Analytically derived wind-wave directional spectrum part 1. Derivation of the spectrum

In contrast with the usual method to obtain the wind-wave directional spectrum by multiplying the frequency spectrum with an empirical directional function, the authors attempt to derive analytically the directional spectrum by adopting proper spectral form and using effective parameters, namely, the zero order momentm₀ of the wind-wave frequency spectrumS(), its peak frequency ₀ and the so-called peakness factorP=₀S(₀)/m₀, where is angular frequency. The directional spectrum is given in a form of frequency spectrum for each direction. The spectral directionality depends on, in addition to frequency, the wind-wave growth status, for the peakness factorP as introduced by the authors previously is a measure of the wave development stage. The salient features of the directional spectrum, comparison with existing formulas and the verification of the spectrum by observational data are to be given in the Part 2 of the paper.Project supported by the National Natural Science Foundation of China.