GIS-Based urban heat island study in University campus

Transcription

1 1127 GIS-Based urban heat island study in University campus N. H. Wong, S. K. Jusuf National University of Singapore, Singapore ABSTRACT Urban development is unavoidable as the result of country s economic growth. Without a careful planning, a city may cause environment destruction. Singapore is known to have the best environmental quality in the world. However, in the previous research on Singapore Urban Heat Island (UHI), it was found that UHI exists with intensity of 4 o C difference between downtown area and nature reserve area. This phenomenon also exists in other countries and many researches have been done proofing its existence. It is because soil and trees are replaced with concrete and high rise building. This study is the extension of the Singapore urban heat island study which focuses more on the microclimate scale. National University of Singapore (NUS) Kent Ridge Campus was chosen for this study. In order to map out temperature condition, field measurement has been done on 26 August 25 September It was found that at the dense greenery area along Kent Ridge Road, the temperature condition can be as low as 3.2 o C as compared with sparse greenery area in Prince George s Park canteen during day time around During night time the temperature difference can be 2 o C around It shows that urban heat island phenomena happened in NUS Kent Ridge Campus and greenery provides a good benefit to reduce the ambient temperature. All of the measurement data was put into the Geographical Information System (GIS) database system and then daily day time and night time temperature maps were produced. By using GIS, temperature profile across the NUS Kent Ridge campus can be generated to provide clearer picture about the temperature distribution and facilitate a further analysis. Keywords: GIS, urban heat island, NUS Campus, field measurement. 1. INTRODUCTION Urban development is unavoidable as the result of country s economic growth. Without careful planning, a city may cause environmental destruction. Singapore is known to have the best environmental quality in the world. However, in the previous research on Singapore Urban Heat Island (UHI), it was found that UHI exists with the intensity of 4 o C (Wong, 2004). At the local scale, basically there are three major urban elements which may influence the temperature condition of a city. They are buildings, greenery and pavement area/streets. The proportion of these elements plays as an important role. Planting of vegetation is one of the methods to reduce the high ambient temperature in an environment. One tree can already generate the coolness, but its impacts are limited only to its surroundings (Jauregui, 1990/91). It has been widely researched about the impact of greenery to the built environment. Greenery provides positive benefits because it dissipates incoming solar heat on building structures through its effective shading; it can reduce long-wave radiation exchange between buildings due to the low surface temperatures created by shading of plants and it can reduce ambient air temperature through evapotranspiration (Wong, et.al, 2003). Chen Yu (2006) concluded that large urban parks can extend the positive effects to the surrounding built environment. Through the field measurement, the built environment, which is close to the park, has a lower temperature of average 1.3 o C. Thus, the more parks are built in an urban area, the lower the urban temperature will be. The temperatures measured within parks also have strong relationship with the density of plants, since plants with higher Lead Area Index (LAI) may result in lower ambient temperatures. Results derived from the energy simulation shows that energy may be saved when buildings are built near to parks with a maximum of 10% reduction in cooling load. The Envi-met simulation (Envi-met, 2006) indicates that park has significant cooling effect on surroundings both during day and night. This study is the extension of the Singapore urban heat island study which focuses more on the microclimate scale. As compared with the previous study on campus area (Wong, et.al, 2006), in this study, more data points were collected with a longer period of measurement and different method of analysis was used. All of the measurement data was put into the Geographical Information System (GIS) database and daily day time and night time temperature maps were produced. Climatic mapping method has become widely used because it is a very useful tool for urban planning. This is because it can provide a clear picture from micro to macro level. By using Geographical Information System (GIS), climatic map can be developed together with PALENC Vol 2.indd /9/2007 1:26:15 µµ

