The time-evolving impact of tree size on nighttime street canyon microclimate: Wind tunnel modeling of aerodynamic effects and heat removal

TL;DR

This study uses wind tunnel modeling to analyze how the growth of urban trees over time affects street canyon microclimate, revealing impacts on ventilation and heat removal during different heat conditions.

Contribution

It introduces a time-evolving perspective on tree impact in street canyons, highlighting how tree size influences airflow and heat dynamics during moderate and extreme heat.

Findings

Large trees reduce canyon ventilation in isothermal conditions.

Trees trap heat at pedestrian level during extreme heat.

Tree foliage management is crucial for urban microclimate control.

Abstract

Urban trees play a crucial role in urban climate in many aspects. However, existing research has not adequately explored the impact from a time-evolving perspective, that is, tree growth over time. To bridge this research gap, this study investigates in a wind tunnel the effects of tree-to-canyon foliage cover and relative height (0.32-1.1 times canyon height), mimicking growth of trees, on conditions in street canyons during moderate and extreme heat. The results reveal that trees may affect canyon-wide ventilation and heat removal in two different scenarios. First, when canyons are in isothermal conditions, medium and large trees, that fill half the canyon height or reach slightly above the canyon, decelerate the shear layer and weaken the vortical flow, as a result reducing the canyon-wide ventilation. Second, in extreme heat conditions, medium and large trees trap heat at the pedestrian level due to the blockage of air entrainment and the suppression of upward buoyancy-driven flow from the ground surface. An air temperature rise that corresponds to 1.5 degree Celsius in a full-scale urban setting is observed in measurements. These observations suggest that urban trees' foliage cover must be managed for a canyon's optimal ventilation and heat removal during nighttime.