Sea breeze, coastal front, and urban environment

Some big cities in Japan, such as Tokyo, Kawasaki, and Yokohama have been developed in the coastal area of Tokyo bay. Approximately 15% of the population in Japan is now concentrated in this area. Tokyo was one of the cleanest cities in the world in 18-19th century. In the period of severe air pollution in 1970's, mesoscale meteorology in the Kanto plain was densely observed by the Japan Meteorological Agency and Agency of Environment in the second half of 1970's. The observation suggested there were two types of local wind system with diurnal variation in summer season, and we noticed interaction between sea breeze and topographically induced flow. Through this analysis, we also noticed the advance of sea breeze front often delayed when it passed over Tokyo. This was one of the reasons to accumulate pollutants around Tokyo in the daytime. Some numerical results revealed that this temporally stop of sea breeze front was due to the urban effects. In early 1980's we found that curious spatial distribution of PM sometimes occurred in this area. There was a clear front line. The concentration of PM was high, temperature was low, and wind was very weak in the north of the line. On the other hand, the concentration was low, temperature was high, and southwesterly wind was strong in the south of the line. This often occurs associated with the passage of a cyclone in the north of Japan. Under such condition, high mountains in the west of Kanto plain block the intrusion of the warm air mass into the Kanto plain and strong inversion layer is generated above the Kanto plain. However it is still very much difficult to predict the location of this front exactly. Recently, air pollution analysis with fine resolution is required for urban environment, for example, for road side diffusion problems. We have developed a multi-layer urban canopy model and a combined model of mesoscale model with CFD model. Our results and future attempt are introduced.