The two steps of 2D convolution and

The two steps of 2D convolution and noise elimination alleviate the sensitivity to parameter settings such as the threshold of merging, spread of frequency and direction, and improving the ability of the method to acquire relevant spectral features objectively. After the separation of wave systems from directional wave spectra, the identification of each wave system will be implemented. In the experiment, the wind sea and swell by the OP method presented in this paper are extracted to compare with that by the WA method with the wave age threshold of 1.5.Figure 4a shows the separation result from the directional wave spectrum at 2133 LST 16 December 2012, in which the dashed line is the directional wave age parabola and the three wave systems with boundaries indicated by gray dots are displayed. The wind speed and direction are 6.3 m/s and 83°, respectively in the meantime. As seen in Fig. 4a, the three wave systems are in the external region of the wave age parabola; that is to say, every peak frequency of the three systems is lower than that of the fully developed wind sea in the same direction, demonstrating that the three systems are all swells.For the OP method, each of the three wave systems has been integrated over direction to obtain the corresponding 1D spectrum. The comparison with the PM spectra of the same peak frequencies are shown in Figs. 4c–e. The peak frequencies of the three wave systems in Figs. 4c–e are 0.12, 0.11, and 0.22 Hz,respectively, and the relevant ratio l proposed in section 2c for the three wave systems are 0.12, 0.02 and 0.55, respectively, in which the overshoot phenomenon is not manifested for the three wave systems. The three fs by Hwang et al. (2012), which are specially used for the 1D spectrum, are shown with the dashed line in the plot. The peak values of the three spectra (solid line) are all lower than that of the PM spectra(dotted line) and the peaks are all in the left position to fs, declaring that the three wave systems are all swells, which yield the same identification effects as the WA method. The example of the directional wave spectrum referenced above involves only swell systems. A wave spectrum consisting of both wind sea and swell at 1557 LST 10 December 2012 is given as a typical example in order to investigate other circumstances, and the identification is shown in Fig. 4b. The wind speed and direction recorded meanwhile are 7.5 m/s and 117°, respectively. There are also three wave systems in the spectrum, and they appear distinctly in both the frequency and direction domains.