Thomas König

An empirical approach for the retrieval of integral ocean wave parameters from synthetic aperture radar data

[1]xa0In this study a new empirical approach to retrieve integral ocean wave parameters from synthetic aperture radar (SAR) data is presented. The idea behind this computationally efficient technique is to estimate integral ocean wave parameters without the intermediate step of retrieving the two-dimensional ocean wave spectrum. The method has the radiometrically calibrated SAR image as the only source of information and is based on a quadratic model function with 22 input parameters. These parameters include the radar cross section, the image variance, and 20 parameters computed from the SAR image variance spectrum using a set of orthonormal functions. The coefficients of the quadratic function were fitted for the estimation of Hs, the mean periods Tm01, Tm02, T−10, the wave power, and the wave heights associated with different spectral bands. The fit procedure is based on a stepwise regression method. A data set of 12,000 globally distributed ERS-2 wave mode image spectra and colocated WAM ocean wave spectra was available for the study. Two separate subsets of 6000 collocation pairs each were used to fit the model and to carry out comparisons of the retrieved wave parameters with numerical model results. Additional comparisons were performed using NDBC buoy measurements. Scatterplots and global maps with the derived parameters are presented. It is shown that the rms of the SAR derived Hs with respect to the WAM Hs is about 0.5 m. For the mean period Tm−10 an rms of 0.72 s with a high-frequency cutoff period of about 6 s is achieved.

Extracting sea ice parameters from SAR imagettes

From raw ERS wavemode data, complex SAR images of size 5 km /spl times/ 10 km - so called imagettes - were computed using the BSAR processor at the German Aerospace Center (DLR). Being collected every 200 km along the orbit, imagettes provide a relatively dense coverage especially over polar oceans up to 84 degrees North. Several statistical measures have been introduced for analysis of imagettes classifying ice free and ice contaminated imagettes as well as deriving sea ice parameters such as ice concentration, deformation energy, and ridge frequency. The results are compared to SSM/I data and sea ice model results. In addition, a wavelet-based edge detector is used to examine the ice surface structure. Using the imagettes in an automatic extraction system can provide global statistics of sea state and sea ice parameters from both upcoming ENVISAT and historic ERS data providing a 10 year dataset.

Global analysis of a 2 Year ERS-2 wavemode dataset over the oceans

Starting in 1991, the ERS-1 and (later) ERS-2 satellites have collected wavemode data over the global oceans whenever no image mode data acquisition was requested. Wave- mode data are full resolution SAR data covering small areas of size 5 km times 10 km every 200 km along the orbits thus forming a dataset giving information from all global oceans daily. In the scope of the WAVEATLAS ESA AO Project, ESA provided two years (Sep. 1998 - Nov. 2000) of raw ERS-2 wave mode data to DLR which were processed into more than one million single look complex images, so-called imagettes, using DLRs BSAR processor. The algorithms CWAVE and LISE were applied to estimate wind and wave parameters such as significant wave height, wind speed and sea surface elevation fields together with several single wave parameters. The paper presents global statistics with emphasis on wave parameters, namely significant wave height, single wave crest height, and wave height. Areal and seasonal distribution of high sea states is discussed and the relevance of the analysis for extreme sea state mapping is pointed out. As far as possible, the results are related to existing theoretical knowledge and compared to observational data (Hogben atlas) as well as model results. The outcome of this processing will be compiled into a new unique atlas on global waves and extreme events. It is planned to extend the atlas back to 1991 when ERS-1 was launched and forward to present times thus covering a more than 15 years period.

Use of tandem pairs of ERS-2 and ENVISAT SAR data for the analysis of oceanographic and atmospheric processes

