Spectral Wave Model Research Articles

COMPUTATIONALLY EFFICIENT TROPICAL CYCLONE PARAMETRIC WIND-WAVE MODEL

Waves generated by tropical cyclones (TCs) can cause damage to offshore structures and coastal settlements and play an important role in the design criteria for engineering projects, risk assessments and insurance purposes. Developing a rapid and simple way to estimate the wave heights associated with TCs has both a social and an economical advantage. The present study aims to develop a parametric TC wave prediction model, based on data generated by numerous simulations with a validated version of the WAVEWATCH III (WW3) spectral wave model (WW3DG, 2019).

Evaluation of Spectral Wave Models Physics as Applied to a 100‐Year Southern Hemisphere Extra‐Tropical Cyclone Sea State

AbstractExtreme significant wave height estimates, and their probability of exceedance, are fundamental offshore and coastal engineering design parameters. These estimates are characterized by uncertainty due to an incomplete understanding of the atmosphere‐ocean energy and momentum exchanges during intense storms. This particularly affects extreme wave statistics of ocean regions exposed to large and frequent synoptic disturbances such as Extra‐Tropical Cyclones (ETCs). In this work, we assessed the performance of global phase‐averaged spectral wave models in representing the 1 in 100‐year sea state generated by a Southern Ocean ETC in April 2021. We collected in situ and remote sensing observations, from the storm generation region to its decaying phase and the impact on South‐East Australian coastlines. We compared the observations with a suite of reanalysis and hindcast global wave model data sets. While comparing well for wind speed up to 20 m/s, the models presented differences in solving the air‐sea momentum exchange between the atmosphere and the ocean for wind speed velocities between 20 and 35 m/s, which are a distinctive characteristic of ETCs. Despite marked differences in the storm generation region, the models converged to a similar representation of the swell systems impacting the South‐East Australian coastlines, as demonstrated by a comparison with deep‐water buoy observations close to the coastlines. Furthermore, we found that the energy of the ERA5 reanalysis, which assimilates satellite wave height measurements is quickly dispersed and, as such, of little advantage in representing the 9.9 m significant wave heights that impacted the South‐East Australian coastlines on 10 April 2021.

EFFICIENTLY FORECASTING 2-DIMENSIONAL SPECTRA INSIDE SHELTERED PORTS USING SPECTRAL AND PHASE-RESOLVING WAVE MODELS

Ports today are grappling with the impacts of increasing ship sizes and increasingly frequent and more extreme weather events. Under these conditions, being able to predict navigation, handling, and mooring risks accurately, is becoming more crucial to safe and efficient port operations. This is especially important in ports subject to complex wave transformations. Traditionally, in order to capture the various wave transformation processes accurately from offshore to nearshore areas, the coupling of a Spectral wave (SW) and Boussinesq wave (BW) model has been required. Unfortunately, the very high computational costs of BW modelling have inhibited running such models in forecast mode. In response to this highly technical challenge, DHI have developed a transformative method for concatenating outputs from SW and BW models, enabling modelers to forecast 2D Spectra accurately, both inside and outside a sheltered port basin in a computationally efficient manner.

STORM TIDE IN A DATA RICH COASTAL EMBAYMENT FINDING THE MISSING SURGE

Numerical modelling of storm tide within Moreton Bay has consistently underestimated peak tide gauge level observations by up to 40 percent during extreme events. This study tests "missing" contribution hypotheses by undertaking a series of integrated hydrodynamic and spectral wave modelling experiments, supported by high quality gridded wind, observed water level and wave data at sites throughout Moreton Bay and the Southeast Queensland coast. The case studies are the regional surge responses during the passages of Ex. Tropical Cyclone Oswald, Ex. Tropical Cyclone Debbie and Tropical Cyclone Oma. Testing of these hypotheses will contribute to an improved understanding of the key processes required to model storm surge accurately in other similar coastal embayments.

