Changes in the daily cycle of urban air temperature

Abstract

The century-long search for the precise mechanisms responsible for urban heat islands continues, while urban warming worsens in many megacities. Most studies have focused on mean temperature, daily and annual temperature ranges and urban heat island intensity. We hypothesize that an analysis of the changes in the characteristics of the whole daily and annual temperature cycles, including not only the mean temperature and temperature ranges (amplitudes), but also the maximum, minimum temperatures and the phases, can provide more information concerning the urban warming phenomena. Through a detailed analysis of long-term observations in Hong Kong, we found that the difference in the daily cycle between urban and rural stations is very distinct while the annual cycle is much more similar, suggesting that the urban environment has a greater effect on the daily cycle than on the annual cycle. The daily phase has shifted a total of 1.77 hours later over the last 130 years (1.36 hours per century) in the urban area of Hong Kong as represented by the Hong Kong Observatory (HKO) data. The annual phase change in HKO reflects the phenomenon that globally observed annual phase advances or seasons onset earlier. Similar results are revealed by studying 670 long-term stations worldwide. The identified large city stations show an average daily phase delay of 0.12 hours, which is 3 times larger than that observed in the rural stations. Such a daily phase delay phenomenon can be explained by the increase in effective daily thermal storage in cities due to human-made structures; the change in annual thermal storage is much smaller. To isolate the impact of thermal storage from human-made structures, we conducted a field measurement in the stone forest, Kunming, which is considered to be a small-scale urban model but without air pollution and anthropogenic heat. Our measured data in the stone forest, exhibited similar characteristics of the annual and daily temperature cycles as the monitored data in Hong Kong, suggesting that the stone forest is a reasonable small-scale field model of the urban thermal environment. And the results indicate that with an increase of compactness, the daily phase delays and daily amplitudes reduces. The observed daytime cool island in different stone forests also reveals the possible mechanisms for the controversial daytime cool island and nighttime heat island phenomena. A conceptual energy balance model is improved and used to investigate the connections between thermal storage with mean temperature, amplitude and phase. Such derived analytical solution for urban air temperature cycles shows that the governing factors for mean temperature, amplitude and phase are different. The amplitude and phase are mainly determined by the time constant which quantifies effective thermal storage. And the time constant does not affect the mean temperature. This is confirmed by the observation from Hong Kong, global stations as well as the stone forest. Such findings not only allow us to draw a new explanation for the notable diurnal asymmetry warming, but also provide new insights into ways of designing thermal climate in a city like a building.