BLANC - Blanc

SIgnature Micro-Onde du Déferlement et de l'Ecume – SIMODE

Micro-wave signature of breaking and foam

The main objective of this project is to better understand and interpret the influence of some phenomena associated to breaking waves in micro-wave ocean active and passive remote sensing. These phenomena encompass various aspects of the ocean surface: non-gaussian waves, steep and breaking waves, foam formation and whitecaps. Incorporating these effects in remote sensing models is essential for the retrieval procedure of oceanic geophysical parameters.

Understanding the influence of breaking phenomena in microwave ocean remote sensing

An important issue for the modelling of climate and global warming is the accurate estimation of geophysical parameters such as sea surface temperature (SST), sea surface salinity (SSS), mass transport, wave elevations and slopes and winds over the ocean surface. The retrieval of these parameters is one of the main objectives of many microwave ocean remote sensing missions. However, the success of these missions necessitates a correct understanding of the interaction of microwaves with the sea surface and a relevant physical description of the latter. In this context, the contribution of breaking waves is significant by strong wind speed . The main objective of this project is to better understand and interpret the influence of breaking on micro-wave ocean active and passive remote sensing which is essential for the retrieval procedure of the oceanic geophysical parameters. Yet, the signature of breaking waves and related phenomena in the microwave domain is not well understood. The difficulty lays in the variety of involved phenomena and their different contribution according to the remote sensing configurations.

The complex issue of the microwave signature of breaking cannot be dealt with in a unified framework but must be addressed separately according to the involved spatial scales, the scattering configuration and the EM frequency band. We identified the following tasks to address this problem:

Airborne measurements of simultaneous optical and radar backscattered signals over the ocean at large scale and coarse resolution. Design of statistical models for the description of breaking and its relation to the radar echo based on these datasets.
Wind-wave tank and stereographic imaging experiments for the analysis of non-gaussian wave fields at high-resolution
Numerical models for the Doppler signature of nonlinear waves at moderate and grazing incidence angles
Use of combined multi-frequency satellite data sets to improve the sea surface scattering models at small and moderate incidence by moderate and strong wind speeds.

Accurate and versatile microwave scattering models for non-linear sea surfaces at arbitrary incidence and moderate wind-speeds
A model for the normalized radar cross section by very strong wind speed based on an effective medium approach for the foam layer at small incidence.
Microwave Doppler signature of steep non-breaking waves at moderate and grazing angles.
Statistical and geometrical description of breaking and some empirical relations to the radar cross-section
Fine scale description of micro-breaking and short gravity-capillarity waves
Improvement of image processing of ocean scenes form airborne observations.

.Les objectifs initiaux ont été majoritairement remplis. La plupart des tâches du projet ont été traitées et la campagne expérimentale principale réalisée.
Des travaux non prévus initialement se sont avérés très pertinents pour le projet en cours de route (analyse de données de soufflerie et de données stéreographiques en mer). L'exploitation du signal radar temporel et du spectre Doppler s'est révélé un outil très utile pour caractériser les vagues proche-déferlement mais n'a pas encore été mise en œuvre expérimentalement.
L'analyse conjointe des données aéroportées (images optiques et signaux radar) s'est révélée d'une très grande complexité technique. Par ailleurs, il ressort que le déferlement simplement défini par son aspect visuel de moutonnement n'est pas une quantité suffisamment discriminante pour expliquer les échos radar. Une approche déterministe consistant à corréler des signaux instantanés (i.e. une image optique et un signal de rétrodiffusion moyenné sur une fraction de seconde) s'est révélée inappropriée. En revanche, une approche statistique basée sur l'analyse de quantités moyennées sur des durées plus longues (1 mn) a permis de dégager quelques premières tendances.
Le sujet est loin d'être clos et mérite d'être poursuivi. Avec le recul de l'expérience, il semble maintenant que des campagnes de mesures dans un environnement mieux contrôlé, typiquement des mesures radar en soufflerie, permettraient de s'affranchir de nombreuses difficultés techniques et aléas rencontrés avec lors de notre campagne en mer tout en illustrant les mécanismes physiques mis en jeu. De telles études pourront être menées dans un avenir proche au laboratoire MIO.

9 articles publiés dans des revues RICL, dont 5 multi-partenaires
10 communications dans des conférences internationales.
Le sujet d'étude ne se prête pas au dépôt de brevet.

The main objective of this project is to better understand and interpret the influence of some phenomena associated to breaking waves in micro-wave ocean active and passive remote sensing. These phenomena encompass various aspects of the ocean surface: non-gaussian waves, steep and breaking waves, foam formation and whitecaps. Incorporating these effects in remote sensing models is essential for the retrieval procedure of oceanic geophysical parameters (sea surface temperature and salinity, winds, currents). Yet, the signature of breaking waves and related phenomena in the microwave domain is not well understood. The difficulty lays in the variety of involved phenomena and their different contribution according to the remote sensing configurations. Today, there is no global hydrodynamic and electromagnetic model to describe the micro-wave interaction with the sea surface in presence of breaking waves. Reaching this goal implies a double challenge : 1) Theoretical approach. We propose a unified theoretical framework which can take into account all the relevant aspects of breaking waves microwave ocean remote sensing at moderate and strong winds. This description will include non-Gaussian wave statistics, the occurrence of breaking and resulting whitecaps. At the same time, it will remain sufficiently versatile from an analytical and numerical point of view. It will be combined with the latest developments in terms of scattering models. The typical configurations of remote sensing experiments will be addressed: active and passive sensing at moderate incidence (relevant for satellites and aircrafts) and active sensing at low-grazing angles (relevant for coastal and shipboard radars). 2) Experimental approach. We propose an experimental validation by co-localizing a maximum number of simultaneous measurements in the different configurations: in situ measurements, satellite data, optical images, radar and radiometric sensing at different bands. This study will rely on several existing datasets: - a recent radar campaign in L band at grazing angles (LSEET) - data from the CETP airborne scatterometer (STORM) and radiometer (CAROLS) - CERSAT satellite data: ASAR ENVISAT images, co-localized data from the QuickScat scatterometer, the WindSat radiometer and TOPEX and JASON altimeters. These data will be completed by an airborne campaign specifically designed for the means of the project. Since this study has a strong potential in spatial applications, it will be restricted to off-shore zones. Finite-depths aspects and bathymetric breaking will not be considered. The proposed team (LSEET, CETP, LOS) meets all required expertises to fulfil this study: hydrodynamic and electromagnetic modelling, spatial remote sensing, sea micro-wave active and passive remote sensing. Key words : ocean microwave remote sensing, breaking, foam, whitecap, sea surface radar cross section, sea surface emissivity.

Project coordinator

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

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Beginning and duration of the scientific project: - 0 Months

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