7TH INTERNATIONAL CONGRESS ON TECHNOLOGY - ENGINEERING & SCIENCE - Kuala Lumpur - Malaysia (2019-08-08)

Mobile Observation of Air Temperature and Humidity Distributions under Summer Sea Breezes in the Central Area of Osaka City
Osaka is the most populous metropolitan area in western Japan and is well known for its severe summertime thermal environment. As of October 2015, the residential populations in Osaka City are 2.69 million. The local government of Osaka City has used the “ventilation path” (in Japanese, Kaze-no-Michi) concept to create a future urban plan for the area that will mitigate its severe summertime thermal environment by utilizing the cooling potential of the sea breezes that come from Osaka Bay. In the areas that sea breezes penetrate, the air temperature distribution near ground level can be used to evaluate their cooling effect. However, the thermal environment near ground level is strongly affected by surrounding features such as buildings, roads, rivers and parks. Especially in urban areas, where buildings are numerous, the urban canopy significantly affects the heat budget at the ground surface. The ventilation efficiency of a city block will strongly depend on its geometry [1], because mean wind speed is weakened and/or intensified by drag forces and building-induced eddies and. The cooling effect of a sea breeze may differ in magnitude even within a small area. The air temperature distribution will therefore show complex patterns. Because such a microscale distribution of local air temperature cannot be captured by a coarse network of meteorological sites, mobile observations are often useful. Mobile measurement of air temperature distribution in an urban area has been carried out by various research groups, including universities, local governments and citizen groups. Automobiles have often been used to cover an objective area when the area was relatively large. Mizuno et al. (2009) [2] used an automobile to make mobile observations on the Osaka Plain. Sahashi (1983) [3] evaluated the various errors in air temperature measurements obtained using automobiles, and made some suggestions to minimize the incidence of observational errors. Some research groups targeting relatively small areas have used bicycles for mobile observation [4, 5]. Since bicycles can go through narrow paths which automobiles cannot access (e.g., back streets, narrow bridges and pathways in green parks), they can capture a more detailed pattern of air temperature distribution. Additionally, the air temperatures measured during a mobile observation using a car can be affected by the heat radiated from the hoods, roofs, and the tailpipes of the car itself and other passing cars. The use of bicycles rather than cars circumvents this problem. Mobile observation with bicycles is therefore effective for detailed research; however, the target area of such a research is often limited to a linear path or a relatively small area. There are few studies involving a large number of observers on bicycles covering different areas simultaneously, because a largescale observation campaign requires a lot of staffs and equipment. In this study, we made a detailed investigation of “street-level” distribution of air temperature and humidity in midsummer in Nakanoshima district and the surrounding area, a central office area in Osaka City, from dense observations obtained using bicycles. The objective area was subdivided into 8 or 13 areas, and mobile observation was executed simultaneously in every area. We also conducted an observation of the upper wind and of the surface temperatures of the rivers sandwiching Nakanoshima district. The relationship between building density and the cooling effect of the sea breezes was discussed. In the observation area, large temperature differences were found even within a relatively small area. The building density seems to affect the mean air temperature because it affects the ventilation efficiency. The temperature of the Nakanoshima district is generally lower than that of its surrounding area because the building density is relatively low. The relative humidity distribution was inversely correlated with the air temperature distribution in general; however, both temperature and relative humidity were high at the confluence of the rivers and green parks. The depth of the sea breeze layer penetrating the Nakanoshima district was found to be about 1 km. The sea breeze direction near the ground is aligned with but opposite to the direction of the rivers surrounding the Nakanoshima district. During the daytime, 0800 to 1800 LST, the surface temperature of the rivers sandwiching the district was lower than the air temperature, and the difference reached approximately 2°C.
Professor Atsumasa Yoshida, Assist. Prof. Ryusuke Yasuda, Assoc. Prof. Sinichi Kinoshita