= Solution
For a ridge-fixed disturbance, $c=0$. If the <Scorer parameter> varies on a height scale much longer than the vertical wavelength, the local <WKB approximation> gives
$$
\widehat w\sim m^{-1/2}
\exp\left(\mathord\pm i\int^z m(s)\,ds\right)
$$
where $m^2=l^2-k^2>0$. The disturbance is vertically oscillatory there and can carry wave activity upward. Where $l^2<k^2$, $m$ is imaginary and the solution is vertically evanescent.
A rigid ridge supplies the lower <boundary condition> and an upper radiation or decay condition selects the physical solution. If $l^2$ decreases through $k^2$, the crossing is a turning level: the wave is reflected or decays above it and is trapped beneath it. Increasing $\overline u$ normally reduces $N^2/\overline u^2$, while decreasing $N^2$ reduces it directly; subject to the curvature term $-\overline u_{zz}/\overline u$, either change therefore promotes <vertical trapping of an atmospheric gravity wave>.
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