Vegetation cover and plant-trait effects on outdoor thermal comfort in a tropical city

Naika Meili,Juan Angel Acero,Nadav Peleg,Gabriele Manoli,Paolo Burlando,Simone Fatichi

doi:10.1016/j.buildenv.2021.107733

Abstract

An increase in urban vegetation is an often proposed mitigation strategy to reduce urban heat and improve outdoor thermal comfort (OTC). Vegetation can alter urban microclimate through changes in air temperature, mean radiant temperature, humidity, and wind speed. In this study, we model how street tree and ground vegetation cover and their structural, optical, interception, and physiological traits control the diurnal cycle of OTC in different urban densities in a tropical city (Singapore). For this purpose, we perform a variance based sensitivity analysis of the urban ecohydrological model UT&C. Model performance is evaluated through a comparison with local microclimate measurements and OTC is assessed with the Universal Thermal Climate Index (UTCI).We find a pronounced daily cycle of vegetation effects on UTCI. Tree cover fraction is more efficient in decreasing UTCI during daytime, while a higher vegetated ground fraction provides more cooling during night. Generally, increasing vegetation cover fractions do not deter OTC, except in certain urban densities during some periods of the day. An increase in tree and ground vegetation fractions provides a higher average UTCI reduction compared to a change in vegetation traits (0.9 – 2.9 °C vs. 0.7 – 1.1 °C during midday, 10 month average). The increase in humidity related to plant transpiration prevents further reduction of UTCI. However, the choice of vegetation traits enhancing tree transpiration can decrease UTCI during hot periods. These results can inform urban planners on the selection of vegetation amount and traits to achieve feasible OTC improvements in tropical cities.

Highlights

The number of people living in urban areas is increasing globally, and especially cities in Asia and Africa are foreseen to grow rapidly in the coming years [1]
Using Urban Tethys-Chloris (UT&C) to simulate outdoor thermal comfort (OTC) in different urban densities, this study aims to answer the following questions: (i) which vegetation properties are the most influential in modifying OTC in humid tropical climates? (ii) Are there substantial differences in the effects of vegetation types and properties across hours of the day or during the hottest periods of the year? The city of Singapore is used as an exemplary case-study; model performance is assessed in five different locations with varying urban density and green cover
The effects of street tree and ground vegetation cover, as well as their structural, optical, interception, and physiological traits on Universal Thermal Climate Index (UTCI) were analysed in different urban densities and for different hours of the day through a variance based sensitivity analysis

Summary

Introduction

The number of people living in urban areas is increasing globally, and especially cities in Asia and Africa are foreseen to grow rapidly in the coming years [1]. Urban vegetation influences the urban climate during daytime as well as nighttime [10], during average climatic conditions as well as during heat waves [12], and requires planting, continuous maintenance, and possibly irrigation — all factors that have associated costs [13]. In several cases, such as in tropical hot humid cities, the effects of vegetation on OTC are not straightforward to predict. Vegetation can shelter and block wind flow, which could further reduce OTC in warm and calm-windless locations [22]

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