Thermal Comfort and Longwave Radiation over Time in Urban Residential Complexes

Abstract

Large cities with a high concentration of high-rise buildings are shaded by urban canyon. This brings a cooling effect compared to the space exposed to the sun, but is not always cool due to the longwave radiation emitted from buildings and the built environment. We tested the micro-scale effects of major external spatial factors, trees, and buildings, under shade on longwave radiation shifts to understand the effects of large shaded areas in megacities. Incoming and outgoing longwave radiations (ILR and OLR, respectively) were found to decrease the overall observation by time zone. Longwave radiation on a micro-scale was also inversely proportional to the tree volume. From mean radiant temperature (MRT) analysis, we found that about a 10% decrease in MRT could be achieved by increasing tree volume by around 50%. Larger tree volumes corresponded to greater blocking effects on longwave radiation. Considering the tree volume, a multilayer urban tree canopy composition can more favorably improve the thermal environment and energy sustainability of a city compared to a single-layer canopy. Larger trees planted with harmonious shrubs are the most effective in reducing longwave radiation.

Is Sensible Heat Flux Useful for the Assessment of Thermal Vulnerability in Seoul (Korea)?

“…Low vulnerability values in the city's core district may have been influenced by high building density or high income, as high-rise buildings exert a cooling effect by providing shade. In some cases, cooling is associated with high plant density and the presence of green infrastructure, parks, or other green areas [24,93]. In other cases, lower levels of impervious surfaces are associated with lower temperatures and reduced thermal vulnerability.…”

Determining Favourable and Unfavourable Thermal Areas in Seoul Using In-Situ Measurements: A Preliminary Step towards Developing a Smart City

“…According to the BCR values and measurement techniques reported previously [86], we designed a net radiation measurement technique. The observation sites were required to be in residential areas with mild wind speed, as the wind speed is often used as an independent variable when measuring heat flux.…”

“…Observation instruments: (a) mobile net radiometer instrument kit, (b) anemometer, (c) thermometer (revised from Park,[87]), and (d) bi-directional radiation sensors (Kwon and Lee,[86]). …”

Influence of landscape interventions on thermal comfort under time-varying building shadow; new Gwanghwamun square case, Seoul, South Korea

“…Most of all, the effectiveness of trees and water has been questioned under deep urban canyon and high urban density area due to blocked ventilation[ 28 , 32 , 35 ]. However, most recently, even in high-density condition, larger tree volumes as well as dense multi-layered plantings were found to lead to greater thermal comfort due to evapotranspiration and long-wave radiation blocking[ 36 ]. The understanding about two shading types of building and tree, and the contextual understanding for cooling effect of plants are important[ 37 ].…”