Jesus Gomez-Enri

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.

Modeling Envisat RA-2 Waveforms in the Coastal Zone: Case Study of Calm Water Contamination

Radar altimeters have so far had limited use in the coastal zone, the area with most societal impact. This is due to both lack of, or insufficient accuracy in the necessary corrections, and more complicated altimeter signals. This letter examines waveform data from the Envisat RA-2 as it passes regularly over Pianosa (a 10-km2 island in the northwestern Mediterranean). Forty-six repeat passes were analyzed, with most showing a reduction in signal upon passing over the island, with weak early returns corresponding to the reflections from land. Intriguingly, one third of cases showed an anomalously bright hyperbolic feature. This feature may be due to extremely calm waters in the Golfo della Botte (northern side of the island), but the cause of its intermittency is not clear. The modeling of waveforms in such a complex land/sea environment demonstrates the potential for sea surface height retrievals much closer to the coast than is achieved by routine processing. The long-term development of altimetric records in the coastal zone will not only improve the calibration of altimetric data with coastal tide gauges but also greatly enhance the study of storm surges and other coastal phenomena.

Measuring Global Ocean Wave Skewness by Retracking RA-2 Envisat Waveforms

For early satellite altimeters, the retrieval of geophysical information (e.g., range, significant wave height) from altimeter ocean waveforms was performed on board the satellite, but this was restricted by computational constraints that limited how much processing could be performed. Today, ground-based retracking of averaged waveforms transmitted to the earth is less restrictive, especially with respect to assumptions about the statistics of ocean waves. In this paper, a theoretical maximum likelihood estimation (MLE) ocean waveform retracker is applied tothe Envisat Radar Altimeter system (RA-2) 18-Hz averaged waveforms under both linear (Gaussian) and nonlinear ocean wave statistics assumptions, to determine whether ocean wave skewness can be sensibly retrieved from Envisat RA-2 waveforms. Results from the MLE retracker used in nonlinear mode provide the first estimates of global ocean wave skewness based on RA-2 Envisat averaged waveforms. These results show for the first time geographically coherent skewness fields and confirm the notion that large values of skewness occur primarily in regions of large significant wave height. Results from the MLE retracker run in linear and nonlinear modes are compared with each other and with the RA-2 Level 2 Sensor Geophysical Data Records (SGDR) products to evaluate the impact of retrieving skewness on other geophysical parameters. Good agreement is obtained between the linear and nonlinear MLE results for both significant wave height and epoch (range), except in areas of high-wave-height conditions.

Coastal Altimetry Products in the Strait of Gibraltar

This paper analyzes the availability and accuracy of coastal altimetry sea level products in the Strait of Gibraltar. All possible repeats of two sections of the Envisat and AltiKa ground-tracks were used in the eastern and western portions of the strait. For Envisat, along-track sea level anomalies (SLAs) at 18-Hz posting rate were computed using ranges from two sources, namely, the official Sensor Geophysical Data Records (SGDRs) and the outputs of a coastal waveform retracker, the Adaptive Leading Edge Subwaveform (ALES) retracker; in addition, SLAs at 1 Hz were obtained from the Centre for Topographic studies of the Ocean and Hydrosphere (CTOH). For AltiKa, along-track SLA at 40 Hz was also computed both from SGDR and ALES ranges. The sea state bias correction was recomputed for the ALES-retracked Envisat SLA. The quality of these altimeter products was validated using two tide gauges located on the southern coast of Spain. For Envisat, the availability of data close to the coast depends crucially on the strategy followed for data screening. Most of the rejected data were due to the radar instrument operating in a low-precision nonocean mode. We observed an improvement of about 20% in the accuracy of the Envisat SLAs from ALES compared to the standard (SGDR) and the reprocessed CTOH data sets. AltiKa shows higher accuracy, with no significant differences between SGDR and ALES. The use of products from both missions allows longer times series, leading to a better understanding of the hydrodynamic processes in the study area.

Satellite Altimetry: sailing closer to the coast

In this chapter we review the history of coastal altimetry. We illustrate the challenges associated with data processing, improvement and exploitation, including: (1) what altimeter data are available today and what are the issues in coastal zones; (2) what efforts are underway to fill the gaps in coastal altimetry and what still needs to be done; (3) how coastal altimetry can be used in support of coastal oceanography. After nearly two decades of data collection near coasts, the planned reprocessing of the multi-mission global record now appears to be necessary for full exploitation of satellite altimetry for coastal oceanography. We will focus on the European research efforts, in particular the main outcomes of the COASTALT project, by showcasing improved corrections (with special emphasis on the wet tropospheric effect), waveform analysis and novel retracking techniques, as well as the structure of the new processor for Envisat RA-2 coastal records. This is of interest to a broad range of data integrators who will be able to use the improved altimeter data in their operational products or services.

