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'Linear' torrential rains double in Japan in 45 years, global warming to blame: study

In this July 7, 2018, file photo, an area in Kurashiki, Okayama Prefecture, is seen flooded after the Odagawa river burst its banks during torrential rains. The torrential rain disaster that struck western Japan in 2018 was caused by a linear precipitation zone. (Mainichi/Tadashi Kako)

TOKYO -- Torrential rains brought on by training "linear precipitation zones" hit Japan in 2020 2.2 times more often than they did 45 years before, and global warming is likely the cause, according to the Japan Meteorological Agency (JMA)'s Meteorological Research Institute (MRI).

    A linear precipitation zone is often defined as a group of well-developed cumulonimbus rain clouds organized in a linear pattern, passing through or remaining in the same location for several hours, with a rain belt stretching 50 to 300 kilometers in length and 20-50 km in width. The majority of torrential rainfalls are considered to stem from these belts, and they cause serious disasters in Japan every year, including landslides.

    In May 2022, the MRI conducted the first detailed analysis of the frequency of torrential rains caused by linear precipitation zones and other phenomena, examining rainfall patterns from 1976 to 2020. The study identified areas that had recorded 130 millimeters or more of rainfall in three hours over that span among 1,178 observation sites nationwide.

    In 1976, the term "linear precipitation zone" did not exist, but there were similar cases of intense rainfall, and MRI researchers compared precipitation volumes and other factors over the ensuing years.

    In 2020, there were about 68 linear precipitation zone torrential rain events, or about 2.2 times more than in 1976. In July, the month when there was a particularly significant increase, the number of torrential rains in 2020 was about 15, roughly 3.8 times that in the same month 45 years earlier. The amount of rainfall has also been increasing since the 2000s.

    This Japan Meteorological Agency image shows linear precipitation zones moving over Japan.

    According to study head Teruyuki Kato, director of the MRI's Department of Typhoon and Severe Weather Research, global warming is behind this greater frequency, as rising sea surface temperatures around Japan put more water vapor into the atmosphere.

    "In the past, torrential rains hit unevenly along the Pacific coast, with its smooth water vapor flows. But in recent years, these rains have been occurring in the Tohoku region and Hokkaido as well. With the advance of global warming, it's becoming increasingly possible that torrential rain will strike anywhere in Japan. We need to be vigilant even in areas where there have been few torrential rain disasters in the past," Kato said.

    (Japanese original by Toshiyuki Suzuki, Lifestyle, Science & Environment News Department)

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