379 research outputs found

    Use of satellite observations for operational oceanography: recent achievements and future prospects

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    The paper gives an overview of the development of satellite oceanography over the past five years focusing on the most relevant issues for operational oceanography. Satellites provide key essential variables to constrain ocean models and/or serve downstream applications. New and improved satellite data sets have been developed and have directly improved the quality of operational products. The status of the satellite constellation for the last five years was, however, not optimal. Review of future missions shows clear progress and new research and development missions with a potentially large impact for operational oceanography should be demonstrated. Improvement of data assimilation techniques and developing synergetic use of high resolution satellite observations are important future priorities

    Synergetic Exploitation of the Sentinel-2 Missions for Validating the Sentinel-3 Ocean and Land Color Instrument Terrestrial Chlorophyll Index Over a Vineyard Dominated Mediterranean Environment

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    [EN] Continuity to the Medium Resolution Imaging Spectrometer (MERIS) Terrestrial Chlorophyll Index (MTCI) will be provided by the Ocean and Land Color Instrument (OLCI) on-board the Sentinel-3 missions. To ensure its utility in a wide range of scientific and operational applications, validation efforts are required. In the past, direct validation has been constrained by the need for costly airborne hyperspectral data acquisitions, due to the lack of freely available high spatial resolution imagery incorporating appropriate spectral bands. The Multispectral Instrument (MSI) on-board the Sentinel-2 missions now offers a promising alternative. We explored the synergetic use of MSI data for validation of the OLCI Terrestrial Chlorophyll Index (OTCI) over the Valencia Anchor Station, a large agricultural site in the Valencian Community, Spain. Using empirical and machine learning techniques applied to MSI data, in situ measurements were upscaled to the moderate spatial resolution of the OTCI. An RMSECV of 0.09 g.m(-2) (NRMSECV = 20.93%) was achieved, highlighting the valuable information MSI data can provide when used in synergy with OLCI data for land product validation. Good agreement between the OTCI and upscaled in situ measurements was observed (r = 0.77, p < 0.01), providing increased confidence to users of the product over vineyard dominated Mediterranean environments.This work was supported in part by the European Space Agency and European Commission through the Sentinel-3 Mission Performance Centre.Brown, LA.; Dash, J.; Lidón, A.; Lopez-Baeza, E.; Dransfeld, S. (2019). Synergetic Exploitation of the Sentinel-2 Missions for Validating the Sentinel-3 Ocean and Land Color Instrument Terrestrial Chlorophyll Index Over a Vineyard Dominated Mediterranean Environment. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 12(7):2244-2251. https://doi.org/10.1109/JSTARS.2019.28999982244225112

