Evaluating the thermal influences of a finite-size forest under varying microclimatic conditions

Increasing urban tree cover is a widely recommended strategy for mitigating urban heat, as trees are expected to cool cities through evaportranspiration and shade provision. In real-world scenarios, both shading and transpirational cooling processes depend on prevailing climate conditions (e.g. radiation, wind, humidity) and tree attributes (e.g. shape and size of greenery space, tree height, leaf-area density, stomatal conductance). Quantifying these factors is crucial for assessing the roles of trees in mitigating the urban heat island effect and shaping local climates. Here, we seek to explore the cooling effect of a finite-size forest in response to changes in local climate conditions using a large-eddy simulation method. Airflow over the finite-size forest experiences abrupt changes in wind, temperature and humidity due to the transition from the sunlit openings to the relatively sheltered forest interior. The vertical velocity changes sign at both the leading and trailing edges, indicating the presence of local recirculations therewith due to the blocking effect imposed by the dense canopy. The dense canopy cover intercepts the incoming solar radiation, resulting in cooler ground temperatures beneath the forest compared to open areas. Additionally, the forest canopy release water vapor to the atmosphere through evapotranspiration, leading to increased humidity levels around the forest. The cooler and more humid air from the forest mixes with the warmer air from the open areas, yielding slightly cooler temperatures in the near-field of forest wake. Simulation results under various scenarios are compared to identify trends and relationships between urban trees and local climate conditions. These findings can be used to inform future field campaigns over forests of finite size with distinct edges and planning strategies aimed at improving microclimate via urban greenery.