During the last years, there has been increasing interest in freak waves and freak forces caused by change in bathymetry. Our research follows the pioneering work of Bitner (Bitner, 1980), who studied how the statistical distribution changed for waves propagating over variable depth. Laboratory data of surface waves from Marin, where waves propagated from deeper to shallower water, were analyzed in Trulsen, Zeng, and Gramstad, 2012. They found a local maximum for kurtosis at the beginning of shallower water. Experimental measurements of Raustl in Trulsen, Raustl, et al., 2020 showed an increase in kurtosis and skewness near the front of the plateau of the shoal, and a minimum in skewness at the lee side. Experimental velocity measurements of Jorde in Trulsen, Raustl, et al., 2020 led to the observation that the velocities had a local maximum and minimum in skewness at the same places as for the surface elevation, but the kurtosis had maximum on the lee side of the shoal. By numerical simulation, the same results were reached by Lawrence, Trulsen, and Gramstad, 2021. This lead us to ask if the forces on a horizontal cylinder would have a similar maximum of kurtosis behind the shoal.

We have done experiments in a wave flume measuring surface elevations and correspond-ing velocities over and behind a shoal, and the resulting forces on a horizontal cylinder behind a shoal, as sketched in fig. 1. Ultrasound probes were placed around the shoal, and an ADV could be moved horizontally over the shoal. The cylinder was equipped with force transducers, and could also be moved. The waves were generated according to the Pierson-Moskowitz spectrum, different from most earlier research that employed the JONSWAP spectrum. In the same way, as for the JONSWAP spectrum in earlier research, we observe the increase in skewness and kurtosis for surface elevation at the front end of the shoal. We also see the increase in kurtosis of the velocity field on the lee side of the shoal. Our force measurements indeed show an increase in skewness and kurtosis on the lee side of the shoal compared to over at the bottom, especially for the horizontal forces. Therefore, there might be reason to be cautious about extreme forces when placing structures behind a shoal.

How to cite: Samseth, K. and Trulsen, K.: Extreme waves over a shoal, and extreme forces on a submerged cylinder behind the shoal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21239, https://doi.org/10.5194/egusphere-egu24-21239, 2024.

A Nonlinear Schrödinger (NLS) equation-based theoretical model is derived in Li & Chabchoub (2024) for deepwater waves in the presence of a background flow. The flow propagates in the horizontal plane with its profile magnitude and direction being depth-dependent (see, e.g., Li & Ellingsen 2019). A new interpretation of the roles of Stokes drift, Eulerian return flow, and background vertically sheared current in the modulational instability (MI) of Stokes waves has been provided. The results in particular show that a current opposing a long-crested wave group can enhance the oblique modulational instability while suppressing completely the modulational instability which arises from sideband waves in the directions parallel to the wave group.  This provides clear physical insights into the roles of a background flow on rogue waves, owing to that the MI has been well recognized as a plausible cause to their generation. Moreover, the relevance of the background flow suppression or enhancement of the modulation instability process to the Craik-Leibobvich type 2 instability in the presence of Langmuir circulation is discussed and quantified. This relevance suggests a plausible physical mechanism for the energy transfers between waves and currents in the open ocean.

References:

Li, Y., Ellingsen, S. ̊A.: A framework for modelling linear surface waves on shear currents in slowly varying waters. Journal of Geophysical Research:Oceans 124, 2527–2545 (2019).

Li, Y., Chabchoub, A.: How currents trigger extreme sea waves. The roles of Stokes drift, Eulerian return flow, and a background flow in the open ocean (2024). (submitted to Geophys. Res. Lett.).

How to cite: Li, Y. and Chabchoub, A.: Extremely large waves atop a depth-dependent background flow, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12073, https://doi.org/10.5194/egusphere-egu24-12073, 2024.

