Researchers say continuous generation of thunderclouds and their slow motion led to deadly record rain across wide areas of western Japan last week.
The Meteorological Agency issued emergency warnings of torrential rain in 11 prefectures. The warnings are normally issued only once in several decades.
But floods and mudslides caused by the rain left more than 180 people dead and forced 5,500 people into shelters as of Thursday.
A group at the National Research Institute for Earth Science and Disaster Resilience led by Chief Researcher Shingo Shimizu has analyzed radar observation data for Friday and Saturday of last week.
The radar data showed that a steady influx of very humid air kept generating thunderclouds or cumulonimbus clouds over Hiroshima, Okayama, Gifu, and other prefectures.
Shimizu says a thundercloud normally survives for about an hour, but this time the continuous piling up of such clouds resulted in heavy rain in the same areas.
In heavily hit Hiroshima Prefecture, a narrow band of thunderclouds stayed over a single area for a long time on Friday evening.
Shimizu also says these thunderclouds were not high enough to be effectively blown away by fast upper atmosphere winds. At about 7 kilometers, they were around half as high as the tallest thunderclouds.
Shimizu says researchers need to develop technology that can predict localized rain more accurately and sufficiently ahead of time.