2 1128 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and the analysis of different information layers. Katzschner and Mülder (2006) developed a regional climatic mapping for sustainable village development in China. Thermal comfort conditions, microclimates and ventilation patterns were studied in seven Chinese villages. The methods for the mapping are, firstly, gathering weather data from the nearby weather stations. The average values of solar radiation and temperatures were calculated by using METEONORM software. The next step was the combination of land use data, topographical information and the climatic data. Through the Geographical Information System (GIS Arc.Info), geographical and land use data were classified and transformed to urban climate functions, such as thermal aspects (i.e. heat and cooling rates), wind classification with air paths and topographically influenced downhill movements of air masses. The building fabric was classified according to roughness length and thermal radiation processes. Matzarakis et al (2004) analyzed the thermal human in Austria for the health and recreation. Physiological Equivalent Temperature (PET) was calculated based on the daily measurements and observations, at various times, of air temperature, relative humidity, wind velocity and mean cloud cover. Statistical and GIS procedures were applied to the PET computation of the single climatic station in order to transfer the point into raster map. At the end it is found that the method used in analyzing the thermal bioclimatic conditions with specific bioclimate diagrams, presents an excellent way of transferring complex scientific information into a form that can be easily understood by decision makers and the general public. Figure 1. Point of measurement location 3. DATA ANALYSIS Figure 2 shows that there is distinct temperature difference between dense greenery area and sparse green area. During the peak hours at 15.00, point 1 (water tank blue dot) experiences the lowest temperature and the hottest area is point 27 (Engineering Auditorium red dot) followed by point 28 (PGP Canteen black dot). The temperature difference can be as high as 3.2 o C. 2. METHODOLOGY National University of Singapore (NUS) Kent Ridge Campus was chosen as the object of study. Temperature measurement was carried out for the period 26 August 25 September The total number of data points is 30, which cover the whole NUS Kent Ridge Campus with dense, less dense and sparse green area. The major instrument used in this study is HOBO meter. The HOBO meters were configured to log data at an interval of every 10 minutes and installed at 2 meters above ground. Location of the data points can be seen in Figure 1. In this study, besides numerical data analysis, Geographical Information System (GIS) was also employed to generate the spatial data and temperature maps. The collected data was compiled and exported to the GIS interface. The data were selected and assigned to the GIS layer file, called point shapefile. By using interpolation method, the temperature map was produced. Figure 2. Temperature profile on 4 September 2006 After sunset around 20.00, temperature in point 28 (PGP Canteen black dot) becomes higher than point 27 (Engineering Auditorium red dot). It is because Engineering Auditorium area is an open area, therefore the heat is easily dissipated. As compared with PGP canteen area which is surrounded by high rise buildings, the heat is trapped in the building during daytime and starts to release it during the night time. Therefore, in this area during the night time, it is still experiencing higher temperature. In the morning around , the heat island intensity reaches the maximum value of 2 o C. The temperature map for 4 September 2006 at is shown in Figure 3. Hot spots are found at point 12 (Guild House), point 21 (SDE 2), point 26 (Lecture PALENC Vol 2.indd /9/2007 1:26:16 µµ

3 1129 Theatre 2), point 27 (Engineering Auditorium), and point 28 (PGP Canteen). The reddest color is shown in point 27 (Engineering Auditorium) as the hottest spot. It is because of the large pavement area which absorbs heat from solar radiation, which in turn increases the ambient temperature. In point 26 (Lecture Theatre 2) and point 28 (PGP Canteen), the hot spots are due to the sparse greenery, with pavements between the buildings. The hot spot in point 12 (Guild House) is suspected to occur due to the antropogenic heat build up from the NUS Alumni House construction process. The yellowish color from point 22 (tennis court) is understandable due to the large hard surface of tennis court nearby. The cool spots are mostly identified along the Kent Ridge Road, point 1 (Water Tank), point 3 (TMSI), point 5 (Bioinformatics Center) and point 19 (King Edward VII Hall). It is because of the large dense greenery area exists along this road. Other cool spots are also identified in the carparks which have large trees covered their areas. They are point 17 (Lecture Theater 11), point 24 (Eusoff Hall) and point 25 (carpark 9). This data provides very strong evidence about the importance of trees, especially mature trees in reducing ambient temperature. The temperature profiles, showing the daytime heat island intensity across NUS Kent ridge Campus are shown in Figure 4. Section A-A cuts through point 27 (Engineering Auditorium), point 26 (Lecture Theatre 2), point 1 (Water Tank), point 13 (PGP Residence) and point 28 (PGP Canteen). In this section, the daytime heat island intensity reaches about 3.2 o C, with the coolest area in point 1 (Water Tank). Section B-B cuts through point 12 (Guild House), point 6 (CJ Koh Law Library), point 1 (Water Tank), point 10 (Multi purpose field) and point 22 (tennis court). In this section the daytime heat island intensity reaches about 2.8 o C. Figure 4. Temperature profile of section A-A and B-B on 4 September 2006 at 15:00 Temperature map on 4 September 2006 at is shown in Figure 5 to study the night time heat island intensity. This figure shows clearly that point 28 (PGP Canteen) turns out to be the hottest area. It is because during the night time the heat which is trapped during daytime dissipates during night time. Dark yellow color are shown for point 7 (S1A), point 20 (Medicine 11) and point 26 (Lecture Theatre 2). The cause is similar with point 28 (PGP Canteen), but they have lower temperatures because they have less congested building area. Figure 5. Temperature map on 4 September 2006 at Figure 6. Temperature profile of section A-A and B-B on 4 September 2006 at 03:00 Figure 3. Temperature map on 4 September 2006 at The cool spots are identified in the points where the greenery exists not only in the points along Kent Ridge Road, but also in the carparks area where they are mostly covered with turfing. It shows that turfing also has a potential impact to the ambient temperature during night time. The temperature profiles, showing the night time heat PALENC Vol 2.indd /9/2007 1:26:16 µµ

