OceanWaveS (Germany)

A method to estimate sea surface elevation maps from marine radar image sequences is presented. This method is the extension of an existing inverse modeling technique to derive wave spectra from marine radar images, which assumes linear wave theory with temporal stationarity and spatial homogeneity of the observed sea surface elevation. The proposed technique to estimate wave elevation maps takes into account a modulation transfer function (MTF), which describes the radar imaging mechanisms at grazing incidence and horizontal polarization. This MTF is investigated and empirically determined by wave measurements and numerical simulations. The numerical simulations show that shadowing is the dominant effect in the radar imaging mechanism at grazing incidence and horizontal polarization. Further comparisons of wave spectra, as well as comparisons of the wave height probability distributions obtained by the wave elevation maps and the corresponding buoy measurements with the theoretical Rayleigh distribution, confirm the applicability of the proposed method.

This work analyses the structure of the different contributions to the image spectrum derived by the three-dimensional Fourier decomposition of sea clutter time series measured by ordinary X-band marine radars. The goal of this investigation is to derive a method to estimate the significant wave height of the ocean wave fields imaged by the radar. The proposed method is an extension of a technique developed for the analysis of ocean wave fields by using synthetic aperture radar systems. The basic idea behind this method is that the significant wave height is linearly dependent on the square root of the signal-to-noise ratio, where the signal is assumed as the radar analysis estimation of the wave spectral energy and the noise is computed as the energy due to the sea surface roughness, which is closely related to the speckle of the radar image. The proposed method to estimate wave heights is validated using data sets of sea clutter images measured by a marine radar and significant wave heights derived from measurements taken by a buoy used as reference sensor.

The importance of directional wave information has been recognised for a long time. However, for decades good measurements of directional spectra were limited almost exclusively to special research campaigns. Collecting directional wave climates from various sites and putting directional wave data to operational use were not a common practice. The situation has now improved. Many new instruments and analysis techniques have been developed for both in situ implementation and for remote sensing. It has become possible to extend the use of directional information over a much wider range, including many practical applications. This has opened up new possibilities. But it has also highlighted the difficulties associated with the directional spectrum. None of the present instruments can provide all the data that is required to calculate the directional spectrum in a robust way. Clever analysis techniques have been introduced that use physical and mathematical constraints to extract the directional spectrum from the limited data available. Each instrument and each analysis method has its own advantages and shortcomings, and the results are not necessarily comparable. If one wants to make use of the directional information, one needs an understanding of the properties of the instruments and the methods as well as the differences between them. COST action 714, supported by the European Commission, had the objective to promote the development of measurement techniques and the use of directional wave measurements. The action was launched in 1996, and, in the end, representatives of ten European countries (Belgium, Finland, France, Italy, Germany, The Netherlands, Norway, Portugal, Spain, United Kingdom) participated in it. Within the action a working group was created with the title Intercomparisons of Spectral Properties of Surface Waves for the purpose of improving the understanding of measurement techniques, methods of analysis, and comparisons between different instruments. The working group decided to publish its work in the form of a book describing the theory of directional wave spectra and the various instruments and analysis methods used to measure them, as well as comparisons between different instruments. The aim of the book is to give a comprehensive and up-to-date review of the instruments and methods of analysis available today for measuring the directional spectrum of ocean waves. In addition to the texts by members of the working group, the book contains contributions by other ocean wave experts.

Abstract This paper proposes a novel algorithm for retrieving the ocean wind vector from marine radar image sequences in real time. It is presented as an alternative to mitigate anemometer problems, such as blockage, shadowing, and turbulence. Since wind modifies the sea surface, the proposed algorithm is based on the dependence of the sea surface backscatter on wind direction and speed. This algorithm retrieves the wind vector using radar measurements in the range of 200–1500 m. Wind directions are retrieved from radar images integrated over time and smoothed (averaged) in space by searching for the maximum radar cross section in azimuth as the radar cross section is largest for upwind directions. Wind speeds are retrieved by an empirical third-order polynomial geophysical model function (GMF), which depends on the range distance in the upwind direction to a preselected intensity level and the intensity level. This GMF is approximated from a dataset of collocated in situ wind speed and radar measurements (~31 000 measurements, ~56 h). The algorithm is validated utilizing wind and radar measurements acquired on the Research Platform (R/P) FLIP (for Floating Instrumentation Platform) during the 13-day Office of Naval Research experiment on High-Resolution Air–Sea Interaction (HiRes) in June 2010. Wind speeds ranged from 4 to 22 m s−1. Once the proposed algorithm is tuned, standard deviations and biases of 14° and −1° for wind directions and of 0.8 and −0.1 m s−1 for wind speeds are observed, respectively. Additional studies of uncertainty and error of the retrieved wind speed are also reported.

