Temporal evolution of sub-daily precipitation extremes in Hong Kong: Dependency on temperature and implications to flash floods

Temporal resolution has strong effects on characterization of precipitation extremes. Under the warming climate, intensification of daily precipitation extremes is generally expected to follow the change of water-holding capacity of the air as temperature increases at a rate of ~6.5% o C-1 (i.e. Clausius-Clapeyron [CC] scaling), while the intensity of hourly extremes is estimated to increase even faster (i.e. super-CC scaling). However, even at the hourly scale, heavy rainstorms shorter than one hour may not be accurately resolved. The characteristics of the temporal evolutions (long-term trends and changes) of precipitation extremes at fine temporal resolutions (e.g. from minutes to hours) and flash floods under global warming are crucial parameters for flood protection infrastructure design and urban planning (e.g. reservoirs and highway), especially for hilly and densely populated territories such as Hong Kong. Our current understanding of the responses of sub-daily extremes (not just intensity, but also duration and total amount) and flash floods to the rising temperature is still limited partly because of the lack of high-quality sub-daily instrumental records with sufficient length and spatial coverage. Furthermore, highly heterogeneous terrain and complex surface hydrological processes also contribute to uncertainties associated with the occurrence and process of flash floods, but past studies mostly related the intensification of flash floods only to the increase in the intensity of sub-daily extremes without consideration of the effects of surface hydrological characteristics and dynamics of river basins.

Hong Kong is the fourth-most densely populated city in the world, with developed areas largely located near steep mountains and rugged terrains. The hilly topography together with the humid subtropical monsoon climate make this international financial center highly vulnerable to flash floods. In our study, we will investigate the temporal evolutions and dependencies of the intensity, duration, and total amount of sub-daily extreme precipitation events using the hourly records for more than 130 years and 5-min observations over 30 years in Hong Kong. The state-of-the-art fully coupled hydrological-atmospheric modeling system WRF/WRF-Hydro will be used to simulate the impacts of changes in sub-daily extremes on flash floods with consideration of surface hydrological processes. This study will substantially contribute to our scientific understanding of changes in sub-daily precipitation extremes and the potential impacts on flash floods under the warming climate. The study results will also carry important practical significance for Hong Kong and other cities to enhance their climate readiness, particularly in terms of flood prevention and management.