Session Overview
UCP10: Influence of urban vegetation I : urban trees
Wednesday, 22/Jul/2015:
9:30am - 10:30am

Session Chair: Oded Potchter, Tel Aiv University
Location: St-Exupéry Amphitheater


Field Observation on Thermal Environment of an Urban Street with Roadside Trees in a Tropical Climate

Hai Jian Toh1, Sheikh Ahmad Zaki1, Aya Hagishima2, Mohamed Sukri Mat Ali1

1Universiti Teknologi Malaysia, Malaysia; 2Kyushu University, Japan

As the urban heat island (UHI) phenomenon is growing in conjunction with rapid urban development, the outdoor thermal environment from low to middle latitudes had since become a major concern on the well-being and health of the citizens. Urban greenery including roadside trees planting is believed to be one of the best remedy to these anthropogenic effects, however, less study and attention has been directed in understanding the cooling effects of roadside trees in a tropical city, hence, this study is carried out in Malaysia. This study aims to provide information on the effects of street trees to the thermal environment for future development and urban planning works in a tropical city. The outdoor field measurements of roadside trees in Kuala Lumpur will be conducted 4 weeks from January to February 2015. The targeted area consists of sparse and dense roadside trees and a reference site without vegetation. In each area of measurement, several sensors of air and globe temperature are recording with sampling interval of 10 minutes at 1.5m above the ground surface, while wind speed and direction along with relative humidity at 1.5m level above ground surface will be observed with sampling interval of 1 minute on selected days during the measurement period. Additionally, road and pathway surface temperature will be recorded at 30 minutes interval. The results of diurnal variation pattern of air and globe temperature are presented. To further analyse the thermal environment, mean radiant temperature (MRT) are computed and compared between different densities of trees. Wind speed reduction inside the canopy, variation of road and pathway surface temperature under different density of trees will be observed and presented. Therefore, from the results of this study, the evidence of cooling effect from roadside trees could be drawn and quantify so that suitable information for planners and designers could be presented for future development work in mitigating the UHI phenomenon in a tropical city.


Effects of Desert Tree Shade and Ground Cover Surface Cover on Human Comfort in an Arid City

Chris Martin

Arizona State University, United States of America

The magnitude of potential for urban vegetation types to mitigate human discomfort across seasonal changes is poorly understood. Further resolution of this relationship was undertaken to elucidate the role of desert tree shade and ground surface cover type on apparent temperature (TAP) within the undercanopy layer during summer dry and summer wet monsoon seasons in Phoenix, Arizona, USA. Micro-meteorological stations recorded diel patterns of air temperature and humidity, wind speed, direct and diffuse radiation, and net radiation at one-minute intervals in a factorial matrix of open full sun exposure or mature hybrid South American mesquite (Prosopis alba x Prosopis chilensis) tree shade and Bermuda grass (Cynodon dactylon) turf grass lawn or decomposing granite (DG) surface mulch cover in co-joining residential yardscapes of similar size and tree and shrub frequencies, but dissimilar landscape designs. TAP was calculated from these data averaged at 30-minute intervals during summer dry (Julian Days 165 to 179) and summer wet (216 to 241) seasons, 2010. During the summer dry season, average maximum TAP was about 15C or 8C higher than air temperature in sun or shade respectively, but was not affected by landscape design. During the summer wet season, average maximum TAP in in the DG covered yardscape was about 2C to 4C higher than in the turf grass covered yardscape in both full sun and under mesquite tree shade. These local differences in TAP were associated with seasonal changes in humidity, radiation balances, and differences in irrigation practices to support yardscapes with different vegetation types. +

Incorporating Resolved Vegetation in City-Scale Simulations of Urban Micrometeorology and Its Effect on the Energy Balance

Brian N. Bailey1, Matthew Overby2, Rob Stoll1, Eric R. Pardyjak1, Pete Willemsen2, Kevin Briggs1, Daniel Alexander1

1University of Utah; 2University of Minnesota, Duluth

Urban vegetation plays a critical role in the exchange of heat and mass across a wide range of scales. Shading and evaporative cooling by trees reduces air and surface temperatures, which can increase human comfort, reduce building energy usage, and mitigate the urban heat island effect. Urban vegetation can also increase air quality through deposition and respiration processes. Despite the clear benefits of urban vegetation, it is difficult to determine how their local effects translate across neighborhood- and city-scales, and the degree to which their benefits offset their costs. The widespread effects of vegetation are extremely difficult to measure directly due to the complicated interactions between micrometeorology and topology. City-scale models are too coarse to represent localized changes in urban form, and building/tree-scale models are too inefficient to feasibly represent whole city-scales. To address these issues, we have developed a new modeling framework that includes highly detailed models for three-dimensionally resolved urban vegetation. Physically robust models were developed for radiation, evapotranspiration, and leaf temperature that can represent complex geometries with arbitrary leaf angle distributions. This level of physical realism comes at a hefty price, which is afforded by performing calculations in parallel on graphics processing units (GPUs). Additional algorithms were incorporated into the physical models to reduce computational cost. The resulting model can resolve urban topologies with hundreds of thousands of buildings/trees, using only a single commodity-level desktop workstation. The model was used to assess several urban design scenarios regarding vegetation or `green infrastructure’. The study focuses on the impacts of species diversity, and the placement of trees relative to buildings, roads, other trees, etc. Model outputs were used to determine how these variables act to influence microclimate in terms of their modification of the local energy balance.