J.C. Nieto Borge

Analysis of directional wave fields using X-band navigation radar

The aim of this work is to present an approach to describe complex sea states including the ones consisting of superpositions of swell and wind sea components, using a nautical radar in X-band as a remote sensing technique. In the present article, the inversion method to obtain the spectral representation of the wave fields is described. The method is applied to analyse data obtained from simulation techniques, as well as from measurements obtained during oceanographic campaigns in the Bay of Biscay and North Sea.

Validation and intercomparisons of wave measurements and models during the EuroROSE experiments

The objective of the EuroROSE (European Radar Ocean Sensing) project was to combine area covering ground-based remote-sensed wave and current data with high-resolution numerical forecast models to provide nowcasts and forecasts for coastal marine operators. Two experiments to test and to demonstrate the system took place: one on the coast of Norway, north of Bergen in March 2000 and the second on the north coast of Spain at Gijon in October–November 2000. Qualitative and quantitative intercomparisons of the wave measurements and wave model products from these experiments are presented. These include measurements using the Wellen Radar (WERA) high-frequency (HF) radar, the WaMoS (Wave Monitoring System) Xband radar, a directional Waverider and output from the WAM wave model. Comparisons are made of the full directional spectra and of various derived parameters. This is the first-ever intercomparison between HF and X-band radar wave measurements and between either of these and WAM. It has provided a data set covering a much wider range of storm and swell conditions than had been available previously for radar wave-measurement validation purposes and has clarified a number of limitations of the radars as well as providing a lot of very useful radar wave data for future model-validation applications. The intercomparison has led to improvements in the data quality control procedures of both WaMoS and WERA. The two radar sytems measured significant wave height with mean biases of 3% and 6%, respectively, and mean direction differences of less than 2j in both cases. Limitations in the WAM model implementation are also discussed. D 2002 Elsevier Science B.V. All rights reserved.

Detection of ocean wave groupiness from spaceborne synthetic aperture radar

[1]xa0In recent years a number of remote sensing techniques capable to provide two-dimensional information of the sea surface have been applied to describe sea states. This information is obtained from the estimation of the directional wave number spectrum. However, the wave number spectrum is not sufficient for a complete description of sea states. The analysis of some phenomena such as wave grouping, detection of individual waves, maximum wave heights, etc., needs a complementary study, which has been carried out historically in the temporal domain from buoy records. In two dimensions (e.g., the spatial domain), the complementary analysis of those phenomena can be carried out in the spatial domain rather than the temporal domain. In this work, two methods to analyze wave groupiness in the spatial domain from sea surface images are presented: The first method is based on the two-dimensional generalization of the Smoothed Instantaneous Wave Energy History (SIWEH) function, which had been originally developed to analyze wave groupiness for wave elevation time series. The second method uses the estimation of the wave envelope in two dimensions to extract wave grouping parameters. Assuming that the wave fields are Gaussian stochastic processes, both methods are applied to ERS-2 synthetic aperture radar (SAR) and ENVISAT advanced SAR (ASAR) images of the sea surface, where only cases with almost range traveling waves are considered in order to avoid strong nonlinearities in the SAR imaging mechanisms. The obtained results shown that spaceborne SAR can be used to extract wave grouping information on global scale.

EuroROSE: a project to support shipping in port approaches

The European Radar Ocean Sensing (EuroROSE) project is presented. The project was designed to monitor and forecast sea state conditions in coastal areas, in particular port approaches. The aim was to provide area-covering information to traffic managers and search and rescue organizations, using ground based radar and numerical models in combination with data assimilation techniques.

SAR ocean wave measurements based on an empirical imaging model

Spaceborne Synthetic Aperture radar is still the only instrument providing continuous wave measurements on a global basis. For more than a decade, the European satellites ERS-1 and ERS-2 have acquired about 1500 globally distributed SAR images daily. It is well known however that strong uncertainties exist in the SAR ocean wave imaging models used so far, which are either purely theoretical or semi-empirical. In this study, a new empirical model for the SAR ocean wave imaging process is proposed. The empirical model is derived based on a global data set of ERS-2 wave mode spectra and collocated two-dimensional ocean wave spectra from the numerical ocean wave model WAM. A quasi-linear model function is fitted using a least-square minimization approach to calculate an optimal estimate of the ocean to SAR transfer function. The empirical transfer function is compared to the theoretical expression used in the literature. The quality of the empirical model is tested by comparison of simulated and observed SAR spectra. Furthermore, the model is used to estimate significant wave heights based on a quasi-linear inversion approach. Statistics as well as global maps of wave parameters are presented. The study is a contribution to the optimization of the operational use of global SAR data for the assimilation of numerical wave forecast models

