Speckle noise is inherent to radar measurements. For applications which need both a high temporal and high spatial resolution, a classical method for the reduction of the speckle noise by filtering the backscattered signal may not be sufficient. In particular, when radar observations are used to estimate ocean wave spectra from relative fluctuations of the radar signal within a given footprint, a method must be implemented to correct for the speckle effect in the Fourier domain (i.e., density spectrum). A theoretical background to model the speckle density spectrum for a radar with near-nadir incidences was proposed by Jackson in 1981 but it is based on a stationary sea surface assumption and ignores the variation of the main factor in the four-frequency moment near the origin. In this article, we revisit this theoretical background to extend this model to a time-varying sea surface and alleviate some assumptions on the Fresnel phase formulation. The results from the model applied in the configuration of an airborne system indicate that not only the displacement of the radar but also the dynamic properties of the sea surfaces have a significant effect on the speckle noise spectrum in certain directions of observations. The effects depend on the radar look direction in azimuth, and on sea surface conditions (wind speed, wind direction with respect to the aircraft route, surface wave spectrum). This new model is validated against observations of the airborne near-nadir incidence scatterometer-Ku-band Radar for Observation of Surfaces (KuROS). We show in particular that the errors between the experimental estimation of the omni-directional speckle noise spectrum from KuROS and the prediction by our model are below 10%.

The CFOSAT (China France Oceanography Satellite) mission launched in 2018 now routinely provides at the global scale, directional spectra of ocean waves. The principle is based on the analysis of the normalized radar cross-section measured by the instrument SWIM (Surface Waves Investigation and Monitoring), a near-nadir pointing Ku-Band real-aperture scanning radar. From the ocean wave spectra derived from SWIM, the principal parameters of ocean wave spectra as significant wave height, peak wavelength, and peak direction are now available to better characterize the sea-state. However, it is known that these principal parameters are not sufficient not fully characterize the distribution of wave energy and understand or validate the physical processes impacting its evolution during growth order decay. Here we show that the parameters characterizing the shape of the wave spectra (e.g directional and frequency spread) can be estimated at the global scale from the SWIM measurements. We also show that they can provide consistent values of the Benjamin-Feir index, an index proposed to estimate the probability of extreme waves. Similarities of differences with the shape parameters of the MFWAM numerical wave model are also discussed.

Transformation of spectral shape during wind wave development and the transition from the spectrum of developing waves to the spectrum of fully developed waves are well documented in measurements, but have so far escaped all modelling, as well as theoretical explanation. Numerical models of long-term wind wave evolution are based on the Hasselmann kinetic equation (KE). The KE predicts strict self-similarity beyond the initial several thousand characteristic periods of wave development, and therefore cannot describe the subsequent change of spectral shape. Instead, it predicts that the self-similar spectral shape, with a steep front and an enhanced peak, holds at arbitrary fetch, notwithstanding the experimental evidence that mature waves are characterised by the much wider Pierson-Moskowitz spectral shape. To resolve the contradiction, we perform long-term modelling of wind wave evolution by direct numerical simulation (DNS), based on the Zakharov equation. We model a particular class of situations when the wave field at hand is generated by a strong quasi-stationary offshore wind jet, which is caused by pressure differences and accelerates passing through a valley into the sea. Examples of such phenomena are the Tehuano event off the Pacific coast of Mexico, and the Mistral in the northern Mediterranean. Modelling results are compared with the airborne observations of waves generated by these winds, collected during GOTEX and HYMEX experiments respectively. In parallel we also perform numerical simulations with the Hasselmann kinetic equation and the generalised kinetic equation. For modelling of waves off the Mexican coast, wind data are taken from measurements during the GOTEX experiment, and the initial conditions from the measured spectrum at the moment when wind waves prevail over swell after a short initial part of the evolution. Waves in the Mediterranean Sea are modelled with constant wind forcing and zero initial condition. We show that the evolution of integral characteristics, e.g. significant wave height and wave steepness, is reproduced reasonably well by all modelling approaches. However, the spectral shape of developed waves demonstrates a large discrepancy between, on the one hand, the measured spectra and the DNS modelling and, on the other hand, spectra modelled by both kinetic equations. At the intermediate and advanced stage of development, both measured spectra and the DNS spectra tend to Pierson-Moskowitz spectral shape, while the modelling based on the kinetic equations invariably predicts spectra with a higher, more pronounced peak. In terms of the parameter of spectral peakedness, a commonly convenient measure of spectral shape, there is a large (of order one) discrepancy. We propose a theoretical explanation of the discrepancy as being due to the neglect of non-gaussianity in the derivation of the kinetic equations, and provide a numerical confirmation of this hypothesis.

