Fig. 1: Surface elevations of maximally steep 3D wave groups differ in space and time.

a–c,g–i, Top and bottom rows show the measured surface elevation of maximally steep non-breaking waves at the time of maximum amplitude with corresponding images above and below. a–c, Directionally spread wave groups (Δθ = 0°) with σ_θ = 0° (a), 20° (b) or 40° (c). g–i, Crossing wave groups (σ_θ = 0°) with Δθ = 90° (g), 135° (h) or 180° (i). d,e, Time series (d) and corresponding frequency spectra (e) measured at the intended point of the linear focus (x = 0 and y = 0) for maximally steep wave groups with fixed Δθ = 0° (top) and fixed σ_θ = 20° (bottom). The line colours go from dark to light as σ_θ and Δθ are increased. f, Visualization of the directional parameter space of our experiments (Δθ = 0°; σ_θ = 0°, 10° or 20°; Extended Data Table 1). Markers with black outlines correspond to the experiments shown in a–c and g–i. In a–c and g–i, the waves travel from left to right.