MEEMD Analysis of the Shallow Water Waves During a Storm

The MEEMD (median ensemble empirical mode decomposition) is used to analyze the nearshore wave surface data of the Qingan Bay in the Qiongzhou Strait during the 16th tropical storm in 2018, and different intrinsic mode functions (IMF) and their contributions to the total variance are obtained, then the collected raw data are compared with corresponding Hilbert marginal spectrum. The results show that, in the growth stage of the storm, the wave energy of each component wave in the wave group structure is greatly enhanced, and the contribution to the wave group energy is mainly in the high and low frequency bands. But in the storm weakening stage, the wave energy distribution in the wave group structure exhibits the characteristics of high frequency with high energy and low frequency with low energy. As the energy of the storm decreases, the energy of the long gravitational wave band in broken wave zone is rapidly attenuated. The results also show that, different distances and azimuths relative to the storm result in different wave frequency and amplitude of each component of the wave structure. The specific manifestations are as follows: As the study area has different azimuths relative to the advancing path of the storm, and different distances from the storm center, the fartheraway low-frequency modal has larger frequencies, and the low frequency modal's energy is smaller; As the distance away from the storm center is the same but the orientation relative to the advancing path of the storm is different, the energy of higher mode component of the waves forced by onshore winds is larger; As the relative azimuth of the advancing path of the storm is same but the distance is different, the modal frequency change exhibits the feature that the frequency of components with closer distance is lower, and the wave energy of all the modal components is larger.