Forecasting Atmospheric Rivers

線状降水帯の発生原因 「大気の川」を予報する

By F. Martin Ralph F. M. ラルフ
English 日本語 日本語
I was eating breakfast on a Monday morning at Sears Fine Food in downtown San Francisco, casually watching the local five-day weather forecast on a television screen behind the counter. A little symbol along the bottom showed a happy-looking sun for the rest of the day. Wednesday had a friendly-looking cloud and a few raindrops, and Thursday had a dark, threatening cloud with heavier drops. I knew Thursday’s conditions would be much rougher than the symbol conveyed. I had been studying detailed satellite data and weather models, and they indicated that a major atmospheric river (AR) was likely to hit the city. The symbol was completely inadequate for communicating the threat of the approaching storm.  ある月曜日の朝,私はサンフランシスコのダウンタウンにあるシアーズファインフードで朝食を食べながら,カウンターの向こうにあるテレビに映る地元の週間天気予報を何気なく見ていた。画面下に小さなマークが並び,左端のニコニコお日様マークは今日一日お天気が続くことを示している。水曜日のマークは白い雲と雨粒が少々,木曜日は黒い雲と雨の斜線だ。
ARs are essentially rivers of water vapor in the sky that are pushed along by strong, low-altitude winds, sometimes at hurricane speeds. The meteorological community formally defined them only in the early 2010s, after improved satellite imaging and science revealed how these storms can form far out over the remote ocean. They can grow to 2,000 miles long, 500 miles wide and two miles deep by the time they strike the western coasts of continents. An average AR brings far greater rainfall than a typical rain or thunderstorm in those parts of the world, transporting enough vapor to equal 25 times the flow rate of the Mississippi River where it pours into the Gulf of Mexico.  大気の川とは,空を水蒸気が川のように流れているもので,湿った空気が低空の強い風,ときにハリケーン並みの強風に乗って流れてくる。気象学界が正式に大気の川を定義したのは2010年代初めになってからだ。これらの嵐がはるか沖合の海上でどのように生じるかが衛星画像と科学の進歩によって明らかになったことによる。
These storms can produce disastrous flooding, including the biggest floods that some areas may see in a century. They can occur in families—a series of storms, as if rolling in on a treadmill. Several times a year they pummel the western coasts of the U.S., Canada, Europe, Africa, South America and New Zealand. They can also reach far inland: the raging terrestrial rivers that tore apart roads in Yellowstone National Park and forced the park to close this past June were fueled in large part by a remarkably strong AR.  大気の川は低気圧の群れとなって到来する。一連の低気圧がベルトコンベヤーに乗って次々にやってくるようなものだ。1年に数回,米国やカナダ,ヨーロッパ,アフリカ,南米,ニュージーランドのそれぞれ西海岸が大気の川に襲われる。内陸深くに達して陸上の川の氾濫を招くこともある。イエローストーン国立公園の道路を寸断して閉鎖に追い込んだ今年6月の水害は,著しく強い大気の川が主な原因だった。
ARs are not always destructive; sometimes they bring welcome rain to parched regions. They can beneficially boost snowpack and help fill natural and human-made water reservoirs. It’s hard to know more than a few days ahead where exactly a storm will make landfall, however. As big as they are, they can be fickle because numerous forces can affect their progression, from ocean-surface temperatures to pockets of cold air aloft.  ただ,大気の川がいつも破壊的であるとは限らない。干ばつで乾き切った地域に恵みの雨をもたらす場合がある。高地の雪塊を適切に増やし,自然の水源や人造の貯水池を満たすのに寄与することもある。だが,正確な上陸地点を予測するのは上陸の2~3日前にならないと難しい。大気の川は巨大ではあるが,海面水温から上空の寒気の塊まで数多くの要因が影響するので進路が変わりやすい。