We present a theoretical study of bichromatic weakly non-collinear Airy-type waves propagating with various amplitudes in water of infinite
depth. The coupled nonlinear Schrödinger equations are invoked to study the behavior of self- and cross-interacting Airy wave packets. We
demonstrate that bichromatic pulses possess the main properties of single Airy wave packets—shape invariance and self-acceleration or selfdeceleration—
when propagating in a dispersive medium. Accounting for nonlinearity leads to a strong dependence of the structure, stability,
and propagation velocity of wave pulses on their amplitude. We show that interacting pulses of Airy-type waves can form giant accelerating
and decelerating waves. The study of accelerating and decelerating bichromatic wave pulses of the Airy type significantly expands the
set of scenarios for the occurrence of rogue waves in various physical media.

How to cite: Shugan, I. and Chen, Y.-Y.: Self-accelerating rogue waves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14285, https://doi.org/10.5194/egusphere-egu24-14285, 2024.

In this talk we examine  a higher order nonlinear Schr\"odinger equation with linear damping and weak viscosity, recently proposed as a model for deep water waves exhibiting frequency downshifting. Through analysis and numerical simulations, we discuss how the viscosity affects the linear stability of the Stokes wave solution, enhances rogue wave formation, and leads to permanent downshift in the spectral peak. The novel results in this work include the analysis of the transition  from the initial Benjamin-Feir instability to a predominantly oscillatory behavior, which takes place in a time interval when most rogue wave activity occurs. In addition, we propose new criteria for downshifting in the spectral peak and determine the relation between the time of permanent downshift and the location of the global minimum of the momentum and the magnitude of its second derivative.

How to cite: Schober, C.: The Effects of Viscosity on the Linear Stability of  Damped Stokes Waves, Downshifting,  and Rogue Wave Generation}, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14258, https://doi.org/10.5194/egusphere-egu24-14258, 2024.

Rogue waves have received considerable attention in recent years, with major advancements in their generation mechanisms having been defined. However, the focus of most investigations has been related to rogue wave occurrence in deep water. In contrast, far fewer results are available in shallower water depths. As such, the present work focuses on exploring the occurrence probabilities of rogue waves in coastal waters, as well as the physical mechanisms that lead to their formation. This is achieved by a thorough analysis of a very extensive experimental dataset of random waves propagating over planar beaches. More specifically, long simulations of realistic JONSWAP spectra arising in intermediate water depths have been generated at the deep end of the Coastal Flume at Imperial College London. These propagate over 3 uniform slopes with inclinations varying between 1:15 and 1:50, while being sampled by a dense array of wave gauges. The fine spatial resolution of wave gauges allows for a detailed description of large wave evolution as they travel towards the shoreline. Importantly, a parametric approach in defining the offshore forcing conditions has been adopted and covers a wide range of sea-state steepnesses and effective water depths. Taken together, 15 different storm conditions, each consisting of approximately 20,000 waves, have been considered for each bed slope configuration.

In analysing these results, the occurrence of rogue waves is examined at all spatial locations across the coastal zone. We observe a considerable increase in rogue wave occurrence for reducing water depths which has not been found previously. This is particularly the case for moderately mild offshore storms. In exploring the shape of rogue waves arising at different water depth regimes, the relative importance of dispersion and nonlinearity is defined. While rogue waves arising at the deeper end of the coast resemble NewWave type events, solitary-type events become more pronounced at the shallower end. The occurrence of rogue waves in shallow water is suppressed once extensive wave breaking arises. While this is expected as a result of depth-induced wave breaking, interesting results arise for the steepest offshore conditions. Evidence suggests that waves breaking at the outer edge of the surf zone, regroup and give rise to rogue waves closer to the shoreline.

Taken together, the rogue wave investigation within the present extensive experimental dataset provides evidence for their increased importance in coastal waters which has not been broadly considered so far.

How to cite: Karmpadakis, I. and Bellos, V.: Rogue wave occurrence over planar coastal bathymetries, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14317, https://doi.org/10.5194/egusphere-egu24-14317, 2024.

