Glasgow/HKU Early Career Mobility Funds 2023-24

Professor Huang, Jianxiang   (Principal Investigator (PI))

458

2023-06-01

50000

Glasgow/HKU Early Career Mobility Funds 2023-24

2310100494

Fellowship Schemes

2023

On-going

Research ObjectivesThe proposed research addresses two objectives: 1) to investigate heatwave events recorded both in the UK and in Hong Kong, and 2) to examine the protective effect of urban greenery in mitigating heat stress at both locations.Key issues and Problems AddressedTrees are considered a time-tested cooling strategy in hot climates. Tree canopies shade pedestrians from direct sunlight, and the evapotranspiration of tree leaves can reduce the ambient air temperature. For this reason, trees are often considered a high priority in heat-resilient urban policy to mitigate weather extremes and urban heat island effects. Examples include multiple policy addresses by the Hong Kong government (HKG, 2022), pledging millions of trees to be planted within the city proper. Similarly, Singapore’s Green Plan 2030 has designated some 200 hectares of urban land as nature parks (SG, 2021). Other municipal policies have established quantitative targets, such as the 25% in tree coverage pledged by the Tree and Shade Master Plan of Phoenix, USA (CP, 2010), or 30% suggested by the 2030-50 Master Plan for Kuala Lumpur, Malaysia (KLCH, 2021) and the 2017-37 Master Plan for Barcelona, Spain (CB, 2017). Melbourne, Australia has set an more ambitious target of 40% (CM, 2012).Existing policies fail to mention where to plant trees. The number of trees allowable is often constrained by ground surface area available for planting and the maintenance budget for irrigation, pest control, trimming, etc. Landscape designers are therefore confronted with the challenge to achieve more cooling with fewer trees, and there is little consensus in research nor practice on how to achieve this. For instance, literature disagree upon whether to concentrate trees in a few locations or to spread them thinly across a larger area for maximum cooling benefits (Morakinyo and Lam, 2016; Zhang et al., 2018; Zhao et al., 2018). To place trees in breezeways, concluded by a recent study by Huang et al. (2022), can further reduce the in-situ pedestrian heat stress in a high-density city compared with those planted elsewhere. Rooftop trees, on the other hand, have a negligible cooling effect at the ground level where pedestrian activities concentrate (Ng et al., 2012). The research findings above often depend on the study context and cannot be readily implemented. In practice, such decisions were often left to rules-of-thumb principles, personal experience, intuitions, or made opportunistically, i.e., to place trees in avoidance of buildings and infrastructure, or away from underground utilities for unhindered root development. Some decisions cited the pre-modern Chinese practices of Fengshui (Mak and Ng, 2005) or its Indian equivalence of Vaastu Shastra (Patra, 2009), with little support from research evidence. There is a need for a systematic approach to identify optimal tree locations for urban cooling practice.Research literature stopped short of providing a design tool to locate trees to maximize their cooling benefits at the micro-scale. The majority of simulation models has missed one or a few components of tree cooling mechanisms, such as shading, evapotranspiration and the modification of localized wind near a tree canopy. Of a handful of attempts made to link the tree simulation model with optimization algorithms (Park et al., 2020; Stojakovic et al., 2020; Wallenberg et al., 2022; Zhao et al., 2017), their results tend to suffer from the lack of various tree cooling components, or their slow computational speed which prevented them from achieving meaningful optimal. Another body of literature, which has recently flourished in the domain of urban climate (Ng et al., 2012; Ng and Ren, 2018; Yuan et al., 2017) and geography (Cheung et al., 2020; Y Li and Song, 2019; Yingnan Li and Song, 2019; Morakinyo et al., 2020; Zhang and Gou, 2021), have studied extensively the cooling benefit of trees in an urban environment, although the spatial resolution of the above models (~1km) were often too coarse to address the challenge of where to plant each individual tree. Questions remain as whether tree locations can be optimized mathematically based on simulation models? And if yes, what lesson does it afford to heat-resilient urban policies and landscape design practices?