at 2 m.

Figure 5. Base model area showing existing tree locations and arrangements.

Source: Author using ENVI-met “Spaces” software

Figure 6. ENVI-met base case 3D model with existing surface and building conditions.

Source: Author using ENVI-met “Spaces” software

Table 2. Summary of area input and configuration parameters for validated simulation

Parameter	Definition	Input value
Meteorological conditions	Maximum air temperature (0 C)	32
	Minimum air temperature (0 C)	28
	Maximum relative Humidity in 2 m (%)	86
	Minimum relative Humidity in 2 m (%)	65
	Constant wind direction at inflow (00: North; 900: East; 1800: South; 2700: West.)	450
	Constant wind speed at inflow boarder (m/s)	0.43
	Cloud cover	0.00
	Roughness length at reference point (m)	0.01

Table 3. Vertical and horizontal surface, water, vegetation, soil information

Code	Material type	Colour	Albedo
Horizontal surface
MH 1	Cobble stone granite	Dark grey	0.27
MH 2	Exposed aggregate concrete paving - cast in-situ	Beige and grey mixed	0.38
MH 3	Concrete paving	Light grey	0.49
MH 4	Concrete paving	Grey	0.34
	Concrete paving used	-	0.20-0.30
MH 5	Glass	-	0.305
MH 6	Water	-	0.08
MH 7	Grass	-	0.25 - 0.30
Vertical surface
MV1	White plaster - building façade	White	0.93
MV2	Glass	-	0.305
Roof material
RM1	Metal roof sheet	Grey	0.30- 0.50

The vegetation data base of ENVI- met package was used to configure the vegetation of the base case. Number of trees, leaf area density (LAD), canopy form and tree height were fixed characteristics and different planting arrangements were proposed. For this study, trees with high LAD values, spherical canopy form and large trees (25m) were taken to investigate the impact of tree planting arrangement on microclimate and OTC. Three scenarios were developed to compare the cooling performance of scattered, clustered and equally distributed planting patterns in a tropical urban square. First, the base case scenario (Figure 5 and Figure 6) is simulated and calculated the PET value to determine the existing comfort conditions. Secondly, scenarios with scattered planting (Figure 7a), clustered planting (Figure 7b) and equally distributed planting (Figure 7c) were simulated.

Figure 7. Proposed scattered, clustered and equally distributed planting arrangements.

Source: Author using ENVI-met “Spaces” software

Results associated with the different arrangements for the entire study area were compared and evaluated. For each scenario air temperature, MRT, wind speed, and relative humidity were simulated for 24 hours at 1.5m height on 26^th October 2021. Finally PET improvement was calculated and compared. Since this research focused in investigating the influence of planting arrangement on microclimate and outdoor human thermal comfort, same tree configuration was used (20m height very dense distinct crown layer) in all simulation scenarios. After calculating the PET value using Bio-met software, the PET improvement of proposed scenarios are calculated using below mentioned formula.

$\Delta PETt=PETveg,t-PETref,t$ …………………… (1)

Where $\Delta PETt$ is the effect of proposed planting arrangement on the mean PET within the square at 1.5m height for 14.00 hour, (t) PET veg, t is the mean PET within the square with proposed planting arrangement at 1.5m height for 14.00 hour (t), and PET ref, t is the mean PET of the base case scenario at pedestrian height for 14.00 hour (t).

Calculation of human thermal comfort

Simulation output including mean radiant temperature (Tmrt) are used as inputs for “Bio-met” software to calculate the PET value using standard Human according to ISO 7730 personal human parameters. PET was calculated with a male in age of 35, weight 75kg, height 1.75m and surface area 1.91m² with static clothing insulation (clo) 0.60 and metabolic rate 86.21W/m² (standing or light activity). Further, the output comfort index distribution patterns are illustrated using “Leonardo” illustrations. This helps to identify the comfort distribution according to proposed planting patterns.

