Xiaoli Deng

Retracking Altimeter Waveforms Near the Coasts

There has been considerable interest in the past few years in addressing some of the long-term technical difficulties associated with retrieving valid measurements from satellite altimeters in coastal areas, where high levels of human activities are putting increasing demand for information about sea level, wind and wave conditions. Developments of altimeter waveform retracking techniques, together with the now-established practice of giving users access to altimeter waveform data, has led to rapid progress in our understanding of the challenges posed by waveform shapes in the vicinity of land. In this chapter, we present observational evidence of the huge diversity and complexity of waveforms seen by contemporary altimeters in coastal areas. We proceed with a review of waveform retracking methods, examining first empirical methods, then so-called physically-based methods, including discussion of some of their implementation intricacies. We proceed with providing examples of the application of waveform retracking methods to coastal altimeter waveforms in coastal regions around the world. Finally, we explore some of the new ideas on how it may be possible to exploit prior knowledge, for example about the statistics or the along-track evolution of ocean properties in the coastal domain, to improve the estimation of geophysical parameters. Innovative schemes, such as iterative retracking or simultaneous batch retracking, are discussed as new ways to yield unbiased parameter estimation for land-contaminated waveforms much closer to the land/water interface than is currently possible.

Validation of Jason-2 Altimeter Data by Waveform Retracking over California Coastal Ocean

We validated Jason-2 satellite altimeter Sensor Geophysical Data Records (SGDR) by retracking 20-Hz radar waveforms over the California coastal ocean using cycles 7–34, corresponding to September 2008–June 2009. The performance of the ocean, ice, threshold, and modified threshold retrackers are examined using a reference geoid based on Earth Gravitational Model 2008 (EGM08). Over the shallow ocean (depth < 200 m), the modified threshold retracker, which is developed for noisy waveforms with preleading edge bump, outperforms the other retrackers. It is also shown that retracking can improve the precision of sea surface heights (SSHs) for areas beyond 2–5 km from the shore. Although the ocean retracker generally performs well over the deep ocean (depth > 200 m), the ocean-retracked SSHs from some of the cycles are found to be less precise when the waveforms do not conform to the Brown ocean model. We found that the retrackers developed for nonocean surfaces can improve the noisy ocean-retracked SSHs. Among the retrackers tested here, the ice retracker overall provides the most precise SSH estimates over the deep ocean in average using cycles 7–34 in the study region.

Climate Impact Risks and Climate Adaptation Engineering for Built Infrastructure

AbstractA changing climate may increase the frequency or intensity of natural hazards, resulting in increased infrastructure damage. The paper will describe how risk-based approaches are well suited to optimising climate adaptation strategies related to the construction, design, operation, and maintenance of built infrastructure. Climate adaptation engineering involves estimating the risks, costs, and benefits of climate adaptation strategies and assessing at what point in time climate adaptation becomes economically viable. Stochastic methods are used to model infrastructure performance, risk reduction, and effectiveness of adaptation strategies, exposure, and costs. These concepts will be illustrated with recent research on risk-based life-cycle assessments of climate adaptation strategies for Australian housing subject to extreme wind events. This will pave the way for more efficient and resilient infrastructure, and help future proof new and existing infrastructure to a changing climate.

Satellite Altimetry for Geodetic, Oceanographic, and Climate Studies in the Australian Region

This chapter provides an overview of recent research applications utilizing satellite altimetry around Australia. Topics covered include improving the quality of altimeter sea surface height (SSH) data in coastal regions, observing and understanding the structure and variability of the major boundary current systems, estimating regional sea-level changes, and determining and verifying the marine gravity field using altimetry. The approaches highlighted in this chapter use altimetry synergistically with all available oceanic data including other remote sensing techniques, drifting buoys, and in situ data such as coastal tide-gauges. The results presented are an integration of altimetric and in situ data with a high-resolution computer model in order to simulate the sea-level changes in Australian coastal and offshore regions. Through such synthesizing research approaches, satellite altimetry continues to make an important contribution to a number of key strategic research areas in the Australasian region.

The importance of coastal altimetry retracking and detiding: a case study around the Great Barrier Reef, Australia

A new approach for improving the accuracy of altimetry-derived sea level anomalies (SLAs) near the coast is presented. Estimation of SLAs is optimized using optimal waveform retracking through a fuzzy multiple retracking system and the most appropriate detiding method. With the retracking system, fuzzy-retracked SLAs become available within 5 km of the coast; meanwhile it becomes more important to use pointwise tide modelling rather than state-of-the-art global tidal models, as the latter leave residual ocean tide signals in retracked SLAs. These improvements are demonstrated for Jason-2 waveforms in the area of the Great Barrier Reef, Australia. Comparing the retrieved SLAs with in situ tide gauge data from Townsville and Bundaberg stations showed that the SLAs from this study generally outperform those from conventional methods, demonstrating that adequate waveform retracking and detiding are equally important in bringing altimetry SLAs closer to the coast.

