= Solution
If $h_0k_T>\tan\theta$, each period contains two critical points satisfying $|h_x|=\tan\theta$, with supercritical intervals around the steepest parts. <Internal-wave ray tracing> sends neighbouring reflected rays towards caustics attached to those critical points; rays can reverse horizontal direction in the supercritical intervals and intersect rays reflected elsewhere on the sinusoid. At equality the inviscid reflected wavelength collapses locally. The physical pattern is therefore a set of intense finite-width beams and mixing regions once viscosity, scalar diffusion, and wave breaking regularize the ray caustics.
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