Japan agency aims to better predict 'linear precipitation bands' as rain disasters worsen
A linear rainband forms when clusters of developed cumulonimbus clouds continuously emerge in a line and move over or remain nearly stationary in the same area. It's a band-shaped zone of intense rainfall that stretches approximately 50 to 300 kilometers in length and 20 to 50 km in width.
The phenomenon was present during the 2014 Hiroshima landslide disaster, and the 2017 northern Kyushu and 2018 Western Japan floods. The Japanese term for "linear precipitation band" gained widespread attention in the mid-2010s and was even nominated for U-Can Inc.'s annual new words and buzzwords award in 2017.
Since 2022, the Japan Meteorological Agency (JMA) has been providing alerts about the likely formation of these rainbands around half a day in advance to facilitate timely evacuations. Previously, such warnings were issued by broad region, such as the "Tokai region" or "Shikoku region," but since May 2024, warnings are now released at the more specific prefectural level.
However, much about how these bands form remains unclear, making accurate forecasting with current technology extremely challenging.
In 2024, the JMA issued 81 such alerts, but heavy rain actually occurred just eight times -- a "hit rate" of roughly 10%. Linear precipitation bands were confirmed 21 times throughout the year, but no alert had been issued on 13 of those. Notably, the agency failed to predict events on the Sea of Japan side, such as July's heavy rain centered on north Japan's Yamagata Prefecture and the September Noto disaster.
To improve accuracy, the agency has expanded real-time observations on the Sea of Japan this year, in addition to the East China Sea and the Pacific. When heavy rainfall is forecast, research vessels now sail the Sea of Japan as well, collecting atmospheric moisture data, alongside upper-level air pressure and humidity measurements to predict the emergence of these rainbands.
Authorities also warn that in summer especially, severe rain disasters can be caused by rapidly developing cumulonimbus clouds even in the absence of linear precipitation bands.
The JMA offers the "Kikikuru" hazard distribution map that displays the risk levels of landslides or flooding, while the Ministry of Land, Infrastructure, Transport and Tourism provides real-time river disaster information, including flood forecasts and evacuation notices.
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