Comparison of spatial and spectral sea state parameters measured by space borne SAR, nautical radar and in situ sensors

Recent techniques to derive sea state parameters in the spatial domain from radar images of the sea surface have been derived. This work describes these techniques in order to derive wave elevation maps from space borne SAR and nautical radar images of the sea surface. These wave elevation maps permits to extract additional sea state information in the spatial domain derived from the statistical analysis of the wave elevation, as well as wave height, spatial distribution.

An empirical SAR imaging model for ocean wave measurements

Spacebore Synthetic Aperture radar (SAR) is still the only instrument providing continuous two-dimensiona (2-D) ocean wave measurements on a global basis. For more than a decade the European satellites ERS-1 and ERS-2 have acquired SAR data over the open ocean operating in wave mode. The ERS acquisitions are currently continued by the ENVISAT ASAR wave mode. It is well known that the derivation of ocean wave parameters from SAR data is not straightforward and different approaches have been proposed. It this study we present a new technique, which is based on an empirical SAR imaging model. The method has the calibrated SAR image as the only input. A data set of 6000 globally distributed ERS-2 wave mode image spectra and collocated ocean wave spectra computed with the numerical model WAM are used to fit a linear model, which relates the SAR spectrum to integral wave parameters like, e.g., the significant wave height. This model is then used for ocean wave parameter retrieval. The radar cross section and the azimuthal cut-off wavelength estimated from the wave mode images are used as additional input variables. The method takes into account the coupling of the different parameters and is based on a least-square minimisation approach. The resulting coupled linear system of equations is solved using a singular value decomposition technique. Disjunct subsets of the collocated data set are used for fitting the model and retrieving ocean wave parameters. Scatterplots and global maps with the derived parameters are presented. It is shown that the standard deviation of the retrieved significant waveheight with respect to the WAM waveheight is in the order of 0.62 m. Other wave parameters, which are of practical relevance like mean wave periods are investigated as well.

Sea state analysis for coastal waters using dual polarization ENVISAT ASAR data

For more than one decade, the scientific developments carried out with spaceborne synthetic aperture radar (SAR) have demonstrated that SAR is a reliable remote sensing technique to analyze wave fields on the open ocean. In their so-called image mode, these SAR systems, such the Advanced SAR (ASAR) ENVISAT, are able to scan ocean areas of 100 x 100 km 2 and more with spatial resolutions about 20 x 20 m 2 . Those SAR images are able to provide information about the spatial variability of 2D wave fields. This work investigates the potential of ENVISAT ASAR capability to scan the ocean surface with dual polarizations. The relevance of these investigations for different practical applications concerning the analysis of ocean waves is discussed.

Analysis of two dimensional sea surface elevation fields using spaceborne SAR

Space borne synthetic aperture radar are able to provide high resolution measurements of ocean waves on a global scale. The present study uses a reprocessed data set of complex SAR images acquired by the European Remote Sensing satellite ERS-2 to estimate different wave parameters relevant for ship security. In addition, a new method is presented to derive two dimensional sea surface elevation fields from complex SAR data. The method permits to analyze wave fields in more detail than conventional SAR wave measurement techniques, which only estimate the wave spectrum. The technique provides parameters like maximum to significant wave height ratios, wave steepness, or the probability of wave breaking. Global maps and statistics of the new parameters are presented.

Comparisons of Wave Measurements from the Eurorose Fedje Experiment

The objective of the EuroROSE project was to combine area covering ground-based remote-sensed wave and current data with high resolution numerical forecast models to provide now- and forecasts for coastal marine operators. The first experiment to test and demonstrate the system took place on the coast of Norway, north of Bergen in March 2000. Qualitative and quantitative intercomparisons of the wave measurements and wave model products from this experiment are presented. These include measurements using the WERA HF radar, the WaMoS X-band radar, a directional waverider and output from the WAM wave model. Comparisons are made of the full directional spectra and of various derived parameters. This is the first ever intercomparison between HF and X-band radar wave measurements and between either of these and WAM.