Abstract For a number of maritime tasks there is a short time period, typically only a few tens of seconds, where a critical event occurs that defines a limiting wave height for the whole operation. Examples are the recovery of fixed and rotary winged aircraft, cargo transfers, final pipe mating in fluid transfer operations, and launch/recovery of small craft. The recovery of a 30-t rescue submersible onto a mother ship in the North Atlantic Treaty Organization (NATO) Submarine Rescue System is a prime example. In such applications short-term deterministic sea wave prediction (DSWP) can play a vital role in extending the sea states under which the system can be safely deployed. DSWP also has great potential in conducting experimental sea wave research at full scale. This report explores the feasibility of using data from an experimental wave profiling radar in achieving DSWP. The report includes theory, simulation, and field testing. Two forms of DSWP are employed: a fixed point system based upon a restricted set of wave directions from which some success is obtained and the other a fully two-dimensional technique that requires further development. The main finding is that using wave profiling radar for DSWP offers promise but requires improvements both to the spatial reliability and the resolution of the wave profiling radar and to the temporal resolution of its sweep before the technique can be considered to be viable as a usable tool.

Abstract. The Southern Ocean has a profound impact on the Earth's climate system. Its strong winds, intense currents, and fierce waves are critical components of the air–sea interface and contribute to absorbing, storing, and releasing heat, moisture, gases, and momentum. Owing to its remoteness and harsh environment, this region is significantly undersampled, hampering the validation of prediction models and large-scale observations from satellite sensors. Here, an unprecedented data set of simultaneous observations of winds, surface currents, and ocean waves is presented, to address the scarcity of in situ observations in the region – https://doi.org/10.26179/5ed0a30aaf764 (Alberello et al., 2020c) and https://doi.org/10.26179/5e9d038c396f2 (Derkani et al., 2020). Records were acquired underway during the Antarctic Circumnavigation Expedition (ACE), which went around the Southern Ocean from December 2016 to March 2017 (Austral summer). Observations were obtained with the wave and surface current monitoring system WaMoS-II, which scanned the ocean surface around the vessel using marine radars. Measurements were assessed for quality control and compared against available satellite observations. The data set is the most extensive and comprehensive collection of observations of surface processes for the Southern Ocean and is intended to underpin improvements of wave prediction models around Antarctica and research of air–sea interaction processes, including gas exchange and dynamics of sea spray aerosol particles. The data set has further potentials to support theoretical and numerical research on lower atmosphere, air–sea interface, and upper-ocean processes.

With a new international agreement on the conservation and sustainable use of marine biodiversity of areas beyond national jurisdiction (BBNJ Agreement) on the horizon, now is the time to start laying the foundation for successful implementation. This paper provides some initial reflections for supporting rapid, effective, and equitable implementation of the BBNJ Agreement in three priority areas: (1) bringing the Agreement into force; (2) establishing the institutional framework, including financial mechanisms; and (3) developing capacity, science, and technology. With reference to selected examples from other international processes, the paper makes suggestions for encouraging wide ratification of the BBNJ Agreement, establishing a Preparatory Commission (PrepCom), mobilizing resources, and building partnerships to advance science and capacity. The growing impacts of climate change and human activities on the global ocean necessitate urgent action, so we must begin to work on the implementation of the BBNJ Agreement as soon as possible to secure ocean health for the benefit of present and future generations.

