High Levels of Winter Air Pollution under the Influence of the Urban Heat Island along the Shore of Tokyo Bay

Abstract

A wintertime small-scale sea breeze associated with high levels of air pollution is described, in which the urban heat island plays an important role. Over a major portion of the Kanto plain, the winter surface air temperature varies diurnally, ranging from 0° to 12°C on average. The water temperature in the innermost part of Tokyo Bay, surrounded by the plain, is maintained at 8°?10°C. The land?sea temperature contrast that generates sea breezes, however, is intensified by two processes: (i) the heat island associated with the Tokyo metropolitan area on the northwestern shore of the bay increases the land air temperature, and (ii) the nocturnal outflow of cool land air, the result of radiative cooling, covers the bay, causing the sea air temperature to be lower than the water temperature during the morning hours. As a result, the sea breeze frequently penetrates into the heavily urbanized area, extending about 20 km from the bayshore. Since the sea breeze is coupled with the heat island, it does not penetrate farther inland. A convergence zone persists over the urbanized area, which in turn results in very high concentrations of pollutants. An analysis of a typical episode is also discussed to more realistically describe the behavior of air pollution than for the averaged case.