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Journal of Heat Island Institute International Vol.7-2 (2012)
Academic Article
Evaluation of Heat Island Intensity in a Coastal Urban Area
Hiroshi. Miyazaki
Faculty of Environmental and Urban Engineering, Kansai University, Suita, Osaka, Japan Email: [email protected]
ABSTRACT
This study describes the characteristics of a heat island in coastal Japanese cities, including monthly mean air temperature in August (2000–2004), sea water temperature in summer (2000–2004), and population size (2000). A correlation analysis of the air temperature and sea water temperature of several small cities yielded a regression equation that was used to calculate the impact of urbanization on the studied areas. Findings of the influence of sea water temperature on air temperature led to the conclusion that a different method is needed that allows these coastal cities to get appropriate and reasonable environmental information.
Introduction
This study describes the characteristics of the urban heat island (UHI) in coastal cities. The degree of the UHI effect is expressed as UHI intensity (i.e., the difference in air temperature between urban and rural areas). Oke (1973) has shown that this index is related to city size. Fukuoka (1983) and Park (1987) demonstrated that population correlates with UHI intensity. On the other hand, in coastal areas, seawater temperature has an influence on land air temperature. To more accurately evaluate the impact of urbanization on environment, and to better calculate UHI intensity in coastal cities, we should take into account the influence of seawater temperature. In this article, a new vision for UHI intensity in coastal cities is described.
Methods
The area studied was the Seto Inland Sea (area:
881.5km2, average depth: 37.3 meters) and its surrounding 41 cities (Table 1, Figure 1). A comparison between air temperature and seawater temperature was made using the following two databases. To further consider the impact of population, national census data for 2000 were used.
Air temperature
The cities studied were located around the Seto Inland Sea, where 41 meteorological telemeters (Automated Meteorological Data Acquisition System; AMeDAS) measure air temperature. Air temperature measurements made around the Seto Inland Sea were monthly mean average, monthly mean maximum, and monthly mean minimum, in August (2000–2004 average).
Seawater temperature
Eight garbage ships belonging to the Regional Development Bureau in the Ministry of Land, Infrastructure, Transport, and Tourism have been measuring the sea water quality quarterly every year since 1982. In the summer, they measure in July and August. The locations for these
measurements are fixed for every investigation. The number of measurement points increased from 179 (2000) to 226 (2003).
Seawater temperature was observed at 2 meters below sea level.
Results
Seawater temperature in summer
Figure 2 shows the distribution of upper seawater temperatures in summer, on average, in 2000–2004.
The Seto Inland Sea is divided into 11 parts (Figure 1). Table 2 shows average, maximum, and minimum seawater temperatures in summer for 2000 through 2004 in each part of the Seto Inland Sea.
The average seawater temperature was 26.7° C during this period. The highest temperature was 30.0°
C ,recorded in Osaka Bay ("A" in Figure 1), while the lowest was 22.1° C ,observed in the northern part of Iyo Nada ("J" in fig. 1). The highest average seawater temperature was observed in Hiuchi Nada ("F" in Figure 1).
Air temperature in the coastal area
The observed monthly mean air temperatures in August during 2000 through 2004 are shown in Table 3. The seawater temperatures observed in the area surrounding the 41 meteorological observatories are also shown in Table 3. With these data, a scatter plot was made showing air temperatures and seawater temperatures. (Figure 3 a–c) In these figures, the sizes of plotted circles represent the population of each telemeter. A regression line was added to each scatter plot using the method of least squares, equation, and regression coefficient. It must be noted that this population did not always reflect the surrounding meteorological conditions measured telemetrically.
Figure 3 thus shows monthly mean average, high and low air temperatures in August (2000–2004), and mean
- 256 - seawater temperatures in summer (2 meters below sea level, average 2000–2004).
Discussion
In general, determinants of air temperature include both geographical factors (e.g., land coverage, land use, and topography) and climatological conditions. In the case of coastal land, the influence of the ocean is included as one of the determining factors.
In this analysis, the correlation coefficient increased to 0.36. This result confirmed the results of prior studies (e.g., Oke 1973). Furthermore, in a coastal region, the influence of seawater temperature is an added determinant of air temperature. The result was a set of data points for small cities that had a linear function by the method of least squares. In this analysis, the regression line was assumed to reflect the relationship between air temperature and seawater temperature in cases in which urbanization had no impact. Air temperature was estimated with this relationship using seawater temperature for 41 subject points. Figure 3 shows the differential in degrees Celsius between estimated air temperature and observed monthly mean average value. Here, the x-axis shows the population in a logarithmic scale.
This scatter plot shows a 0.39 correlation coefficient.
This result indicates a small correlation. It should be noted, however, that this simple analysis used autonomous population data, which may not reflect surrounding urban conditions measured telemetrically, and did not use topographical information.
In conclusion, studies of UHIs in coastal regions should include the influence of seawater temperature. Seawater temperature is a basic thermal, or climatological, condition for each coastal city. To calculate UHI intensity, the selection of rural or urban area is important, but sometimes this may be difficult (e.g. Osaka or Kobe). The method of a relation equation between air temperature and seawater temperature may be used as a substitute for UHI intensity.
References
1. Fukuoka, Y., 1983: Physical climatological discussion on causal factors of urban temperature, Memoirs of the Faculty of Integrated Art and Science, (8), Hiroshima Univ., 157-178.
2. Fukuoka, Y. 1995. Air, Water & Soil in City (or City Environment), Asakura-shoten (Japanese)
3. Oke, T. 1973.City Size And The Heat Island. J.
Human-Environment System. 1(7):769-779.
4. Park, H., 1987: Variations in the urban heat island intensity affected by geographical environments, Environ. Res. Center Papers, 11, Tsukuba Univ., 1-79
5. Miyazaki, H., 2008: Research on relation between seawater temperature and air temperature in the coastal area around the Seto Inland Sea, Summaries of Technical Papers of Annual Meeting Architectural Institute of Japan, D-1, 863-864
(Received Feb 9, 2012, Accepted Oct 10, 2012)
Table 1. Characteristics of studied cities in the coastal area
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Table 2. Seawater temperature data for the Seto Inland Sea area
Table 3. Monthly mean air temperatures and seawater temperatures in the studied area, summers 2000–2004
- 260 - Figure 1. Area studied
Figure 2. Distribution of seawater temperature
Figure 3 a. Relationship between seawater temperatures and monthly mean air temperatures, August, 2000–2004
Figure 3 b. Relationship between seawater temperatures and monthly mean maximum air temperatures, August, 2000-2004
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Figure 3 c. Relation between seawater temperatures and monthly mean minimum air temperatures, August, 2000–2004
Figure 4. Scatter plot of differential between estimated air temperature and observed air temperature
