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The Japan Meteorological Agency will study developments of certain cloud formations off Kyushu to provide more accurate heavy-rain warnings and buy additional time for possible evacuations.

Observation vessels will be sent to the East China Sea around the rainy season in June to reinforce the JMA’s monitoring ability of weather patterns.

The new system is designed to foresee the formation of linear rainbands, which comprise rows of cumulonimbus clouds stretching dozens to hundreds of kilometers. Regions beneath their wind paths suffer heavy rain.

If the JMA mechanism detects a higher risk of the line-shaped rainbands forming, it can issue earlier warnings for torrential rainfall.

The JMA uses radar equipment to check for rainfall, but it is difficult under the current system to predict formations of rainbands.

In the downpour that hit Kumamoto Prefecture and surrounding areas in summer last year, the forecast made half a day earlier was for up to 128 millimeters of rain over three hours. The amount of rain that actually fell was 330 mm.

One big problem is the lack of available data on vapor volumes on the windward side, although such information is essential for weather forecasts. Monitoring is notably insufficient in Kyushu because winds there come from the East China Sea and other areas above the ocean.

In an attempt to overcome the challenge, the JMA and the Meteorological Research Institute last summer deployed the Ryofu Maru observation vessel to the East China Sea 400 km west of Kagoshima Prefecture.

Vapor levels and other weather conditions were measured as a test at the time of the Kumamoto rainfall. The collected data was not reflected in the estimate.

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A downtown area in Hitoyoshi, Kumamoto Prefecture, is flooded on July 4, 2020, with the Kumagawa river seen on the right. (Asahi Shimbun file photo)

But when the measured values were taken into account after the vessel’s return to port, the prediction was 202 mm of rain over three hours, bringing the forecast much closer to the actual figure.

For that reason, the JMA decided to station two monitoring vessels in the East China Sea and elsewhere for prolonged periods in the rainy season around June this year or later.

Monitoring data will be sent in real time for weather reporting. Under the plan, the JMA will be able to announce possible formations of linear rainbands.

The program set to begin this summer aims to offer information on a limited scale when a certain level of data precision is confirmed. From 2022, the JMA intends to issue warnings for areas where downpours deriving from linear rainbands and other factors are expected.

The JMA will develop a meteorological satellite to cover a wider area over the sea for vapor measuring with a higher degree of accuracy. It will be combined with the Fugaku supercomputer and artificial intelligence so that production of line-shaped rainbands can be predicted nationwide half a day earlier by 2030.

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