Assessing ESA Climate Change Initiative data for the monitoring of phytoplankton abundance and phenology in deep lakes: investigation on Lake Geneva

Lake water quality assessment requires quantification of phytoplankton abundance. Optical satellite imagery allows us to map this information within the entire lake area. The ESA Climate Change Initiative (ESA-CCI) estimates Chl-a concentrations, based on medium resolution satellite data, on a global scale. Chl-a concentrations provided by the ESA-CCI consortium were analyzed to assess their representativeness for water quality monitoring and subsequent phenology studies in Lake Geneva. Based on vertically resolved in-situ data, those datasets were evaluated through match-up comparisons. Because the underlying algorithms do not take into account the vertical distribution of phytoplankton, a specific analysis was performed to evaluate any potential biases in remote sensing estimation, and consequences for observed phenological trends. Different approaches to data averaging were performed to reconstruct Chl-a estimates provided by the remote sensing algorithms. Strong correlation (R-value > 0.89) and acceptable discrepancies (rmse ∼ 1.4 mg.m−3) were observed for the ESA-CCI data. This approach permitted recalibration of the ESA CCI data for Lake Geneva. Finally, merging satellite and in-situ data provided a consistent time series for long term analysis of phytoplankton phenology and its interannual variability since 2002. This combination of in-situ and satellite data improved the temporal resolution of the time series, enabling a more accurate identification of the timing of specific spring events characterising phytoplankton phenology

Apparent surface-to-sky radiance ratio of natural waters including polarization and aerosol effects: implications for above-water radiometry

Above-water radiometry (AWR) methods have been developed to provide “ground-truth” (or fiducial) measurements for calibration and validation of the water color satellite missions. AWR is also an important tool for environmental survey from dedicated field missions. Under clear sky, the critical step of AWR is to retrieve the water-leaving radiance from radiometric measurements of the upward radiance that also includes the reflection of the direct sunlight and diffuse skylight reflected by the wind ruffled water surface toward the sensor. In order to correct for the surface reflection, sky radiance measurements are performed and converted into surface radiance through a factor often called “sea surface reflectance factor” or “effective Fresnel reflectance coefficient”. Based on theoretical and practical considerations, this factor was renamed surface-to-sky radiance ratio, Rss, to avoid misuse of the term reflectance as often encountered in the literature. Vector radiative transfer computations were performed over the spectral range 350–1,000 nm to provide angular values of Rss for a comprehensive set of aerosol loads and types (including maritime, continental desert and polluted models) and water surface roughness expressed in wave slope variances or in equivalent Cox-Munk wind speeds, for practical use. After separating direct and diffuse light components, it was shown that the spectral shape and amplitude of Rss are very sensitive to aerosol load and type even for extremely low values of the aerosol optical thickness. Uncertainty attached to Rss was computed based on propagation of errors made in aerosol and surface roughness parameters demonstrating the need to adapt the viewing geometry according to the Sun elevation and to associate concurrent aerosol measurements for optimal AWR protocols

DataSheet1_Apparent surface-to-sky radiance ratio of natural waters including polarization and aerosol effects: implications for above-water radiometry.PDF

Above-water radiometry (AWR) methods have been developed to provide “ground-truth” (or fiducial) measurements for calibration and validation of the water color satellite missions. AWR is also an important tool for environmental survey from dedicated field missions. Under clear sky, the critical step of AWR is to retrieve the water-leaving radiance from radiometric measurements of the upward radiance that also includes the reflection of the direct sunlight and diffuse skylight reflected by the wind ruffled water surface toward the sensor. In order to correct for the surface reflection, sky radiance measurements are performed and converted into surface radiance through a factor often called “sea surface reflectance factor” or “effective Fresnel reflectance coefficient”. Based on theoretical and practical considerations, this factor was renamed surface-to-sky radiance ratio, Rss, to avoid misuse of the term reflectance as often encountered in the literature. Vector radiative transfer computations were performed over the spectral range 350–1,000 nm to provide angular values of Rss for a comprehensive set of aerosol loads and types (including maritime, continental desert and polluted models) and water surface roughness expressed in wave slope variances or in equivalent Cox-Munk wind speeds, for practical use. After separating direct and diffuse light components, it was shown that the spectral shape and amplitude of Rss are very sensitive to aerosol load and type even for extremely low values of the aerosol optical thickness. Uncertainty attached to Rss was computed based on propagation of errors made in aerosol and surface roughness parameters demonstrating the need to adapt the viewing geometry according to the Sun elevation and to associate concurrent aerosol measurements for optimal AWR protocols.</p

