TOKYO -- The seasonal rain front caused torrential rains to strike the southwestern Japan prefecture of Kumamoto on July 4 as well as the northern Kyushu region from July 6 to 7, flooding several areas mainly in Oita and Fukuoka prefectures. The Kyushu region has been hit multiple times in the past with deadly torrential rains caused by stationary fronts toward the end of the rainy season. But why has the damage gotten more severe in recent years?
The torrential rains battering southwest Japan were caused by a seasonal rain front that developed between relatively cold, dry air from continent of China north of Kyushu and the Pacific high-pressure system to the south.
According to the Japan Meteorological Agency (JMA), the front stayed near Kyushu from around July 4, stuck between high-pressure areas to the north and south. Warm, humid air traveled around the edge of the high-pressure system from the south and kept merging into the front, which made it easy for cumulonimbus clouds to form. Linear rainbands formed several times as the front moved north and south between the early hours of July 4 and the evening on July 6. Heavy, localized rains were triggered in a short amount of time, resulting in severe damage across Kyushu.
Early July marks the end of the rainy season for the Kyushu region. Compared to the beginning of the rainy season, it is warmer and pressure from the Pacific High to the south increases. Therefore, warmer, more humid air is likely to join the front. An active seasonal rain front was similarly the cause of the deadly torrential rains that struck northern Kyushu in 2017 and western Japan in 2018.
Where heavy rain falls depends on where the front stays based on the power balance between the high-pressure systems in the north and south. It is known, however, that Kyushu is susceptible to torrential rain as it is the first area to be hit by seasonal winds holding large amounts of water vapor from the East China Sea.
Such heavy rains are believed to be occurring more often with increased severity as a result of global warming. According to JMA data recorded at 51 spots across Japan over the last 120 years or so, the number of days in a year which saw 200 millimeters or more rain has been increasing by about 0.05 days per 100 years. Two hundred millimeters of rain in a day is equal to the amount of rain that usually falls in Tokyo for the whole month of September.
When the sea surface temperature rises due to global warming, more water vapor becomes suspended in the air, and when the air temperature rises, air can hold much more water vapor. Hiroaki Kawase, a senior researcher at the JMA's Meteorological Research Institute, and other researchers used a supercomputer to find that in the 2018 torrential rains in western Japan, rainfall increased by about 6% due to the recent rise in temperature.
Kawase explained, "Western Kyushu is especially prone to the effects of global warming, and there is a tendency for us to see a notable increase in how often the area is hit by torrential rain in July."
The JMA expects that if carbon dioxide is emitted across the world at the current pace, Japan will see by the end of this century at least twice as many days with daily rainfall of 200 millimeters or more compared to the late 20th century. Kawase said, "There's no doubt that the amount of rain falling over a single occasion will increase. Measures to control flooding must be altered accordingly."
(Japanese original by Tomoko Mimata and Shuichi Abe, Science & Environment News Department)