Physiological equivalent temperature (PET)

PET is the mostly implemented (Potchter, Cohen et al., 2018) and advanced index which define as equivalent to the air temperature in a typical indoor setting at which the heat balance of the human body is maintained, with core and skin temperatures equal to those under the conditions being assessed (Höppe, 1999; Matzarakis, Mayer et al., 1999). PET is derived based on the human energy balance principle, which means that, like PMV, it inherits the steady state limitations when used in outdoor environments. Different studies have applied varied PET ranges with regional calibrations in OTC assessments. Table 4 shows the variation of thermal comfort ranges according to different climatic contexts. PET range for Singapore and related climate zones are newly added to the chart developed by Lucchese, Mikuri et al. (2016). Accordingly, acceptable PET range for this study is 24°C to 30°C considering the PET range applied in Singapore. However, future research is needed to conduct specifying particular PET ranges for different climatic zones as classified in Köppen-Geiger climate classification. Thus, it would be more reliable assessing local OTC levels employing a unique PET index.

Table 4. The comparison of the variation of thermal comfort (PET) ranges according to different climatic contexts

Sensation	Central Europe (°C)	Taiwan (°C)	Vitória/Brazil (°C)	Campo Grande/ Brazil (°C)	Singapore (°C)
Very cold	PET ≤ 4	PET ≤ 14	-	PET ≤ 11	-
Cold	4 < PET ≤ 8	14 < PET ≤ 18	18 < PET ≤ 20	11 < PET ≤ 15	-
Cool	8 < PET ≤ 13	18 < PET ≤ 22	-	-	-
Slightly cool	13 < PET ≤ 18	22 < PET ≤ 26	20 < PET ≤ 22	15 < PET ≤ 21	20 < PET ≤ 24
Neutral	18 < PET ≤ 23	26 < PET ≤ 30	22 < PET ≤ 30	21 < PET ≤ 27	24 < PET ≤ 30
Slightly warm	23 < PET ≤ 29	30 < PET ≤ 34	30 < PET ≤ 34	27 < PET ≤ 32	30 < PET ≤ 34
Warm	29 < PET ≤ 35	34 < PET ≤ 38	-	-	34 < PET ≤ 38
Hot	35 < PET ≤ 41	38 < PET ≤ 42	34 < PET ≤ 46	PET >32	38 < PET ≤ 42
Very hot	PET > 41	PET > 42	PET > 46	-	PET > 42
Reference	Matzarakis and Mayer (1996)	Lin and Matzarakis (2008)	Trindade da Silva and Engel de Alvarez (2015)	Lucchese, Mikuri et al. (2016)	Yang, Wong et al. (2013)
Climate Zone	Cold, no dry season, warm summer (Dfb)	Temperate, Dry winter, hot summer (Cwa)	Tropical savannah (Aw)	Tropical savannah (Aw)	Tropical rainforest (Af)

Results

As this research intends to compare how the planting pattern affect the microclimate and outdoor thermal comfort, the simulation results were compared at the hottest time of the day (14.00 h) for all scenarios at the pedestrian level (1.5m) including base case scenario. Outdoor environment of the independence square was selected as the study domain and calculated the mean air temperature of entire east and west courtyards.

Outdoor microclimate comparison

Figure 8 shows the results of mean air temperature. All three proposed scenarios have air temperature increase. Comparing all three scenarios, the clustered planting arrangement has provided the lowest mean air temperature increase (0.44 ⁰C air temperature rise compared to the base case scenario). The reason is the clusters are placed in west and east sides which helps to reduce afternoon incoming solar radiation. Equally distributed and scattered planting arrangement have provided 0.45 ⁰C and 2.15 ⁰C mean air temperature rise respectively. However, the difference of air temperature rise of linear and clustered arrangement is very low (0.01 ⁰C). Scattered planting arrangement is the worst case scenario in terms of air temperature cooling benefits. Nevertheless, the clustered planting arrangement provides the least canopy coverage to the hard paved areas, while the equally distributed planting arrangement provides homogeneous canopy coverage and least direct sunlight to the hard paved areas.

