| Internal waves introduced by tide take place continually at northeastern South China Sea (SCS). Sea surface wave-height spectrum can be modified by the subsurface current. Long quasi-linear bands of internal waves propagating at the sea surface can be observed by satellite radars. However, there are some disadvantages for observing internal waves using satellites, such as itself operation cycle and bad sea state, so it captures internal waves uncertainly. Shipboard X-band radar has the advantage of high-resolution, and it can measure surface weak signals of internal waves. There is only a little research on internal waves by X-band radars, and a lot of work needs to be further studied in the future. Additionally, numerical simulation is more often used to study the vertical structure of internal waves than surface signals of internal waves (SSIW).In this thesis, the SSIW are studied, one day's data from the field investigation cruise of the"Key program of Knowledge Innovation"project of the Chinese Academy of Sciences in the SCS is analyzed. The SSIW are simulated by Lamb model and ROMS model. Some important scientific results have been acquired and summarized as follows:Based on the Radon Transform (RT) technology, the phase velocity of internal solitary waves with more than 100 meters at K106 station around DongSha islands on 24th Jun. is calculated from X-band backscatter images. Except of the effect of sea current, the phase speed of it is equal to 3.04m/s, and the translation direction is about 297 degree.There is an internal wave packet including three nonlinear internal waves in X-band radar images. Some internal wave parameters, such as propagation speed and direction of internal waves are obtained from these two images based on RT technology. Using mixing layer depth (MLD) formulas with two layer and limited depth sea hypothesis, it is first to calculate the depth of MLD from shipboard X-band radar images. The result is consistent with temperature chains and CTD data in situ.Shipboard radar observes an internal wave packet with more than four wavelets at 11:47 on 25th Jun., which the fourth wavelet is biggest with most powerful surface signal. Additionally, an interesting phenomenon is found that there is no surface signal with about 20 meters amplitude internal wave at front of the first solitary wave at 22:43 on 24th, but there are distinct surface signals with about 5 meters amplitude internal waves at front of the third solitary wave at 11:47 on 25th. Based on in situ ADCP and CTD data, we found that background current and MLD have important effect on sea surface signals power.Six simulation tests are done by Lamb model with a nonhydrostatic hypothesis. The result shows that background current has important effect on the vertical structure of internal waves. Along with background current decreasing, internal waves can change from nonlinear to linear form, its propagation speed may become slow, and disturb mixing region may become small. In addition, background current can affect the SSIW. The index named MASUG is used to describe the effect. Along with background current increasing, the gradient of surface current changes bigger, the SSIW strengthen. If the depth of MLD increases, the SSIW are weaken.Based on the mode splitting technology of ROMS, the sea level height simulated has high and low bands with the effect of tide, the results show consistent with SAR image at time and space. The internal waves'development character on time and space is shown based on simulated SSIW, which originate from Babuyan island and Batan island separately. The"hand in hand"phenomenon is also simulated well. The translation velocity from simulated SSIW agrees with results of previous studies.
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