The Wave Monitoring System WaMoS II was developed for real time measurements of directional ocean waves spectra to monitor the sea state from fixed platforms in deep water or coastal areas as well as from moving vessels. The system is based on a standard marine X-Band radar used for navigation and ship traffic control. WaMoS II digitises the analogous radar signal and analyses the sea clutter information to obtain directional wave spectra from the sea surface in real time even under harsh weather conditions and during night. Spectral sea state parameters such as significant wave height, peak wave period and peak wave direction both for wind sea and swell are derived. Within the EU funded project ‘MaxWave’ and the German project ‘SinSee’ new algorithms were developed to determine sea surface elevation maps from radar images which are used to investigate the spatial and temporal evolution of single waves simultaneously. In this paper a short overview describes the calculation of surface elevation maps and the detection of individual waves. Considering two case studies, the results of spatial single wave detection and corresponding temporal single wave properties are compared and discussed. Individual wave parameters derived from radar images are compared to individual waves measured by a buoy. An application of the method to characterise extreme sea states is discussed.

Abstract The ocean wave signatures within conventional noncoherent marine X-band radar (MR) image sequences can be used to derive near-surface current information. On ships, an accurate near-real-time record of the near-surface current could improve navigational safety. It could also advance understanding of air–sea interaction processes. The standard shipboard MR near-surface current estimates were found to have large errors (of the same order of magnitude as the signal) that are associated with ship speed and heading. For acoustic Doppler current profilers (ADCPs), ship heading errors are known to induce a spurious cross-track current that is proportional to the ship speed and the sine of the error angle. Conventional mechanical gyrocompasses are very reliable heading sensors, but they are too inaccurate for shipboard ADCPs. Within the ADCP community, it is common practice to correct the gyrocompass measurements with the help of multiantenna carrier-phase differential GPS systems. This study shows how a similar multiantenna GPS-based ship heading correction technique stands to improve the accuracy of MR near-surface current estimates. Changes to the standard MR near-surface current retrieval method that are necessary for high-quality results from ships are also introduced. MR and ADCP data collected from R/V Roger Revelle during the Impact of Typhoons on the Ocean in the Pacific (ITOP) program in 2010 are used to demonstrate the MR currents’ accuracy and reliability.

OceanWaveS GmbH has been developing a prototype system, based on standard non-coherent X-band navigational radars, capable of predicting future sea surface elevations. Combined with a vessel hydrodynamic simulator, this system will forecast ship motions. Applications for such a system include offshore operations, e.g. crane lifts, LNG, cargo and personnel transfers. The forecasts may be assimilated into automated control systems; e.g. floating wind turbines or dynamic positioning systems, or used within Decision Support Systems. This article presents correlation analysis results between the predicted sea surface elevations and vessel-mounted reference sensors for three independent sea trials. Despite neglecting vessel hydrodynamics, correlations of forecasts to measured references of 80% are achieved for T+60 seconds predictions for all sea trials. The prediction horizon is observed up to 180 seconds of forecast.

WaMoS II is an operational wave monitoring system based on a standard marine X-Band radar generally used for navigation and ship traffic control. The system was basically developed in the 80' ties and is commercially available since 1995. WaMoS II can be operated from fixed platforms as well as from moving vessels. The system is type approved by Det Norske Veritas for the use on board ships travelling with up to 40 knots. In this paper operationally collected WaMoS II wave data are compared to conventionally measured wave data. The measurements are from a deep water off-shore oil platform (Ekofisk, Norway), from an oil shuttle (Navion Oceania, Norway), and from a shallow water coastal station (Helgoland, Germany). For the main sea state parameters, such as significant wave height, peak wave period and peak wave direction, error statistics are applied. Within MaxWave, an EU project funded under the 5th framework programme, a new method to obtain single wave information from nautical radar images will be developed. Here a method to derive partial wave information from nautical radar images is presented. Phases of partial waves are derived from single pixel. This spatial varying information, represents the local variability in the observed wave field. The resulting phase maps show significant wave crest patterns. These wave crest patterns exhibit the modulation of the ocean waves as they propagate through the observation area.

