Observational Analysis of Urban Heat Island Effect of Haikou in 2022

Using 315 days of hourly observation data from 19 automatic weather stations in 2022, the stations are classified into three categories, namely urban (6 stations), rural (8 stations), and coastal stations (5 stations), to analyze the diurnal variation characteristics of urban heat island (UHI) and the effects of wind speed, precipitation and humidity on UHI in Haikou City, northern Hainan Island. The results show that: (1) The annual average temperature of the three station categories shows a pattern of rural station > urban station > coastal station from morning to afternoon, reversing to coastal station > urban station > rural station from night to early morning. The UHI presents a "V"-shaped diurnal pattern with the two turning points at 09:00—10:00 and 17:00. A weak "urban cool island (UCI)" (0.6 °C) phenomena is observed during the day, while a pronounced UHI (1.2 °C) phenomena occurs at night. The appearance of UCI is closely related to the effect of sea breeze. (2) The UCI intensity varies seasonally between spring (?1.1 °C), winter (?0.6 °C), summer (?0.4 °C), and autumn (?0.2 °C). In contrast, the strongest UHI intensity occurs in autumn (1.6 °C), followed by summer (1.4 °C), and weak in winter (1.1 °C) and spring (1.0 °C). Additionally, the onset of UHI in the late afternoon during summer and autumn occurs 2 h earlier than in winter and spring. (3) The UHI (or UCI) becomes more pronounced under Level 1 wind and no-rain conditions, with maximum intensities reaching 1.5 °C (?1.0 °C) and 1.9 °C (?0.9 °C), respectively. Under wind speeds of Level 2—4 and Level 5—7, the intensities decrease to around 1.2 °C and 0.9 °C, respectively, while the maximum UHI intensity stabilizes around 1.2 °C under other precipitation conditions. Notably, the duration of the UHI increases significantly as the increase of wind speed and precipitation. (4) The UHI and UCI are most pronounced under comfortably dry conditions, with intensities reaching approximately 2.5 °C and 0.9 °C, respectively. The UHI intensity decreases by about 0.3℃with each increase in humidity level. After reaching the humid level, the UCI intensity also decline with further increases in humidity. (5) During the daytime, the maximum amplitude of temperature difference between urban and coastal stations is generally slightly larger than that of the UCI, and the increase in this temperature difference may provide some indication of sea breeze precipitation. (6) A more detailed analysis of wind speed, precipitation, and humidity conditions, categorized into high and low thresholds, shows that UHI intensity is most pronounced when all conditions are at low thresholds (i.e., low wind speed, no rain, and low humidity). As more conditions shift to higher thresholds, the UHI intensity gradually decreases. (7) Compared to non-sea breeze days, UHI and UCI characteristics are more pronounced on sea breeze days, with UHI occurring later. (8) The spatial distribution characteristics indicate that high UHI intensity areas are mainly concentrated along the coastal regions during the night. It is also noted that the temperature at the northwest coastal stations is similar to that of coastal urban stations. In the summer and autumn seasons, the UCI intensity differences among urban stations are relatively small.