![]() One representative type of extreme heat events was considered in this study: three consecutive hot nights with two very hot days in between (2D3N). Twenty-two weather stations in Hong Kong were classified into four categories, namely urban, urban oasis, suburban, and rural, with reference to the local climate zone (LCZ) scheme, to analyze UHI and UCI phenomena during extreme heat and non-extreme heat situations. In this study, warm periods (May to September) of 2000–2018 were examined to acquire a comprehensive understanding of the UHI and UCI characteristics for the case study of Hong Kong, China. An increasing trend of extreme heat events has been observed over the last few decades and is expected to continue in the foreseeable future. Urban heat island (UHI) and cool island (UCI) effects are well-known and prevalent in cities worldwide. Both the cloud-top radiative cooling effect and the enhanced radiative cooling over inland cloud-free areas also play roles in the development of the morning rainfall peak over Hong Kong. Linked to the inverse relationship between monsoon intensity and the land-sea thermal contrast, the diurnal cycle of precipitation is strengthened when the SCS monsoon is active and weakened when the land-sea thermal contrast is high. The overlying weak divergence could be ascribed to the nocturnal-early morning acceleration of southerly flow over southeast China. The convergence may be attributed to the prevalence of the southerly monsoonal flow over the South China Sea (SCS) and to the northerly land breeze induced by the land-sea thermal contrast in the morning. The morning peak of precipitation over Hong Kong is coincident with deep rising motion, linking to near-surface convergence and overlying weak divergence. The diurnal cycle exhibits a notable seasonal dependence, with the strongest signal in the summer. ![]() Precipitation unrelated to tropical cyclones (TCs) dominates the diurnal variation of precipitation, especially in the summer. The precipitation in Hong Kong is characterized by a spatially uniform diurnal cycle with the peak at about 0800 local time (LT), with rather weak dependence on local terrain. Hourly rainfall from automatic weather stations and reanalysis data from MERRA-2 are used to investigate the diurnal variation of precipitation in Hong Kong, a site along the southeast China coast with strong interactions between the monsoonal circulation and the land-sea breeze. (5) Currently, the number of hot days and extremely hot days in the two cities has increased significantly compared with that a century ago, indicating that climate warming has an adverse impact on human settlements. This result may have led to the declines in the maximum temperature experienced by both cities. (4) At a certain stage of urban development, owing to the shading effect of new high-rise buildings, the solar shortwave radiation reaching the Earth’s surface decreased, and anthropogenic heat generated by the energy consumption of buildings and urban human activities at that time was not sufficient to make up for the reduced shortwave radiation. (3) The diurnal temperature ranges (DTRs) of the two cities decreased a certain phase of the decreases in DTR in the two cities was caused by decreases in the maximum temperature. ![]() The fluctuation ranges of minimum temperature were smaller, whereas the linear increases were higher than those for the mean temperature. Moreover, in both cities, the annual mean maximum temperature decreased during a phase of more than a decade. (2) The fluctuation ranges of maximum temperature in the two cities were considerably higher than those of mean temperature. The statistical results reveal the following: (1) The average temperatures of the two cities underwent fluctuating increases during the past 120 years, with linear warming rates of 0.23 ☌/decade in Shanghai and 0.13 ☌/decade in Hong Kong. Using surface air temperature observations from 1901 to 2020, this study compared the warming trends of Shanghai and Hong Kong over a period of 120 years.
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