Artificial intelligence and machine learning are known to be excellent at empirical modelling of complex systems. These empirical models, however, usually can only provide limited physical explanations about the underlying systems. With a “knowledge discovery” scheme in machine learning, instead of being constrained by fitting the coefficients, can we now discover an equation that can shed light on the underlying physics? In this presentation, we will focus on a real fluid mechanics challenge as an example to demonstrate the potential of such a scheme – modelling wave breaking evolution.

In this ongoing study, we use symbolic regression to discover the equation that describes the wave breaking evolution from a large dataset of Direct Numerical Simulations of breaking waves. We found a new boundary equation that approximates the surface elevation (water-air interface) to evolve forward in time even during the breaking-in-progress stage, whereas traditional potential flow type equations will eventually become unstable when the overturning jet touches the crest. Unlike empirical models where the underlying dynamics are hidden in coefficients/matrixes, the physical meaning of each term of the discovered equation can be revealed successfully through math derivation and simulation. The new boundary equation suggests a new characteristic of breaking waves in deep water – a decoupling between the water-air interface and the fluid velocities. The current model is only limited to unidirectional spilling breaking waves in deep water but can be easily extended to accommodate more complex breaking behaviour.

How to cite: Tang, T., Chen, Y., Taylor, P., and Adcock, T.: Discovering equations that govern wave breaking evolution using scientific machine learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12541, https://doi.org/10.5194/egusphere-egu24-12541, 2024.

Figure 1: Underwater vortical structures generated during the breaking: vortex-tubes and vortex-sheets are drawn in yellow and gray, respectively.

The breaking of ocean waves is of significant interest due to its implications in various physical, chemical, and biological processes that occur at the ocean-atmosphere interface.  Wave breaking generates free-surface turbulence, dissipates wave energy, and enhances momentum, heat, and gas transfer between air and water. Over the years, a number of review articles and monographs have been published on the subject (Banner & Peregrine 1993; Melville 1996; Duncan 2001; Babanin 2011; Kiger & Duncan 2012; Perlin, Choi & Tian 2013; Lubin & Chanson 2017; Deike 2022), and these works call for more research into nearly every aspect of wave breaking. For these reason, the flow generated by the breaking of free-surface waves in a periodic domain is simulated numerically by means of a gas-liquid multiphase Navier Stokes solver. The solver relies on the Volume-of-Fluid (VOF) approach, and interface tracking is carried out by using a novel algebraic scheme based on a tailored TVD limiter (Pirozzoli et al., 2019). The solver is proved to be characterized by low numerical dissipation, thanks to the use of the MAC scheme, which guarantees discrete preservation of total kinetic energy in the case of a single phase. Both two- and three-dimensional simulations have been carried out, and the analysis is presented in terms of energy dissipation, air entrainment, bubble fragmentation, statistics and distribution. Particular attention is paid to the analysis of the mechanisms of viscous dissipation. For this purpose, coherent vortical structures (Horiuti and Takagi, 2005), are identified and the different behaviour of vortex sheets and vortex tubes are highlighted, at different Re. The correlation between vortical structures and energy dissipation demonstrates clearly their close link both in the mixing zone and in the pure water domain, where the coherent structures propagate as a consequence of the downward transport. Notably, it is found that the dissipation is primarily connected with vortex sheets, whereas vortex tubes are mainly related to flow intermittency.

In order to highlight the connections between air entrainment and viscous dissipation with vortical structures, in fig. 2, slices taken in the longitudinal symmetry plane are drawn. The results display a very close correlation between viscous dissipation and the vortex sheet indicator. Also, it is worth noticing that viscous dissipation is not confined about the free surface, but it is spread within the bubble cloud. Within the high-dissipation regions, marked by the vortex sheet indicator, vortex tubes also form in zones with high vorticity.

Figure 2: Longitudinal sections of the solutions computed at Re = 10000 (top) and Re = 40000 (bottom). The coloured contours denote the local values of the normal-to-plane vorticity components (left), and the local dissipation rate (right). The black solid lines in left and right figures denote the vortex-tube and vortex-sheet iso-lines, respectively. Note that the solutions in the water domain are shown only.