THE PHYSICAL PROCESSES ACTIVE IN TROPOCAL CYCLONE WAVE GENERATION

In tropical and subtropical regions tropical cyclones represent the major extreme meteorological events, generating winds in excess of 40m/s and significant wave heights above 10m. Such systems are characterized by a well-formed translating vortex wind field with a calm eye and winds that spiral in toward the center of the storm. Despite the apparent complexity of the forcing wind field, observations of waves in tropical cyclones show that there are many similarities to waves generated in relatively simple cases with approximately constant unidirectional winds (Tamizi and Young 2020. It has often been speculated (Tamizi and Young 2020) that this is because nonlinear wave–wave interactions play a dominate role in defining the tropical cyclone directional wave spectrum and hence the significant wave height, as they do for more simple wave generation cases. Despite this speculation and the many observational studies of tropical cyclone wave spectra, no study has yet been able to demonstrate the role of nonlinear wave–wave interactions in tropical cyclones. Firstly, the observational database from buoys and remote sensing systems are summarized. A third generation spectral wave model is then used to model typical tropical cyclones and the role played by each of wind input, dissipation and nonlinear interactions is investigated in detail.

WAVE TRANSFORMATION ON CORAL REEFS: COMPARISON OF A CFD MODEL AND PHYSICAL MODEL TESTS OF A MALDIVIAN REVETMENT

Designing optimized coastal structures on islands bordered by coral reefs has proven to be a challenging task due to the complex reef and wave interaction (Jensen, 1991; Jensen, Sloth and Jacobsen, 1998). Often the reefs in these tropical regions have a steep seaward profile until a certain depth, whereafter the reef suddenly becomes flat with a mild slope. When offshore waves reach the reef, it can result in plunging wave breaking, a wave set-up, and energy transfer from the peak frequency to lower frequencies resulting in long period wave motions. Under these conditions, it is difficult to predict the rate of overtopping as the complex hydrodynamics are not covered in standard overtopping formulas. Furthermore, the standard spectral wave models, on which the overtopping formulas often are based, cannot describe the transformation of offshore waves over the reef as the models are not able to resolve the physics of this violent water motion. In connection with the hydraulic study for a detailed design of a revetment on Fuvahmulah in the Maldives, the OpenFOAM CFD model is applied to obtain the hydrodynamic design parameters. The wave dynamics on the reef, the revetment overtopping rates, and hydrodynamic forces derived from the numerical model are compared with results from a physical model conducted by the Danish Hydraulic Institute (DHI).

Numerical modeling of return period waves based on non-linear Boussinesq wave models to support tidal flood studies in the Kedungsepur area

A number of rapid developments and negative effects of climate change in coastal areas have forced its equilibrium system. As a result, tidal floods, high waves, and storms occur more frequently. The goal of this research is to examine nearshore hydrodynamic conditions by using results from the hindcasting procedure. A non-linear Boussinesq (BW) and Spectral Wave (SW) models are used in this study. The models use the manning coefficient, bathymetry data (BATNAS), significant wave height, and wave period calculated by using the hindcasting method. This research commenced by processing wind data from the European Center for Medium-Range Weather Forecasts (ECMWF) from the years 2009 to 2021 with an interval of 3 hours. Then, wave height for return periods 1-, 50- and 100-years are calculated using Gumbel and Fisher-Tippett type I distribution, respectively. The aforementioned approach produced wave heights of 4.23; 6.16; and 6.49 m for return periods 1, 50, and 100 years, respectively. By using this series of wave data, the models predicted that 1.2 m waves propagate close to the nearshore regions.

Comparative analysis of the long-term extreme VWBM calculated according to IACS Rec. No 34 rev 1 and rev 2

After facing some criticism regarding assumptions contained in IACS Rec. No. 34 Rev 1 (2001), efforts have been put in by IACS members resulting in a new revision of this document, which provides a procedure for computation of the long-term extreme wave loads on ocean-going ships. The main changes include a new scatter diagram, modification of the wave spectral model and energy spreading function, as well as probability level of design load. Preliminary, the reduction of extreme loads from 10% to 30%, depending on the vessel and response type, is estimated. As this reduction could have serious implications for ship structural design, the present research aims to further investigate consequences on the most important load component, i.e., vertical wave bending moment (VWBM) at midship. Transfer functions of VWBM are calculated using closed-form expressions, which are convenient for comparative studies where many computations are required. Ships of different types and sizes are subjected to the long-term VWBM analysis comparing extreme values based on the procedures proposed in Rev 1 and Rev 2. Although this study is not intended to provide extreme VWBM in absolute terms, the comparison with IACS UR 11 is nevertheless presented as a reference. It is found that Rev 2 long-term VWBMs never exceed corresponding IACS Rule values, which can’t be stated for Rev 1. Results also indicate that differences in scatter diagrams between Rev 1 and Rev 2 could explain a large decrease in the extreme VWBM.

