URBAN HEAT ISLAND IN SEOUL

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1 URBAN HEAT ISLAND IN SEOUL Jong-Jin Baik *, Yeon-Hee Kim ** *Seoul National University; ** Meteorological Research Institute/KMA, Korea Abstract The spatial and temporal structure of the urban heat island intensity of Seoul, Korea, is investigated using tempertaure data measured at 31 automatic weather stations (AWSs) in the Seoul Metropolitan area for the oneyear period of March 21 to February 22. Analysis results reveal noticeable spatial and temporal variabilities in the urban heat island of Seoul. The urban heat island intensity is pronounced in the nighttime, tends to be strong in fall, is weak in summer. An empirical orthogonal function (EOF) analysis is performed to find dominant modes of variabilities in the urban heat island of Seoul and physical interpretation to each mode is given. Key words: urban heat island, Seoul, empirical orthogonal function (EOF) 1. INTRODUCTION Korea Meteorological Administration (KMA) operates 9 automatic weather stations (AWSs) over the country for purposes of providing real-time weather information and making better weather forecasts through data assimilation. The AWS measures air temperature, wind speed and direction, and precipitation amount. Since its first installment in 19, the number of AWSs has increased and the quality of AWS data has become more reliable. The density of installed AWSs is high in the Seoul metropolitan area, providing a good opportunity for studying the spatial and temporal structure of the urban heat island of Seoul. This motivates present study. 2. DATA The data used in this study are near-surface air temperatures measured at 31 AWSs in the Seoul metropolitan area for the one-year period of March 21 to February 22. The data are saved in one-hour intervals. Figure 1 shows AWS location and topography. Twenty-four stations are located within Seoul and seven stations are located just on the outskirts of Seoul. Seoul is overall flat, especially in the east-west direction. Mt. Bukhan with a highest peak of 37 m is located in the north and Mt. Gwanak with a highest peak of 632 m is located in the south. The fraction of areas covered by mountains in Seoul is, however, small. 3. RESULTS AND DISCUSSION 3.1. Structure of urban heat island Seoul is the capital city of Korea and one of the most densely populated cities in the world. Seoul has a population of about 1 millions, about 2% of the total population in Korea. The Han river bisects the city into northern and southern parts. Seoul belongs to the temperate climate zone with distinct daily and seasonal temperature variations. In Seoul, the annual mean temperature is.2 C and the annual precipitation amount is 13 mm. In summer, weather in Seoul is hot and humid and most of rainfall is concentrated. In winter, Seoul has cold and dry weather. Figure 2 shows the distribution of air temperature averaged for the one-year period of March 21 to February 22. The spatial pattern of an idealized urban heat island is such that the temperature decreases outwards from a city center, thus exhibiting nearly concentric isotherms. The urban heat island of Seoul considerably deviates from the prototype mainly due to the topography and the location of main commercial and industrial sectors (that is, land-use type). Relatively warm region extends in the east-west direction and relatively cold region is observed near Mt. Bukhan and Mt. Gwanak. Near the borderline of Seoul, the temperature is, as expected, relatively cold, except near the southwestern and southeastern borderlines, where the sprawling expansion of urbanization is already progressed. Several warm cores are observed. Particularly pronounced are a warm core around station 1, whose surroundings are characterized by an industrial complex, and a warm core around station 2, whose surroundings are highly commercialized with high-story buildings and heavy traffic. Figure 2 indicates that the distribution of near-surface air temperature within a city is closely linked to that of landuse type therein. Figure 3 shows the distribution of air temperature averaged for each season of March 21 to February 22. The spatial pattern of the seasonal temperature distribution (Fig. 3) is in general similar to that of the yearly * Corresponding author address: Jong-Jin Baik, School of Earth and Environmental Sciences, Seoul National University, Seoul , Korea; jjbaik@snu.ac.kr