ENVISAT Radar Altimeter Tracker Bias

In the past, errors in the determination of the orbit were dominant in radar altimeter missions, but technical advances have improved the orbit accuracy and hence, other sources of error have become more important. Sea-state bias is now the main source of error and can be divided into three sea-state dependent errors: skewness, electromagnetic bias, and tracker bias. We estimated the magnitude of the third term, by retracking ocean waveforms from ENVISAT RA-2. The retracking algorithm used is based on Maximum Likelihood Estimation. Tracker bias shows a seasonal and geographical dependence related to the distribution of significant wave height (SWH) and time origin differences. We estimated a mean value of 0.13 ± 0.07 %SWH. Temporal and regional dependent errors are introduced when using a linear retracker processing approach.

Validation of High Spatial Resolution Wave Data From Envisat RA-2 Altimeter in the Gulf of Cádiz

Extending the applications of satellite altimetry to the coastal zone requires validated, quality controlled data. We present a validation study in the Gulf of Cádiz (SW Iberian Peninsula), an area of relevant social, economic, and strategic importance. We compare against in-situ data seven years (Dec. 2002-Jan. 2010) of significant wave height (SWH) measurements by the Envisat RA-2 altimeter from two sources: 1) standard geophysical data records (GDR, 1 Hz) and 2) the processor developed in the COASTALT (development of radar altimetry data processing in the coastal zone) project (18 Hz). The comparison is made along two Envisat passes (one descending, i.e., north to south, one ascending). For the descending pass (land to ocean transition) the COASTALT processor improves SWH retrieval in the coastal fringe. In particular, the COASTALT SWH product displays accuracies in the sub-coastal strip (12-20 km from the coastline) of a very similar magnitude to those further offshore, representing a clear improvement over GDR. The bias and standard deviation of the difference with regard to the in-situ measurements in the coastal fringe is about 60% and 40% lower, respectively, when COASTALT data are used instead of the standard GDR product. For the ascending pass, the differences in the ocean to land transition are less marked, probably due to the altimeter keeping a good lock on the sea surface until relatively close proximity to the coastline. This validation case shows that this new coastal-oriented product gets closer to the shoreline than before, while also making available higher-resolution along-track estimates.

COASTALT: improving radar altimetry products in the oceanic coastal area

Fifteen years of global altimetry data over the coastal ocean lie, largely unexploited, in the data archives, simply because intrinsic difficulties in the corrections and issues of land contamination in the footprint. These data would be invaluable for studies of coastal circulation, sea level change and impact on the coastline. Amongst some initiatives, we describe here the COASTALT Project, funded by ESA. The main objective of the COASTALT Project is to contribute towards making the status of pulse-limited coastal altimetry operational. In this paper we will first illustrate the first project phase, based on the assessment of user requirements, and summarize those requirements. Then we will describe the COASTALT methodology and objectives. Finally, we will illustrate and discuss the various options for coastal waveform retracking, and present a plan for the validation of the retracked data. The first results in the radar altimeter waveform analysis show the complexity of the coastal signals due to land contamination and calm/rough waters.

Improving coastal altimeter products by a new retracking approach

Satellite altimetry has proved successful as a global tool for monitoring sea surface height, significant wave height and wind speed. Nevertheless, a global archive of 17 years of raw data from a series of missions is presently unexploited around the world coastline. This huge amount of unused data can be re-analyzed, improved and more intelligently exploited, possibly promoting coastal altimetry to the rank of operational service. Operational users interested in monitoring sea level change and wave conditions in the coastal zone (e.g. for coastal erosion, sediment/pollutant transport applications) still rely on sparse (and expensive) in situ monitoring stations or poor models. In this work we present a new approach in the exploitation of altimeter data in the coastal zone (currently impeded by unsuitable waveform retracking scheme and coarse along-track spatial sampling in the coastal zone, among others). The objective of this paper is to show how a new, robust, retracking algorithm is able to retrieve with high accuracy physical ocean parameters from altimeter waveforms in the coastal zone. The main focus lies on retrieving sea surface height in the coastal zone with the same precision as is achieved in the open ocean. In addition, the retrieval of more accurate altimeter-derived wave products in the coastal zone is also important as waves are more directly relevant to many operational applications in the coastal zone.

Seasonal and interannual patterns of chlorophyll bloom timing in the Gulf of Cádiz

Seasonal and interannual variability of the spring bloom in the Gulf of Cadiz, western North Atlantic, has been investigated using remote sensing chlorophyll-a (chl-a) concentration between 1997 and 2007. Variability in both the timing and magnitude of the spring bloom in the basin has been evaluated according to physical properties in the water column such as mixed layer depth (MLD), sea surface temperature (SST), sea level anomaly (SLA), absolute dynamic topography (ADT) and wind forcing. The analysis indicated that the timing, size and duration of the phytoplankton bloom in this zone is influenced by meteorological and oceanographic conditions, which means that it can vary widely from one year to another. These results in conjunction with previous studies performed in the area show that the timing of the spring bloom affects markedly the development of zooplankton, the survival of juvenile fish and the seasonality of the biological carbon pump in the Gulf of Cadiz.