This study investigated the impacts of climate change on the urban heat island (UHI) and the number of very hot (maximum temperature >35°C) and very cold days (minimum temperature <5°C) in the central business district (CBD) of Melbourne city in Australia. A station located in Laverton (less urbanised area), which is 17 km southwest of Melbourne CBD, was selected as the reference station for the computation of UHI intensity in Melbourne CBD. Using daily minimum/maximum temperatures at the two stations, nocturnal/diurnal UHI intensities in Melbourne CBD were computed for the period 1952–2010. It was found that in Melbourne CBD, nocturnal UHI intensities show a clear rising trend over the period 1952–2010 unlike the diurnal UHI intensities. For the analysis of nocturnal UHI intensities in Melbourne CBD, under changing climate, for each calendar month statistical models based on the gene expression programming (GEP) technique were developed for downscaling general-circulation model (GCM) outputs to monthly average minimum temperature at Melbourne CBD and Laverton. Using the outputs of HadCM3, GFDL2.0 and ECHAM5 pertaining to the A2 greenhouse gas emission scenario on the downscaling models, projections of monthly average minimum temperature were produced for the two stations over the period 2000–2099. In each season, at both stations, the ensemble average of monthly minimum temperature gradually increased over the period 2000–2099. The ensemble-average UHI intensity in Melbourne CBD projected into the future was higher for all seasons in comparison to that of period 1952–1971. Downscaling models based on the GEP technique were developed for each calendar month for projecting the number of very hot days in November–March and very cold days in May–September in Melbourne CBD. It was found that, in the future, summer weather will spread to early autumn, and winter weather will move to early spring, in Melbourne CBD.
Figures 1 and 2 display the monthly climatology of hourly UHI intensity from 1990–2015 at HKO and LFS, respectively. The grey background in the clocks represent the nighttime and the white background for the daytime. The daytime here is the total amount of sunshine. Information for sunrise and sunset times for each month is from the Civil Aviation Department Hong Kong (2015). At the HKO station, which is the main station in the HK area, the UHI clocks show clear positive UHI intensity in the nighttime. For example, in January, positive UHI intensity occurred from 5 pm–11 am the next day, and in July, it occurred from 6 pm–8 am the next day. The UHI intensity maximum value in one day usually appeared during the nighttime, especially before sunrise (in May–September from 3 am–6 am, in other months from 5 am–8 am). The UHI intensity therefore shows apparent seasonal variability. UHI intensity is higher during the late autumn and winter seasons (i.e. November–January) and relatively lower during the late spring and summer seasons (i.e. March–August). For instance, in December, UHI intensity is greater than 3 °C for 9 hours per day. However, in May, the maximum UHI intensity was only approximately 1.5 °C. Our results corroborate previous studies from HK, which found higher UHI intensity in winter. Leung et al (2004) mentioned that during 1947–2002, the UHI intensity of annual mean temperature in winter was 0.21 °C and in summer was 0.12 °C. Chan and Ng (1991) also pointed out that for minimum temperature, UHI intensity was 3.2 °C in winter and 1.8 °C in summer. The seasonal variations in UHI over Hong Kong are related to the seasonal climate characteristics of the region (Memon et al2009, Wong et al2011). During the summer monsoon, humid air arrives in the HK region. However, during the winter, the climate is relatively dry. This could explain the higher UHI intensity observed in the winter.
The diurnal and seasonal variations in positive UHI intensity at the LFS station are generally consistent with those at the HKO station. Positive UHI always occurred in the nighttime. Relatively high UHI intensity was observed in the colder months. Maximum UHI intensity was approximately 0.7 °C in July but was twice as large in December at approximately 1.5 °C. However, in general, the magnitude of UHI intensity of LFS is less than that of HKO. For instance, maximum UHI intensity at LFS reached only about 1.5 °C, which occurred in December from 7–8 am. The maximum UHI intensity values from February–September at LFS were almost all less than 1 °C. These values were less than half the UHI intensity at HKO during the same time period.