Figure 8. Boxplots of outdoor air temperature comparison of different planting patterns at 1.5m height, compiled by author (The upper and lower bounds of the boxplots indicates the lower and upper quartile (Q1 and Q3) of the values, the whiskers 5th and 95th percentiles

Mean radiant temperature

The mean radiant temperature (MRT °C) numerically explains how human being experience the energy budget of long wave and short wave radiation fluxes (Tan, Wong et al., 2013). MRT is one of the most significant factor affecting human thermal comfort (Alfano, Dell’Isola et al., 2013) which is the result of direct, diffuse and reflected thermal and solar radiation by outdoor surfaces. Figure 9 shows how the MRTs vary according to the planting arrangement changes. All the proposed scenarios have improved MRT values compared to the base case scenario. Scattered planting arrangement has provided the highest MRT improvement (6.00 ⁰C) and linear individual and cluster planting arrangement has provided 3.71 ⁰C and 3.55 ⁰C increase in MRT respectively.

Figure 9. Boxplot of MRT comparison of different planting arrangement

Wind speed

Figure 10 shows how the wind speed varies according to the planting arrangement scenarios at 14.00 pm afternoon. According to the results all the proposed scenarios have improved the wind speed within the case study area. Scattered planting arrangement has improved mean wind speed by 0.074 m/s as the best wind speed improvement. Equally distributed and clustered planting arrangement have improved wind speed by 0.07 m/s and 0.067 m/s respectively. The clustered planting arrangement has the lowest wind speed increase as it has high planting density. Therefore, increased planting density may block the wind flow with overlapped canopies and tree branches than the scattered individual planting arrangements. Although the planting arrangement is scattered in both scattered and equal interval planting patterns randomly planted trees allow more wind flow through the case study area than formal equal interval planting arrangement.

Figure 10. Boxplots of wind speed comparison of different planting arrangement

Relative humidity

In warm-humid climates, relative humidity should not be neglected in microclimatic assessments. Figure 11 shows how relative humidity (RH) has varied at 14.00pm afternoon in different planting arrangements. Unlike other parameters, humidity levels have been considerably reduced due to proposed planting arrangements. The highest RH (78.6%) has resulted in the base case scenario in which the wind direction (from north-east) is blocked by the existing vegetation. All the proposed scenarios have reduced RH value inside the square. The lowest relative humidity (59.9%) has resulted in randomly scattered planting pattern. Compared to base case scenario, the difference of humidity reduction between randomly scattered and equally distributed planting arrangement is almost same (1.9%). The most densified planting arrangement has resulted the least RH decrease (2.1%) compared to existing (base case) situation.

Figure 11. Boxplots of relative humidity comparison of different planting arrangement

Outdoor thermal comfort comparison

Physiological equivalent temperature (PET) was calculated using simulated air temperature, relative humidity, wind speed and mean radiant temperature (MRT) for base case and proposed planting scenarios at pedestrian level (1.5m). Figure 12 shows the outdoor thermal comfort (mean PET) values of different scenarios at 14.00 hr. Compared to the existing situation, scattered and clustered planting arrangements create negative impact on outdoor thermal comfort levels and the mean PET values have been increased by 1.32 ⁰C and 2.24 ⁰C respectively. The existing planting arrangement (base case) is slightly similar to a scattered planting arrangement, but more trees can be found in north-east (inflow wind direction) and south-west directions. Therefore, the mean PET value is close to the base case. Linear individual trees with equal distribution has provided a significant comfort improvement (-9.8 ⁰C PET) at pedestrian level reducing the mean PET from 44.39 ⁰C to 34.58 ⁰C. Cluster planting arrangement is the worst case scenario with (2.24 ⁰C) PET value increase.

Figure 12. Boxplot of PET comparison of different planting arrangement

Leonardo illustrations of the distribution of PET values of each scenario at 14.00 hour has shown in Figure 13, 14, 15 and 16 to understand the comfort level distribution arrangements. Equally distributed planting arrangement has a homogeneous PET distribution throughout the site area as the best thermal comfort improvement scenario compared to the base case. It has significantly improved the thermal comfort levels from 44.4 ⁰C (very hot conditions) to 34.6 ⁰C (warm conditions) according to the PET index while the scattered and clustered patterns maintain the same comfort conditions as the existing one. When the PET levels of very immediate surrounding of the building is considered, only the equally distributed planting arrangement has created lower PET levels than the mean PET value. The reason is that the linear arrangement offers homogeneous canopy coverage, allows wind flow throughout the study area and the building shadow also has a positive impact on thermal comfort levels in the immediate surroundings.