Currently the European satellites ERS-2 and ENVISAT are flying on the same orbit with a time separation of 30 minutes. In this presentation pairs of the respective synthetic aperture radar data are analyzed with respect to different atmospheric and oceanic processes. The presented results were obtained in the framework of the ESA AO project COTAR. The tandem configuration exists since the launch of ENVISAT in 2002. In the presentation an overview will be given of the available image pairs acquired over the ocean on a global scale. Combinations of ERS-2 SAR data with both image mode and wide swath mode scenes provided by the ENVISAT ASAR are considered. The two SAR images enable the analysis of the change of radar cross section within half an hour. This temporal separation is very interesting for oceanographic applications because there are many processes like atmospheric fronts, convective cells, ocean tides, etc., which are detectable on this time scale. In the presentation tandem pairs acquired over the research platform FINO west of the island Borkum in the North Sea will be presented. The area is of high practical interest because of the planned Offshore Windpark Borkum West. The platform provides wind measurements at different heights and additional oceanic information to support the planning activities for this windpark. The dynamics of atmospheric structures is analysed. It is well known that the near surface wind field is a dominating factor for the normalized radar cross section of the sea surface. For this reason SAR scenes are well suited to study atmospheric effects with high spatial resolution. It is shown that the use of tandem pairs enables the study of processes like the propagation of an atmospheric fronts or the evolution of convective cells. Both effects are illustrated with different examples using additional information from in situ measurements. Furthermore existing techniques for the estimation of the wind field in 10 m height from SAR data are applied to both images. The evolution of the spatial structure of both wind speed and wind direction is analysed. The observations are related to some theoretical issues like, e.g., the Taylor hypothesis. Particular emphasize is put on the connection between the spatial and the temporal structure of the wind field. This topic is of high practical relevance, e.g., in the context of offshore windenergy exploitation. Additional applications like ship tracking, oil spill detection and the study of ocean wave field dynamics are briefly discussed as well.

Global analysis of ocean wave systems from SAR wave mode data

One of the major goals of the European Remote Sensing Satellites ERS-1 and ERS-2 were applications in ocean wave research and wave forecasting. For the first time, two-dimensional spectral information on the sea state globally, continuously, and in quasi real time are provided in the so-called SAR wave mode. To use not only the spectral but also the image information of these data, a global set of single look complex (SLC) synthetic aperture radar (SAR) images (imagettes) was processed from ERS-2 wave mode raw data, using the BSAR processor developed at DLR. SAR imagettes are used to analyze recently developed algorithms for wind and wave measurements. As the new European Satellite ENVISAT will provide cross spectra on a global and continuous basis, interest in the described techniques is growing. Two-dimensional ocean wave spectra are derived from SAR imagettes by quasi-linear inversion of the SAR imaging mechanism. Individual wave systems are detected by a so-called partitioning algorithm. The resulting wave parameters are analyzed and compared to model data. To take into account the complete nonlinear SAR imaging mechanisms, wave model spectra are simulated forward into cross spectra and compared to observations. Using this approach cases of significant inconsistencies between SAR and model data can be detected. With our 3 week data base in 1996 (about 30,000 imagettes) statistics are calculated. Finally, we derive daily maps showing areas with significant deviations between simulated and observed cross spectra.

An Empirical Approach for the Retrieval of Ocean Wave Parameters from Synthetic Aperture Radar Data

Spacebore Synthetic Aperture radar (SAR) is still the only instrument providing continuous two-dimensional (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.

SAR wave mode data: a new source for ocean and sea ice related climate research

From raw ERS wavemode data, complex SAR images of size 5 km x 10 km - so called imagettes - are computed using DLRs BSAR processor. Being collected over oceans every 200 km along the orbit, imagettes provide a patchy daily global coverage of the worlds oceans. This paper presents DLRs processing chain and results for a three week testing period covering the detection of ocean slicks and sea ice, the use of cross spectra to derive wave spectra over open ocean, the detection of one or several ocean wave systems together with its wave length and its propagation direction, and the estimation of single wave heights. In sea ice covered areas, statistical parameters derived from the SAR are well correlated to physical measures such as the ice deformation energy and ridge frequency. Over ice free areas, comparisons between the SAR results and the ECMWF analysis as well as model results show good agreement in most cases. Observed deviations between SAR measurements and model results are discussed. As a new application of wave mode data statistics are presented on the occurence of extreme waves. As soon as ESA will make available the daily global raw wavemode data for the whole lifetime of ERS-1 and ERS-2, it will be possible to DLR to derive unique statistics on sea ice and wave parameters for a period of at least 10 years, which may be even extended by the use of ENVISAT data in future. Together with near surface wind fields which are also derivable from the imagettes, this work will be of extreme use for climate research applications.