How to cite: Di Giorgio, S., Pirozzoli, S., and Iafrati, A.: On coherent vortical structures in wave breaking, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16319, https://doi.org/10.5194/egusphere-egu24-16319, 2024.

Linear theory of water waves is reasonable to use only in the case of small waves. As the waves’ steepness increases, nonlinear effects start to play a role too significant to be neglected. One of the widely used and commonly accepted methods of calculating water wave kinematics is Fenton’s method (Fenton 1985). It allows one to find free surface elevation and velocity potential up to 5^th order in wave steepness.

In case of wave-related phenomena, among quantities of interest is wave-induced mass transport as its knowledge is necessary to find tracer transport, like oil pollution, algae bloom, or plastic. Cieślikiewicz & Gudmestad (1994) introduced a method of calculating mass transport induced by harmonic and random water waves. A key contribution in this study was taking into account the emergence effect: in the Eulerian frame of reference, a fixed point in space in the vicinity of the free surface emerges and submerges under the water.

The primary objective of the present study was to integrate Fenton’s kinematics into the Cieślikiewicz & Gudmestad methodology for more accurately calculating the wave-induced mass transport. I used the perturbation scheme and the technique of transformation of random variables (Huang et al. 1983). The results demonstrate strong agreement with previous approaches.

Cieślikiewicz, W. & Gudmestad, O. T. (1994). Mass transport within the free surface zone of water waves. Wave Motion, 19(2), 145–158. https://doi.org/10.1016/0165-2125(94)90063-9

Fenton, J. D. (1985). A Fifth-Order Stokes Theory for Steady Waves. Journal of Waterway, Port, Coastal, and Ocean Engineering, 2(111), 216–234

Huang, N. E., Long, S. R., Tung, C.-C., Yuan, Y., & Bliven, L. F. (1983). A Non-Gaussian Statistical Model for Surface Elevation of Nonlinear Random Wave Fields. Journal of Geophysical Research, 88(C12), 7597–7606; Papoulis, A., & Pillai, S. U. (2002). Probability, Random Variables and Stochastic Processes. McGraw-Hill

How to cite: Grzonka, L. and Cieślikiewicz, W.: Mass transport induced by 5-th order nonlinear water waves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1149, https://doi.org/10.5194/egusphere-egu24-1149, 2024.

We are interested in the interaction of ocean waves with steep coastal topography such as encountered in some coastal profiles for example in the United States, New Zealand and Norway. In previous works, it has been shown that under such conditions, shoaling ocean waves may experience significant amplification in the last 50 to 100 meters before they run up on the shore, leading to potentially hazardous run-up events even under relatively calm conditions. In the present work, we are reporting on a remotely accessible observational system which was deployed on the Norwegian Coast near the city of Haugesund. We report on the data analysis and statistical correlation of large run-up events with certain sea states and weather conditions.

References:

[1] Bjørnestad, M. and Kalisch, H., 2020. Extreme wave runup on a steep coastal profile. AIP Advances, 10(10).

[2] Kalisch, H., Lagona, F. and Roeber, V., 2023. Sudden wave flooding on steep rock shores: a clear but hidden danger. Natural Hazards, pp.1-21.

How to cite: Kalisch, H., Blandfort, D., Buckley, M., Bödewadt, J., Bjørnestad, M., Lagona, F., Derriey, A., Björqvist, J.-V., and Horstmann, J.: Wave run-up detection at the Norwegian coast, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17231, https://doi.org/10.5194/egusphere-egu24-17231, 2024.