Figure 13. PET distribution of existing situation (base case) at 14.00 hr.

Source: Author using ENVI-met “Leonardo” software

Figure 14. PET distribution of scattered planting arrangement at 14.00 hr.

Source: Author using ENVI-met “Leonardo” software

Figure 15. PET distribution of cluttered planting arrangement at 14.00 hr.

Source: Author using ENVI-met “Leonardo” software

Figure 16. PET distribution of equally distributed planting arrangement at 14.00 hr.

Source: Author using ENVI-met “Leonardo” software

Discussion

Planting trees can greatly influence in regulating micrometeorological conditions of urban open spaces (Davtalab, Deyhimi et al., 2020). The results of this simulation study has also identified that the changes in planting arrangement directly affect the air temperature, wind speed, mean radiant temperature and relative humidity and finally it can alter the microclimate in urban outdoors. Further, the equally distributed shade (canopy cover), the planting location (distance from the building), and orientation (west side planting) provide more air temperature cooling benefits reducing the amount of afternoon solar radiation to the hard surface and the building. Although the higher planting densities provide more cooling benefits in terms of PET reduction than outdoor air temperature reduction in semi-arid climate (Zhao, Q., Sailor et al., 2018), this study explored that in warm-humid climates, densified planting arrangement contributes to reduce air temperature but not to reduce PET. The reason is that the clustered or densified planting arrangement reduces the wind speed and decrease the evaporation rate of people’s skin, which will have a detrimental effect on human thermal comfort (Hsieh, Jan et al., 2016). Current research has also proven that cluster planting arrangement (most densified) has the least impact on wind speed improvement while more scattered planting arrangement have higher impact on wind speed improvement. Furthermore, more scattered planting patterns or more densified planting patterns are not effective improving thermal comfort (PET) in warm-humid climates while equally distributed canopy coverage contributes the highest outdoor thermal comfort improvement reducing PET values. Planting arrangement with clustered (densified) patterns can increase the cooling effect of one tree if it creates large area of shade, especially aligned and without canopy overlaps (De Abreu-Harbich, Labaki et al., 2015). Based on this argument, clustered planting arrangement could not provide positive cooling benefits because it is arranged with canopy overlaps and not aligned. Therefore, if the proposed planting pattern is clustered, the trees should be aligned and canopies should not overlap in order to maximize the cooling effect.

In warm-humid climates, wind speed plays a vital role in microclimate improvements. According to Zhao, Q., Sailor et al. (2018), in a hot-humid environment, both shading and ventilation are important factors to be considered and it has been proven by the results of the current study. It is identified that densified (clustered) planting arrangement is the worst case scenario for wind speed, MRT and PET, but not for the air temperature. The air temperature increase is high when planting trees are randomly scattered. Therefore, it is clear that MRT and the wind speed are the most important factors for thermal discomfort in warm-humid outdoors compared to air temperature. Similarly, several studies have previously stated that the MRT is the most important factor influencing outdoor human thermal comfort due to tree-shading effect and radiative fluxes (Gatto, Buccolieri et al., 2020; Morakinyo, Kong et al., 2017; Rui, Buccolieri et al., 2018; Zhao, Q., Sailor et al., 2018). As understood by this study, De Abreu-Harbich, Labaki et al. (2015) has also found that changes in MRT and wind speed are the paramount controlling parameters of microclimate and urban thermal environment, especially in tropics. Therefore, the changes of planting arrangement considering the wind direction can significantly improve the thermal comfort levels in urban outdoors (Abdi, Hami et al., 2020). Furthermore, as explained by (Abdollahzadeh and Biloria, 2021), wind speed has the most significant impact on OTC particularly in coastal regions of subtropics. Thus, Colombo as a coastal city in tropical rainforest climate, above statement is applicable in terms of OTC improvements. According to the results, planting arrangement make a significant impact on outdoor relative humidity. When the wind speed and humidity levels are compared in different planting arrangements, it is clear that the planting arrangement directly affect to change the wind speed thus altering the humidity levels due to evapotranspiration. The highest relative humidity reduction (59.9%) has resulted in the scattered planting arrangement where the wind speed is highest as there is enough space for the wind flow. The most scattered planting arrangement showed the highest humidity reduction while the most densified arrangement showed the lowest (2.1%). There is a 16.6% difference in humidity reduction between clustered and scattered planting arrangements, thus the more scattered planting arrangement is the more decrease in humidity level. Therefore, the planting arrangement should avoid blocking prevailing wind direction and densifying vegetation near buildings to improve the thermal comfort levels, especially in warm-humid climates.