Modelling swell propagation across the Pacific

Ocean wave swell generated in the vicinity of Campbell Island in the Southern Ocean is tracked along Great Circle paths across the Pacific Ocean. Data from a wave buoy at Campbell Island provides data on the directional spectrum in the generation region. The swell is measured at locations along a series of 19 Great Circle paths across the Pacific using Sentinel-1 SAR and CFOSAT satellite data. The WAVEWATCH III spectral wave model is used as a diagnostic tool to investigate the physical processes active in the swell propagation and decay. The results indicate that present day spectral wave models over-estimate the decay rate of swell. Although these models contain source terms to represent swell decay and negative wind input, these terms still largely remain tuning parameters. The data indicates that further research is required to adequately represent the observed magnitudes of the swell decay. In addition, the data show that currents have only a small impact on the observed swell decay and that islands can have a major impact. Such island impacts are poorly represented by spectral wave models.

Regional wave climate projections forced by EURO‐CORDEX winds for the Black Sea and Sea of Azov towards the end of the 21st century

AbstractWave phenomena impact high commercial value coastal and offshore activities, infrastructures and transportation. The knowledge of future wave conditions allows for consistent long‐term planning and decision‐making. The present study aims to provide robust, reliable projections of the potential future wave conditions of the Black Sea under the influence of climate change. For this purpose, an eight‐member dynamic wave climate ensemble that accurately represents the Black Sea's present wave climate has been produced, and future projections are assessed and analysed here. The wave climate ensemble was obtained by forcing the Simulating Waves Nearshore spectral wave model with eight regional wind fields from the EURO‐CORDEX. The future simulations of the regional wind fields are based on the RCP8.5 high‐emission scenario. The historical wave climate of the ensemble was evaluated against ERA5 reanalysis data. Projected changes in the mean wind and wave characteristics are examined by comparing historical and future simulations. In addition, the projected trends in the annual means during the future period of the wave simulations for significant wave height (Hs) and wave energy flux are also analysed. The projections for Hs and wind speeds are pronounced with significant decreases down to −10% across the basin, especially in the eastern region of the Black Sea. Changes in the Hs 99% percentiles of up to 16% are projected to occur. The projected changes in the annual mean of the wave energy flux are close to the projected changes in Hs means, while the seasonal changes (between −15% and 12%) are expected to be higher.

Assessment of input and dissipation source terms in the spectral wave model during tropical cyclones of varying intensity in Bay of Bengal

The vulnerability of the tropical cyclonic activities in the Bay of Bengal (BoB) domain are analyzed in terms of its destructive potential. The wave characteristics due to cyclones of varying intensities: Vardah, a Very Severe Cyclonic Storm (VSCS); Phethai, a Severe Cyclonic Storm (SCS); and Amphan, the Super Cyclonic Storm (SuCS) in BoB during 2016, 2018, and 2020 causing loss to human life and wealth, are investigated. The input-dissipation source term packages in the spectral wave models under tropical cyclones are studied through (i) integrated parameter, significant wave height, and (ii) individual source term analysis. In addition to the default linear drag coefficient in the wind input source term of van der Westhuysen et al. (2007), two nonlinear formulations of the drag coefficient are included to understand its influence on the source terms. The integrated parameters such as significant wave height are compared with buoy measurements, and the performance of the packages is ascertained through statistical error estimates. Energy contribution from the different source terms under various packages to generate the extreme significant wave height over the domain during the cyclonic period is established. The influence of the drag coefficient on the input source term is found to be pronounced.