The daytime cooling (negative UHI intensity) in the daytime occurs both in the heavily developed station HKO and developing station LFS. The negative UHI in urban areas during daytime could be explained by surface energy balance, sea breezes, and/or humidity. Firstly, Siu (2011) found that a high density of buildings in urban areas allows more storage of the heat flux during the daytime as the construction materials normally have higher thermal admittance than rural land. Furthermore, Siu (2011) also pointed out that the lower sky view factor results in reduced long wave radiation loss. Secondly, in the case of HK, land–sea breeze can also be a factor to reduce the UHI effect. During the daytime, the sea wind brings cooler air to urban areas. Previous studies have shown how sea breeze effects can reduce temperature (Oda and Kanda 2009). Both HKO and LFS stations are located in coastal areas. Thirdly, humidity could be another significant factor, as the sea wind also brings moisture from the sea surface. Memon et al (2009) found that in the HK area, the UHI effect has a negative relationship with relative humidity. Figures 1 and 2 show that at both stations, summer daytime cooling periods are longer than in winter. In HK, the rainy and warm seasons are in the same period. In tropical and sub-tropical cities, the humidity (dry/wet) is more important than the temperature (cool/warm) for the UHI effect (Roth 2007). The cooling impact can be stronger during the wet season (summer), leading to longer cooling periods.
There are also some differences between the two stations. HKO daytime cooling is stronger than that at LFS in the spring. One of the reasons could be that in the spring, wind rarely comes from the sea. Wind patterns in HK are largely determined by the monsoonal pattern (Hong Kong Observatory 2015); the wind comes from the southwest (the same direction as the sea) in the summer, from various directions in the autumn, and from the northeast (from inland) in the winter and spring. However, because of massive urbanization in Shenzhen (one of the largest cities in China), the sea breeze is enhanced during the winter, affecting the western part of HK where LFS is located (Lu et al2010). As a result, it is only during the spring when there is little wind from the southwest (from the sea). This could be one of the reasons explaining the lower daytime cooling effect during spring (March, April, May).
We defined UHI duration as the number of hours in a day with positive UHI intensity values. Figures 1 and 2 show the durations of the UHI effect per month at two stations (i.e. the number of the red bars in a clock). At HKO, in January, the UHI duration started at 5 pm and ended at 11 am the next day and lasted approximately 18 hours. At HKO, UHI duration was longest during the winter season (e.g. 18 hours for January) and shortest during the summer season (e.g. 14 hours for July). Like the HKO station, UHI duration at LFS showed clear seasonal variability. The longest UHI durations were observed in March, April, January and December (e.g. 18–19 hours), whereas the shortest UHI duration was observed in July (e.g. 13 hours).
In the previous section, based on the climatology of UHI intensity, we obtained the climatology of UHI duration for the period 1990–2015. Here, to see the changes in UHI duration over time, we chose two time periods: 1990–1999 and 2006–2015. Figure 3 shows the UHI durations for the two time periods and the differences in UHI duration between them. At HKO, UHI duration apparently did not change for all months, suggesting stabilized diurnal variations in UHI over time. For instance, in winter, UHI duration during the earlier period (1990–1999) (e.g. 19 hours) is mostly the same as that during the later period (2006–2015). However, as opposed to the HKO station, a difference in UHI duration at the LFS station between the two time periods is noticeable. Changes in UHI duration for 6 months in a year show a statistically significant difference. For example, the UHI duration increased by more than 7 hours in February, by about 6 hours in March, and by nearly 5 hours in May and June. These results suggest that diurnal variations in UHI intensity at the LFS station have changed over time. Specifically, the duration of extra warming in urban areas has increased over time.
To understand the cause of UHI duration changes, we compared the changes in the start and end times of UHI duration of two time periods (figure 4). At HKO (figure 4(a)), both the start and end times of UHI duration difference values were near zero. However, in contrast to the HKO station, the start and end times of UHI duration at LFS changed over time. In February, the start time advanced by 2.9 hours, and the end time was delayed by 4.6 hours, thus lengthening the UHI duration by 7.5 hours. Deviations also existed; in March, for example, the duration increased about 6 hours, but on average, the start time was nearly 6 hours earlier and end time more than 2 hours later, which would represent an increase of more than 8 hours. The error line presenting the standard deviation of each value is also shown. Both an earlier start time and later end time led to the longer duration in developing urban areas. Larger changes occurred in February, March and during the summer compared with other months, which is consistent with the result shown in figure 3(b).