Generation of a local front and high levels of air pollution on the Kanto plain in early winter

Abstract

Mesoscale meteorological analyses of air pollution were carried out using data from late autumn to early winter when air pollutant concentrations increased to high levels in urban areas of the Kanto Plain in Japan. Concentrations of NOx(NO2 + NO) and SP (suspended particles) often become very high when the region lies between an anticyclone and the warm area of a cyclone, the centre of which is moving over the northern part of Japan. Under this meteorological pattern, winds from SW/WSW blow over the southern part of the Kanto Plain and a local front, called the ‘Boso Front’, often appears across Tokyo Bay in a direction from WSW-ENE. High levels of pollutants occur in a wide area in the northern part of this front, where a strong inversion near the ground surface is formed and the air is stagnant. This local front remains until the winter monsoon begins to prevail over the Kanto Plain. Qualitatively speaking, the geostrophic wind from SW/W should be blocked by the central mountains of Honshu Island and a stagnant region should be formed downstream of the mountains, on the northern part of the Kanto Plain. On the southern part of the Plain the wind can blow because there are no obstacles upstream. The local front, a line of discontinuity of temperature and wind, may occur between these two air masses. Using aerological data at Tateno located at the eastern part of the Kanto Plain, we found that this local front appears when the wind direction at 900 hPa is between 210 and 310°, the temperature difference between heights of 900 and 1000 hPa is larger than about −3°C and the wind speed at 900 hPa is larger than 5 ms−1. The Froude number estimated from these values is in a limited range, 0.3–1.3 but it changes with wind direction.