Conference Agenda

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Session Overview
S1: Waveforms and Retracking
Tuesday, 21/Feb/2017:
14:10 - 15:50

Session Chair: Marcello Passaro
Session Chair: Walter H. F. Smith
Session Chair: Pierre Thibaut

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14:10 - 14:30

From Deep Ocean to Inland Water: Homogeneous Retracker Solution for Continuous Observations

Pierre Thibaut1, Jean Christophe Poisson1, Thomas Moreau1, Fanny Piras1, Sophie Le Gac2, François Boy2, Nicolas Picot2

1CLS, France; 2CNES, France

Until now and for all altimetry missions, operational ground processing chains are implementing retracking solutions different for inland water and deep ocean surfaces. Physical retrackers are used for deep ocean (Hayne, Samosa). Threshold retrackers (OCOG or derivatives) are used for hydrology. Similar discrepancy is also observed between deep ocean and sea iced surfaces. The consequence of that dual processing schema is the introduction of potential water topography biases between areas upstream and downstream estuaries.

New physical model accounting for water surface roughness, associated with new numerical techniques provide us with a powerful solution, homogeneous whatever the overflown water surface. The aim of this talk is to illustrate the benefits of such a solution over selected estuaries and coastal regions.

14:30 - 14:50

ALES Coastal Processing Applied to ERS: Extending the Coastal Sea Level Time Series

Marcello Passaro1, Francisco M Calafat2

1Deutsches Geodätisches Forschungsinstitut der Technischen Universität München, Germany; 2National Oceangraphy Centre Liverpool, UK

The Adaptive Leading Edge Subwaveform (ALES) retracked dataset of Envisat and Jason time series, together with updated geophysical corrections, tidal and mean sea surface models, has shown that coastal altimetry is now in a mature stage in which it can be used for sea level variability studies. However, the relatively short period (2002-2015) covered by the current ALES dataset still represents a limitation to trend and long-term variability analyses. This study is dedicated to the application of the ALES concept to the ERS-1 and -2 satellite missions, which extends the time series up to 10 years, guaranteeing more than 20 years (1991-2012 along the same Envisat ground tracks) of coastal altimetry. The reference dataset, on which the reprocessing is based, is the ESA REAPER project, which provided updated ERS data with state-of-the-art waveform fitting (retracking) and corrections.

The first part is dedicated to the retracking strategy. The ALES algorithm selects only a portion of the altimetric signal (waveform), in order to estimate the distance between the satellite and the sea surface (range) while avoiding the noise in the tail of the signal. The ALES design based on the relation between estimated sea state, achievable precision and width of the subwaveform is adapted to ERS sampling characteristics. The dataset obtained with the adapted algorithm is then validated against tide gauge observations and the noise performances are analysed. Preliminary results show the possibility to improve the sea level estimation compared to the REAPER reprocessing currently available, but a specific compromise has to be found between the desired precision and the portion of the signal considered in the retracking, given the lower sampling frequency of ERS compared to Envisat and Jason.

In the second part, joint Envisat+ERS time series are built in the North Sea and in the Mediterranean Sea to investigate seasonal variability and trends in sea level at a regional scale. Estimates of the annual cycle and trends from the altimetric time series are used in combination with tide gauge observations to characterize the magnitude and the geographic variability of these two components of sea level. The new coastal altimetry dataset enables us to explore coastal sea level changes in regions where very few tide gauge stations are available such as along the North African coast in the Mediterranean Sea. Furthermore, by providing sea level observations closer to the coast, it allows us to investigate differences between coastal and open ocean sea level along the entire coast.

14:50 - 15:10

A New Retracking Technique for Brown-Peaky Altimetric Waveforms

Fukai Peng, Xiaoli Deng

School of Engineering, The University of Newcastle, New South Wales, Australia

Waveform retracking technique has been developed for many years to process the corrupted altimetric waveforms which do not conform to the Brown model. The peaky waveform shows a combination of the Brown-like waveform with single or multiple peaks. It usually appears near coastlines when land topography is partly covered by the altimeter footprint. Many attempts have been tested to reduce the effect of the peak on waveforms, such as the modified Brown model and sub-waveform retrackers. Here, we propose a new retracking method to retrieve high quality estimates of altimeter ranges from peaky waveforms. The method first detect the location of the peak(s), and then retracks the waveform using the weighted least squares estimation. The corrupted waveform gates are assigned to lower weights, from which the effect of peaks can be effectively reduced. When taking an accuracy criteria of the 1 cm tolerance with respect to the full waveform retracker for the Epoch estimates by Passaro et al. (2014), the new retracker can achieve the required accuracy with up to 30 gates corresponding to the width of the peak assigned to lower weights.

