Jérôme Benveniste

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.

SAR Altimeter Backscattered Waveform Model

The backscatters power single-look waveform recorded by a synthetic aperture radar altimeter is approximated in a closed-form model. The model, being expressed in terms of parameterless functions, allows for efficient computation of the waveform and a clear understanding of how the various sea state and instrument parameters affect the waveform.

ACE2: The New Global Digital Elevation Model

Detailed accurate Digital Elevation Model (DEM) data have historically not been available on other than a regional scale, and often have uncertainties in both vertical and horizontal precision.

Annual sea level variability of the coastal ocean: The Baltic Sea‐North Sea transition zone

The annual cycle is a major contribution to the non-tidal variability in sea level. Its characteristics can vary substantially even at a regional scale, particularly in an area of high variability such as the coastal ocean. This study uses previously validated coastal altimetry solutions (from ALES dataset) and the reference ESA Sea Level Climate Change Initiative dataset to improve the understanding of the annual cycle during the Envisat years (2002-2010) in the North Sea - Baltic Sea transition area. This area of study is chosen because of the complex coastal morphology and the availability of in-situ measurements. To our knowledge, this is the first time that the improvements brought by coastal satellite altimetry to the description of the annual variability of the sea level have been evaluated and discussed. The findings are interpreted with the help of a local climatology and wind stress from a reanalysis model. The coastal amplitude of the annual cycle estimated from ALES altimetry data is in better agreement with estimations derived from in-situ data than the one from the reference dataset. Wind stress is found to be the main driver of annual cycle variability throughout the domain, while different steric contributions are responsible for the differences within and among the sub-basins. We conclude that the ALES coastal altimetry product is a reliable dataset to study the annual cycle of the sea level at a regional scale and the strategy described in this research can be applied to other areas of the coastal ocean where the coverage from the tide gauges is not sufficient.

Measurements and Observations in the XXI century (MOXXI): innovation and multi-disciplinarity to sense the hydrological cycle

ABSTRACT To promote the advancement of novel observation techniques that may lead to new sources of information to help better understand the hydrological cycle, the International Association of Hydrological Sciences (IAHS) established the Measurements and Observations in the XXI century (MOXXI) Working Group in July 2013. The group comprises a growing community of tech-enthusiastic hydrologists that design and develop their own sensing systems, adopt a multi-disciplinary perspective in tackling complex observations, often use low-cost equipment intended for other applications to build innovative sensors, or perform opportunistic measurements. This paper states the objectives of the group and reviews major advances carried out by MOXXI members toward the advancement of hydrological sciences. Challenges and opportunities are outlined to provide strategic guidance for advancement of measurement, and thus discovery.

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.

On the Azimuthally Anisotropy Effects of Polarization for Altimetric Measurements

We have investigated the effect of the radar-altimeter antenna polarization on European Remote-sensing Satellite and Envisat observations of the media penetrable by a radar microwave such as ice sheets. This effect is due to the complex interaction between the radar wave, the subsurface backscatter, and the antenna polarization direction. It is modulated by the angle between the antenna polarization and the direction of the anisotropy of the target. Thus, it depends on both the anisotropy direction and the interaction between the radar wave and the reflecting surface. This effect leads to one of the most complex and least understood errors of radar altimetry over ice sheets and can be clearly identified when looking at the crossover differences between ascending and descending satellite tracks. The crossover differences are as large as a few decibels for a backscattering coefficient and a few meters for height, and affect more strongly the Ku-band than the S-band. This causes limitations and difficulties for the processing of altimetric observations, for instance when comparing time series from different satellites whose polarization geometry differs. This will be the case when a new altimeter will fly on a different orbit, as planned for CryoSat. Nevertheless, the ability of both the roughness anisotropy direction and the subsurface modulation to be inverted with satisfactory precision by using simultaneous observations at crossover points between two different satellites is demonstrated here. Thus, it offers a unique way of describing this error accurately to correct for it

Global Analysis of Envisat RA-2 Burst Mode Echo Sequences

The Envisat RA-2 burst echoes are being gathered throughout the mission; however, these data are only now being made generally available by the European space agency. Considerable work has been necessary to turn these engineering-level data into a useable altimeter product. This paper documents the processing steps undertaken to generate usable data and presents the first extensive analysis of this unique dataset, using over 75 000 burst sequences with a global distribution. The results show that the burst echo data from Envisat are of extremely low noise, which is particularly evident over non-ocean surfaces, and contain a wealth of detailed information from both land and ocean surfaces. Examples illustrate the complexity of surface response from targets such as inland water and rough terrain. These unique data clearly have the potential to inform future instrument design as well as to improve the understanding of existing altimeter datasets.

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.

Global Analysis of EnviSat Burst Echoes Over Inland Water

Satellite radar altimeters have an established and increasingly vital role in monitoring the Earths surface inland water resources. This paper analyzes two years of EnviSat burst echoes (which have 3.9-m nominal along-track separation) over rivers, lakes, and ephemeral water globally to assess the increase in monitoring potential afforded by the higher pulse repetition frequency (PRF) of the next generation of synthetic aperture radar altimeters and investigate spatial burst correlation. Burst echoes at 1800 Hz are successfully retracked with no waveform averaging. The conclusion is that the higher PRF allows detection and measurement of water bodies on a far finer spatial scale because water is a very bright reflector at Ku-band and land in general is relatively poor, with pools of water a few tens of meters across being successfully identified.