    Ocean surface currents reconstruction from microwave radiometers measurements

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    Premi Extraordinari de Doctorat, promoció 2014-2015. Àmbit d'Enginyeria de les TICOcean currents are a key component to understanding many oceanic and climatic phenomena and knowledge of them is crucial for both navigation and operational applications. Therefore, a key problem in oceanography is the estimation of the synoptic velocity field. Currently, global ocean surface velocities are routinely estimated from Sea Surface Height (SSH) measurements provided by altimeters. However, the separation between passes, as well as and the limited number of available altimeters leads to errors in the accurate location of oceanic currents when these measurements are used exclusively. Contrarily, satellite images of Sea Surface Temperature (SST) provide a good qualitative view of the location of ocean patterns, which has encouraged the investigation of alternative methodologies to reconstruct the velocity field based on these observations. This Ph.D. thesis has assessed the capability of SST microwave radiometers observations to retrieve ocean surface currents. The reconstruction of the ocean surface currents from SST observations can be expressed in terms of a transfer function notation, that allows to convert SST maps into SSH, and thus into currents. Because under geostrophic balance, the slope of SSH is proportional to ocean surface currents. This transfer function can be theoretically derived using the Surface Quasi-Geostrophic equations (SQG). Two different approaches were analyzed at a global scale: on one side, the analysis of the validity of the SQG approach has been performed, and on the other, an approach based on the synergetic properties between simultaneous SST and SSH observations has been analyzed. Both approaches have been compared with ocean surface currents retrieved from merged altimetric observations. The study has been focused on the period from October 2002 to May 2005, since during that period there were available four different altimeters, and the quality of the merged altimetric observations was enhanced. The analysis of the validity of SQG at a global scale revealed that this dynamical model is valid near the major extratropic current system such us the Gulf Stream, the Antartic Circumpolar Current, Kuroshio currents. Besides, the potential of MW SST observations to reconstruct ocean surface currents was analyzed using a synergetic approach: the combination of the SST phase with the SSH spectra. Actually, we explored under which environmental conditions the phase of the MW SST is close to the SSH phase. Results showed that the phase of the MW SST can be used to retrieve ocean currents during winter, near the major extratropical current systems, which are characterized by an intense mesoscale activity and the presence of strong thermal gradients, and deep ML. Furthermore, the reconstruction of the velocity fields from an ideal transfer function built up from simultaneous SST and SSH observations revealed that the SQG approach can be enhanced. The spectral properties of this ideal transfer function derived from simultaneous SST and SSH observations were characterized at a global scale. The analysis of spectral properties of the transfer function between SST and SSH observations revealed that despite daily spectral can be flatter or steeper than the k^{-1} predicted by SQG theory, in mean eSQG is a good statistically approach to retrieve ocean currents, when no simultaneous observations of SSH and SST are available.Las corrientes oceánicas son clave en muchos procesos oceánicos y climáticos, y su conocimiento es crucial para aplicaciones operacionales y de navegación. Por lo tanto, un aspecto importante en oceanografía es la estimación de campos sinópticos del campo de velocidades superficiales del mar. Actualmente, las velocidades superficiales el mar se estiman rutinariamente a partir de medidas del nivel del mar proporcionadas por altimetros, denotadas a partir de ahora con sus siglas en inglés SSH. Sin embargo, la llocalización de las corrientes puede no ser la correcta si solo se utilizan este tipo de medidas para su estimación, debido a la separación entre trazas del satélite. Por contra, las imágenes de temperatura superficial del mar, SST, proporiconan una visión cualitativa de la localización de las estructruas oceánicas. Este hecho ha motivado la investigación de metodologías alternativas para reconstruir los campos de velocidades superficiales del mar basados en estas observaciones. Esta tesis doctoral ha investigado la capacidad de las observaciones de SST proporcionadas por radiometros de microondas para recuperar las corrientes oceánicas superficiales. La reconstrucción de estas velocidades a partir de observaciones de SST se puede expresar en términos de una función de transferencia que relacione las observaciones de SST con las observaciones de SSH. Con lo que la estimación del campo de velocidades es directa, puesto que bajo la condición de equilibrio geostrófico la pendiente de la SSH es proporcional a las corrientes oceánicas. Esta función de transferencia se puede derivar teóricamente mediante las equaciones superficiales cuasi-geotróficas, denotadas con sus siglas en inglés SQG a partir de ahora. Una pregunta clave, es si las ecuaciones de este modelo dinámico son válidas. En esta tesis, se han llevado a cabo dos aproximaciones diferentes para la reconstrucción del campo de velocidades superficiales del mar: por un lado, el análisis de la validez de las ecuaciones SQG, y por otro, una aproximación basada en las propiedades espectrales de medidas simultáneas de SST y SSH. El estudio se ha centrado en el período comprendido entre Octubre del 2002 y Mayo del 2005, puesto que durante este período había disponibles hasta cuatro altmímetros, y consecuentemente la calidad de las observaciones es mayor. El análisis de la validez de SQG a escala global reveló que este modelo dinámico es válido en las regiones cerca de los sistemas de corrientes extratropicales, como la corriente del Golfo, la Corriente Circumpolar Antártica (ACC), o la Kuroshio. Además, el potencial de las observaciones de SST en el rango de las microondas para la recuperación del campo de velocidades superficiales del mar, ha sido analizado utilizando un método que combina la fase de la SST con el espectro de SSH. En realidad, se ha investigado bajo que condiciones la SST y SSH están en fase. Los resultados mostraron que la fase de la SST de microondas puede utilizarse para para la reconstrucción en invierno, cerca de los sistemas de corrientes extratropicales, caracterizados por una intensa actividad de mesoscala y la presencia de fuertes gradientes termales, así como de capas de mezcla profundas. Asimismo, la reconstrucción del campo de velocidades a partir de una función de transferencia ideal, construida a partir de imágenes simultaneas de SST y SSH, reveló que la aproximación SQG puede ser mejorada. Las propiedades espectrales de esta función de tranferencia ideal han sido estudiadas., así como su variabilidad temporal. Este análisis desveló que para escalas pequeñas y zonas enegéticas, la aproximación SQG es una buena aproximación, al menos, desde un punto de vista estádistico.Award-winningPostprint (published version

