Summer air temperature distribution and thermal environment in urban areas of Japan

Transcription

1 ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 9: (2008) Published online 16 June 2008 in Wiley InterScience ( Summer air temperature distribution and thermal environment in urban areas of Japan Masahide Aikawa,* Takatoshi Hiraki and Jiro Eiho Hyogo Prefectural Institute of Public Health and Environmental Sciences, Kobe, Hyogo , Japan *Correspondence to: Masahide Aikawa, Yukihira-cho, Suma-ku, Kobe, Hyogo , Japan. Masahide hyogo.lg.jp Received: 24 January 2008 Revised: 17 March 2008 Accepted: 12 May 2008 Abstract The air temperature distribution was measured, and the thermal environment was studied in August (summer season) in an urban area of approximately 10 km 15 km in Hyogo Prefecture, Japan. We introduced a new index to evaluate the thermal condition. Combining the newly introduced index with other indices enabled us to show the thermal conditions in more detail. The inland thermal conditions were milder than those in the coastal areas when the air temperature was lower than 30 C. In contrast, the inland thermal conditions were more severe than those in the coastal areas when the air temperature was higher than 30 C. Copyright 2008 Royal Meteorological Society Keywords: air temperature; thermal condition; urban heat island 1. Introduction The thermal environment in urban areas has become severe for humans due to the urban heat island phenomenon. This phenomenon has been studied worldwide with the objective of limiting thermal pollution in urban areas, (e.g. Oke, 1973; Oke and Maxwell, 1975; Gotoh, 1993; Saitoh et al., 1996; Yamashita, 1996; Oke et al., 1999). The Hyogo Prefectural Government of Japan settled on an action plan for the heat island phenomenon and measured the air temperature to verify the effects of the action plan. The action plan aims to mitigate the phenomenon by introducing measures to decrease artificial waste heat, improve the urban earth s surface, reform the urban form, and bring about a change of lifestyle. Aikawa et al. (2006) studied the accumulated air temperature data in a 15 km 15 km urban area of Hyogo Prefecture and clearly defined the growing heat island phenomenon in the area. The data set used by Aikawa et al. (2006) was obtained from the six stations used to monitor air pollution in the relevant area. In addition, Aikawa et al. (2007) studied the spatial distribution of the air temperature based on air temperature data acquired from 24 sites in the relevant area. The air temperature data measured by the Hyogo Prefectural Government in 2005 were analyzed in detail, and the characteristics of the thermal environment were studied. The findings are reported below. 2. Experimental 2.1. Air temperature measurement The air temperature was measured at an elementary and a junior high school located within the 10 km 15 km area in Hyogo Prefecture. The area lies between Osaka City (population density: people/ 222 km 2 ) and Kobe City (population density: people/551 km 2 ). The southern part of the area is characterized by intensive industrial development and dense populations, which categorize it as an urban area. In contrast, urbanization has been progressing rapidly in the northern part of the area, which is developing into satellite cities of Osaka and Kobe (Aikawa et al., in press b). The location of the site is shown in Figure 1. The air temperature was measured by a thermometer calibrated with a thermostat bath at two temperatures 5 and 35 C. The thermometer was installed in a naturally ventilated thermometer shelter (about 1.5 m above the ground). The air temperature data were stored in a data logger. The air temperature was measured at the survey site every 15 min; hourly data, measured on the hour were used for the evaluation. The air temperature measured in August (summer) 2005 has been analyzed in the study Survey site characteristics The current survey area can be classified into three categories: (1) the highly urbanized area along the coast, (2) the suburban area, primarily in the southern part of the study area, and (3) the residential area being developed as satellite cities, primarily in the northern part of the study area. This categorization is reported in detail in another article (Aikawa et al., 2007) Geographic information system A geographic information system (ArcView) was used for the spatial analysis of the air temperature. An inverse distance-weighted method was employed to draw the distribution of the parameters, such as the air temperature. Copyright 2008 Royal Meteorological Society

