Urban moisture island phenomenon in a subtropical high-density city

Abstract

Urbanisation has brought great changes on urban climate. Previous studies tend to report the effect of rapid urbanisation on temperature, but not on humidity. In this thesis, we aimed to address this research gap by examining urban humidity variation as well as its implications in a subtropical high-density city, Hong Kong. The average urban air moisture concentration exceeds that in rural areas for both day and night, confirming the existence of an urban moisture island (UMI) in Hong Kong. A new lumped moisture model is developed to predict the hourly urban air humidity using weather data from the operational suburban weather stations, to better understand and quantify urban moisture environment. The coupled thermal-moisture model can capture the main characteristics of diurnal profiles of urban air temperature and humidity in Hong Kong. The modelling results demonstrate that weakened city ventilation tends to lead to an increase in urban air moisture. Furthermore, natural surfaces reduce urban heat stress by evapotranspiration. Besides, the large wall areas in the compact high-rise areas function as both sinks and storage sites, contributing to a noticeable diurnal trend in the urban air humidity profile. In brief, the significant urban moisture island phenomenon may result from enhanced evapotranspiration, restrained moisture sinks, intensive human activities and a weakened wind environment. Effective mitigation methods are needed for relieving urban heat and moisture excess. Introducing urban tree is among the most feasible mitigation methods. However, there exists large difference in the tree performance among different species and planting patterns. A new vegetation sub-model has been added to the heat-moisture coupled model. The result shows tree species with higher leaf area index (LAI) and larger crown present a better capability of transferring sensible heat to latent heat. In terms of planting patterns, the sparse trees provide a greater cooling and humidification effect, compared with dense planting. Besides, impact of trees is also discussed on urban area with different densities. Extreme climates are observed to become more frequent as the impact of both global and local climate change. Both extreme hot day and extreme wet day present a rising trend in Hong Kong, and it increases at significant level over urban area. Besides, the compound hot and wet days are also observed to rise. Compound extreme weather events could additionally bring negative burden to human bodies and worsen the public health conditions. Finally, to explore the impact of UMI on building energy consumption, a generic 40-storey office building model was used to run simulations at the local scale for 10 years (2006–2015). The local-scale simulations included six weather stations representative of the different local climate zones (LCZ) in Hong Kong. Latent cooling load accounted for a considerable fraction of sensible cooling load in urban station, suggesting that urban humidity excess plays a significant role in building loads. This thesis aims to reveal the mechanism behind UMI and show the impacts brought by urban humidity change. The works hope to provide insights in sustainable urban planning for effectively mitigating temperature and humidity excess.