    A SWOT analysis for offshore wind energy assessment using remote-sensing potential

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    The elaboration of a methodology for accurately assessing the potentialities of blue renewable energy sources is a key challenge among the current energy sustainability strategies all over the world. Consequentially, many researchers are currently working to improve the accuracy of marine renewable assessment methods. Nowadays, remote sensing (RSs) satellites are used to observe the environment in many fields and applications. These could also be used to identify regions of interest for future energy converter installations and to accurately identify areas with interesting potentials. Therefore, researchers can dramatically reduce the possibility of significant error. In this paper, a comprehensive SWOT (strengths, weaknesses, opportunities and threats) analysis is elaborated to assess RS satellite potentialities for offshore wind (OW) estimation. Sicily and Sardinia-the two biggest Italian islands with the highest potential for offshore wind energy generation-were selected as pilot areas. Since there is a lack of measuring instruments, such as cup anemometers and buoys in these areas (mainly due to their high economic costs), an accurate analysis was carried out to assess the marine energy potential from offshore wind. Since there are only limited options for further expanding the measurement over large areas, the use of satellites makes it easier to overcome this limitation. Undoubtedly, with the advent of new technologies for measuring renewable energy sources (RESs), there could be a significant energy transition in this area that requires a proper orientation of plans to examine the factors influencing these new technologies that can negatively affect most of the available potential. Satellite technology for identifying suitable areas of wind power plants could be a powerful tool that is constantly increasing in its applications but requires good planning to apply it in various projects. Proper planning is only possible with a better understanding of satellite capabilities and different methods for measuring available wind resources. To this end, a better understanding in interdisciplinary fields with the exchange of updated information between different sectors of development, such as universities and companies, will be most effective. In this context, by reviewing the available satellite technologies, the ability of this tool to measure the marine renewable energies (MREs) sector in large and small areas is considered. Secondly, an attempt is made to identify the strengths and weaknesses of using these types of tools and techniques that can help in various projects. Lastly, specific scenarios related to the application of such systems in existing and new developments are reviewed and discussed

    Spaceborne L-Band Synthetic Aperture Radar Data for Geoscientific Analyses in Coastal Land Applications: A Review

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    The coastal zone offers among the world’s most productive and valuable ecosystems and is experiencing increasing pressure from anthropogenic impacts: human settlements, agriculture, aquaculture, trade, industrial activities, oil and gas exploitation and tourism. Earth observation has great capability to deliver valuable data at the local, regional and global scales and can support the assessment and monitoring of land‐ and water‐related applications in coastal zones. Compared to optical satellites, cloud‐cover does not limit the timeliness of data acquisition with spaceborne Synthetic Aperture Radar (SAR) sensors, which have all‐weather, day and night capabilities. Hence, active radar systems demonstrate great potential for continuous mapping and monitoring of coastal regions, particularly in cloud‐prone tropical and sub‐tropical climates. The canopy penetration capability with long radar wavelength enables L‐band SAR data to be used for coastal terrestrial environments and has been widely applied and investigated for the following geoscientific topics: mapping and monitoring of flooded vegetation and inundated areas; the retrieval of aboveground biomass; and the estimation of soil moisture. Human activities, global population growth, urban sprawl and climate change‐induced impacts are leading to increased pressure on coastal ecosystems causing land degradation, deforestation and land use change. This review presents a comprehensive overview of existing research articles that apply spaceborne L‐band SAR data for geoscientific analyses that are relevant for coastal land applications