2 one (Fig. 2). The temperature distribution with multiple warm cores is present in spring, summer, and fall. In spring, the warm core area in eastern Seoul, including stations 9, 1, 1, and, is enlarged compared with other seasons. In summer, the warm cores near or south of the Han river appear to merge. In fall, multiple warm cores reappear. In winter, a warm sector is pronounced in southwestern Seoul, including station 1. As already mentioned, the region including station 1 is characterized by an industrial complex from which anthropogenic heat release can be especially large. The effect of anthropogenic heat release on air temperature seems relatively large in winter (Ichinose et al. 1999) because the thermal diffusion tends to become smaller in a more stable cold environment. It is also observed that the temperature gradient in the region including station 1 is largest in winter. Figure 3 well documents the seasonal variations of the urban heat island of Seoul. These seasonal variations are linked to the seasonal variations of weather and anthropogenic heat release and the land-use type. Figure shows the distribution of air temperature averaged for each hour of the one-year period. At 3, 6, and 9 local standard time (LST), warm sectors stand out in southwestern Seoul. As office hours start, warm sectors appear in some southern and northeastern parts of Seoul, where daytime commercial activities are high. Near business-close hours (at 1 LST), the temperature distribution is fairly homogeneous in the region near or south of the Han river except for the southern mountain area. At 21 LST, the warm sector reappears in the industrial and residential area of southwestern Seoul. The diurnal march of the urban heat island of Seoul in Fig. is closely linked to the diurnal cycle of human activities as well as meteorology characterizing daytime and nighttime Urban heat island intensity To investigate the temporal characteristics of the urban heat island intensity in Seoul (shown here) and the influences of meteorological elements on the urban heat island intensity (not shown here), Seocho (station 2) is selected as an urban site and Sanung (station 26), Nunggok (station 29), and Goyang (station 3) as suburban sites. The air temperature averaged for the one-year period of March 21 to February 22 is highest at stations 2, 16, and 1 (Fig. 2). Station 16 is very close to the Han river and station 1 is located in an industrial complex. Therefore, station 2 might be a natural choice for a representative urban site. There are seven AWSs on the outskirts of Seoul (Fig. 1). Among those, four stations (25, 27, 2, and 31) are situated in already urbanized regions. Therefore, the temperature data at the remaining three suburban stations (Sanung, Nunggok, and Goyang) are utilized to represent suburban temperature. Here, an arithmetic mean of temperatures at the three stations is taken. The difference between temperature at Seocho and the three-station averaged temperature can be roughly regarded as the maximum urban heat island intensity of Seoul, which is plotted in Fig. 5 as the time series at 3, 9, 15, and 21 LST. The average maximum urban heat island intensity over the one-year period is 2.2 C. The urban heat island intensity is stronger in the nighttime (3, 21 LST) than in the daytime (9, 15 LST), exhibiting a diurnal cycle. The average maximum urban heat island intensity is strongest at 3 LST (3. C) and weakest at 15 LST (.6 C). At 15 LST, there are a lot of days in which the urban heat island intensity is negative, that is, the suburban temperature is higher than the urban temperature. The reversed urban heat island intensity is, however, very weak. Figure 5 also indicates that the urban heat island intensity tends to be strong in fall and is weak in summer, exhibiting a seasonal cycle. These results are similar to those of Kim and Baik (22) based upon a traditional two-station anaysis. In a recent review article, Arnfield (23) summarized extensive studies of urban heat islands made in the last 2 years, which confirm empirical generalizations offered by Oke (192). These include the followings. 1) The urban heat island decreases with increasing wind speed. 2) The urban heat island decreases with increasing cloud cover. 3) The urban heat island intensity is best developed in summer or warm half of the year. ) The urban heat island intensity is greatest at night. 5) The urban heat island may disappear by day or the city may be cooler than the rural environs. All of the above generalizations are found in Seoul except for 3). In Seoul, the urban heat island is least developed in summer. In Korea, days with rain and high cloudiness is much more frequent in summer than in other seasons due to the influences of the Asian summer monsoon and locally developed cloud and precipitation systems. Also, the incoming solar radiation is strongest in summer (highest temperature in summer) and hence the energy consumption is not so much different regionally. The atmospheric stability in boundary layer is on average weaker in summer than in cold seasons, leading to a greater thermal mixing and thus less horizontal contrast in temperature. These factors help to explain why the urban heat island of Seoul is least developed in summer. Ichinose et al. (1999) estimated anthropogenic heat flux in Tokyo, Japan, whose results show its seasonal and diurnal variations. The anthropogenic heat flux in central Tokyo exceeded W m -2 in the daytime and the maximum value was 159 W m -2 in winter. These large values of anthropogenic heat flux and its seasonal and diurnal variations directly affect local air temperature and its seasonal and diurnal variations. We expect that the pattern of the seasonal and diurnal variations of anthropogenic heat flux and its magnitude in Seoul are not much different from those in Tokyo because both cities are megalopolis located in the far east Asia and the energy consumption is similar. A future work of constructing maps on the spatial and temporal distribution of anthropogenic heat flux in Seoul would be beneficial to a work of quantitatively connecting it with the spatial and temporal structure of the urban heat island of Seoul.

