10月21日未明、マーシャル諸島付近の太平洋で台風28号が発生した。日本接近の恐れがある27号のあとを追うように西に進む見込み。tenki.jpによると、二つの台風が近づくと、それぞれの動きに影響を与える可能性があり、注意が必要だ。

気象庁によると、10月21日午後3時現在、台風28号の中心の気圧は990ヘクトパスカル、中心付近の最大風速は25メートル、最大瞬間風速は35メートル。この後、発達しながら27号の後を追うように速度を早めながら北西に進む見込み。24日には強い勢力となり、進路を次第に北寄りに変えると見られるという。一方、非常に強い台風27号は、今週半ば以降に強い勢力で日本に近づく恐れがあるという。

■台風27号と28号が接近で「藤原効果」?

気象予報士の斎藤恭紀さん(@saitoyasunori)は自身のツイッターで、台風27号と28号が干渉し合う「藤原効果」が生じる可能性に言及している。

「藤原効果」は、戦前、当時の中央気象台長だった藤原咲平氏が提案した効果で、「二つの台風が接近(その距離は台風の大きさに依存するが、ふつう1000km以内)すると、ある点を中心に反時計回りに回転運動をする」こと(kotobankより引用)。THE PAGEによると、大きくわけて二つのパターンがある。

(1) 2つの台風が同じくらいだった場合
一定の距離(典型的な台風の直径である1000キロ程度が目安)までは近づこうとするのですが、それ以上は近づくことは出来ず、お互いに影響し合うのだといいます。2つの台風を結んだ点を中心に、2つの台風が反時計回りに動き、進路が複雑な動きをすることがあるといいます。
(2) 2つの台風の勢力がまったく違う場合
勢力の小さい方の台風が、大きい方の台風に吸い寄せられ、吸収されることがあるといいます。

気象庁によると、勢力的には同じ程度ということで、上記(1)のパターンに当てはまる可能性があるとのことです。
(THE PAGE「2つの台風が接近 「藤原の効果」って何?」 2013/10/21 13:05)

気象予報士の村上繁朗さんによると、27号と28号が藤原効果により干渉し合うと、普通とは異なる複雑な動きをするため、進路予想が難しくなるという。

台風が必ず予報円の中心を進む訳ではないが、仮にその中心を進んだとすると、26日9時には、二つの台風の中心が1000km以内に近づく。台風は巨大の空気の渦。1000km以内に接近すると干渉しあい、ダンスのごとくお互いに反時計回りに動くなど、普通とは異なる複雑な動きをする。
(「気象予報士 村上繁郎のブログ」 2013-10-21 13:21:01)

また、斎藤さんは今年の台風被害の特徴を次のように分析。台風27号は広範囲で丸二日に渡って大雨になる「雨台風」だと指摘し、「西日本は木・金曜日、東海・関東・東北は金・土曜日に大雨の警戒が必要」と注意を促している。

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