    The HOAPS Climatology - Evaluation and Applications

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    Empirical Relationship Between the Doppler Centroid Derived From X-Band Spaceborne InSAR Data and Wind Vectors

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    One of the challenges in ocean surface current retrieval from synthetic aperture radar (SAR) data is the estimation and removal of the wave-induced Doppler centroid (DC). This article demonstrates empirically the relationship between the dc derived from spaceborne X-band InSAR data and the ocean surface wind and waves. In this study, we analyzed over 300 TanDEM-X image pairs. It is found that the general characteristics of the estimated dc follow the theoretically expected variation with incidence angle, wind speed, and wind direction. An empirical geophysical model function (GMF) is fit to the estimated dc and compared to existing models and previous experiments. Our GMF is in good agreement (within 0.2 m/s) with other models and data sets. It is found that the wind-induced Doppler velocity contributes to the total Doppler velocity with about 15% of the radial wind speed. This is much larger than the sum of the contributions from the Bragg waves (~0.2 m/s) and the wind-induced drift current (~3% of wind speed). This indicates a significant (dominant) contribution of the long wind waves to the SAR dc. Moreover, analysis of dual-polarized data shows that the backscatter polarization ratio (PR=σ⁰VV/σ⁰HH) and the dc polarization difference (PD=|dcVV|-|dcHH|) are systematically larger than 1 and smaller than 0 Hz, respectively, and both increase in magnitude with incidence angle. The estimated PR and PD are compared to other theoretical and empirical models. The Bragg scattering theory-based (pure Bragg and composite surface) models overestimate both PR and PD, suggesting that other scattering mechanisms, e.g., wave breaking, are involved. In general, it is found that empirical models are more consistent with both backscatter and Doppler data than theory-based models. This motivates a further improvement of SAR dc GMFs

    Retrieval of Ocean Surface Currents and Winds Using Satellite SAR backscatter and Doppler frequency shift

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    Ocean surface winds and currents play an important role for weather, climate, marine life, ship navigation, oil spill drift and search and rescue. In-situ observations of the ocean are sparse and costly. Satellites provide a useful complement to these observations. Synthetic aperture radar (SAR) is particularly attractive due to its high spatial resolution and its capability to extract both sea surface winds and currents day and night and almost independent of weather.The work in this thesis involves processing of along-track interferometric SAR (ATI-SAR) data, analysis of the backscatter and Doppler frequency shift, and development of wind and current retrieval algorithms. Analysis of the Doppler frequency shift showed a systematic bias. A calibration method was proposed and implemented to correct for this bias. Doppler analysis also showed that the wave contribution to the SAR Doppler centroid often dominates over the current contribution. This wave contribution is estimated using existing theoretical and empirical Doppler models. For wind and current retrieval, two methods were developed and implemented.The first method, called the direct method, consists of retrieval of the wind speed from SAR backscatter using an empirical backscatter model. In order to retrieve the radial current, the retrieved wind speed is used to correct for the wave contribution. The current retrieval was assessed using two different (theoretical and empirical) Doppler models and wind inputs (model and SAR-derived). It was found that the results obtained by combining the Doppler empirical model with the SAR-derived wind speed were more consistent with ocean models.The second method, called Bayesian method, consists of blending the SAR observables (backscatter and Doppler shift) with an atmospheric and an oceanic model to retrieve the total wind and current vector fields. It was shown that this method yields more accurate estimates, i.e. reduces the models biases against in-situ measurements. Moreover, the method introduces small scale features, e.g. fronts and meandering, which are weakly resolved by the models.The correlation between the surface wind vectors and the SAR Doppler shift was demonstrated empirically using the Doppler shift estimated from over 300 TanDEM-X interferograms and ECMWF reanalysis wind vectors. Analysis of polarimetric data showed that theoretical models such as Bragg and composite surface models over-estimate the backscatter polarization ratio and Doppler shift polarization difference. A combination of a theoretical Doppler model and an empirical modulation transfer function was proposed. It was found that this model is more consistent with the analyzed data than the pure theoretical models.The results of this thesis will be useful for integrating SAR retrievals in ocean current products and assimilating SAR observables in the atmospheric, oceanic or coupled models. The results are also relevant for preparation studies of future satellite missions
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