Determination of sea surface wind speed using the polarimetric and multidirectional properties of satellite measurements in visible bands

International audienceThe reflection of the direct sunlight onto the rough sea surface (sun glint) generates a strong signal which is informative on wind speed. An original method is described to determine the wind speed values and their associated uncertainty over the ocean using multidirectional and polarimetric data measured by a passive satellite sensor in the visible/near infrared bands, namely PARASOL sensor. The method is able to derive wind speed values for almost 80% of a cloud-free scene. Comparisons with buoys and with the operational wind product of the AMSR-E sensor (NASA) show a satisfactory agreement (coefficient of correlation r > 0.84). This study demonstrates that passive satellite sensors that are able to measure the polarization and multidirectionality features of the radiation at solar wavelengths can be relevant alternative approaches to quantify the wind speed at a spatial resolution at least four times higher than that currently obtained using passive or active microwave sensors. Citation: Harmel, T., and M. Chami (2012), Determination of sea surface wind speed using the polarimetric and multidirectional properties of satellite measurements in visible bands, Geophys. Res. Lett., 39, L19611, doi:10.1029/2012GL053508

Estimation of the sunglint radiance field from optical satellite imagery over open ocean: Multidirectional approach and polarization aspects

International audienceRadiometric satellite measurements over the ocean are greatly affected by the contribution of the direct sunlight reflected on the ruffled ocean (so-called sunglint). Sunglint produces radiance that can far exceed the radiance scattered by both the atmosphere and ocean layers. Knowledge of the sunglint radiance is required in many remote sensing applications using radiance and polarization information (e.g., retrieval of aerosol or hydrosol optical properties, sensor calibration). The Cox and Munk model is currently used for estimating sunglint signal, but its accuracy is mainly limited by the mandatory use of wind speed data sets. An algorithm (so-called polarization-based atmospheric correction glint) was developed based on the original multidirectional and polarization radiometric measurements of the Polarization and Anisotropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar satellite mission. The method enables to accurately estimate the radiance and the polarization terms of the sunglint signal. The strength of the algorithm is to quantify the sunglint radiation using the Polarization and Anisotropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar data without any a priori information on the actual sea state A relevant application of the algorithm is proposed to better detect the pixels influenced by clouds provided that ancillary data of wind speed are used

Influence of polarimetric satellite data measured in the visible region on aerosol detection and on the performance of atmospheric correction procedure over open ocean waters

An original atmospheric correction algorithm, so-called multi-directionality and POLarization-based Atmospheric Correction (POLAC), is described. This algorithm is based on the characteristics of the multidirectional and polarimetric data of the satellite PARASOL (CNES). POLAC algorithm is used to assess the influence of the polarimetric information in the visible bands on the retrieval of the aerosol properties and the water-leaving radiance over open ocean waters. This study points out that the use of the polarized signal significantly improves the aerosol type determination. The use of the polarized information at one visible wavelength only, namely 490 nm, allows providing estimates of the Angstrom exponent of aerosol optical depth with an uncertainty lower than 4%. Based on PARASOL observations, it is shown that the detection of the fine aerosols is improved when exploiting polarization data. The atmospheric component of the satellite signal is then better modeled, thus improving de facto the water-leaving radiance estimation. (C) 2011 Optical Society of Americ