To maximize the PET improvements in urban outdoors in warm-humid climates, equally distributed shade coverage to hard surfaces and buildings, blocked west side direct solar radiation, reduced planting densities near the buildings, open space without trees in the immediate surrounding of the buildings and improved effective ventilation are suggested. Moreover, shade should be provided to both east and west outdoor environments with equally distributed planting pattern without canopy overlaps to receive the maximum benefit of a planting design. These findings correspond with the results of Zhao, Q., Sailor et al. (2018). Homogeneous canopy coverage (linear individual trees with equal intervals) could provide a significant comfort improvement (-9.8 ⁰C PET) at pedestrian level reducing the mean PET from very hot to warm condition while densified planting arrangement effect negatively (2.24 ⁰C) on PET. Further, it is important to note that keeping a no tree space in the immediate surrounding of a building could enhance thermal comfort of the same space significantly. It is lower than the mean PET value compared to the rest of the areas. In hot-humid climates, more vegetation coverage does not contribute towards more cooling benefits for built environment and previous research have found that 20–30% coverage ratio is the most efficient and effective threshold, but this could be varying with the climate background and seasons (Grimmond and Oke, 2002; Ouyang, Morakinyo et al., 2020).

Increased air temperature and humidity are detrimental in tropical urban spaces such as Colombo (Johansson and Emmanuel, 2006). Therefore, mitigation of OTC can be significantly achieved by planting trees (Bartesaghi-Koc, Osmond et al., 2020) and the planting arrangement should be carefully considered to reduce humidity, enhance the shade and to increase the wind speed at the same time. These planting recommendations might not be effective in another climatic zone. Even in the same climate conditions, the planting arrangement may vary according to different urban morphologies and distance from the sea (Johansson and Emmanuel, 2006) and cultural preferences (Dalman, Salleh et al., 2013) availability of water bodies and hard surface coverage (Jacobs, Klok et al., 2020; Manteghi, Mostofa et al., 2020), the type of public space and the specific morphological features such as aspect ratios, street orientation and sky view factor (Qaid, Lamit et al., 2018) for OTC improvements.

This research is limited to one type of public spaces (public square) and the results are context specific. The onsite climate data collection was limited to daytime, but night time data also should be considered for more precise results and recommendations. Further, it is recommended to conduct the simulation on a hottest day in the hottest month of the year for future studies. This study was limited to simple forcing meteorological simulations and full forcing simulations are suggested for future research. ENVI-met vegetation database should be updated according to local species and conduct further studies for results with more accuracy. This research is limited to basic three types of planting arrangements (scattered, clustered, and equally distributed) which basically define the amount of shade coverage in a small-scale landscape design. Since the number of trees should be constant in the limited open space available within the square, and the time limitations for the number of simulations, the current study could not compare particularly the impact of planting densities by conducting more scenarios. However, further research can be conducted to find more specific results comparing quantified densities of planting arrangements.

