Mitigating urban heat island effects : An analysis of precinct ventilation performance and its impact on urban heat islands and outdoor thermal comfort

Given the increasing frequency and intensity of extreme weather events and its increasingly severe impact on human settlements, there is an urgent need for climate change mitigation in the built environment. Many cities are suffering the severe impacts of urban climate change via urban heat islands (UHI) effects. Wind is a critical factor for urban temperature regulation. However, the potential of utilising natural wind for UHI mitigation is significantly constrained by the limited knowledge on urban ventilation at the precinct scale. Many existing studies have focused on the building, city and regional scales but very little research has been conducted at the precinct scale. Furthermore, wind is affected by numerous urban morphological characteristics; yet, there is a lack of studies on the relationship between wind and morphological characteristics at the precinct scale and on the potential of wind to alleviate UHIs and improve outdoor thermal comfort in precincts. To fill such gaps, this research first develops a framework for analysing precinct ventilation performance and its impact on UHIs and outdoor thermal comfort. The framework addresses the relationships among external meteorological conditions, precinct morphological characteristics, precinct ventilation performance, UHIs and outdoor thermal comfort. This research subsequently proposes a precinct morphology classification scheme using a three-component protocol of building height, street structure and compactness for examining the relationship between precinct ventilation and precinct morphological characteristics. The precinct morphology classification scheme is then applied to Greater Sydney, Australia, with the resulting identification of 20 representative precinct ventilation zones. In the course of this research, empirical studies were conducted in three representative precinct ventilation zones: open low-rise gridiron (OLG), open midrise gridiron (OMG) and compact high-rise gridiron (CHG) on extreme hot days. According to the relative wind velocity ratio (RWvR), the OMG precinct exhibited the best precinct ventilation performance, followed by OLG and then the CHG precinct. In the OLG and CHG precincts, the precinct ventilation created by the sea breeze alleviated UHIs, with every 0.1 increase in RWvR leading to a 0.09-0.12 °C reduction in UHI intensity. Meanwhile, in the OMG precinct, the precinct ventilation created by the sea breeze made no difference for UHIs. However, the precinct ventilation of the OMG precinct still partially exhibited UHI alleviation potential with every 0.1 increase in RWvR leading to a 0.06-0.1 °C reduction in UHI intensity depending on the approaching wind temperature and shading conditions. Only the precinct ventilation of the OLG precinct leads to outdoor thermal comfort improvement with every 0.1 increase in RWvR leading to 0.29 °C and 0.50 °C physiological equivalent temperature reductions under sea breeze and varying wind conditions, respectively. The results also indicate that within ‘gridiron’ precincts, street orientation is not critical to precinct ventilation performance and its impact on UHIs and outdoor thermal comfort. Under wind conditions, trees do not always alleviate UHIs and improve outdoor thermal comfort as trees can block sea breeze penetration and inhibit wind cooling potential. These key findings will serve to inform urban heat island mitigation strategies and future planning and design decisions in the built environment. This research addresses the grand challenge facing the built environment with a focus on urban heat island effects and wind cooling potential. It employs a multidisciplinary approach to tackling the complexity of the inter-relationships between precinct morphology, precinct ventilation performance, urban heat island effects and outdoor thermal comfort. The research findings will serve to advance the existing knowledge on the impact of wind on UHIs and on outdoor thermal comfort at the precinct scale.

Grantor dc:publisher

UNSW, Sydney

Year dc:date

2020

Author dc:creator

• He, Baojie

• Precinct ventilation performance
• Urban heat island
• Urban ventilation
• Urban morphology
• Outdoor thermal comfort

Statement dc:rights

• open access
• CC BY-NC-ND 3.0
• free_to_read

Licence

• https://purl.org/coar/access_right/c_abf2
• https://creativecommons.org/licenses/by-nc-nd/3.0/au/

Language dc:language

EN

Identifier

https://doi.org/10.26190/unsworks/22202

OAI identifier oai:identifier

oai:unsworks.library.unsw.edu.au:1959.4/70446