Ocean turbulence at meso- and submesocales affects the propagation of surface waves through refraction and scattering. This induces spatial modulations in wave energy, with implications for air–sea exchanges, the likelihood of extreme waves, and remote sensing. We develop a theoretical framework that relates modulations in significant wave height (SWH) to the currents that induce them. We exploit the asymptotic smallness of the ratio of typical current speed to wave group speed (which holds for wavelengths above 10 m or so) to derive a linear map – the U2H map – that relates SWH anomalies to the surface current velocity. This map is a convolution, non-local in space but expressible as a product in Fourier space and, crucially, independent of the magnitude of the Fourier vector. The properties of the map show how the SWH anomaly responds differently to the vortical and divergent parts of the currents, and how the anisotropy of the wave spectrum is key to large current-induced SWH anomalies. Analysing the U2H map, in particular for swell-like, highly directional waves enables us to explain a series of earlier numerical observations. We implement the U2H map numerically and test its predictions against  WAVEWATCH III numerical simulations for both idealised and realistic current configurations. Our framework can be straightforwardly extended to relate characteristics of the wave field other than SWH such as Stokes drift to the currents.  

How to cite: Vanneste, J., Villas Bôas, A. B., Wang, H., and Young, W. R.: Impact of submesoscale currents on surface waves: the U2H map, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1374, https://doi.org/10.5194/egusphere-egu24-1374, 2024.

Improving wave forecasting in the polar oceans is crucial for coupled earth system and climate monitoring. There is still a strong uncertainties on wave variability in the Marginal Ice Zone (MIZ) and polar oceans. The wave scatterometer SWIM of CFOSAT, provide directional wave spectra, which are very useful to improve the wave forecast in the MIZ and the validation of using wave/ice interactions source term in the MFWAM model. The aim of this work is firstly to assess the impact of using the ice probability products provided by CFOSAT in the MFWAM wave model, and secondly to calibrate and validate the source term for wave attenuation induced by sea ice based on Yue et al (2022) implemented in the MFWAM model. Several MFWAM model simulations have been performed in a global configuration during boreal and austral winter and summer seasons. Different ice probability or fraction forcings provided by the IFS atmospheric system and CFOSAT have been tested in the MFWAM model, while sea ice thickness is provided by the Copernicus Marine Service global ocean reanalysis GLORYS. Significant Wave height (SWH) validation of MFWAM model simulations have been carried out using Sentinel-3 altimetry data, which has good coverage of polar regions. The results show a significant improvement in the bias and scatter index of SWH in Antarctica for Weddell and Ross Seas. The assimilation of SWIM wave spectra enhances the improvement of SWH in the polar oceans, particularly in the Ross Sea, Weddell Sea in Antarctica and Beaufort Sea in the Arctic ocean.

In this work we also analyzed wave attenuation by sea ice. Validation with Sentinel-3 in the Weddell Sea during the boreal summer shows a good performance of the MFWAM model with the wave/ice ineractions term compared with the simulation without interactions. The analysis of wave attenuation by sea ice was carried out in the Arctic in the Sprtizbergen archipelago area, where observations from drifting buoys (Open Met buoys) have been used to validate the MFWAM model performance. The results show good consistency between the MFWAM model and the drifting buoys. Further analysis regarding to the impact of using wave/ice interactions on ocean circulation has been conducted with ocean mixed layer model.

More discussions and conclusions will be summarized in the final presentation.

How to cite: Aouf, L., Bedossa, E., Law Chune, S., Rabault, J., Carasco, A., and Hauser, D.: On the validation of wave/ice interactions in the model MFWAM : Analysis with CFOSAT data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18430, https://doi.org/10.5194/egusphere-egu24-18430, 2024.

