High-resolution regional modeling of urban moisture island: mechanisms and implications on thermal comfort

The urban moisture island (UMI) can aggravate the thermal stress due to the urban heat island (UHI) in subtropical and tropical cities. In this study, we investigated the spatiotemporal variation patterns of UMI in Hong Kong, a subtropical coastal city, using the fine-resolution mesoscale Weather Research and Forecasting (WRF) model by integrating local climate zone (LCZ) maps based on the World Urban Database and Access Portal Tools (WUDAPT). Our results show that at regional scale, the UMI phenomenon tends to occur in coastal areas, possibly owing to rich moisture sources from sea breeze and inhibited moisture penetration due to barrier effects of mountains. Specifically, an all-day UMI effect was found in coastal low-density low-rise areas (LCZ5&8&10), while a nocturnal UMI effect and a daytime urban dry island (UDI) effect were found in coastal high-density high-rise areas (LCZ1&2). The UDI effect at daytime can be attributed to strong vertical moisture convection associated with intensive surface sensible heat fluxes in a strongly mixed urban boundary layer (UBL). The UMI effect at night can be attributed to blocked ventilation aisle, inhibited dewfall due to UHI, and weakened upward motion in a stable UBL. On the other hand, UMI can increase regional heat risks with additional 37.5% neighbourhoods in Extreme caution level and additional 6.1% neighbourhoods in Danger level. In addition, the impact of UMI on human thermal stress was found to be dominant at daytime in coastal low-density low-rise areas (LCZ5&8&10) and at nighttime in coastal high-density high-rise areas (LCZ1&2).