Ocean tides from satellite altimetry and GRACE

Satellite altimetry and GRACE observations carry both the signature of ocean tides and have in general complementary potential to resolve tidal constituents. It is therefore straightforward to perform a combined estimation of a global ocean tide model based on these two data sources. The present paper develops and applies a three step procedure for generating such a combined ocean tide model. First, the processing of multi-mission altimetry data is described along with the harmonic analysis applied to derive initially a pure empirical ocean tide model. Then the capability of GRACE to sense particular tidal constituents is elaborated and an approach to estimate tidal constituents from GRACE is outlined. In a third step a combination strategy with optimal stochastic data treatment is developed and applied to the altimetry-only tide model EOT08a and four years of GRACE observations, leading to the combined model EOT08ag. The differential contributions of GRACE to EOT08ag remain small and are mainly concentrated to the Arctic Ocean, an area with little or poor altimetry data. In comparison with other tide models, EOT08ag is validated by K-band range residuals, the impact on gravity field modelling and on precise orbit determination and by variance reduction of crossover differences. None of these comparison exhibits a significant improvement over the altimetry-only tide model except for a few areas above 60°N. Overall the improvements of the combination remain small and appear to stay below the current GRACE baseline accuracy.     

Possible differences between mean sea-levels from satellite altimetry

Summary The first attempt has been made at determining the differences between mean sea-levels, using the recent satellite altimetry data [1].     

Estimating thermohaline variability of the equatorial Pacific Ocean from satellite altimetry

The vertical thermohaline structure in the western equatorial Pacific is examined with a Gravest Empirical Mode (GEM) diagnosis of in-situ mooring measurements. The poor GEM performance in estimating deep thermohaline variability from satellite altimetry confirms a lack of vertical coherence in the equatorial ocean. Mooring observation reveals layered equatorial water with phase difference up to 6 months between thermocline and sub-thermocline variations. The disjointed layers reflect weak geostrophy and resemble pancake structures in non-rotating stratified turbulence. A coherency theorem is then proved, stating that traditional stationary GEM represents in-phase coherent structure and can not describe vertically out-of-phase variability. The fact that stationary GEM holds both spatial and temporal coherence makes it a unique tool to diagnose vertical coherent structure in geophysical flows. The study also develops a non-stationary GEM projection that captures more than 40% of the thermohaline variance in the equatorial deep water.     

联合卫星测高和模式资料研究海水热含量变化

本文在考虑洋底压力变化的情况下,利用2003~2008年融合多颗卫星的测高资料估计了全球和中国近海的海水热含量变化.顾及洋底压力(OBP)变化以及热膨胀系数随海水深度变化的影响,提出的改进方法提高了对中高纬度地区热含量变化的估算精度.在OBP变化较为明显的北太平洋区域I(30°N~50°N,170°E~190°E)、南印度洋区域II(40°S~60°S,100°E~120°E)和南太平洋区域III(40°S~60°S,100°W~120°W),改进方法的均方差较传统方法分别降低了16.3%、60.5%和48.4%.同时研究表明,卫星测高的精度以及盐度变化是影响中高纬度地区热含量估计精度的重要因素.在中国近海地区,东海和黄海的热含量主要表现为周年变化;南海区域的热含量除周年变化外,还存在半周年项和年际变化项,且南海的海水热含量近年有增加的趋势.     

Arctic sea ice thickness variations from CryoSat-2 satellite altimetry data

Arctic sea ice plays an important role in Earth's climate and environmental system. Sea ice thickness is one of the most important sea ice parameters. Accurately obtaining the sea ice thickness and its changes has great significance to Arctic and global change research. Satellite altimeters can be used to derive long-term and large-scale changes in sea ice thickness. The leads detection is vital in sea ice thickness estimation by using satellite altimetry. Different leads detection methods are compared with remote sensing images, and results show that the detection method that uses waveform parameters can obtain improved results. The model for the conversion of freeboard to thickness is optimized by considering the incomplete penetration of snow for radar altimeters. We derive the estimates of the Arctic sea ice thickness for November 2010 to December 2019 by using the CryoSat-2 altimetry data. The sea ice thickness from the IceBridge and draft data from the upward-looking sonar are used to validate our thickness results. Validations show that the accuracy of our thickness estimates is within 0.2 m. Variations in the Arctic sea ice thickness are analyzed using the PIOMAS model and air and sea surface temperatures. A sharp increase in sea ice thickness is found in 2014.     

Scale factor for lengths of the geopotential model

Summary Using the data in[1], the scale factor for lengths is derived of the geopotential model R ₀ =GM/W ₀ (W ₀ is the potential on a generalized geoid). The resulting value, R ₀ ==6 363 672.9 m, which is2 m less than the original value[5], is practically the same as that in[6].     

