Summer squalls

... a result of urbanisation?

Last Updated: Saturday 04 July 2015

in recent years, the rainfall pattern in midsummer in the densely populated Tokyo and Yokohama in Japan has changed from gentle to tropical squall-type rainfall, involving sudden, violent winds with torrential downpours. Researchers at Tokyo University's Centre for Climate System Research believe it is due to the "heat island phenomenon", where the temperature rises abnormally in areas where the density of population is very high. It is a result of increasing areas of land being concreted over and emissions from air conditioners and heating systems.

The research team led by Naoki Sato compared the amount of comparatively strong rainfall of more than 10 millilitres per hour for two periods - between 1976-1986 and 1987-1997.

The results show the only areas that saw drastic increase in heavy rain were Tokyo and Yokohama. The percentage increased from 19.8 per cent (of the total rainfall) in the first period to 50.3 per cent in the second for Setagaya Ward, Tokyo. In the case of Yokohama, it rose from 16.7 per cent to 45.8 per cent.

While the total volume of rainfall in the two periods was more or less same, the rainfall pattern had shifted from gentle to heavy rain. Given that the drastic changes were limited to areas near the Tokyo metropolitan residential zone, the change in the pattern of rainfall is likely to be attributed not to global warming, which influences the whole of Japan, but to the urbanisation of limited areas, Sato said.

"The atmosphere in summer is basically in an unstable condition, which often causes showers in the evening. The heat island effect seems to accentuate the trend. As a result, the rainfall pattern changed to the tropical squall type," Sato said.

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