Research On Ocean Tides Modeling Using Satellite Altimetry

Ocean tides, resulting mainly from the gravitational attractions of the Moon and the Sun, are one of the most fascinating natural events in the world with a significant importance for commerce and science over hundreds of years. In oceanography and geophysics, tides have strong influence on modeling of coastal or continental shelf circulations, play a significant role in climate due to its complex interactions between ocean, atmosphere, and sea ice, dissipate their energy in the ocean and solid Earth, and decelerate the Moon’s mean motion, there is a clear need for more accurate ocean tide predictions. With the rapid development of the technologies of the Earth observation from the space, almost all high accuracy measurements of Geo-science contaminated with the deformations and loading effects due to ocean tides, also require to be de-tided using global ocean tide models in order to reach the scientific goals, respectively. The advent of altimetry has been totally revolutionary; it overcomes the limitation of traditional tide gauge pattern and provides the globally sampled record of sea surface height. Advances in satellite altimetry in the past two decades, especially the launch of Topex/Poseidon satellite and its follow on mission Jason-1/2, can construct a continuous long term sea level change time series and enable numerous scientific studies or discoveries, including improved global ocean tide modeling. The approach (harmonic constants or orthoweights) to tidal analysis based on satellite altimeter data is a direct, efficient and convenient way for ocean tides modeling. However, the altimeter satellites fly over the same location on the ground track once every repeat period. Because the repeated periods are of the order of ten days to a month, far larger than the most energetic tides, such as semi-diurnal and diurnal constituents with frequencies near2and1cycle per day, therefore, this kind of short-period variability clearly cannot be resolved in altimeter observations and consequently aliases to the lower frequencies. The aliasing and the question of how to separate two tides that alias to nearly the same time are the major problems to estimate the ocean tide from satellite altimetry. Owing to the distinct orbit design, the coverage and resolution of each altimeter decided by inclination and exact repeat period (ERP) is too sparse to form a global ocean tide model. All these could be largely improved by combining several kinds of altimeter data, and it is helpful to improve the ability of monitoring sea surface and investigate the water driving dynamic mechanism in shallow water.Based on the status of the present researches on ocean tide modeling, the aim of this thesis is mainly to develop a global ocean tide model with an accuracy of better than2cm in the deep oceans from multi-satellite altimeter data and a more accurate regional tide solution to representer method by means of assimilation altimeter data and tide gauge observations. The main research word and contributions of this dissertation include:1. Starting with the historical context of classical tidal theories, we summarize and review the realistic significance of commerce and science on study of tides. Reviewing the history and present status of satellite altimetry and traditional tidal measure pattern, as well as the global ocean tides modeling from the aspects of the methodology. The necessity of global ocean tides modeling according to principle, the present situation and applications of the existing ocean tide models are also summarized. At last, pointing out the author’s opinions.2. Research on the fundamental theories of tides, including hydrodynamics, hydrostatic equilibrium, expansion of the tide generating potential, and empirical tidal analysis.3. Altimeter satellites are designed to monitor the sea level variation at same ground track every ERP. It is necessary to accomplish an ERP for investigating the time-variant signal, such as tidal current, ocean circulation and any other seasonal cycles. With these repeat orbits, diurnal and semi-diurnal ocean tides are aliased into periods much longer than a day or half a day. For three major satellite altimetry missions until now. i.e. T/P, ERS-2and GFO, the alias periods as well as the Rayleigh periods over which the aliased tides decorrelate have been identified. And prove these correlation problems can largely be solved by the differences of the tidal phase advances on crossing satellite ground tracks.4. Introducing the harmonization in multi-satellite altimeter data pro-processing and the fundamental principle to transform sea surface height from different missions in a certain reference frame and ellipsoids. The response method with orthotide formulation as well as harmonic method has been used to analyze the along track T/P observations, including20years initial orbit data and6years interleaved orbit data. The results show that the most accurate tidal solutions are obtained with the response method, which offers the possibility to infer a number of smaller tides from the dominant tides. The harmonic constants derived from the unevenly sampling pattern at crossover point are better than these from along track.5. The most complete altimeter coverage of the polar oceans is provided by the ERS-1/2and Envisat-1series of satellites. Unfortunately, their sun-synchronous orbits severely limit their usefulness for measuring solar tides. Nonetheless, with a decade-long time series from these satellites now in hand, and with other sun-synchronous satellite altimeters planned for the future, there is a need for detailed studies addressing what can be learned about tides from such data. The present work examines an empirical type analysis of ERS data which combine harmonic method and response method with orthotide formulation, validate the accuracy with the T/P response solution at dual-satellite crossover points and in-situ observation. The result show that the solution derived from the ERS series keep the same level as T/P response solution except the phase lag of solar tides. When we use multi-satellite altimeter data to develop a global ocean tide model, the ability of crossing ground tracks from other satellites could mitigate the decorrelation and aliasing problem.6. Investigating the methodology of ocean tide modeling in detail. Generally, ocean tide models can be classified into three groups, including hydrodynamic model by numerical simulation, assimilation model by data assimilation method and empirical model by direct tidal analysis on altimetry observations. After the launch of T/P, almost all ocean tide models are determined through the data assimilation or via the use of altimeter data in an empirical modeling solution. However, ocean tide model accuracy is still much worse, up to an order of magnitude, in the coastal regions or over partially or permanently sea-ice or ice-shelf covered polar ocean, than that of models in the deep ocean.7. A purely empirical ocean tide models with0.25°×0.25°spatial resolution has been determined using improved multi-satellite altimetry data from TOPEX, Jason-1/2, ERS-2, Envisat, and GFO, based on a Gauss function for weighting inverse proportional to the spherical grid node distance. Compared with contemporary ocean tide models using assessment from tidal constants of tide gauges and from variance reduction studies using crossover discrepancies. Meanwhile, a region ocean tide model with0.125°×0.125°is determined by means of representer method through assimilation of altimeter along track data and tide gauge data. The results show that:the ocean tides model is improved, particularly M2constituents in East China Sea and K1tide in South China Sea, the values of the other constituents are close to the uncertainty expected from the comparison of the global ocean tide models. It is indicated that assessing further improvements in tidal model accuracy will require development of a higher quality validation data set.