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“Never Before Experienced” Rains Hammer Japan During Early July

“We’ve never experienced this kind of rain before. This is a situation of extreme danger.” — The Japan Meteorological Agency

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During recent days as much as 25 inches of rain has fallen over parts of Japan shattering previous all time precipitation records for parts of the island nation. The resulting floods have spurred a major emergency response by 54,000 personnel, taken the lives of more than 125 people, and forced more than 2.8 million to evacuate.

(Rising global surface temperatures increase atmospheric water vapor levels — providing liquid fuel that spikes the most powerful rainfall events to even greater extremes.)

On July 3, Typhoon Prapiroon swept over southwestern Japan bringing with it a spate of heavy rains. Over the following days, Prapiroon got caught up in stationary front even as a high pressure system to the east continued to circulate tropical moisture into the region. Beneath that eastern high, sea surface temperatures ranged between 2 and 3.5 degrees Celsius above normal. Meanwhile, warmer than normal ocean surfaces dominated a region east of the Philippines. These large, abnormally warm zones produced excess evaporation which helped to feed even more moisture into the region.

The result was a historic and devastating rain event for Japan. Isolated locations received more than 39 inches (1000 mm) of rain over a three day period. With one hour rainfall exceeding 3 inches in a number of locations. Motoyami received one day rains of 23 inches. With Mount Ontake seeing more than 25 inches over three days.

(Warmer than normal ocean surfaces, as shown in yellow and red in this sea surface temperature anomaly map, helped to fuel Japan’s recent extreme rainfall event. Image source: Earth Nullschool.)

Rising global temperatures increase overall atmospheric moisture loading by approximately 8 percent for each degree Celsius of global temperature increase. Water vapor provides fuel for storms both through enhancing convection and by engorging clouds with moisture. Recent scientific studies have found that climate change can greatly enhance the peak intensity of the most severe storms in this way. And the U.S. National Climate Assessment has identified a historical trend of increasing instances of heavy precipitation.

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