La compréhension et la prévision des vagues relèvent d’un enjeu économique et sociétal d’une part, notamment pour la navigation, les activités pétrolières et côtières, et d’un enjeu climatique d’autre part car les vagues participent aux échanges entre l’océan et l’atmosphère. Ces échanges se font par le biais de processus physiques complexes. L’observation des vagues par les instruments in situ et par télédétection a permis d’améliorer la représentation des vagues dans les modèles. Cependant, l'amélioration des concepts instrumentaux et l’amélioration de la représentation des processus physiques liés à l’évolution des vagues dans les modèles nécessitent de poursuivre des travaux de recherche en physique de la mesure par télédétection radar de la surface océanique. Le radar SWIM, à bord de la mission CFOSAT, est un nouveau concept dédié à la mesure des spectres de vagues. Ce système permet de fournir des informations détaillées sur les vagues à l’échelle globale. Afin de préparer la mission CFOSAT, un radar aéroporté a été développé par le CNRS-LATMOS avec le soutien du CNES. Ce radar, dénommé KuROS, utilise un principe et une géométrie de mesure comparables à ceux relatifs au radar SWIM. Les objectifs de cette thèse sont doubles : d’une part comprendre les performances et les limites du radar aéroporté KuROS, et d’autre part caractériser l’apport de l’information spectrale observée à moyenne échelle par un tel concept. La première partie de la thèse traite de la comparaison des paramètres spectraux des vagues mesurées par KuROS, lors de deux campagnes de mesure, avec des données issues de modèles de vagues. On montre en particulier que les situations pour lesquelles la longueur d’onde des vagues est inférieure à 200 m et leur hauteur significative est inférieure à 4 m sont les plus appropriées à la mesure des spectres de vagues par KuROS. Un simulateur a ensuite été développé afin de quantifier les performances des mesures en fonction de l’état de la surface marine et de la géométrie d’observation. Les résultats de ce simulateur, comparés aux observations, nous permettent de confirmer les conditions dans lesquelles les paramètres spectraux sur les vagues issus de KuROS sont obtenus avec une bonne précision et de préciser quels sont les facteurs qui impactent le plus la dégradation de la précision de mesure dans certaines conditions d’état de mer.

In situ observations, satellite observations, and regional observations from airborne remote sensing are very useful to characterize sea state evolution and related physical processes, improve numerical modeling, and contribute to climate variable survey. Directional wave spectra describe the complexity of sea state and give access to parameters such as directional parameters (mean direction and directional distribution of energy) and frequency parameters (peak frequency, frequency spread). In this paper, directional ocean wave spectra and their parameters, retrieved from observations carried out with the airborne radar system KuROS during two field campaigns, are analyzed. These campaigns provide a very rich variety of meteorological conditions: high wind conditions either fetch-limited cases or mature sea conditions and moderate wind conditions and sea state dominated by swell. The objective of this paper is to compare the KuROS data set with numerical wave model outputs and buoy observations. This comparison aims first at assessing the performances on main wave parameters (significant wave height, mean direction at the peak, peak frequency) retrieved from KuROS in different conditions (wind sea, swell, mixed seas), and then, to discuss on parameters characterizing the shape of the wave spectra, namely the frequency and the directional spread. Results of the comparisons show that, due to the size of the KuROS radar footprint, ocean waves with dominant wavelengths lower than 200 m are the most appropriate situations for wave retrieval. They also show an overestimation of the model frequency spread and an underestimation of the model directional spread compare to KuROS and buoy data for both campaigns.

This paper is on the retrieval of spectral properties of surface ocean waves from the real-aperture radar KuROS carried on an airplane. The system was designed to prepare the satellite mission CFOSAT and to contribute to its validation. In addition, KuROS provides Doppler measurements which are analyzed as Doppler velocity spectra co-located with the wave height spectra derived from intensity measurements. Data have been acquired from 2013 to 2017 in various circumstances including fetch-limited, swell, or mixed sea conditions and under moderate to high wind conditions. In this paper we present results on the directional spectra obtained and on the associated integral parameters such as significant wave height, peak direction, peak wavelength (or frequency), directional and frequency spreads. Comparisons with outputs of the wave model hindcast (MFWAM model) are presented. We also show some results on the Doppler fluctuation spectra.