2 210 M. Aikawa, T. Hiraki and J. Eiho Hyogo Prefecture JAPAN Site B 200 m a.s.l. 100 m a.s.l. Site A Figure 1. Location map of the research site. Sites A and B are discussed in Section Results and discussion 3.1. Distribution of mean air temperature Figure 2 shows the distribution of the monthly mean air temperature in August The air temperature in Figure 2 was corrected by the altitude of the sites and a temperature-lapse rate of 0.6 C/100 m. High air temperature was observed 5 10 km inland from the coast. The distributions of the monthly mean maximum and minimum air temperatures were reported in detail in an earlier article (Aikawa et al., 2007) Distribution of the index for an urban heat island Miyazaki et al. (2006) introduced an index for an urban heat island that shows the total number of hours during which the air temperature was higher than a threshold value. Miyazaki et al. (2006) adopted the threshold values of 25 and 30 C. Figure 3(a) and (b) shows the distribution of the index for an urban heat island introduced by Miyazaki et al. (2006) for 25 and 30 C, respectively, where air temperature data that are not corrected by the altitude of the sites are used. Figure 3(a) shows that the index near the coastal line was larger than that inland when the threshold value was 25 C, showing that the condition near the coastal area was more severe than that in the inland area under the condition with a threshold value of 25 C. In contrast, Figure 3(b) shows that the index in the eastern and 5 10 km inland area was larger than that in other Figure 2. Distribution of the monthly mean air temperature ( C) in August areas and the index in the northern area (20 km inland) was the smallest when the threshold value was 30 C, showing that the condition in the eastern and 5 10 km inland area was the most severe; in addition, that in the

3 Summer air temperature distribution in urban Japan 211 northern part (20 km inland) was milder than that in the southern part of the study area under the condition, with a threshold value of 30 C. Miyazaki et al. (2006) reported that hot air masses moved in the daytime and identified a statistically significant correlation between the movement of a hot air mass and wind direction in Kobe City located to the west of our study area. The movement of the hot air mass found by Miyazaki et al. (2006) was explained by winds from the west and west-southwest. The index in the eastern area was larger than that in the western area in the case of the 30 C threshold value (Figure 3(b)), showing that our results corresponded to the findings by Miyazaki et al. (2006); Aikawa et al. (in press a), in fact, showed the predominance of the southwest wind in our study area, demonstrating that the hot air mass would be transported from the western to the eastern area in the study area Distribution of index for thermal condition The index introduced by Miyazaki et al. (2006) should be one of the indices of a thermal condition. Here, we would like to introduce a new index to evaluate a thermal condition. The index is defined as the sum of the difference between the observed air temperature and a threshold value when the observed air temperature is higher than the threshold value. The index is expressed as follows: Index = (Ti T ) when Ti is higher than T, where Ti and T are the hourly measured air temperature and a threshold value, respectively. Figure 4(a) and (b) shows the distribution of the newly introduced indices for the threshold values of 25 and 30 C, respectively, where air temperature data, which are not corrected by the altitude of the sites are used. The distribution of the newly introduced index with a threshold value of 25 C (Figure 4(a)) was similar to the result shown above (Figure 3(a)). In contrast, the distribution of this index with a threshold value of 30 C (Figure 4(b)) showed a different pattern from the result shown in Section 3.2 (Figure 3(b)). In Figure 4(b), the eastern and 5 10 km inland areas showed the largest index value, which is the same as the result shown in Figure 3(b). In contrast, the newly introduced index in the northern area (20 km inland) was not the smallest but, rather, was larger than or comparable to that in the southern area, while the index in the northern area was the smallest in the above distribution (Figure 3(b)). In other words, the area with the smaller newly introduced index was widely distributed in the south, showing that excessively high air temperature occurred less in the south than in the north, although the total number of hours during which the air temperature was higher than 30 C was larger in the south than that in the north Comparison of the southern and northern areas Figure 3. Distribution (in h) of the index for urban heat island introduced by Miyazaki et al. (2006) for 25 C (a) and 30 C(b) Observed air temperature The thermal conditions of the southern and northern areas are compared and discussed. We selected sites A and B to represent the southern and the northern areas, respectively (Figure 1). Figure 5 shows the correlation of the air temperatures observed at sites A and B. The air temperature at site A was higher than that at site B when the air temperature was lower than 30 C. On the other hand, the air temperature at site A was lower than that at site B when the air temperature was higher than 30 C, showing that the thermal condition at site

