The peak of the wave system with th

The peak of the wave system with the boundaries indicated by the gray dots is in the internal region of the directional wave age parabola in Fig. 4b; hence,this wave system is identified as wind sea by the WA method. The remaining two systems are considered swell because they not “included” by the parabola.The identification in Fig. 4b using the OP method presents the same result as that by the WA method,and the corresponding 1D spectra are delineated in Figs. 4f–h. The peak frequencies of the three wave systems in Figs. 4f–h are 0.13, 0.11, and 0.23 Hz, respectively, and the relevant ratio l for the three wave systems are 0.44, 0.07, and 2.26, respectively, in which the overshoot phenomenon appears only in the third wave system, plotted in Fig. 4h. The identification also coincides exactly with the results of Hwang et al.(2012). A typical example of the directional wave spectrum at 2315 LST 10 December 2012, which has a different identification result, is displayed in Fig. 5a. The wave system with boundaries indicated by gray dots is swell and the other is wind sea by the WA method. But Figs. 5b,c exhibit another result, in which the two systems are both considered to be swells by the OP method. The separation by Wang and Hwang (2001) supports the WA method, indicating there is probably something wrong with the OP method. In addition to the results of the two methods used in one directional wave spectrum, detailed identification is implemented by adopting the directional wave spectra from 0100 LST 4 December 2012 to 2000 LST 19 December 2012 during which the time series of directional wave spectra and wind velocity are both linearly interpolated every hour. Figures 6–8 show the comparison of significant wave height (Hs), and mean period and mean direction of wind sea and swell by the WA and OP methods separately. During the observation, the wind speed and direction at the height of 10 m MSL are plotted in Fig. 9. The wind directions are less than 1308 at all times during the observation, indicating the wind always blows toward the six antennas. The two sequences of significant wave height of swell by the two methods both have larger value than that of wind sea from Fig. 6, with the same characteristics of the value of the mean period in Fig. 7. This phenomenon illustrates that swell has larger energy than wind sea during the observation. As for the mean direction, the result of wind sea in Fig. 8a by the OP method systematically has larger value than that by the WA method, demonstrating larger error than that of swell in Fig. 8b. The error statistics of the wave parameters of wind sea and swell by the OP method compared with that by the WA method are shown in Table 1. The significant wave heights of both wind sea and swell calculated by the two methods have a high correlation coefficient and small RMSE. The correlation coefficients of the mean period of wind sea and swell are relatively lower than that of wave height. In regard to the mean direction, the correlation coefficient of wind sea is only 0.30, far less than the value of 0.92 for swell. The RMSE presents a worse value of 21.348 as well. This is probably because of the different energy of wind sea and swell in the data used here. During the observation, wave systems that are always below the wave age parabola are generally identified as swells for both the WA and OP methods. These swells have large energy in most cases. Wave systems with higher peak frequencies often give different identification results by the WA and OP methods, and they usually have lower energy than swells. So these “unstable” wave systems will have a smaller influence on swells than wind seas, manifesting the consistent statistic values of swells in Table 1. However, the mean direction of wind sea is calculated by averaging the weighted energy over the direction cell. So, it could be affected largely by the unstable identification result.