Combination of Local Wind Systems under Light Gradient Wind Conditions and Its Contribution to the Long-Range Transport of Air Pollutants

Abstract

The meteorological structure and transport mechanism of long-range transport of air pollutants from the coastal region to the mountainous inland region were investigated using joint field observation data. The observations were conducted during 26?30 July 1983 over a wide area from the coastal region around Tokyo Bay to the mountainous region located in central Japan. Air, polluted around the Tokyo Bay area during the early morning remained there until noon, being transformed by photochemical reactions. It was then transported inland, at first by the extended sea breeze and thereafter by the large-scale wind system, toward the thermal low generated in the mountainous central region. At about 1600 Japan Standard Time (JST), polluted air penetrated into the mountainous region, and after sunset it drained down along the valley in the form of a gravity current. The path of the polluted air was in good agreement with the air trajectory started Tokyo at an altitude of 100 m, calculated from pilot-balloon data. Its transport speed reached a maximum value (7.5 m s?1) at about 1500 JST, when the thermal low was most fully developed. Daytime temperatures over the mountainous central region (Saku) were higher than those over the Kanto Plain (Takasaki), and the thermal effects of the mountainous central region (average height; 1200 m) extended to more than 3000 m in height. The vertical temperature profiles at these locations showed a linear variation in the daytime. At the 700 m level, the temperature difference between these locations attained 3.6°C when the thermal low was most fully developed. The pressure difference between the mountainous central region (Matsumoto) and the Kanto Plain (Kumagaya) was calculated using the vertical temperature profiles observed at 1500 JST. The pressure difference between these locations decreased with altitude and became almost zero at an altitude of 1900 m, indicating the vertical extent of the thermal low. From these results it became obvious that the creation of the thermal low and the wind blowing toward the thermal low was due to the difference in temperature between the air above the plain and that on the plateau.