Vessels and floating structures are subject to wave induced motions affecting and often even preventing their operation. A system to predict short periods of quiescent vessel motions a few minutes in advance could reduce the costly ‘Waiting on Weather’. The Joint Industry Project ‘On board Wave and Motion Estimator (OWME)’ developed a system capable of predicting the vessel motions on board in real time up to two minutes in advance. It measures remote wave profiles by means of nautical radar, uses linear wave theory to propagate the waves in time and space and estimates the ship response to the predicted wave field. This paper describes the concept and shows results of a first field trial aboard an offshore support vessel in mild sea conditions.

The dynamic wave field in a high-energy coastal environment is investigated using frequency direction wave spectra obtained by nautical X-band radar imagery. Nautical radars are generally used for navigation and ship traffic control. Under various conditions (wind speed > 3m/s, significant wave height > 0.5m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> ), signatures of the sea surface (sea clutter) become visible in the near range (less than 3 nautical miles) of nautical radar images. Swell and wind sea waves become visible in nautical radar images as they modulate the sea clutter signal. Since standard X-band nautical radar systems scan the sea surface with high temporal and spatial resolution, they are able to monitor the sea surface in both time and space. The combination of the temporal and spatial wave information allows the determination of unambiguous directional wave spectra. Here, wave data collected from February-October 2005 at the US Army Corps of Engineers Field Research Facility (USACE-FRF) in Duck, North Carolina is presented. For the radar wave measurements the Wave and surface current Monitoring System WaMoS II was connected to a Furuno FR-7112 X-Band radar with a 6 feet open antenna and an update rate of 2.5s (24 rpm). The radar covers a range from 240m 2160m from the antenna with a spatial resolution of 7.5m. The wave analysis was carried out over an area of 3.7 km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> located in relative homogeneous bottom topography, off the near shore breaker bar system, in a water depth of 8m -10m. The WaMoS II wave measurements were compared to those obtained from a pressure gauge array located in the same area. Earlier WaMoS II validations provide a general indicator of the quality of the measurement performance as they were carried out for standard integral wave properties over all existing wave systems such as mean or peak wave parameters. Here the XWaves ocean wave field analysis toolbox is used to compare data sets by means of a wave spectral partitioning analysis. This approach provides a more detailed validation especially for bi-and multi modal sea states, allows for a comparison of the heights, periods and directions of individual wind sea and swell components, and tracking the evolution of specific wave systems. Such analysis methods have been successfully applied in a variety of wave model validations. The data comparison was carried out for different sea state and wind conditions. Preliminary results of the data comparison show that the WaMoS II system captures the temporal evolution of the individual wind sea and swell wave components entering the surf zone. A statistical error analysis of the isolated wind sea and swell wave systems provides a quantitative assessment of WaMoS II performance in a coastal setting.

Wave induced motions limit the working time and safety of many offshore operations, e.g. crane lifts, LNG transfers, and helicopter landings. This paper details recent efforts to deterministically predict ocean surface waves using measurements from the WaMoS II. The WaMoS II derives full 3-dimensional sea surface elevation maps from nautical X-band radar images, yielding both sufficient resolution and range for a useful prediction horizon. Validation of these surface elevation fields shows they are in close agreement with sea surface elevation timeseries from wave buoys and motion reference units. Results from a simple propagation model are presented.

Real-time quality-controlled surface current data derived from X-Band marine radar (MR) measurements were evaluated to estimate their operational reliability. The presented data were acquired by the standard commercial off-the-shelf MR-based sigma s6 WaMoS® II (WaMoS® II) deployed onboard the German Research vessel Polarstern. The measurement reliability is specified by an IQ value obtained by the WaMoS® II real-time quality control (rtQC). Data which pass the rtQC without objection are assumed to be reliable. For these data sets accuracy and correlation with corresponding vessel-mounted acoustic Doppler current profiler (ADCP) measurements are determined. To reduce potential misinterpretation due to short-term oceanic variability/turbulences, the evaluation of the WaMoS® II accuracy was carried out based on sliding means over 20 min of the reliable data only. The associated standard deviation σ W a M o S = 0.02 m/s of the mean WaMoS® II measurements reflect a high precision of the measurement and the successful rtQC during different wave, current and weather conditions. The direct comparison of 7272 WaMoS® II/ADCP northward and eastward velocity data pairs yield a correlation of r ≥ 0.94 , with | b i a s Δ | ≤ 0.06 m/s and σ S = 0.05 m/s. This confirms that the MR-based surface current measurements are accurate and reliable.

The dynamics of the Rhine outflow plume in the proximity of the river mouth is investigated by using remote sensing data and numerical simulations. The remote sensing data consist of 41 synthetic aperture radar (SAR) images acquired by the First and Second European Remote Sensing satellites ERS-1 and ERS-2 over the outflow region of the river Rhine. Most of them show sea surface signatures of oceanic phenomena, for example, surface current and wind variations, ship wakes, and oil slicks. In particular, in 36 of these images pronounced frontal features are visible as narrow zones of mainly enhanced, sometimes enhanced/reduced radar backscatter that can be associated with the Rhine surface front. Within the area enclosed by the frontal line, large zones characterized by a lower radar backscatter than in the outer area are often visible. The analysis of the ERS SAR images suggests that the form and the location of the frontal features are mainly linked to the semidiurnal tidal phase in the outflow region, although their variability suggests also that they weakly depend on river discharge, residual currents, and neap-spring tidal cycle. In order to test this observational hypothesis, the results obtained from the analysis of the ERS SAR images are compared with the results obtained from the numerical simulation of the hydrodynamics of the Rhine outflow region carried out using a two-layer, frontal model, which is based on the nonlinear, hydrostatic shallow-water equations on an f plane. The model is forced by prescribing tidal and residual currents and river discharge at the open boundaries. Several simulations are performed by varying the values of these forcing parameters. The numerical results corroborate the observational conjecture: It is found that the form and the location of the simulated interface outcropping lines in the proximity of the river mouth are mainly determined by the semidiurnal tidal phase in the outflow region and that river discharge, residual currents, and neap-spring tidal cycle contribute only secondarily to their determination. Inserting the simulated surface velocity field into a simple radar-imaging model that relates the modulation of the backscattered radar power to the surface velocity convergence in radar look direction, narrow, elongated bands of enhanced radar backscatter emerge near the model frontal line while patches of low radar backscatter appear within the simulated Rhine plume area. The consistency of the model results with the results obtained from the analysis of the SAR images enables one to infer a mean spatial and temporal evolution of the Rhine outflow plume over a semidiurnal tidal cycle from the analysis of spaceborne SAR images acquired during different tidal cycles over the Rhine outflow area and suggests the possibility of using numerical modeling, in conjunction with the analysis of spaceborne measurements, for monitoring the oceanic variability in the Rhine outflow area.

The wave and current monitoring system WaMoS II is a remote sensing system based on a nautical X-Band radar generally used for navigation and ship traffic control. It has been used in recent years to monitor sea state information from moored platforms, coastal sites and moving vessels. A nautical radar can scan the sea surface over a large area (~ 10 km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) with a high spatial (~7.5 m) and temporal resolution (~2s). Directional wave spectra and standard sea state parameters such as significant wave height, peak wave period and direction can be derived by analyzing the sea surface image sequences. Using the temporal and spatial evolution of the sea surface wave images it is also possible to determine high resolution current and bathymetry information. In the paper a brief introduction into the measuring principle of WaMoS II is given and results of a high resolution current and bathymetric mapping technique for shallow water areas (<20 m) are presented. For validation these results are compared with model data and in-situ measurements.

Nautical X-Band radars used for navigation can also be used to determine spectral and individual wave properties. The sea surface reflects the incident radar beams and wave fronts become visible as stripe like pattern of high back scatter on the radar screen. When connected to a conventional nautical X-Band radar, the Wave Monitoring System WaMoS II exploits this imaging of waves to detect full directional wave spectra and to derive statistical sea state parameters as well as surface currents. WaMoS II is continuously improved with new features being developed. In particular, the sea surface elevation maps derived by WaMoS II allow to investigate and describe the spatial and temporal development of 3D ocean surface waves. The European Joint Industry Project `On board Wave and Motion Estimator (OWME)' used this measurement technique to provide the wave information that is required to predict periods of quiescent vessel motions. A task that offers valuable support for various offshore operations like e.g. the tensioning of a tanker or the landing of a helicopter. This contribution gives an overview of the overall OWME system design showing first validation results, focusing on the method to derive wave trains using the WaMoS II system.

During the Antarctic summer season 2008/2009 the wave radar system WaMoS II was installed onboard of the German research vessel “Polarstern.” The purpose was to collect quasi-in situ data for the comparison with satellite-borne SAR and altimeter instruments (Envisat, TerraSAR-X, Jason). The experiment was part of the German research project DeMarine-Security. On 7 March 2009 in the central South Atlantic Ocean, “Polarstern” was heading towards Punta Arenas against a rough cross sea. In the night, a sudden event of heavy rolling, i.e., an oscillation about its length axis, hit the vessel and lasted for a few minutes. Using WaMoS II data, as well as ENVISAT and wave model data, we investigate the conditions under which the event occurred. It is shown that the rolling was caused by a “parametric” resonance when the period of encounter came close to one half of the vessels's natural rolling period. We conclude that an onboard wave radar can be helpful in diagnosing and forecasting critical conditions.

Coastal waters are characterised by complex wave fields that are influenced by inhomogeneous bathymetries, and changing tidal- and wind-induced currents. Understanding these interactions is important for coastal engineering and environmental management. Remote sensing techniques, such as radar for flow field data collection, increase the amount of available information. Unlike in-situ techniques (e.g. buoys, or moored current meters), remote sensing can provide continuous observation of several parameters (waves, currents and bathymetry) simultaneously across a wide area. The wave and current monitoring system, WaMoS II, is a remote sensing system using standard nautical X-Band radars generally used for navigation and ship traffic control. Nautical radars are designed to monitor the sea surface continuously over a relative large area (~ 10 km2) with high spatial (~7.5 m) and temporal resolution (~2 s). Under various conditions, signatures of the sea surface itself become visible in the near range (less than 3 nautical miles) of such radar images. These signatures include spatial and temporal information of the sea surface waves (wind/sea and swell), currents and in shallow water also about the local water depth. In recent years, development has focussed on retrieving current and wave data at that high resolution on an operational basis (Hessner et al., 2007 [1]; Hessner and Bell, 2009, Hessner et al. 2014). In this paper, a brief introduction will be given to the high resolution current and water depth measurement principles of WaMoS II. WaMoS II current and wave data will be shown from the Sizewell test site, where a system has been installed since September 2013. This site is located on the East coast of England, an area of coastline which has been intensively studied over many decades. The hydrodynamics of this area are characterised by strong prevailing tidal currents with current magnitudes between 1-1.5m/s and a strongly bi-directional wave climate. The WaMoS II system at this site, operates in connection with a Kelvin Hughes (Manta Digital) with a horizontally polarized 8 ft antenna and a radar repetition rate of 1.34s. The antenna is mounted 66m above sea level. The area of radar observation ranges from 150m to 4000m, off shore (0-180° relative to N). The system delivers standard sea state measurements with an update rate of 2 min and high resolution current and depth information in a range up to 4 km with an update rate of 20 minutes and a spatial resolution of about 180m. The observation area is characterised by a straight North-Southward aligned coastline and an offshore sandbank with varying water depth between 5-15m. The localised effect of the bank on the wave and flow fields is thought to have significant impact at the shoreline. The complex hydrodynamics and spatially varying currents would be impossible to monitor at the appropriate scale with point measurements only. Here typical tidal states and sea state conditions will be discussed. For the validation of the WaMoS II wave and current measurements, reference point measurements acquired by ADCP sensors at 3 different locations in the radar view field were used. Data acquired within the working range of WaMoS II (Hs > 0.5m, wind speed > 5m/s) show an excellent agreement with reference data but also spatial wave and current variation due to the bathymetry. During the observation period the prevailing wave direction was East North East, but periods of South-East waves were also observed. The radar data revealed the complexity of the wave and flow field with respect to the interaction between tides, waves and bathymetry.