Infragravity waves at the Hambantota port located in the North Indian Ocean

Infragravity (IG) waves are the primary cause of harbor oscillations. The Hambantota port located in the North Indian Ocean is taken as an example to study the IG waves in the vicinity and interior of a complex-shaped real harbor and to reveal the energy transfer of IG waves toward the harbor. The research employs a combination of in-situ wave measurement and numerical simulations conducted using the spectral wave model WAVEWATCH III. The latter could provide a long-term estimation of the level of IG wave energy over a broad geographical region, though it cannot reflect the complex mechanism of IG waves near/at the shore. Results indicate that, at the sensor near the harbor entrance, the free IG wave heights HFIG generally exceeds the bound IG wave height HBIG. However, during energetic sea states (significant wave height HS > 2.5 m in this study), HBIG increases significantly and occasionally exceeds HFIG. The spatial distributions of HS, HBIG, and HFIG indicate that the narrow continental shelf surrounding Sri Lanka has a marginal effect on attenuating swell energy. Consequently, the Hambantota port experiences a substantial impact from both swell and accompanying IG waves. If solely considering the maximum acceptable HS, the downtime for large ships would be greatly underestimated. The process of wave propagation into the harbor enables the energy transfer from bound IG waves to free IG waves that finally excite harbor oscillations. Albeit the tidal range is low, the tidal level has a noteworthy impact on harbor oscillations. The IG spectral density during low tides is generally greater than that during high tides due to the enhanced nonlinear interactions of short waves under low-tide conditions.

Coastal Flood at Gâvres (Brittany, France): A Simulated Dataset to Support Risk Management and Metamodels Development

Given recent scientific advances, coastal flooding events can be modelled even in complex environments. However, such models are computationally expensive, preventing their use for forecasting. At the same time, metamodelling techniques have been explored for coastal hydrodynamics, showing promising results. Developing such techniques for predicting coastal flood information (e.g., inland water depths) requires large enough learning datasets providing such inland information. However, detailed inland coastal flood observations are scarce and—when available—only correspond to a limited number of events. This paper aims at demonstrating how we can fill this gap by introducing a publicly available dataset, presenting its setup, and providing examples of use and recommendations. It has been built for the site of Gâvres (France), relying on the joint use of spectral wave (WW3) and non-hydrostatic wave-flow (SWASH) models, accounting for wave overtopping. It compiles 250 scenarios (defined by time-varying forcing conditions; including real and stochastically generated events) and the resulting maximal flooded areas and water depths (on 64,618 inland points). Its construction required the equivalent of 2 years of simulations on 48 cores. The examples of use of the dataset focus on method developments (metamodelling, forecast), local knowledge, and risk management.

Assessment of Spectral Wave Model Performance Using Three Wind Speeds in the Eastern Mediterranean Sea

In the study, the performance of the MIKE 21 SW (Spectral Wave) model using the three different wind fields, namely ERA-Interim, ERA5, and CFSR was evaluated in the Eastern Mediterranean Sea. Model results were calibrated with four buoy measurements by tuning physical model parameters. Wave simulations showed a strong sensitivity to the whitecapping parameter (Cds). Calibrated MIKE 21 SW model run to validate at two buoy measurements. Considering the statistical error measures: (i) ERA-I predicted lower significant wave heights and wave periods than ERA5 and CFSR, (ii) statistical error measures (bias, RMSE, and SI) obtained with CFSR were calculated slightly higher than those obtained using ERA5 (iii) ERA5 performed slightly better in the hindcast of Eastern Mediterranean wave properties than ERA-I. As a result of all evaluations, the highest correlation coefficient (R), relatively low statistical error measures, and slightly better accuracy were provided by the model settings forced with ERA5 wind fields.