In this laboratory investigation, a range of wind and wave conditions was sampled to visualize the airflow and to quantify the mechanisms through which the momentum is transferred towards a wavy surface. The experiments were conducted in the SUrge STructural AIr-sea INteraction facility located at the University of Miami. Three distinct background wave conditions were subjected to wind forcing U ₁₀  of 7 and 14 m/s, to investigate the effects of wave amplitude, frequency, as well as wind forcing on the airflow regime and momentum transfer. We report that under the milder wind forcing (U ₁₀ = 7 m/s ) and the least steep wave, no sign of sheltering was observed. The airflow streamlines follow the shape of surface waves, resulting in a wide area of lower-than-average pressure above the wave crest. In this regime, more than 90% of the air-sea momentum transfer comes from the viscous drag at the surface due to the smooth airflow tightly following a smooth wave surface. Meanwhile, such pressure distribution above the waves is mostly the result of the Bernoulli Effect due to the wave shape, with almost-symmetrical low pressure above the wave crest and high pressure above the wave trough. However, a sole increase in wave frequency, while maintaining the amplitude and the wind forcing, is enough to induce airflow sheltering on the leeward side of the wave due to the steepened wave crest. Further, an increase of wind forcing over the same steepened crest did not alter the airflow regime or the pattern of the airflow pressure distribution, while doubling the momentum transfer magnitude. Here, the sheltered airflow regime is further evidenced by the enhanced turbulent kinetic energy observed on the leeward side of the wave. In these conditions, the viscous drag weakens, and the form drag rises to become the dominant mechanism for the air-sea momentum flux, accounting for over 50% of the total stress. In this regime, the pressure distribution is the result of aerodynamic sheltering, with high pressure on the windward side and low pressure on the leeward side. The results of this work will serve as the first step within a larger effort to develop a new formulation for the wave model wind input function, which will account for the airflow separation physics.

How to cite: Tan, P., Savelyev, I., Haus, B., and Matt, S.: The impact of the Airflow Separation on the Wind-Wave Momentum Flux, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2266, https://doi.org/10.5194/egusphere-egu24-2266, 2024.

Sea spray spume, droplets generated through the interactions of wind and waves, can critically alter the heat, momentum, mass and gas exchanges between the ocean and atmosphere during extreme wind conditions. Most operational forecasting models, however, do not consider sea spray spume physics in their models as the uncertainty in sea spray parameterizations is simply too large (roughly three orders of magnitude). While this uncertainty is in part caused by the extreme complexity in the droplet generation physics, considerable uncertainty comes from the difficulty in measuring sea spray and, consequently, the near absence of field observations.

Here, we present a new method to measure sea spray spume droplets in extreme winds based on acoustics. Specifically, hydrophones are positioned in the air flow laden with droplets to record droplet impact acoustics. The hydrophones were initially exposed to monodisperse free-falling droplets in the absence of wind in a first set of experiments. We find that both the magnitude and duration of the acoustic response to droplet impact are a function of the droplet diameter and the impact velocity. A second set of experiments were performed in a high-speed wind tunnel to validate the hydrophone’s response in extreme winds and under continuous exposure of droplets. Droplets ranging from less than 100 µm to 3 mm were released using a spray nozzle in winds up to 30 m/s, and their speed and diameter were independently measured using multiple exposure photography. Droplet impact measurements in the wind tunnel were found to be consistent with the initial experiments of free-falling droplets in stagnant air and provide estimates of the accuracy of the developed method. The range of droplet sizes that can be measured was found to depend on the size and sensitivity of the hydrophone, and wind speed. The results show that this new method provides significant opportunities in measuring sea spray spume droplets in situ at close proximity to the ocean surface. Field experiments to do so are currently in planning.

How to cite: Voermans, J.: An acoustic method to measure sea spray spume droplets in-situ, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14457, https://doi.org/10.5194/egusphere-egu24-14457, 2024.

How to cite: Buckley, M. and Gade, M.: Laboratory measurements of turbulence below wind-generated waves, with and without surfactants, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19211, https://doi.org/10.5194/egusphere-egu24-19211, 2024.

