Satellite altimetry has been successfully applied to monitoring water level variation of global lakes. However, it is still difficult to retrieve accurate and continuous observations for most Tibetan lakes, due to their high altitude and rough terrain. Aiming to generate long-term and accurate lake level time series for the Tibetan lakes using multi-altimeters, we present a robust strategy including atmosphere delay corrections, waveform retracking, outlier removal and inter-satellite bias adjustment. Apparent biases in dry troposphere corrections from different altimeter products are found, and such correctios must be recalculated using the same surface pressure model. A parameter is defined to evaluate the performance of the retracking algorithm. The ICE retracker outperforms the 20% and 50% threshold retrackers in the case of Ngangzi Co, where a new wetland has been established. A two-step algorithm is proposed for outlier removal. Two methods are adopted to estimate inter-satellite bias for different cases of with and without overlap. Finally, a 25-year-long lake level time series of Ngangzi Co are constructed using the TOPEX/Poseidon-family altimeter data from October 1992 to December 2017, resulting in an accuracy of ~17 cm for TOPEX/Poseidon and ~10 cm for Jason-1/2/3. The accuracy of retrieved lake levels is on the order of decimeter. Because of no gauge data available, ICESat and SARAL data with the accuracy better than 7 cm are used for validation. A correlation more than 0.9 can be observed between the mean lake levels from TOPEX/Poseidon-family satellites, ICESat and SARAL. Compared to the previous studies and other available altimeter-derived lake level databases, our result is the most robust and has resulted in the maximum number of continuous samples. The time series indicates that the lake level of Ngangzi Co increased by ~8 m over 1998–2017 and changed with different rates in the past 25 years (-0.39 m/yr in 1992–1997, 1.03 m/yr in 1998–2002 and 0.32 m/yr in 2003–2014). These findings will enhance the understanding of water budget and the effect of climate change.

Water level variations in four natural lakes, Poyang, Dongting, Tai, and Chao, within the Yangtze River basin are studied using ENVISAT GDRs. The GDRs were edited using simple editing criteria and appropriate geophysical corrections applied. Altimeter-derived lake level variation time series were then generated and analyzed. The results of this study, which is the first of its kind in using data from ENVISAT missions over the Yangtze River basin in China, reveal that the water level changes in these four lakes directly reflect the water level of the Yangtze River and contribute to the floods and their associated disasters that usually occur in the middle and lower reaches of the Yangtze River.

Global mean sea level (GMSL) has not only significant secular trend and seasonal variations, but also inter-annual and decadal variations. This paper reconstructs the time series of GMSL variations between 1948 and 2007 by combining satellite altimeter measurements and tide gauge observations. Based on the time series, the acceleration of GMSL rise in the second half of 20th century is estimated to be 0.010±0.009 mm/a 2 , and multi-scale low frequency sea level oscillations including decadal variations are detected, and the high-rate of GMSL rise during 1993-2003 is locate in the ascending phase of low frequency oscillation. Then, using the reconstructed GMSL time series after removing the secular trend from satellite altimetry and tide gauges measurements, it shows that low frequency signal of sea level variation has strong correlations with the index of El Niño/Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). But in several time periods, they have large difference due to effects of both high frequency occurrence of El Niño and La Niña phenomenon and short term switch of PDO events. , the GMSL rise is an indisputable fact, but whether its rate is faster or not, is the problem still need to be studied further.In the past few decades, there appears strong amplitude of sea level rise between the western Pacific and eastern Indian Ocean. The large inter-annual variability is associated with the El Niño events. By using satellite altimeter measurements, there have been a few results about the correlation of sea level variation with El Niño and La Niña events in the Pacific [5][6][7]. However, the frequency of these two events is not regular, which happens once in approximately every 2-7 years. What is more, there are still much lower frequency sea level variations, such as the low frequency oscillations in the period of 50-70 year lead by sea surface temperature anomalies in the PDO events, which cannot be estimated by shorter satellite altimeter measurements. Although the long-endured and stable tide gauges provide nearly 150 years' observations that can be used for the analysis of decadal sea level variations, the tide gauges are sparse in the earlier years and these parts of records only represent the mean sea level variation within the localized area. Therefore, it is difficult to get the time series of GMSL variation, e.g. the rate of sea level variation can be 5 times of the global mean within some sea areas. Multi-satellite altimetry can not only collect the nearly global coverage high resolution sea level observations, but also accurately display sea level variations of different regions, and make up for the shortage of spatial distribution for tide gauges. By combining the advantages of high spatial sampling rate of satellite altimetry and the high temporal sampling rate of tide gauges, it can get more detailed characteristics of sea

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Gravity anomalies on a 2.5 x 2.5 arc-minute grid in a non-tidal system were derived over the South China and Philippine Seas from multi-satellite altimetry data. North and east components of deflections of the vertical were computed from altimeter-derived sea surface heights at crossover locations, and gridded onto a 2.5 x 2.5 arc-minute resolution grid.EGM96-derived components of deflections of the vertical and gravity anomalies gridded into 2.5 x 2.5 arc-minute resolutions were then used as reference global geopotential model quantities in a remove-restore procedure to implement the Inverse Vening Meinesz formula via the 1D-FFT technique to predict the gravity anomalies over the South China and Philippine Seas from the gridded altimeter-derived components of deflections of the vertical. Statistical comparisons between the altimeter-derived and the shipboard gravity anomalies showed that there is a root-mean-square agreement of 5.7 mgals between them.

The quality of altimeter data and ocean tide model is critical to the recovery of coastal gravity anomalies. In this contribution, three retracking methods (threshold, improved threshold and Beta-5) are investigated with the aim of improving the altimeter data over a shallow water area. Comparison indicates that the improved threshold is the best retracking method over China Sea. Two ocean tide models, NAO99b and CSR4.0, are analyzed. Results show that different tide models used in the processing of altimeter data may result in differences more than 10 mGal in recovered coastal gravity anomalies. Also, NAO99b is more suitable than CSR4.0 over the shallow water area of China Sea. Finally, gravity anomalies over China Sea are calculated from retracked Geosat/GM and ERS-1/GM data by least squares collocation. Comparison with shipborne gravimetry data demonstrates that gravity anomalies from retracked data are significantly superior to those from non-retracked data. Our results have the same order as the other two altimeter-derived gravity models: Sandwell&Smith(V16) and DNSC08.

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