Ocean Surface Gravity Wave Evolution during Three Along-Shelf Propagating Tropical Cyclones: Model’s Performance of Wind-Sea and Swell

Despite recent advancements in ocean–wave observations, how a tropical cyclone’s (TC’s) track, intensity, and translation speed affect the directional wave spectra evolution is poorly understood. Given the scarcity of available wave spectral observations during TCs, there are few studies about the performance of spectral wave models, such as Simulating Waves Nearshore (SWAN), under various TC scenarios. We combined the National Data Buoy Center observations and numerical model hindcasts to determine the linkages between wave spectrum evolution and TC characteristics during hurricanes Matthew 2016, Dorian 2019, and Isaias 2020. Five phases were identified in the wave spectrogram based on the normalized distance to the TC, the sea–swell separation frequency, and the peak wave frequency, indicating how the wave evolution relates to TC characteristics. The wave spectral structure and SWAN model’s performance for wave energy distribution within different phases were identified. The TC intensity and its normalized distance to a buoy were the dominant factors in the energy levels and peak wave frequencies. The TC heading direction and translation speed were more likely to impact the durations of the phases. TC translation speeds also influenced the model’s performance on swell energy. The knowledge gained in this work paves the way for improving model’s performance during severe weather events.

Wave climate and energy resources in the Mariana Islands from a 42-year high-resolution hindcast

A high-resolution wave hindcast from 1979 to 2020 was developed for Guam and the Commonwealth of the Northern Mariana Islands, which are influenced by mesoscale and synoptic weather systems from the western Pacific Rim to the Central Pacific. The spectral wave modeling utilized structured global and unstructured regional grids with improved resolution in nearshore waters, where wave resources development is most feasible. The hindcast demonstrates its capability in capturing multi-modal seas through reproduction of bulk and partitioned wave parameters derived from altimetry and buoy measurements. The local wave climate is influenced by the year-round trade winds and tropical cyclones as well as seasonal monsoons, frontal systems, and mid-latitude high- and low-pressure systems. The strong intra-annual variation of the significant wave height and power flux reflects transitions between local and distant weather systems, which account for the diverse wave conditions comprised of short-period wind waves, intermediate-period seas, and long-period swells around the islands. Among the Pacific island regions, spectral partitioning is particularly important for describing the dominant wave components by period and direction with pertinent information conducive to energy resources characterization and coastal zone management in the Mariana Islands.

Spatial characteristics of frazil streaks in the Terra Nova Bay Polynya from high-resolution visible satellite imagery

Abstract. Coastal polynyas around the Antarctic continent are regions of very strong ocean–atmosphere heat and moisture exchange that are important for local and regional weather, sea ice production, and water mass formation. Due to extreme atmospheric conditions (very strong offshore winds, low air temperature, as well as humidity) the surface ocean layer in polynyas is highly turbulent, with mixing due to combined Langmuir, wind-induced, and buoyancy-driven turbulence. One of the visible signs of complex interactions between the mixed-layer dynamics and the forming sea ice are frazil streaks, elongated patches of high ice concentration separated by areas of open water. In spite of their ubiquity, observational and modelling analyses of frazil streaks have been very limited largely due to the fact that their significance for heat flux and ice production is only just becoming apparent. In this study, the first comprehensive analysis of the spatial variability of surface frazil concentration is performed for the Terra Nova Bay Polynya (TNBP). Frazil streaks are identified in high-resolution (pixel size 10–15 m) visible satellite imagery, and their properties (surface area, width, spacing, and orientation) are linked to the meteorological forcing (wind speed and air temperature). This provides a simple statistical tool for estimating the extent and ice coverage of the region of high ice production under given meteorological conditions. It is also shown that the orientation of narrow streaks tends to agree with the wind direction, suggesting the dominating role of the local wind forcing in their formation. Very wide streaks, in turn, deviate from that pattern, as they are presumably influenced by several additional factors, including local water circulation and the associated convergence zones. An analysis of peak wavelengths and directions determined from the images, compared to analogous open-water wavelengths computed with a spectral wave model, demonstrates a significant slow-down in the observed wave growth in TNBP. This suggests an important role of frazil streaks in modifying wind-wave growth and/or dissipation in polynyas.

