  The design of urban areas and building that utilizes the microclimatic
effects of trees is a promising approach for reducing the severe heat stress
caused by urban heat islands and global warming. Although trees can reduce heat
stress through solar shading during the daytime, their influence on the air
temperature under and around them during the nighttime, which is important for
nighttime thermal comfort, has not yet been fully elucidated. In this study, we
investigated the nocturnal cooling effect of trees in a physical urban space by
the coupled numerical simulation of longwave radiative transfer and
computational fluid dynamics. To represent the spatial structure of an actual
urban space, airborne LiDAR-derived three-dimensional data of leaf area density
distribution and building shape were used. The species-specific convective heat
transfer coefficient was also considered. An analysis of the calculated
sensible heat flux shows that both leaf area density and sky view factor are
important factors in the production of cool air. According to the calculated
distributions of air temperature and velocity, even under the condition of a
certain degree of incident flow, the cooled air can flow down to the space
under the crown, accumulate, and then diverge when the wind speed is
sufficiently low in the crown owing to the crown drag. Buildings contribute to
both the accumulation and dissipation of cool air. The findings of the present
study suggest that cool spots can be produced during nighttime by trees planted
near streets by devising a suitable arrangement and morphology of trees and
buildings.