The Retracking Technique on Multi-Peak and Quasi-Specular Waveforms for Jason-1 and Jason-2 Missions near the Coast

This article presents the waveform retracking technique for two predominant waveform classes near the coast: quasi-specular and multi-peak echoes. The technique retracks the truncated sub-waveform rather than the full-waveform. Sub-waveforms, which are only based on returns reflected from the water surface, are extracted and retracked using the Brown model. The retracker is applied to simulated waveforms and Jason-1 and Jason-2 waveforms in the Great Barrier Reef. When using it as a supplement to the full-waveform retracker, results show that retracked SSHs can be extended further to the coastline, up to ∼2–6 km for Jason-1 and ∼1–7 km for Jason-2.

Marine Gravity Anomaly from Satellite Altimetry: a Comparison of Methods over Shallow Waters

Gravity anomalies over shallow waters are useful in many geodetic and geophysical applications. This work compares three methods of gravity anomaly derivation from altimetry over shallow waters near Taiwan: (1) compute gravity anomalies by LSC using along-track, differenced geoidal heights and height slopes, (2) compute gravity anomalies by least-squares collocation (LSC) using altimeter-derived along-track deflections of vertical (DOV), and (3) grid along-track deflections of vertical by LSC and then compute gravity anomalies by the inverse Vening Meinesz formula. A nonlinear filter with outlier rejection is applied to along-track data. We used altimeter data from Seasat, Geosat, ERS-1, ERS-2 and TOPEX/POSEDION missions. Retracked ERS-1 waveforms are shown to improve the accuracy of estimated gravity anomalies. For the three methods, the RMS differences between altimetry-derived gravity anomalies and shipborne gravity anomalies are 9.96 (differenced height) and 10.26 (height slope), 10.44 and 10.73 mgals, respectively. The RMS differences between shipborne gravity anomalies with gravity anomalies from retracked and non-retracked ERS-1 waveforms are 11.63 and 14.74 mgals, indicating retracking can improve the accuracy.

Geodesy – Introduction to Geodetic Datum and Geodetic Systems

Introduction.- Geodetic Data Collection Techniques.- Geodetic datum and Geodetic Control Network.- Geoid and Height System.- Reference Ellipsoid and Geodetic Coordinate System.- Gauss and UTM Conformal Projection and Plane Rectangular Coordinate System.- Establishment of Geodetic Coordinate System.

Assessment of Geoid Models Offshore Western Australia Using In-Situ Measurements

Abstract In Western Australia, coastal dynamics are influenced by a major ocean boundary current system, the Leeuwin Current, which is characterised by mesoscale features. To fully understand the Leeuwin Current using satellite altimeter measurements, we must have a precise (1–2 cm) and full-spatial-scale (<100 km) geoid model. This paper focuses on a comparison between two mean dynamic ocean topography models derived from independent hydrographic climatologies, and an altimeter-observed mean sea surface referenced to recently released geoid models offshore of Western Australia (20°S to 45°S, 108°E to 130°E). The geoid models used include combined global geopotential models from the GRACE satellite mission and AUSGeoid98. The estimated mean dynamic ocean topography models are compared with independent dynamic ocean topography from CSIROs Atlas of Regional Seas (CARS) climatology. The results show that the EIGEN_GL04C and GGM02C + EGM96 global geopotential models to degree and order 360 give the best comparisons against CARS in the Leeuwin Current region, suggesting that they should be used in the future for computing ocean transport, surface current velocities, and dynamic topography, and be used as a reference field for future computations of regional marine geoid models.

An iterative coastal altimetry retracking strategy based on fuzzy expert system for improving sea surface height estimates

This paper improves the accuracy of altimeter-derived sea level anomalies (SLAs) near coast through an iterative waveform retracking system. The principle of this system is twofold. First is to reprocess the altimeter waveforms using the optimal retracker, which is searched base on the analysis from a fuzzy expert system. Second is to minimize the relative offset in the retrieved SLAs when switching from one retracker to another, using a neural network. The system reprocesses 20-Hz waveforms from Jason-2/OSTM in the Great Barrier Reef, Australia. When compare the retrieved SLAs with tide gauge data from Townsville and Bundaberg stations, results show the SLAs from this study generally outperform SLAs from MLE4 and Ice retrackers. It yields higher correlations (≥0.8) and smaller root mean square errors (≤16.6 cm) than those of MLE4 (≤0.78 and ≤19 cm) and Ice (≤0.78 and ≤18.7 cm) retrackers.