The accuracy of global circulation models partly relies on understanding the dynamics at the coupled atmosphere-ocean interface. Related current research efforts focus on a range of aspects, including the quantification of energy budgets, an understanding of wave growth mechanisms as well as other processes relating to the interaction between waves and ambient turbulence of the upper ocean. The latter is particularly important for the understanding of gas transports close to the surface in the presence of old and long waves (swell). Reported efforts are based on a previously introduced hybrid scale-resolving, numerical method that fully resolves both the air and water phase at realistic Reynolds numbers [Phys. of Fluids, Vol. 35 (7): 072108]. In this context, we enhanced the model by a passive scalar transport procedure to observe the transport of an arbitrary number of passive scalars close to the surface within the upper ocean layer. The presentation will address the treatment of numerical issues, primarily related unintended numerical diffusion through the interface, and explain the layout of a method to obtain accurate results for different wave-turbulence scenarios. Data processing follows experimental approaches [Journal of Fluid Mechanics, Vol. 962: R1], thereby supporting future joint numerical/experimental studies. Results display wave effects on both the turbulence and the scalar transport close to the surface. In particular, enstrophy and turbulent kinetic energy (TKE) are affected in the vicinity of surface waves. The developed two-phase flow model proves to be a promising approach for future collaborative experimental/numerical studies of transport processes in this highly dynamic domain.

How to cite: Loft, M., Ellingsen, S. Å., Hearst, R. J., Rømcke, O., Gautam, P., and Rung, T.: Simulation of scale-resolved mixing of passive scalars in waves and turbulence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18606, https://doi.org/10.5194/egusphere-egu24-18606, 2024.

In tropical and sub-tropical regions, tropical cyclones represent one of the most important sources of extreme meteorological events. The extreme ocean waves generated by such events have important engineering, oceanographic and societal impacts. This presentation outlines the results of the application of a computationally efficient parametric tropical cyclone ocean wave prediction model to each of the world’s tropical cyclone basins. The parametric model is based on present understanding of wind-wave physics in such systems and is formulated through more than 300 simulations of the Wavewatch III model covering the parameter range of typical tropical cyclones. The model is applied to synthetic tropical cyclone tracks for both historical and future projected periods. In each case, the equivalent of 1000 years of synthetic tropical cyclones is simulated for each basin. Extreme value analysis of the resulting data is used to estimate the 100-year return period significant wave height distribution across each basin. The results are explained in terms of the key tropical cyclone parameters. In addition to providing a comprehensive analysis of present day tropical cyclone wave extremes, the analysis describes how such extremes are projected to change under future climate change scenarios.

How to cite: Young, I. and Grossmann-Matheson, G.: Global Tropical Cyclone Extreme Wave Climatology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1403, https://doi.org/10.5194/egusphere-egu24-1403, 2024.

Human interaction with ocean surface waves occurs mainly in the nearshore region. Waves propagating towards the coast over gradually sloping bathymetry undergo fundamental transformations, resulting in statistics and spectral energy distributions that are substantially different to those of the incoming wave field in deep water. This is of particular importance to the generation of individual extreme waves.

This presentation will address our recent observational studies of spectral wave properties and surface elevation statistics at various nearshore locations ranging from normalized water depth of kH > 5 (deep water) to kH < 0.1 (beach-water interface). Data were obtained by surface following wave buoys, bottom-mounted pressure sensors, and an acoustic current profiler. The data reveal the dependence of skewness, kurtosis, and wave groupiness on normalized water depth, and on the position within the surf zone relative to the onset of depth-induced breaking. In the surf zone, skewness and groupiness are modulated coherently, whereas modulations of the kurtosis seem to be more random. In addition, the spectral change of the wave energy across the surf zone including the emerging infragravity wave signal will be discussed.

How to cite: Gemmrich, J.: Wave statistics and spectral shape in the nearshore region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6797, https://doi.org/10.5194/egusphere-egu24-6797, 2024.