Lateral density anomaly distribution in the lower mantle obtained from satellite low order geopotential coefficients

In this paper, we attempt to use satellite gravity data and a new inversion method to study the lateral density anomaly distribution in the mantle. First, density difference Δρ(τ,θ,φ) is expanded in terms of a three—dimensional orthogonal function system, the coefficients of the expansion are to be determined. Then, a set of observation equations is established from the relationship between density anomaly and disturbing geopotential. In the equations the unknown vector contains the coefficients of density anomaly expansion, the observational vector is obtained by computing geopotential perturbations using the potential coefficients of GEM10B, and a filtering process is done for the observational values by properly selecting the harmonic degrees of geopotentical. Finally, the lateral density variations in the lower mantle (670 km toCM boundary) are investigated. In this case, the degrees of disturbing geopotential are selected as 2–11, the truncated degrees of density anomaly expansion are taken asL=6 andK=4, and the damping least squares method is used to solve the observation equations. The resulting model shows the high level of density perturbations at 670 km discontinuity and core — mantle boundary, a high — density zone circumscribing the Pacific and a lower—density region under the center of Pacific. These features are in agreement with the three—dimensional seismic velocity variation features by Dziewonski (1984). In the Antarctic region and some parts of Atlantic and Indian Ocean, however, the resulting density anomalies are negatively correlated with the seismic velocity anomalies, the cause resulting in these phenomena is preliminarily analysed in this paper. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 53–65, 1991. The principle and method represented in this paper can also be suitable to study the lateral density anomaly distribution in the earth’s crust and the upper mantle.     

Investigating the uncertainty of satellite altimetry products for hydrodynamic modelling

Satellite altimetry products are increasingly used in many hydraulic applications, and recent studies demonstrate their suitability for the calibration of hydraulic models. The study investigates the effect of satellite‐data uncertainty on the calibration of a quasi‐two‐dimensional (quasi‐2D) model of the middle‐lower portion of the Po river (~140 km). We refer to extended (~16 years of observations) ERS and ENVISAT altimetry products (i.e. River and Lake Hydrology data, RLH) to investigate the effect of (i) record length (i.e. number of satellite measurements at a given satellite track) and (ii) data uncertainty (i.e. altimetry measurements errors) on the calibration of the quasi‐2D model. We first present an assessment of ERS and ENVISAT altimetry errors and then perform the investigations in a Monte Carlo framework by generating datasets of synthetic altimetry products. The results of our analysis further emphasize the suitability of satellite data for the calibration of hydraulic models, providing also a quantitative assessment of the effect of the uncertainty of altimetry products. The analysis highlights the higher accuracy of ENVISAT data, which ensures a stable calibration with ~1.5 years of data (Mean Absolute Error, MAE, lower than 0.4 m, ~0.2 m of which results directly from the uncertainty of ENVISAT data). ERS‐based calibrations become stable with longer series (~3.5–5 years of data), and the negative effect of uncertainty in ERS data is higher (i.e. MAE of 0.6–0.9 m, of which 0.4–0.6 m results from the uncertainty of ERS measurements). Copyright © 2015 John Wiley & Sons, Ltd.     

On the potential of mapping sea level anomalies from satellite altimetry with Random Forest Regression

Ocean Dynamics - The sea level observations from satellite altimetry are characterised by a sparse spatial and temporal coverage. For this reason, along-track data are routinely interpolated into...     

Vertical crustal motions from differential tide gauge observations and satellite altimetry in southern Italy

Our goal is to determine vertical crustal movement rates from tide gauge and satellite altimetry measurements. Tide gauges measure sea level, but as they are fixed to the crust, they sense both sea surface height variations and vertical crustal movements. The differential sea level rates of sufficiently nearby stations are a good means to determine differential crustal movement rates, when sea level height variations can be assumed to be homogeneous. Satellite altimetric measurements determine sea surface height variations directly and can be used to separate the crustal signal from the sea surface height variations in tide gauge measurements. The correction of the tide gauge sea level rates for the sea surface height contribution requires collocation of the satellite pass and the tide gauge station. We show that even if this is not the case, the satellite altimetric observations enable correction of differential tide gauge rates for the effects of sea surface rate inhomogeneities.     

星间加速度解算模式的模拟与分析

低低卫星跟踪卫星的观测量是两低轨卫星的星间距离或星间速度,星间加速度由星间速度通过数值微分导出,用星间加速度作为观测量可以避免解算卫星运动的变分方程,简化观测方程的建立,但数值微分会使观测噪声放大,从而影响重力位的解算精度.为了定量给出星间加速度观测模式的精度,本文分析并模拟验证了数值微分公式计算星间加速度的精度,导出了基于星间加速度的一般形式的观测方程,模拟计算了基于星间加速度的重力位模型.结果表明,采用星间加速度观测模式的解算精度要明显低于星间速度观测模式的解算精度.     

卫星测高混合边值问题的球谐级数解法

研究了球界面下卫星测高问题的解法,利用有限逼近方法得到了下列结论：若陆地部分是球冠,则卫星测高问题的解可以转换成关于球谐级数位系数的线性方程组,并且位系数的阶和次是以分离形式出现的,从而确保该解法具有实用意义. 利用重力场360阶模型进行模拟计算的结果表明：该解法得到的位系数的相对精度达到了10－１１. 同时证明了常用的Stokes问题、Dirichlet问题、Neumann问题可以看成卫星测高问题的特殊情况.     