4 212 M. Aikawa, T. Hiraki and J. Eiho Figure 5. Correlation of the air temperatures observed at sites A and B. Sites A and B are shown in Figure 1. of conditions with excessively high air temperatures, in which the vertical axis shows the difference in the air temperature between the observed air temperature and 30 C. The difference in the air temperature when the observed air temperature was lower than 30 C is plotted as zero. The air temperature at site A was lower than that at site B when the air temperature was higher than 30 C, showing that the thermal condition was more severe at site B than at site A when the air temperature was higher than 30 C. Figure 4. Distribution ( C) of the newly introduced indices for the threshold value of 25 C (a) and 30 C (b) Newly introduced index = (Ti T) when Ti is higher than T, whereti and T are the hourly measured air temperature and threshold value, respectively. B was more severe than that at site A when the air temperature was higher than 30 C, while the thermal condition at site B was milder than that at site A when the air temperature was lower than 30 C Time series of conditions with excessively high air temperatures Excessively high air temperatures were defined as those higher than 30 C. Figure 6 shows the time series 3.5. Meaning of this research and directionality of measures against heat island phenomenon Excessively severe thermal conditions frequently appeared in the inland area in the given study. In addition, the excessively severe thermal conditions appeared in the daytime. On the other hand, Habara et al. (2005) clarified that the anthropogenic waste heat had a greater influence on the air temperature at nighttime than during daytime. Taking into account our observation as well as the study by Habara et al. (2005), the severe thermal condition observed in the current study is due to the strong solar radiation, indicating that it is difficult to make effective plans to counter the severe thermal condition in the inland area. Finally, it is more effective to counter the heat island phenomenon for the 5 10 km inland area than for the inland area, when we consider countermeasures for the anthropogenic waste heat. 4. Conclusions The air temperature distribution and the thermal condition were studied in an urban area of Japan. A new index was introduced to evaluate the thermal condition. By combining the newly introduced index with

5 Summer air temperature distribution in urban Japan 213 Figure 6. Time series of the excessively high air temperature condition at sites A and B. The vertical axis shows the difference of the air temperature between the observed air temperature and 30 C. the other index, we evaluated the thermal conditions in greater detail. We showed the thermal conditions from the viewpoint of the number of hours when the air temperature was higher than the threshold value and the conditions with excessively high air temperatures. The characteristics of the thermal conditions in the study area are summarized as follows: 1. The number of hours during which the air temperature was higher than the threshold value (30 C) in the inland area (the northern area) was smaller than that in the coastal area (the southern area). 2. The air temperature increased considerably in the inland area when compared with that in the coastal area once the air temperature became high, showing that conditions with excessively high air temperatures in the inland area were more severe than those in the coastal area. Acknowledgements The authors are grateful to Prof. Dr Masakazu Moriyama of Kobe University for his support with the calibration of the thermometer. References Aikawa M, Hiraki T, Eiho J, Miyazaki H Characteristic air temperature distributions observed in summer and winter in urban area in Japan. Environmental Monitoring and Assessment 131: Aikawa M, Hiraki T, Eiho J. 2008b. Grouping and representativeness of monitoring stations based on wind speed and wind direction data in urban areas of Japan. Environmental Monitoring and Assessment 136: Aikawa M, Hiraki T, Eiho J, Miyazaki H. 2008a. Air temperature variation with time and thermally evaluated atmospheric conditions correlated with land use change in urban areas of Japan. International Journal of Climatology 28: Aikawa M, Hiraki T, Sumitomo S, Eiho J Distribution and variation of the air temperature from 1990 through 2003 in urban areas of hyogo prefecture from the aspect of heat Island Phenomenon. Bulletin of the Hyogo Prefectural Institute of Public Health and Environmental Sciences 2: 1 9. Gotoh T Relation between heat islands and NO 2 pollution in some Japanese cities. Atmospheric Environment 27B: Habara K, Narumi D, Shimoda Y, Kondo A, Mizuno M Effect of anthropogenic waste heat upon urban heat island phenomenon. In Papers of Environmental Engineering Symposium of the Japan Society of Mechanical Engineers, Muroran, Japan, Vol. 15; , (in Japanese). Miyazaki H, Niimoto M, Kyakuno T, Tahara N Urban heat island investigation in Kobe using natural ventilation screen shelter - the formation of mass of hot air in summer and the effect of current of air. Humans and Nature 16: 21 33, (in Japanese with English abstract). Oke TR City size and the urban heat island. Atmospheric Environment 7: Oke TR, Maxwell GB Urban heat island dynamics in Montreal and Vancouver. Atmospheric Environment 9: Oke TR, Spronken-Smith RA, Jauregui E, Grimmond CSB The energy balance of central Mexico City during the dry season. Atmospheric Environment 33: Saitoh TS, Shimada T, Hoshi H Modeling and simulation of the Tokyo urban heat Island. Atmospheric Environment 30: Yamashita S Detailed structure of heat island phenomena from moving observations from electric tram-cars in metropolitan Tokyo. Atmospheric Environment 30: