Characterizing urban heat island and its effects in Hong Kong

Abstract

Urban Heat Island (UHI) becomes a big concern, especially in a subtropical region. UHI not only changes the environment but also affect human life. More energy consumption, high disease transmission rate, and poor air quality are the possible consequences of heat island. In this study, four approaches were employed to 1) investigate the existence of heat island effect, 2) explore a linkage between atmospheric and surface heat island studies and 3) examine the impact on heat budget due to a land cover change. The four adopted approaches were mobile transverse measurements, a remote sensing technique, fixed station method and a 1-dimensional heat budget measurement. First, a series of 20 mobile transverse measurements were conducted in 2003 - 2005. Two vehicles with temperature sensors installed were employed to be driven mainly in urban areas in order to quantify heat island intensity. Second, four satellite images were captured in a night of winter, a day of winter, a night of summer and a day of summer in year 2007 and 2008. A ground mobile in-situ temperature measurement team was simultaneously conducted when the satellite passed over Hong Kong in order to explore the temperature correlation between atmospheric and surface heat island intensities.Third, a long-term UHI intensity between two fixed stations was analyzed for 19 years (1989 - 2007). The characteristic of UHI in a subtropical city was generalized based on the data from two typical meteorological stations. Fourth, a 1-year intensive heat budget measurement was conducted at Ta Kwu Ling in 2007/2008. A comparison of heat flux of net radiation, soil heat, sensible heat and latent heat was made over grass and concrete surfaces. There are three key contributions: (1) the characteristic of heat island in Hong Kong, (2) a useful tool for converting satellite-derived temperature to ambient temperatures and (3) impacts on heat budget, emissivity and albedo when a land cover changes from grass to concrete surfaces. Regarding the characteristic of heat island, both diurnal and seasonal variations of average heat island intensity were demonstrated. The average of daily maximum heat island intensity was around 2.7°C. One hourly maximum heat island intensity of 11.5°C occurred in the early morning of winter which agreed with the surface temperature measurement results. This finding suggests that topography and urban design can contribute to heat island effect in a subtropical city. A useful tool for converting satellite-derived temperature to ambient temperatures in a winter night was introduced. A good correlation between nighttime surface temperatures and nighttime ambient temperatures was found. Two empirical equations in urban and suburban areas were generated and these equations were recommended to apply in surface heat island studies in future case studies on winter night. The last key findings is that a land cover changing from grass surface to concrete surface can cause an significant increase in sensible heat on clear sky days, a reduction in emissivity value by at least 6.4%, a decrease in albedo by around 6% during hot seasons and an increase in albedo by about 27% during cold seasons. Overall, the findings of this study were able to examine the characteristic of heat island intensity in Hong Kong and justify the impact of a land surface change on heat budget, albedo and emissivity. Further research studies are recommended to fill the gaps of heat island studies.