3 3.3. EOF analysis For an empirical orthogonal function (EOF) analysis, the domain-averaged temperature is subtracted from air temperature at each station. Figures 6 and 7 show the temporal behavior and spatial display of the first and second empirical orthogonal functions (EOFs), respectively, for air temperatures averaged for each hour of March 21 to February 22. The first EOF explains.6% of the total variance. The temporal coefficient of the first EOF (Fig. 6) is positive from ~9 LST to ~19 LST and negative from ~19 LST to ~9 LST. That is, the temporal coefficient is positive in the daytime and negative in the nighttime. The spatial structure of the first EOF (Fig. 7a) exhibits negative values in the broad area of Seoul including central, northern (near Mt. Bukhan), southwestern, and southeastern Seoul and positive values in northeastern, southern (near Mt. Gwanak), westernmost, and easternmost Seoul. Notice that the sprawling expansion of urbanization near and beyond the southwestern and southeastern borderlines is already progressed. Therefore, in the broad area of Seoul, the air temperature anomaly is positive from ~19 LST to ~9 LST and negative at other times of a day. Recalling that the urban heat island of Seoul is strong in the nighttime and is weak in the daytime, the first EOF can be called the major diurnal mode in the urban heat island of Seoul. The second EOF explains 16.% of the total variance. The temporal coefficient of the second EOF is negative from ~2 LST to ~1 LST and positive at other times of a day. A minimum temporal coefficient of.31 C is observed at 9 LST (near the beginning of office hour) and a maximum temporal coefficient of.35 C is observed at 19 and 2 LST (near the end of office hour). A main feature in the spatial structure of the second EOF (Fig. 7b) is such that the structure is positive in the eastern part of Seoul and negative in the western part. It is noticed that a positive region including stations 16, 19, and 2 is intruded into the negative western part. Therefore, in the eastern part of Seoul, the air temperature anomaly is negative from ~2 LST to ~1 LST and positive at other times. On the other hand, in the western part of Seoul, the air temperature anomaly is positive from ~2 LST to ~15 LST and negative at other times. In Seoul, commercial areas are more concentrated in the eastern part than in the western part. Anthropogenic heat release from commercial areas has a diurnal cycle, starting at ~9 LST and ending at ~19 LST. Hence, the cumulative heat is maximal at ~19 LST and its effect is most diminished at ~9 LST, as implied in Fig. 6. In this regard, the second EOF mode may be called the landuse type mode in the urban heat island of Seoul. The total variance explained by the two leading EOFs is 96.6%.. ACKNOWLEDGMENTS The first author was supported by the Climate Environment System Research Center sponsored by the SRC Program of the Korea Science and Engineering Foundation. The first author was also supported by the Brain Korea 21 Project. The second author was supported by the Project Prediction of the Urban Atmospheric Characteristics and Development of Their Applied Techniques of the Meteorological Research Institute/KMA. References Arnfield, A. J., 23, Two decades of urban climate research: A review of turbulence, exchanges of energy and water, and the urban heat island. Intl. J. Climatol., 23, Ichinose, T., K. Shimodozono, and K. Hanaki, 1999, Impact of anthropogenic heat on urban climate in Tokyo. Atmos. Environ., 33, Kim, Y.-H., and Baik, J.-J., 22, Maximum urban heat island intensity in Seoul. J. Appl. Meteor., 1, Oke, T. R., 192, The energetic basis of the urban heat island. Quart. J. R. Meteor. Soc., 1, latitude ( o N) longitude ( o E) (meter) Figure 1. Location of automatic weather stations (AWSs) in the Seoul metropolitan area. The boundary between Seoul and its surrounding regions is marked by a thick solid line and topography is shaded with different scales. Figure 2. Distribution of air temperature averaged for the one-year period of March 21 to February 22. The contour interval is.2 C.

4 (a) spring (b) summer (a) 3 LST 3/1/1 6/1/1 9/1/1 /1/1 3/1/2 (c) fall (d) winter (b) 9 LST 3/1/1 6/1/1 9/1/1 /1/1 3/1/2 Figure 3. Distribution of air temperature averaged for each season of March 21 to February 22. The contour interval is.2 C. (a) 3 LST (b) 6 LST (e) 15 LST (f) 1 LST (c) 15 LST 3/1/1 6/1/1 9/1/1 /1/1 3/1/2 (d) 21 LST 3/1/1 6/1/1 9/1/1 /1/1 3/1/2 date Figure 5. Time series of difference between air temperature at Seocho and three-station averaged temperature at (a) 3, (b) 9, (c) 15, and (d) 21 LST. The three stations are Sanung, Nunggok, and Goyang o C. (c) 9 LST (g) 21 LST -.2 (d) LST (h) 2 LST first mode (.6 %) second mode (16. %) hour Figure 6. Temporal behavior of the first and second empirical orthogonal functions (EOFs) for air temperatures averaged for each hour of March 21 to February 22. (a) first mode (.6 %) (b) second mode (16. %) Figure. Distribution of air temperature averaged for each hour of March 21 to February 22. The plots are given in 3-hour intervals. The contour interval is.2 C. Figure 7. Spatial display of the first and second empirical orthogonal functions (EOFs) for air

5 temperatures averaged for each hour of March 21 to February 22. The contour interval is.1.

J17.3 Impact Assessment on Local Meteorology due to the Land Use Changes During Urban Development in Seoul

J17.3 Impact Assessment on Local Meteorology due to the Land Use Changes During Urban Development in Seoul Hae-Jung Koo *, Kyu Rang Kim, Young-Jean Choi, Tae Heon Kwon, Yeon-Hee Kim, and Chee-Young Choi