Laurent Roblou

Post-processing Altimeter Data Towards Coastal Applications and Integration into Coastal Models

Altimetry missions in the last 16 years (TOPEX/Poseidon, ERS-1/2, GFO, Jason-1 and ENVISAT) and the recently-launched Jason-2 mission have resulted in great advances in deep ocean research and operational oceanography. However, oceanographic applications using satellite altimeter data have become very challenging over regions extending from near-shore to the continental shelf and slope (Cipollini et al. 2008). In these regions, intrinsic difficulties in the corrections (e.g., the high frequency ocean response to tidal and atmospheric loading, the mean sea level, etc.) and issues of land contamination in the radar altimeter and radiometer footprints result in systematic flagging and rejection of these data. Forthcoming altimeter missions (SARAL/AltiKa, SWOT, Sentinel-3, etc.) are designed to be better-suited for use in the coastal ocean. However, a number of studies have dealt with the problem of re-analysing, improving and exploiting the existing archive to monitor coastal dynamics. The early encouraging results (Vignudelli et al. 2005; Bouffard et al. 2008, Birol et al. submitted J Mar Syst 2009) support the need for continued research in coastal altimetry, with the opportunity of providing input and recommendations for future missions.

X-track, a new processing tool for altimetry in coastal oceans

Oceanographic applications using satellite altimeter data become very challenging when leaving the deep ocean for the coastal regions. Close to the coast, altimeter observations are often of lower quality for a number of reasons, including land contamination of the satellite footprints or inaccurate resolution of the corrections of the high frequency ocean response to tidal and atmospheric loading. This paper presents a new processing toolbox, called X-TRACK, to derive improved altimeter products, such as sea surface heights (SSH), mean sea surface heights (MSSH) or sea level anomalies (SLA), dedicated for coastal applications. Starting from classical geophysical data records (GDR) products, particular attention is made to recover a maximum amount of exploitable data (dedicated data editing, interpolation of missing corrective terms). Where possible, local modelling of the high frequency response of the ocean to the tidal and atmospheric loading is applied instead of standard, global corrections given in the GDRs. In addition, orbit errors are reduced by a stability criterion and a high resolution mean sea surface consistent with the altimeter data set is computed along the satellite ground track. This poster presents promising results obtained in various coastal areas.

15 Years of Altimetry at Various Scales over the Mediterranean

The present article reviews the application of altimetry in the Mediterranean Sea, to study the circulation and sea surface height variability, both at basin scale and in specific regions. The improvements needed to fully exploit the 15-year record of data close to the coast are also discussed. These range from improved tidal models, to specialized atmospheric corrections, to ad hoc screening of data in proximity of the coast. Some of these improvements are already underway while others are the focus of forthcoming programs.

Exploiting satellite altimetry in coastal ocean through the ALTICORE project

Altimeter-derived information on sea level and sea state could be extremely important for resolving the complex dynamics of the coastal ocean. Satellite altimetry was not originally conceived with coastal ocean in mind, but future missions (AltiKa and CryoSat-2) promise much improved nearshore capabilities. A current priority is to analyze the existing, under-exploited, 15-year global archive of coastal altimeter data to draw recommendations for these missions. There are intrinsic difficulties in processing and interpretation of the data, e.g.: the proximity of land, control by the seabed, and rapid variations due to tides and atmospheric effects. But there are also unexploited possibilities, including higher along track data rates and multi-altimetry scenarios that need to be explored. There are also difficulties of accessing and manipulating data from multiple sources, many of which undergo regular revision and enhancement. In response to these needs, the ALTICORE (ALTImetry for COastal REgions - www.alticore.eu) project started in December 2006, funded for two-years by the European INTAS scheme (www.intas.be). The overall aim of ALTICORE is to build up capacity for provision of altimeter-based information in support of coastal ocean studies in some European Seas (Mediterranean, Black, Caspian, White and Barents). ALTICORE will also contribute to improved cooperation between Europe and Eastern countries and enhance networking of capacity in the area of satellite altimetry. This paper discusses the approach, summarizes the planned work and shows how the coastal community should eventually benefit from better access to improved altimeter-derived information.

Satellite radar altimetry from open ocean to coasts: challenges and perspectives

The history of satellite radar altimetry stems from the need to capture a global view of the surface topography of the oceans. As altimeters are specifically designed for global observations, they encounter major problems in coastal regions, such as relatively poor sampling and inaccuracy of the corrections, so measurements are generally discarded. Nevertheless, a global archive of 15 years of raw data from a series of missions is presently available. The huge amount of unused data in coastal regions can be re-analyzed, improved and more intelligently exploited, possibly promoting coastal altimetry to the rank of operational service. This paper outlines the obstacles limiting the use of the data, discusses some areas of improvement, shows the lessons learned from a case-study in the Mediterranean Sea, and shows that the improved coastal altimetry concept can be extended to other regions, e.g. along the coasts of India. This paper also explores the implications of adopting the emerging vision of the Internet infrastructure in the coastal altimetry context: a collection of unstructured information becomes a network of linked data and software, necessary to perform the specialized on-the-fly processing of the raw data to provide ready-to-use geophysical parameters such as sea level and significant wave height.

Reprocessing Altimeter Data Records along European Coasts: Lessons Learned from the Alticore Project

A coastal-oriented processing strategy has been developed in the Northwestern part of the Mediterranean Sea and has showed that improved altimetry in the coastal ocean is feasible and could be extended to other regions. In this work, we will provide an overview of current capabilities and challenges of existing altimetry products in Black, Caspian, White and Barents seas, in the prospect of increasing the quantity and quality of data in these regions. With respect to the work done in the project called ALTICORE (ALTImetry for COastal REgions - www.alticore.eu), the obstacles limiting the use of the data and the possible areas of improvement are highlighted and discussed.

ALTICORE - A consortium serving european seas with coastal altimetry

In this paper, we describe the ALTICORE (value added satellite ALTImetry in COastal REgions) initiative, a consortium aiming at providing high quality coastal altimetry over some European seas. Taking the Ligurian Sea in the NW Mediterranean as an example, which acts as a test zone for this work, we show the improvement in availability and quality of ENVISAT data, through our processing, when compared with the official altimetric products delivered by AVISO. We also introduce the building concepts of solutions for data search, extraction, update and delivery based on web-services. This grid-type infrastructure is being designed within ALTICORE.

Interactions aérosols-rayonnement-climat en région méditerranéenne : Impact de l'effet radiatif direct sur le cycle de l'eau

An experimental campaign, coupled with three-dimensional modeling, was conducted in the western Mediterranean during the summer of 2013 to study the impact of aerosols on the radiative balance and climate of this region. In situ observations were obtained on the ground, aboard two research aircraft and balloons to characterize the physico-chemical and optical properties of particles and their vertical stratification. This campaign was mainly characterized by moderate events of desert aerosols. During these episodes, strong vertical stratification was observed and the measurements of the optical properties reveal moderate absorbing particles in the visible spectrum. Climate simulations indicate a significant impact of aerosols in particular by changing the surface temperature of the sea, the ocean-atmosphere fluxes and consequently seasonal precipitation.