4 1130 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and island intensity are shown in the Figure 6. Section AA cuts through the point 27 (Engineering Auditorium), point 26 (Lecture Theatre 2), point 1 (Water Tank), point 13 (PGP Residence) and point 28 (PGP Canteen). In this section, the night time heat island intensity reaches about 2oC. Section B-B cuts through point 12 (Guild House), point 6 (CJ Koh Law Library), point 1 (Water Tank), point 7 (S1A) and point 11 (football field). In this section the night time heat island intensity reaches about 1.8oC. Temperature maps were also produced based on the average daytime (31 maps) and night time (31 maps) temperature. Only some of the maps are discussed here. They are maps on 31 August (rainy day) and 12 September (hot day temperature) for daytime average temperature and on 31 August (rainy night) and 22 September (hot night temperature). It is to represent both extreme cases. It is very interesting to observe that two different weather conditions create different heat island characteristics. During day time (comparison between Figures 7 and 8), in the rainy day (Figure7), point 27 (Engineering Auditorium) which has a large pavement area is not hotter than point 28 (PGP Canteen). It is because not much solar radiation was received by the surface. When the ambient temperature decreases due to rain, the heat can be released in a short period. However, in PGP area, solar radiation is received and absorbed by the building. Since buildings have a large heat capacity, the heat is released to the environment in a slow pace which leads to a hot spot in that particular area. Figure 7 shows that in rainy day, yellowish color is evenly distributed in less dense greenery built area and the area along Kent Ridge Road is still the coolest area in NUS. In the hot day, as shown Figure 8, the dense greenery shows its positive impact to the environment, bringing the cooling effect. Figure 8. Average daytime temperature distribution on 12 September 2006 (represented as hot day) Figure 9. Average night time temperature distribution on 31 August 2006 (represented as rainy night) Figure 10. Average night time temperature distribution on 22 September 2006 (represented as hot night) Figure 7. Average daytime temperature distribution on 31 August 2006 (represented as rainy day) PALENC Vol 2.indd 1130 During night time (comparison between Figures 9 and 10), point 28 (PGP canteen) is shown as the hottest area. In the rainy day, as seen in Figure 9, less dense green built areas, such as in Faculty of Engineering, Faculty of Arts and Faculty of Science, show a yellowish color. During the night with normal weather (Figure 10), the 7/9/2007 1:26:17 µµ

5 1131 dense greenery shows a very good cooling effect from the evapotranspiration process of plants. 4. CONCLUSIONS In order to map out temperature condition in NUS Kent Ridge Campus, field measurement has been done on 26 August 25 September 2006 with 30 data points. At the dense greenery area along Kent Ridge Road, the temperature condition can be lower by 3.2 o C as compared with sparse greenery area in PGP canteen during day time around During night time the temperature difference can be 2 o C around It shows that urban heat island phenomena happened in NUS Kent Ridge Campus and greenery provides a good benefit to reduce the ambient temperature. In an open pavement area with sparse greenery area, it is easily heated up during day time and also easily cooled down during night time. In contrary, at the place with highly dense building arrangement with sparse greenery area, it is heated up during day time due to lack of greenery, but it does not easily release the heat due to the heat capacity of the building. From this study, it shows that temperature mapping is very useful as a tool since it can spatially and graphically map out the temperature distribution. The analysis can be made in correlation with the geographic characteristic of the place. In the near future, the correlation between temperature and building characteristics; temperature and pavement characteristics temperature; temperature and greenery condition in terms of Green Plot Ratio value (Ong, 2003) will be conducted. ACKNOLEDGEMENT PGP UCC UH SDE SH SLSS SRC YIH REFERENCES : Prince George s Park : University Cultural Centre : University Hall : School of Design and Environment : Shears Hall : Singapore Synchrotron Light Source : Sport & Recreation Centre : Yusof Ishak House Chen Yu and Wong Nyuk Hien. (2006). Thermal benefits of city parks. Energy and Buildings, 38, ENVI-met software, Jauregui, E. (1990/91). Influence of a large urban park on temperature and convective precipitation in tropical city. Energy and Buildings, 15-16, Katzschner, L. and Mülder, J. (2006). Regional climatic mapping as a tool for sustainable development, Journal of Environmental Management, accepted for publication. Matzarakis, A., Zygmuntowski, M., Koch, E. and Rudel, E. (2004) Mapping the thermal bioclimate of Austris for health and recreation tourism. In Press. Ong, B.L. (2003). Green plot ratio: an ecological measure for architecture and urban planning. Lansdcape and Urban Planning, 63, Wong Nyuk Hien, Chen Yu, C L Ong and A Sia. (2003). Investigation of thermal benefits of rooftop garden in the tropical environment. Building and Environment, 38, Wing Nyuk Hien. (2004). Study of Urban Heat Island in Singapore. Research report. National University of Singapore, Singapore. Wong Nyuk Hien, Steve Kardinal Jusuf, Aung Aung La Win, Htun Kyaw Thu, To Syatia Negara and Wu Xuchao. (Accepted for publication). Environmental Study of the Impact of Greenery in an Institutional Campus in the Tropics. Building and Environment. This research is supported by Department of Building, National University of Singapore. I would like to send my great appreciation to Office of Estate and Development, especially for Ms. Lina Goh and Mdm. Helen Yip in providing all of the supporting data. ABBREVIATIONS FASS : Faculty of Art & Social Science GIS : Geographical Information System IMRE : Institute of Materials Research and engineering I2R : Institute for Infocomm Research KE7 : King Edward VII KRH : Kent ridge Hall LT : Lecture Theater NUS : National University of Singapore OED : Office of Estate and Development PALENC Vol 2.indd /9/2007 1:26:17 µµ