计算测高卫星的正常点及其平均海面高

利用测高卫星的冻结轨道特征计算了正常点轨迹,利用三次B样条函数和Stacking技术内插计算了T／P、ERS-1／2和Geosat测高数据在正常点的平均海面高,改善了测高数据的精度和分辨率．最后,利用正常点上的平均海面高反演了多种分辨率的海域重力异常,其中所反演的海域重力异常的标准差为mGal级．     

利用多年卫星测高资料研究南海上层环流季节特征

利用10年高精度卫星测高海面高异常网格资料,联合EGM96稳态海面地形模型,构成南海海域合成海面地形的时间序列,并计算了各个时期的南海表层地转流场. 利用卫星跟踪漂流浮标观测结果与相应时期南海地转流场进行对比验证,结果显示本文结果可以很好地反映南海海域一些中小尺度的环流特征. 根据南海各季节多年平均表层环流场结构,对南海环流周年变化规律和季节特征进行了初步的探讨. 研究结果表明,南海表层环流始终处在不断演变过程之中,在时间和空间上都表现出明显的多尺度特征.     

Determination of polar motion from satellite observations

Doppler observations of U.S. Navy Navigation Satellites have been used to determine the Earth's pole position for the period 1967 to July 1972. Recent results are competitive in accuracy with astronomic results and exhibit about 0".02 agreement with astronomic pole positions. The differences are no larger than those between independent calculations of pole positions based on astronomic data (BIHvs. ILS). For a two-day span of observations made by 18 Doppler stations the uncertainty in computed pole position corresponding to random errors in observation is 0".002. However, the results have systematic errors with various frequencies which are primarily due to uncertainties in the gravity coefficients. Since these errors are independent of those producing systematic errors in astronomic results, the two techniques are complementary. Recent results of computation of polar motion based on laser observations of artificial Earth satellites have also produced results having accuracies comparable to Doppler and astronomic results. The analysis of the laser observations is based on the effect of pole position on the apparent inclination of satellite orbits; on the other hand, since the Doppler observations are made on polar orbiting satellites, they are more sensitive to errors in the component of pole position which lies in the orbit plane. As a result of this difference, biases in Doppler and laser results may be different in size or character.     

Using satellite altimetry data to augment flow estimation techniques on the Mekong River

Satellite altimetry is routinely used to provide levels for oceans or large inland water bodies from space. By utilizing retracking schemes specially designed for inland waters, meaningful river stages can also be recovered when standard techniques fail. Utilizing retracked waveforms from ERS‐2 and ENVISAT along the Mekong, comparisons against observed stage measurements show that the altimetric measurements have a root mean square error (RMSE) of 0·44–0·65 m for ENVISAT and 0·46–0·76 m for ERS‐2. For many applications, however, stage is insufficient because discharge is the primary requirement. Investigations were therefore undertaken to estimate discharges at a downstream site (Nakhon Phanom (NP)) assuming that in situ data are available at a site 400 km upstream (Vientiane). Two hypothetical, but realistic scenarios were considered. Firstly, that NP was the site of a de‐commissioned gauge and secondly, that the site has never been gauged. Using both scenarios, predictions were made for the daily discharge using methods with and without altimetric stage data. In the first scenario using a linear regression approach the altimetry data improved the Nash‐Sutcliffe r² value from 0·884 to 0·935. The second scenario used known river cross‐sections while lateral inflows were inferred from a hydrological model: this scenario gave an increase in the r² value from 0·823 to 0·893. The use of altimetric stage data is shown to improve estimated discharges and further applications are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Contribution of satellite altimetry data in the geophysical investigation of the Red Sea Region, Egypt

Successful development of geodetic satellite missions has aroused new interest in determining global and regional gravity field based on satellite data. Satellite altimetry data enable direct determination of the geoid over sea regions. In Egypt, where land and marine geophysical data are inadequate because of rough topography and economic reasons, the use of satellite altimetry data is of special importance. The northern Red Sea region has been selected as a site for case study of the current research, after applying spectral analysis to reveal near-surface structure, the residual geoid of the studied region shows a good correlation with the known geologic features. Moreover, satellite-based gravity data enhance small-scale features and agrees well with land and marine gravity data. Thus, geoid undulation and satellite gravity data can be a complementary source of data to determine near-surface and deep structures.     

Bottom pressure tides along a line in the southeast Atlantic Ocean and comparisons with satellite altimetry

Seafloor pressure records, collected at 11 stations aligned along a single ground track of the Topex/Poseidon and Jason satellites, are analyzed for their tidal content. With very low background noise levels and approximately 27 months of high-quality records, tidal constituents can be estimated with unusually high precision. This includes many high-frequency lines up through the seventh-diurnal band. The station deployment provides a unique opportunity to compare with tides estimated from satellite altimetry, point by point along the satellite track, in a region of moderately high mesoscale variability. That variability can significantly corrupt altimeter-based tide estimates, even with 17 years of data. A method to improve the along-track altimeter estimates by correcting the data for non-tidal variability is found to yield much better agreement with the bottom-pressure data. The technique should prove useful in certain demanding applications, such as altimetric studies of internal tides.