First statistical analysis of a one-year global dataset of re-processed ERS-2 imagettes for oceanic applications

Since 1991, the ERS satellites have collected high resolution SAR data over the oceans on a global and continuous basis. Operating in wave mode the ERS SAR yields a patchy coverage of the oceans with about 1500 images of 10 by 5 km size each day. These data have recently been shown to be able to deliver wind speed and wave spectra as well as information on individual waves. In the scope of the ESA AO WaveAtlas, the ERS-2 raw data acquired in 1999 and 2000, one year i.e. approximately half a million of reprocessed single look complex imagettes were used for a first statistical evaluation providing global ocean maps of mean intensity, standard deviation, modulation, inhomogeneity and their regional change during all seasons of a year for the first time. Furthermore the paper demonstrates the consequences of these results for different oceanographic applications. Special emphasize is put on wind, wave and sea ice measurements. In addition atmospheric features can be analyzed. The data in particular enable the estimation of two-dimensional sea surface elevation fields which are of high practical relevance for ship design or offshore operations. Mean wave parameters like the mean period derived from SAR wave mode data can be used for assimilation of numerical wave models thus helping to improve wave forecast. The reprocessing of wave mode data is planned to be extended to the full lifetime of ERS-1 and ERS-2, which is at least 19912004. This will allow the derivation of wave climatologies on a decadal basis in particular enabling the analysis of climate trends. As wave mode data are also available from the ENVISAT mission a consistent update of these climatologies is possible.

A new historic ERS wave mode data set for oceanographic applications

Since 1991, the ERS satellites have collected high resolution SAR data over the oceans on a global and continuous basis. Operating in wave mode the ERS SAR yields a patchy coverage of the oceans with about 1500 images of 10 by 5 km size each day. These data have recently been shown to be able to deliver wind speed and wave spectra as well as information on individual waves. In the scope of the ESA AO WaveAtlas, the ERS-2 raw data acquired in 1999 and 2000 are reprocessed to single look complex imagettes. This paper describes the necessary steps for the reprocessing of historic wave mode data including an assessment of the effort required to reprocess all available ERS wave mode raw data archived at ESA. Furthermore the paper demonstrates the potential of these data for different oceanographic applications. Special emphasize is put on wind, wave and sea ice measurements. In addition atmospheric features can be analyzed. The data in particular enable the estimation of two-dimensional sea surface elevation fields which are of high practical relevance for ship design or offshore operations. Mean wave parameters like the mean period derived from SAR wave mode data can be used for the assimilation of numerical wave models thus helping to improve wave forecast. Global maps of mean and individual wave parameters are presented. The reprocessing of wave mode data is planned to be extended to the full lifetime of ERS-1 and ERS-2, which is at least 1991-2004. This allows the derivation of wave climatologies on a decadal basis in particular enabling the analysis of climate trends. As wave mode data are also available from the ENVISAT mission a consistent update of these climatologies is possible

Severe Weather Applications over the Oceans using ERS SAR Wavemode Data

Due to the relatively small amount of in situ data available for the open oceans, particularly during extreme events, under such conditions remote sensing techniques take an important role in the retrieval of geophysical information. Up to now the only remote sensing system capable of providing information on two dimensional sea state on a global and continuous scale and under all weather conditions is the Synthetic Aperture Radar (SAR). In the scope of the project Wave Atlas, ESA provided a two years wave-mode dataset of ERS-2 SAR raw data, mainly collected during 1999 and 2000, which was reprocessed to single-look- complex imagettes at DLR using the BSAR processor. In this study the data were used for a statistical evaluation resulting in global ocean maps of different basic image parameters and oceanic parameters like wind speed, significant wave height, mean period and their respective regional and temporal variability during the seasons of the year using the empirical algorithms CWIND1.0 and CWAVE1.0. Global statistics are given for the time frame of 1998 to 2000 and examples of severe storms are analysed in detail. In future, the reprocessing of wavemode data is planned to be extended to the full lifetime of ERS-1 and ERS-2, which is at least 1991-2006. As wave mode data are also available from the ENVISAT mission, there is the possibility for future expansion.