Estimation of daily photosynthetically active radiation (PAR) in presence of low to high aerosol loads: application to OLCI-like satellite data

Estimation of daily photosynthetically active radiation (PAR) is of primary importance for monitoring the ocean primary production and the subsequent production of carbon by phytoplankton at global scale from remote sensing ocean color sensors. On the other hand, aerosol abundance and composition play a critical role in the modulation of PAR. In this study, an original algorithm, so-called OLCIPAR, is proposed for routinely determining the daily PAR from optical satellite sensors such as the OLCI sensor aboard Sentinel-3 (ESA). The OLCIPAR algorithm has been developed to overcome some of the limitations of the current existing methods. In particular, multiple scattering effects induced by the atmospheric layer are taken into account based on exact radiative transfer calculations. Another advantage of OLCIPAR method is to consider a great variety of aerosol models to better account for their optical variability as observed in real world conditions. The OLCIPAR algorithm was applied to the archive of MERIS data, whose sensor is similar to OLCI. The validation of the retrieved daily PAR was carried out based on comparison with the time series acquired by the BOUSSOLE oceanographic buoy moored in the Mediterranean Sea. Results show a regression slope of 1% and an accuracy within 10% which confirms the robustness of the algorithm. The comparison of OLCIPAR retrievals with the products routinely distributed by NASA shows that estimates of PAR differ by up to 20% in the subtropical Atlantic Ocean where important amounts of dust aerosols are present. The improvements brought by OLCIPAR method for deriving the daily PAR could thus permit to better assess the impact of aerosols on reduction of PAR with implications on the estimation of oceanic primary production. (C) 2016 Optical Society of Americ

Invariance of polarized reflectance measured at the top of atmosphere by PARASOL satellite instrument in the visible range with marine constituents in open ocean waters

The influence of oceanic constituents on the polarized reflectance measured at the top of atmosphere (TOA) over open ocean waters in one visible band is investigated. First, radiative transfer modelling is used to quantify the effects of biomass concentration on the TOA polarized signal for a wide range of observation geometries. The results showed that the TOA polarized reflectance remains insensitive to variations in the chlorophyll a concentration whatever the geometrical conditions in oligotrophic and mesotrophic waters, which represent about 90% of the global ocean. The invariance of the polarized signal with water content is explained by the prevailing influence of both atmospheric effects and skylight reflections at the sea surface on the polarization state of the radiation reaching the top of atmosphere level. The simulations also revealed that multidirectional and polarized TOA reflectances obtained in the visible spectrum are powerful tools for the discrimination between the aerosol optical properties. In the second part of the paper, the theoretical results are rigorously validated using original multiangle and polarized measurements acquired by PARASOL satellite sensor, which is used for the first time for ocean color purposes. First, a statistical analysis of the geometrical features of PARASOL instrument showed that the property of invariance of the TOA polarized reflectance is technically verified for more than 85% of viewed targets, and thus, indicating the feasibility of separating between the atmospheric and oceanic parameters from space remotely sensed polarized data. Second, PARASOL measurements acquired at regional and global scales nicely corroborated the simulations. This study also highlighted that the radiometric performance of the polarized visible wavelength of PARASOL satellite sensor can be used either for the aerosol detection or for atmospheric correction algorithms over open ocean waters regardless of the biomass concentration. (c) 2008 Optical Society of America

Apport des mesures directionnelles et polarisées aux corrections atmosphériques au-dessus des océans ouverts (Application à la mission PARASOL)