Hybrid modelling to improve operational wave forecasts by combining process-based and machine learning models

Operational wave forecasting plays an important role in ensuring safe navigation and in the prediction of tidal windows for harbour approach channels. The underlying nearshore process-based wave models need to be accurate for a wide range of different conditions, from more common mild wave conditions to the occasional high energy (storm) conditions. In this work, an innovative hybrid modelling approach is proposed to improve the accuracy of operational wave forecasts. An operational wave model is combined with a machine learning model which is trained using wave measurements within the wave model domain. This hybrid modelling approach is applied to the Dutch North Sea, covering four major harbour approach channels.The final hybrid operational wave model results in a significant average error decrease compared to just the process-based model, amounting to 21.7% for the wave energy density and 25.3% for the wave direction. The error reduction for the spectral wave parameters is even larger, with a 33.3% smaller error in spectral wave height and a 38.8% smaller error in spectral wave period. As this approach is generically applicable to spectral wave models, it contains the potential for significant improvements in operational modelling.

The Met Office operational wave forecasting system: the evolution of the regional and global models

Abstract. The Met Office operational wave forecasting modelling system runs four times a day to provide global and regional forecasts up to 7 d ahead. The underpinning model uses a recent development branch of the third-generation spectral wave model WAVEWATCH III® (version 7.12) that includes several updates developed at the Met Office. These include the spherical multiple-cell (SMC) grid, a rotated pole grid formulation for mid-latitudes, enhancements to OASIS coupling and updates to the netCDF postprocessing. Here we document the technical details behind the system with a view to further developments. The operational system includes a global forecast deterministic model (GS512L4EUK) and two regional models nested one-way covering the Northwest (NW) European shelf and UK waters (AMM15SL2, where AMM is for Atlantic Margin model) in addition to an Atlantic wave ensemble (AS512L4EUK). GS512L4EUK and AS512L4EUK are based on a multi-resolution four-tier SMC 25-12-6-3 km grid. The regional AMM15SL2 configuration uses a two-tier SMC 3−1.5 km grid and is run operationally both as a standalone forced model (includes wave–current interactions) and as the wave component of a two-way ocean–wave coupled operational system. Model evaluation is focused on the global and regional baseline configurations. Results show evidence of resolution-dependent differences in wave growth, leading to slightly overestimated significant wave heights in coastal mid-range conditions by AMM15SL2 but an improved representation of extremes compared to GS512L4EUK. Additionally, although a positive impact of the surface currents is not always shown in the overall statistics of the significant wave height due to a larger spread in the observation–model differences, wave–current effects help to better capture the distribution of the energy in terms of frequency and direction near the coast (>20 % improvement), which has implications to beach safety, coastal overtopping risk and shoreline evolution. Future system developments such as the use of sea point wind forcing, the optimisation of the models in line with model resolution and the utilisation of SMC multi-grids are discussed.

Erosion-deposition Prone Assessment Along the Kelantan and Terengganu Coasts Due to Sea Level Rise

The erosion-prone zone is characterised by nearshore sand formed by the combined action of tides, wind, and recurring waves crashing on the beach. By running perpendicular to the coastline and bathymetry, 51 cross-shore sections were chosen from a total of 54 to study longshore transport along the beaches of Kelantan and Terengganu. The hydrodynamic model was used to determine water level, current speed, and spectral density, while the Spectral Wave Model and LitDrift were used to construct boundary wave variables and Net Transport across each sector. The model output was compared to previously published erosion-prone zones in the NCES Report (2015), and the results were agreed. The net transfer varies based on the angle of the coastline, the direction of the waves, and the beach profile. The net transport ranges from -693,000 m3/year to 444,000 m3/year depending on the beach profile, wave direction, and angle of the coastline. Net transit for each section was also calculated for 2030, 2050, and 2100, taking into consideration sea level rise. The most recent IPCC assessment (AR6) was applied to generate SLR forecasts for year 2030, 2050, and 2100. According to the statistics, all sections are expected to increase in year 2030, whereas only 53% and 67% are expected to develop in year 2050 and 2100, respectively. From 2030 to 2050 and 2100 to 2020, total net transport along the Kelantan and Terengganu beaches grows by 9.5%, 10%, and 4.5%, respectively.Net transportation is expected to grow until 2050, then steadily decline until 2100. However, by using a better anticipated wave model, the results of this inquiry can be improved.