The urban thermal environment is getting more and more attention in recent decades because of rapid urbanization and global climate change. The heterogeneous landscape and different levels of human activities jointly lead to the large variation of the thermal environment across a city. As a result, studies focusing on the urban thermal environment have shifted from the traditional city-scale urban heat island towards the sub-kilometer scale neighborhood microclimate. The main purpose of this dissertation is to advance the understanding of the urban thermal environment at the sub-kilometer scale from two aspects: underlying mechanisms and mitigation strategies.

Observation directly provides firsthand data to study real-world phenomena and explore their underlying mechanisms and can be fu...[ Read more ]

The urban thermal environment is getting more and more attention in recent decades because of rapid urbanization and global climate change. The heterogeneous landscape and different levels of human activities jointly lead to the large variation of the thermal environment across a city. As a result, studies focusing on the urban thermal environment have shifted from the traditional city-scale urban heat island towards the sub-kilometer scale neighborhood microclimate. The main purpose of this dissertation is to advance the understanding of the urban thermal environment at the sub-kilometer scale from two aspects: underlying mechanisms and mitigation strategies.

Observation directly provides firsthand data to study real-world phenomena and explore their underlying mechanisms and can be further used to support the evaluation of numerical simulations. To satisfy the demand of studying the high-resolution urban climate at the sub-kilometer scale, dense weather sensor networks are being built among megacities over the world. The dissertation first proposed a cluster-based design for an urban climate monitoring network and examined its potential applications in Beijing and Hong Kong by using weather simulation data as ground truth. Utilizing the cluster-based design strategy, we optimized the urban climate monitoring networks by rearranging sensor locations and expanding the network.

The dissertation then focused on evaluating the impact of landscape and anthropogenic heat, which are two key factors that govern the urban thermal environment. Based on measurements from fixed weather stations, our analyses found that the built-up urban landscapes are 0.51 ºC warmer than the low plant area on an annual average over China. We then conducted mobile measurements in Hong Kong and revealed that the green-blue spaces could consistently enhance thermal comfort in the evening. This suggests that open high-rise neighborhoods are preferred over compact mid-rise and low-rise neighborhoods for sustainable city development. The key points for designing urban parks for hot and humid regions are shading and ventilation. As the second largest portion of the anthropogenic heat, vehicle heat increases the urban canyon temperature by more than 0.30 ºC in both summer and winter in Hong Kong. The warming impact will be stronger in the winter, but weaker in the summer, under global warming. To mitigate the urban heat stress caused by vehicle heat, EV adoption can offset 25.5% of the global warming signal at the city scale in winter, and the benefit is most distinct during the evening for Hong Kong.

This dissertation advances the observation and simulation methods for urban thermal environments at the sub-kilometer scale and provides useful guidance for landscape design and green transportation technology in urban heat mitigation.