= Closed-streamline threshold for a Gaussian disturbance in uniform flow
{title2=$|A|/(|U|a)=\sqrt{e/2}$}
For total <streamfunction> $\Psi=-Uy+Ae^{-(x^2+y^2)/a^2}$, the maximum opposing zonal disturbance speed is $\sqrt{2/e}|A|/a$. Below the displayed threshold the zonal velocity never changes sign and streamlines cannot close. Above it two stagnation points appear on the symmetry axis: one has definite Hessian and is a center, the other has indefinite Hessian and is a saddle. Closed contours surround the center. Equality is a degenerate onset, not a finite-area closed-cell regime.
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