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Accurate numerical computations of wave trains with small modulations that grow are described. The range of wave steepness and modulation lengths that give breaking waves are determined. The number of wave crests is reduced for a time at greatest modulation but frequency downshifting is not observed. Near-breaking wave-group structure is always similar, with energy concentrated into one or two wavelengths.
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Dold et al. (1986) studied this question.