The function γ is the peak enhancement factor, which
modifies the interval around the spectral peak making it
much sharper than in the Pierson-Moskowitz spectrum.
Otherwise, the shape is similar. The general form of the
JONSWAP spectrum is illustrated in Figure 1.17.
Using JONSWAP results, Hasselmann et al. (1976)
proposed a relation between wave variance and peak
frequency for a wide range of growth stages. Transforming
their results into terms of Hm0 and fp we get: