Heinz Günther

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.

Projection of Global Wave Climate Change toward the End of the Twenty-First Century

AbstractWind-generated waves at the sea surface are of outstanding importance for both their practical relevance in many aspects, such as coastal erosion, protection, or safety of navigation, and for their scientific relevance in modifying fluxes at the air–sea interface. So far, long-term changes in ocean wave climate have been studied mostly from a regional perspective with global dynamical studies emerging only recently. Here a global wave climate study is presented, in which a global wave model [Wave Ocean Model (WAM)] is driven by atmospheric forcing from a global climate model (ECHAM5) for present-day and potential future climate conditions represented by the Intergovernmental Panel for Climate Change (IPCC) A1B emission scenario. It is found that changes in mean and extreme wave climate toward the end of the twenty-first century are small to moderate, with the largest signals being a poleward shift in the annual mean and extreme significant wave heights in the midlatitudes of both hemispheres, more...

Spectral wave modelling with non-linear dissipation: validation and applications in a coastal tidal environment

Abstract A spectral wave model with non-linear dissipation is validated and applied in wind-wave investigations in the Sylt–Romo Bight. The model was developed for applications in small-scale shallow-water environments. Numerical experiments on wind waves in the bight demonstrate the applicability of the model in small-scale systems with time-varying water levels and currents. A 1-month hindcast of wind in the Sylt–Romo Bight is used to successfully validate the model against field data. The influence of currents on wave parameters is reproduced quantitatively. It is shown that inclusion of currents distinctly improves the hindcast skill for wave periods. Case studies for prescribed wind situations reveal a significant complex interaction of tide- and wind-driven currents on wind waves.

Turbulent Mixing due to Surface Waves Indicated by Remote Sensing of Suspended Particulate Matter and Its Implementation into Coupled Modeling of Waves, Turbulence, and Circulation

This paper studies the impact of the surface waves on the turbulent mixing. The satellite observations of suspended particulate matter (SPM) at the ocean surface as an indicator of turbulent quantities of the flow are used. In a water column, SPM builds a vertical profile depending on settling velocities of the particles and on vertical mixing processes; thus, SPM is a perfect marker to study the turbulent quantities of the flow. Satellite observations in the North Sea show that surface SPM concentrations, in locations of its deposition, grow rapidly and build plume-shaped, long (many kilometers) uninterrupted and consistent structures during a storm. Also, satellites reveal that SPM rapidly sinks to the seabed after the storm peak has passed and wave height decreases (i.e., in the absence of strong turbulence). The nonbreaking wave-induced turbulence has been discussed, parameterized, and implemented into an equation of evolution of turbulent kinetic energy (TKE) in the frame of mean-flow concept, which can be used in existing circulation models. The ratio between dissipated and total wave energy is used to describe the influence of wave damping on the mean flow. The numerical tests reproduce experiments in a wave tank very well and are supported by observations of SPM in the North Sea. Their results show that the motion of an individual nonbreaking wave includes turbulent fluctuations if the critical Reynolds number for wave motion is exceeded, independent of the presence of currents due to wind or tides. These fluctuations can produce high diffusivity and strongly influence mixing in the upper water layer of the ocean.

A semi-operational comparison of two parametrical wave prediction models

In recent years several numerical wave prediction models have been developed. Two such models HYPAS and GONO are based on very similar concepts i. e. a parametrical wind/wave description and a characteristic ray method for swell propagation. The models are discussed briefly with emphasis on the differences. Rather important differences are found in the treatment of shallow water effects. For wind sea the HYPAS model uses the concept of a depth dependent self similar water wave spectrum. GONO has an explicit bottom reduction algorithm and adjusts spectral parameters, but in shallow water it assumes the same spectral shape as in deep water. To compare the semi-operational performance of both models, wave calculations for the southern North Sea were made by both models for a three month period with the same wind fields. The results of this comparison are discussed.

Similarity of the Wind Wave Spectrum in Finite Depth Water Part 2: Statistical Relations between Shape and Growth Stage Parameters

This paper continues the description of surface waves on finite depth water started in Bouws et al. [1985]. We present relationships between the parameters of the wind wave spectra similar to the correlation found in deep water between total energy and spectral peak frequency. In contrast to deep water the peak frequency is not the most convenient parameter to describe spectral development. The wave number of the spectral peak, however, is connected with other spectral parameters by relations that are independent of water depth or site.

Comparison of Remotely Measured and Modelled Currents in Coastal Areas of Norway and Spain

Data from two six-week current measurement campaigns at the coasts of Norway and Spain are presented. Spatial coverage of surface currents was obtained by an HF (High Frequency) radar. Subsurface currents were measured by bottom-mounted and ship-borne ADCPs (Acoustic Doppler Current Profiler). The HF data were assimilated by a fine gridded model with the aim of predicting currents for about 6 h. The objective of this article is twofold, to investigate the performance of the instruments and the model, and to show the high temporal and spatial variability of currents in the coastal zones under investigation. Several strong storms occurred during the experiments with significant waveheights exceeding 11 m (Norway) and 8 m (Spain), respectively. High waves affected both the WEllen RAdar (WERA) (reduced ranges) and the ADCP (unreliable near-surface current velocities). The assimilation algorithm of the model worked well. The comparisons of measured and modelled maps of the surface-current velocity, time series...

Wave climatology and extreme value analysis for the Baltic Sea area off the Warnemünde harbour entrance

A numerical wave model was used to determine the wave climate in the southern Baltic Sea. The basic data derived from wave hindcasts were evaluated statistically. Climate graphs were developed on the basis of continuous computations for the years 1988 through 1993. Extreme value statistics were derived from the results of hindcast computations for 40 storms in the years 1956–1993. The evaluation was made primarily with a view to redesigning the Warnemunde harbour entrance, but the data are also useful with respect to other areas of the Baltic Sea.

Shallow water wave modelling with nonlinear dissipation

In this paper a new shallow water wave model is described which uses nonlinear dissipation derived from turbulent diffusion as damping mechanism. The source functions of the model are presented in detail. Analytical results of the dynamical equation for simple cases illustrate basic features of the model. Academic test runs in deep and shallow water are performed. The designed cases are identical to the ones used in previous wave model intercomparison studies and thus allow comparison with other wave models. Results of a hindcast of a North Sea storm event illustrate the model behaviour in nonuniform real shallow water systems. In this case we can compare with field data and with the community wave model WAM cy. 4, which has been run parallel to our model. Our study shows that the concept of wave modelling with nonlinear dissipation is consistent with common knowledge of wave evolution in oceanic and shelf sea applications.

Joint along-across track interferometry of ocean waves

In the framework of the EuroROSE project high resolution ocean wave, wind and current fields were measured at the North Coast of Spain by simultaneous data acquisitions of spaceborne, airborne, and in situ instruments. The objective of the project was to provide a guidance system for ships entering the harbour of Gijon, by monitoring and forecasting wind, waves and currents. This paper presents first results of a new airborne three antenna interferometric synthetic aperture radar (InSAR) system which provides information on both sea surface elevation and motion. The system transmits and receives signals with a combination of three antennas, whereby two are placed along flight direction and the third in across flight direction. This configuration allows, for the first time, to acquire along and across track InSAR data simultaneously. The system was flown in different flight patterns with simultaneous measurements of directional wave spectra, wind and currents fields taken by the HF-radar WERA, the radar based wave monitoring system WaMoSII, the ERS-2 SAR and a waverider buoy.