The new retracker has been applied to the simulated and real Jason-1 waveforms. The simulated results show that not only the Epoch estimate can achieve 1cm accuracy level, but also the estimated significant wave height (SWH) is in the order of several centimetres. The results also show that the ALES (Passaro et al. 2014) cannot handle selected sub-waveforms affected by the peak, resulting in the estimation accuracy degraded significantly. The results of retracked Jason-1 waveforms have been compared to those from MLE4 and ALES, as well validated against in-situ tide-gauge observations. Four tide gauges (Darwin, Burnie, Lorne and Broome) are chosen around the Australian coastal zone. The comparison results with MLE4 and ALES show that all retrackers have similar performance over open oceans with the correlation coefficient of ~0.7 between altimetric and tide-gauge sea-level anomalies (SLAs). While near the coast, especially within ~7km to the coast, the performance of our new retracker is superior to MLE4 and ALES for that it can extend high correlated SLAs to 2.5km off the coast at the study area.

15:10 - 15:30

Mission-Independent Classification of Altimeter Waveforms for Applications in the Open Ocean, at the Coastal Zone and Over Land

Christian Schwatke, Denise Dettmering

DGFI-TUM, Germany

Altimeter waveforms are the basis for range and water level determination, which are estimated by waveform retracking. But waveforms also contain additional information about the surface type covered by the footprint of the radar measurement. Depending on the surface, the waveform shapes vary strongly between brown-like, brown-like with peaks, specular, etc. The knowledge of the waveform shape can be very helpful for different applications. For example, this information can be used for detecting sea-ice areas in the open ocean. In coastal regions, the classification can be used to distinguish between ocean, land, and corrupted ocean waveforms. Over land, the waveform classification can be used to identify inland water bodies such as lakes, reservoirs, rivers, and wetlands.

In this contribution, we present our new approach for a mission-independent classification of altimeter waveforms that separates different waveforms by its shapes into more than 50 different classes such as ocean-like, specular, etc. Hereby, a combination of function fitting and statistical criteria is applied for classifying the altimeter waveforms. An advantage of our classification method is that it works for all altimeter missions because it is independent from the waveform length and the measurement band. We demonstrate our approach by using waveforms of classical altimeter missions such as Envisat, SARAL, and Jason-3 but also input data from the SAR missions Cryosat-2 and Sentinel-3A. The performance of our classification will be shown in detail for different study cases such as ocean, coast, lakes, rivers, etc.

15:30 - 15:50

High-Rate Radar Altimeter Waveform Signatures of Internal Solitons in Tropical Marginal Seas

Jose Carlos Da Silva, Jorge Manuel Magalhaes, Ana Fernandes Cerqueira, Eliana Vieira

University of Porto, Portugal

We report efforts for detection and recognition of short-period internal waves, also many times described in the literature as Internal Solitary Waves (ISWs), in high-rate along-track waveform records of the Jason-2/3 altimeters and SARAL Altika. A synergetic observation approach is developed for the identification of ISWs in altimeter data, based on validation with imaging radars and high-resolution optical sensors (measuring sun-glint patterns on the water surface). Geophysical parameters obtained from available SGDRs were processed and analyzed for regions of tropical marginal Seas where the existence of high-amplitude ISWs is known, namely: Andaman Sea, Sulu Sea, Celebes Sea, South China Sea, Red Sea, Northwest Australian Shelf, and Strait of Gibraltar. Evidence of modulation of several geophysical parameters is presented, namely: the “off-nadir-angle” available from the MLE4 retracking algorithm; sigma0 as retrieved from MLE4; significant wave height (SWH); and the differenced mean square slope calculated from the dual-band Jason-2/3 sigma0 measurements. The ISW signatures are sometimes recognized as parabolic-like shape sigma0 anomalies in the along-track radargram. These anomalies are mostly recognized as sigma0 positive anomalies which are related to short-event sigma0 blooms that have been reported in the literature (Mitchum et al., 2004; Tournadre et al., 2006; Dibarboure et al., 2014). On some occasions however, sigma0 negative anomalies with parabolic-like shape have also been observed back-to-back with the positive anomalies. We suggest that these consecutive negative/positive anomalies are associated to enhanced and decreased surface roughness produced by ISWs. This is consistent with some of our records of differenced mean square slope calculated from the dual-band Jason-2/3 altimeters. It is suggested that the improved MLE4 Brown model provides the capability to absorb the waveform distortion as ‘‘off-nadir angle’’ and sigma0, since the true pointing of Jason-2/3 is very good, so the retracked off-nadir angle is only apparent. With MLE4 the slope of the trailing-edge parameter and the sigma0 estimate are absorbing the bulk of the backscattering ISW event, for the case of pulse-limited altimeters, when the outer rings of the waveform footprint are affected. Hence, oceanography users interested in short-period internal wave signals may find useful information in 20-Hz rate Jason-2/3 current altimeter products. Development of better editing and postprocessing algorithms on the 20-Hz rate of current products is needed if we want to account for ISWs in coastal regions. New SARAL altimeter waveforms are compared with Jason-class high-rate altimeter waveforms related to ISWs.

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