  Accurate prediction of the onset and strength of breaking surface gravity
waves is a long-standing problem of significant theoretical and applied
interest. Recently, Barth\'el\'emy et al (https://doi.org/10.1017/jfm.2018.93)
examined the energetics of focusing wave groups in deep and intermediate depth
water and found that breaking and non-breaking regimes were clearly separated
by the normalised energy flux, $B$, near the crest tip. Furthermore, the
transition of $B$ through a generic breaking threshold value $B_\mathrm{th}
\approx 0.85$ was found to precede visible breaking onset by up to one fifth of
a wave period. This remarkable generic threshold for breaking inception has
since been validated numerically for 2D and 3D domains and for shallow and
shoaling water waves; however, there is presently no theoretical explanation
for its efficacy as a predictor for breaking. This study investigates the
correspondence between the parameter $B$ and the crest energy growth rate
following the evolving crest for breaking and non-breaking waves in a numerical
wave tank using a range of wave packet configurations. Our results indicate
that the time rate of change of the $B$ is strongly correlated with the energy
density convergence rate at the evolving wave crest. These findings further
advance present understanding of the elusive process of wave breaking.