Copernicus Sentinel-1 Synthetic Aperture Radar (SAR) mission systematically acquires data in Interferometric Wide swath mode over European land and water. This study investigates the potential of this data processed into Level-1 (L1) Single Look Complex (SLC) to compute meaningful image cross-spectra over the ocean to retrieve ocean surface waves parameters. First, each L1 SLC is processed relative to a "tile'', a prescribed spatial division of the burst (a unitary acquisition sequence of Sentinel-1 TOPS mode) and SAR image cross-spectra are defined within the burst and between two adjacent bursts. Then, using a neural network approach based on SAR image cross-spectra and on the WaveWatch III® wave model, an algorithm is proposed to provide estimates of significant wave height, fractions of wave height due to wind sea, and mean wave period. As obtained, the algorithm yields to promising performances when validated against in situ and satellite data.

How to cite: Maillard, L., Grouazel, A., Nouguier, F., Accensi, M., Marquart, R., Delouis, J.-M., and Mouche, A.: Potential of synthetic aperture radar wide swath acquisitions to map sea state variability of European seas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10064, https://doi.org/10.5194/egusphere-egu24-10064, 2024.

The nonlinear Fourier transform (NFT) has been recently used to analyze the measured rogue waves in shallow water (Teutsch et al., Nat. Hazards Earth Syst. Sci., 2023). When analyzing field measurement data from the buoy, measurement errors are unavoidable due to various factors. The drag forces on the buoy can e.g. result in low-frequency measurement noise (Ashton and Johanning, Ocean Eng., 2015), while the sensors employed on the buoy are limited in bandwidth and contribute noise themselves (Yurovsky and Dulov, Ocean Eng., 2020). It is still uncertain how the noise generated during the buoy measurement influences the nonlinear Fourier spectrum computed by the NFT. In this study, we discuss the impact of measurement errors on the nonlinear Fourier spectrum of water waves. We first generate random-phase time series from typical wave spectra such as the Pierson-Moskowitz (PM) spectrum for various significant wave heights. We will then artificially incorporate measurement errors into the generated time series. The impact of the errors is studied by comparing the nonlinear Fourier transforms of the time series with and without measurement errors.

How to cite: Lee, Y.-C. and Wahls, S.: Impact of Errors in Buoy Measurements on Nonlinear Fourier Spectra, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20078, https://doi.org/10.5194/egusphere-egu24-20078, 2024.

Validation of the global model long-term wind wave hindcasts against wave observations from Voluntary Observing Ships (VOS) are of a high demand, as VOS observations provide separate estimates of wind sea and swell for a long period of about several decades. However, VOS data suffer from spatial and temporal inhomogeneity of sampling and direct comparisons with model output are difficult as there is no way of quantifying differences associated with model setting and with sampling uncertainties. In this study, we perform a validation of the 40-yr long (1980-2019) wave model hindcast performed with WAVEWATCH-III spectral wave model forced with ERA5 reanalysis against provisionally corrected global VOS wind wave archive, accounting for sampling density in VOS data. For this purpose, model output was subsampled according to the VOS observations using the space-time collocation technique. This allows for estimating the total sampling uncertainty of significant wave heights (SWH), wind sea and swell. Further, this allows for comparison of model data with VOS observations which is not influenced by sampling biases.

Sampling uncertainties in SWH are largely driven by sampling uncertainties in the wind sea, being quite close to each other. Total sampling error variability in SWH and wind sea provide considerable reduction of uncertainties along the major ship routes where sampling errors drop by several times compared to the areas outside of the dense ship traffic. The smallest sampling uncertainties are identified in the North Atlantic subtropics where relatively weak short-term variability of wind waves is collocated with the moderately high sampling associated by dense ship traffic between Europe and the North America. Estimates of sampling errors are separately developed for sea and swell providing hints on the accuracy of wea and swell portioning in spectral wave model. We also estimated the impact of sampling onto extreme wind waves and this impact regionally is not necessarily correlated with the effect of sampling on means. Finally analysis is performed for patterns of interannual variability, including long-term trends.

This study is funded by RSF project # 23-47-00030.

How to cite: Sharmar, V., Grigorieva, V., and Gulev, S.: Comparative assessment of VOS and model wind waves over the global oceans accounting for sampling effects, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2553, https://doi.org/10.5194/egusphere-egu24-2553, 2024.