Terrain-induced air stagnation over the southern Kanto Plain in early winter

Abstract

In the southern Kanto Plain of Japan including Tokyo, the highest air pollution concentrations occur under stagnant conditions in early winter. A series of 3-dimensional observations of these episodes have been performed since 1989, three of which are analysed in this note. In every case, a surface of discontinuity covers the stagnant region, the height of which is several hundred meters, even during the daytime. A terrain-induced local front (LF) provides this structure, but in one case a synoptic-scale stationary front acts in its place.

A much warmer and cleaner southwesterly flow prevails on the southern side of the LF. However, upper measurements at a mountainous site indicate that the southwesterly winds entrain pollutants from the lower stagnant layer due to very large vertical wind shear.