Pour l estimation de la biomasse phytoplanctonique de l océan par satellite, l atmosphère est un élément perturbateur majeur qu il est nécessaire d éliminer. La qualité des paramètres océaniques estimés par satellite est directement reliée à la précision des algorithmes de correction atmosphérique. Les méthodes actuellement utilisées pour les capteurs satellite du type SeaWiFS (NASA) se sont révélées parfois infructueuses sur certaines zones avec des erreurs conduisant à des réflectances marines négatives dans le bleu. Dans ce cas, l exploitation des mesures spatiales pour estimer les paramètres marins est rendue impossible. Il est donc nécessaire d améliorer les algorithmes existants. Le satellite PARASOL effectue des mesures originales de l état de polarisation de la lumière de façon multidirectionnelle. Prochainement, deux autres satellites possèderont des caractéristiques similaires. Dans ce contexte, nous avons étudié l apport des mesures multidirectionnelles et polarisées aux corrections atmosphériques au-dessus des océans ouverts. Un premier travail a conclu que l information présente dans les mesures de polarisation dans le visible est strictement atmosphérique. Sur la base de ce résultat, un algorithme original de correction atmosphérique a été développé. Il permet d estimer à la fois les aérosols et les luminances marines à partir des images PARASOL. Ces paramètres ont été comparés à des mesures effectuées depuis le sol et en mer, ainsi qu aux produits des missions spatiales actuelles. Ces comparaisons ont permis de valider l algorithme, de quantifier le bénéfice de l information polarisée et de définir les possibilités d amélioration future.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Apport de la télédétection haute résolution spatiale pour le suivi phytoplanctonique du Léman

The work consisted in selecting and parametrizing the best adapted algorithms to improve the spatial resolution ofLake Geneva’s water quality surveillance by satellite. The satellite data came from Sentinel‐2 and the in‐situchlorophyll‐a in‐situ data were compiled based on a variety of sources, including LéXPLORE, OLA, and occasionalobservations at three additional sites on satellite passage dates. The O’Reilly algorithms perform best, with highlysignificant correlations between in‐situ and satellite measurements, but sometimes underestimate the chlorophyll‐aconcentrations. The results indicate a strong influence of the milieux’s optical conditions, which may explain thenotable deviations between in‐situ and satellite observations. Given that optical conditions vary in space and time, useof Sentinel‐2 for regular monitoring will require additional work taking into account the spatiotemporal variability ofthe optical properties of water in the calculation of chlorophyll‐a concentrations.De précédents travaux ont été effectués à partir de l’imagerie « moyenne résolution spatiale » (e.g., Sentinel‐3 à 300mde résolution) pour cartographier l’hétérogénéité spatiale et les variations temporelles de la biomassephytoplanctonique dans le Léman. Il est d’intérêt, pour un suivi plus fin de la distribution du phytoplancton et l’étudedes impacts locaux, d’analyser l’imagerie « haute‐résolution » de l’imagerie de la mission Sentinel‐2 (ESA/Copernicus)à une résolution de 20 m. Ce rapport présente les résultats obtenus à partir de la chaine de traitement OBS2CO duCNES et de la collecte de données in‐situ pour analyser les performances des algorithmes actuels. Premièrement, lesalgorithmes les plus adaptés ont été sélectionnés et implémentés pour la production de séries temporelles de laconcentration en chlorophylle‐a. Ces données satellitaires ont ensuite été confrontées aux données provenant desmesures in‐situ de diverses sources dont la station LéXPLORE, la base de données OLA et des observations ponctuellesen 3 sites supplémentaires spécialement effectuées lors des dates de passage du satellite. Les algorithmes développéspour les eaux oligotrophes sont en général les plus performants, avec des corrélations significatives entre les mesuresin‐situ et satellitaires, mais ont parfois tendance à sous‐estimer les concentrations en chlorophylle‐a. Les résultatsindiquent une forte influence des conditions optiques du milieu, ce qui peut expliquer les écarts notables entreobservations in‐situ et satellites. Étant donné que les conditions optiques varient dans l'espace et le temps, l'utilisationde Sentinel‐2 pour un suivi régulier nécessitera un travail supplémentaire pour prendre en compte la variabilité spatio‐temporelle des propriétés optiques de l'eau dans le calcul des concentrations en chlorophylle‐a