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% Chapter 4   Atmospheric Waves: Part II
%             4-6 BAROTROPIC EFFECTS IN THE ATMOSPHERE
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% 2002/12/16  森川 靖大   作成
% 2002/12/18  森川 靖大   完成
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\markboth{5-6 大気の順圧効果}{5-6 大気の順圧効果}
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\section{大気の順圧効果}

\begin{itembox}[l]{概要}
 \Dparskip \vspace{-6mm}
 この節では, この章でここまでおこなった順圧不安定に関しての説明と,
 実際での話をまとめている.
\end{itembox}

この章での解析では, 絶対渦度の勾配の符号が変化する場合には
順圧不安定が起こることが示された
\footnote{
 これは4-3節の(5-14)で示されたことである.
 絶対渦度は$\eta = \overline{\zeta} + f$である.
 $\overline{\zeta} = - dU/dy$(5-2節参照), $df/dy=\beta$より,
 絶対渦度の勾配は$d \eta / dy = - d^2 U / dy^2 + \beta$である.
}.
$\beta$の存在は中緯度での西風のジェット風を安定にするが,
東風のジェットは$\beta$の存在によって特定の条件下では不安定になる.
不安定な波が存在しないならば, 連続スペクトル効果
によって
擾乱は弱まる.
擾乱は風の基本場と同じ方向に傾く場合には弱まり,
風の基本場と反対方向に傾く場合は増幅されることは
エネルギーの式によって示された
\footnote{
 (5-20)により示されたことである.
}.
中緯度ジェットの南部では, ほとんどの擾乱は南西-北東に傾く.
そのため擾乱は順圧的にそのエネルギーを減らし, 風速の基本場にその
エネルギーを渡す.
これはモーメントのフラックス$\overline{u' v'}$の観測からも示されている.
このことから, 中緯度での擾乱は順圧的に弱まり,
そして擾乱のエネルギーは失われて基本場の流れになることによって, 
摩擦に逆らって基本場の流れを維持していることがわかる.
しかし, ジェットが非常に鋭くなると, 順圧不安定は短期間に必ず起こる.
熱帯では順圧不安定はより一般である.
それは, 東風のジェットがより不安定であることや,
加熱(風の基本場を生み出すもの)が小さい水平スケールを持つ傾向に
あることなどから言えることである.




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