Generally, numerical simulations of CFD modelling have been validated by field measurements in majority of the studies (Duarte, Shinzato et al., 2015) to ensure the output variables and gain useful understanding about the model accuracy, but this research has not conducted a validation. Nevertheless, according to the ENVI-met model limitations highlighted by (Krüger, Minella et al., 2011), model accuracy is reduced when the wind speed is higher than 2 m/s and this study did not exceed the particular wind speed limit. Moreover, only the tree database of ENVI-met software package were used to compare the cooling effects of three different planting arrangements. Therefore, future research may assess various planting arrangements using different native tree species (Zhao, Q., Sailor et al., 2018) considering vegetation parameters such as crown geometry, tree height, leaves type and shape De Abreu-Harbich, Labaki et al. (2015) and Trunk height, Crown height, Leaf area index (Morakinyo, Lau et al., 2018) for more realistic planting arrangements to assess the impact of urban vegetation on outdoor thermal comfort and microclimate.

Conclusion

Trees can enhance the microclimatic conditions and thermal comfort in warm-humid urban public spaces, and planting arrangements should be considered to offer maximum cooling benefits. Since the space for planting is limited and hard surface coverage is high in urban squares, the cooling benefits should be provided using a limited number of trees. Therefore, planting arrangement is a critical factor which requires serious consideration. This research conducts microclimate simulations to understand how planting arrangement affects microclimate and outdoor thermal comfort at pedestrian level in an urban square. Numerical models (ENVI-met) can simulate and compare OTC for a wide range of planting arrangements according to the research purposes. In this study, dense, scattered and equally distributed planting arrangements are proposed to compare the impact on microclimate. Changes in planting arrangements affect the air temperature (Ta), wind speed (WS), mean radiant temperature (MRT) and relative humidity (RH), and finally, it can alter the microclimate and OTC in urban outdoors. Changes in planting arrangements have a significant impact on PET. The homogeneous canopy coverage provides the highest comfort improvement (9.8 ⁰C PET reduction) at the pedestrian level, reducing the mean PET from very hot to warm conditions. The following recommendations are made to maximize the comfort improvements; layout trees without canopy overlaps for equally distributed shade coverage to hard surfaces buildings, improve effective ventilation, reduce planting densities near the buildings and keep free space adjacent to the buildings, and avoid densified planting arrangements, use scattered tree planting especially for the inflow wind direction. Accordingly, canopy coverage and distribution, planting location and orientation of the buildings are important factors. Mean radiant temperature (MRT) and wind speed are the most critical parameters for thermal comfort in warm-humid outdoors. Planting arrangements which reduce humidity, enhance the shade and increase the wind speed are suggested to minimize the thermal discomfort. Future research where the simulation results are validated with field measurements is useful for more precise results and valid recommendations.

This approach and methodology can be applied to assess the thermal comfort levels in different regions. However, the PET ranges should be selected according to the climate classification of the particular region to define the thermal sensation and the ranges of PET index is also should be redefined considering more specific climate characteristics. Specifically, these findings are applicable in tropical (Af) countries and not applicable to the climates with seasonal changes since the microclimate is distinctively context specific. Finally, future research may assess the impact of native tree species with particular vegetation parameters to benefit the pedestrian level OTC and microclimate. The results of this research are helpful in guiding urban designers and landscape architects in urban vegetation policy-making process to improve outdoor thermal comfort in warm-humid cities.

Author Contributions

Conceptualization, Clarence Dissanayake., and Weerasinghe UGD.; methodology; software; investigation; resources, Clarence Dissanayake.; data curation, Kawshalya LWG.; writing—original draft preparation, Clarence Dissanayake.; writing—review and editing, Clarence Dissanayake., and Weerasinghe UGD.; supervision, Weerasinghe UGD. All authors have read and agreed to the published version of the manuscript.

Ethics Declaration

The authors declare that they have no conflicts of interest regarding the publication of the paper.

Acknowledgments

This work is supported by the Accelerating Higher Education Expansion and Development (AHEAD)—DOR Grant affiliation with Ministry of Higher Education & University Grants Commission Sri Lanka.

Funding Statement

This research was funded by the World Bank. Grant NO; Credit/Grant #: 6026-LK/8743-LK (AHEAD/DOR/52).

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