Wave Model Simulating WAves Nearshore Research Articles

Atmospheric circulation and storm events in the Baltic Sea

Abstract The storm events in the Baltic Sea are examined in connection with the main weather patterns grouped into the circulation types (CTs), and their changes in present climate. A calendar of storms was derived from results of wave model SWAN (Simulating WAves Nearshore) experiments for 1948-2011. Based on this calendar, a catalogue of atmospheric sea level pressure (SLP) fields was prepared for CTs from the NCEP/NCAR dataset. SLP fields were then analyzed using a pattern recognition algorithm which employed empirical orthogonal decomposition and cluster analysis. For every CT we conducted an analysis of their seasonal and interannual changes, along with their role in storm event formation. An increase of the storm CTs’ frequency in the second part of the 20th century was shown to be in a close agreement with teleconnection circulation patterns such as the Arctic Oscillation, North Atlantic Oscillation and the Scandinavian blocking.

Analysis of extreme wave events on the southern coast of Brazil

Abstract. Using the wave model SWAN (simulating waves nearshore), high waves on the southwestern Atlantic generated by extra-tropical cyclones are simulated from 2000 to 2010, and their impact on the Rio Grande do Sul (RS) coast is studied. The modeled waves are compared with buoy data and good agreement is found. The six extreme events in the period that presented significant wave heights above 5 m, on a particular point of interest, are investigated in detail. It is found that the cyclogenetic pattern between the latitudes 31.5 and 34° S is the most favorable for developing high waves. Hovmöller diagrams for deep water show that the region between the south of Rio Grande do Sul up to a latitude of 31.5° S is the most energetic during a cyclone's passage, although the event of May 2008 indicates that the location of this region can vary, depending on the cyclone's displacement. On the other hand, the Hovmöller diagrams for shallow water show that the different shoreface morphologies were responsible for focusing or dissipating the waves' energy; the regions found are in agreement with the observations of erosion and progradation regions. It can be concluded that some of the urban areas of the beaches of Hermenegildo, Cidreira, Pinhal, Tramandaí, Imbé and Torres have been more exposed during the extreme wave events on the Rio Grande do Sul coast, and are more vulnerable to this natural hazard.

Wave-energy dissipation by bottom friction in the English Channel

The energy dissipation by bottom friction of wind-generated surface-gravity waves is evaluated in relation to the seabed roughness magnitude in the English Channel (western Europe). The investigation is based on the phase-averaged wave model SWAN (Simulating WAves Nearshore) modified to account for a new parameterisation of the wind-drag coefficient at high wind speeds. Two formulations of the bottom-drag coefficient are evaluated: (1) the default constant empirical values derived from the JONSWAP experiment and (2) the eddy-viscosity model of Madsen et al. (1988) integrating the hydrodynamic conditions and the bottom roughness length scale considered successively constant and parametrised according to the grain size of bed sediments. Model performances are evaluated by comparing predictions with available measurements of the significant wave height and the peak period at (1) three offshore lightships and (2) two nearshore wave buoys off Le Havre and Cherbourg harbours. The heterogeneous bottom roughness length scale associated with the grain size of seabed sediments improves globally numerical estimates. Mappings of coastal regions influenced by bottom friction are produced exhibiting significant energy dissipation in areas of pebbles and gravels of the Normano-Breton Gulf and the surroundings of the Isle of Wight exposed to the incoming waves from the North-Atlantic ocean.

Spatial variability of waves within a marine energy site using in-situ measurements and a high resolution spectral wave model

A high resolution spectral wave model is used to quantify the spatial wave climate on geographical scales relevant to intra-site variability for marine renewable energy installations. For the first time, results are compared to in-situ data from an array of four floating wave buoys, and demonstrate the ability of the spectral wave model SWAN (Simulating WAves Nearshore) to resolve spatial differences in the wave climate. Examination of the model source terms highlights bottom friction and refraction as the primary processes contributing to the observed differences across the site. Wave models for climate assessments for marine renewable energy are not commonly operated at sufficient spatial resolution to accurately resolve intra-site variability. This study demonstrates that high spatial resolution spectral wave models, nested into a larger model domain, have the potential to provide an accurate and detailed prediction of the spatial variability of wave conditions across a marine renewable energy site. As such, they could be implemented to provide a more accurate resource assessment for wave energy array deployments, but also for engineering assessments of other marine energy technologies.

Atmospheric circulation and storm events in the Black Sea and Caspian Sea

Abstract Extreme sea storms are dangerous and a potential source of damage. In this study, we examine storm events in the Black Sea and Caspian Sea, the atmosphere circulation patterns associated with the sea storm events, and their changes in the present (1961–2000) and future (2046–2065) climates. A calendar of storms for the present climate is derived from results of wave model SWAN (Simulating WAves Nearshore) experiments. On the basis of this calendar, a catalog of atmospheric sea level pressure (SLP) fields was prepared from the NCEP/NCAR reanalysis dataset for 1961–2000. The SLP fields were subjected to a pattern recognition algorithm which employed empirical orthogonal decomposition followed by cluster analysis. The NCEP/NCAR reanalysis data is used to evaluate the occurring circulation types (CTs) within the ECHAM5-MPI/OM Atmosphere and Ocean Global Circulation Model (AOGCM) for the period 1961–2000. Our analysis shows that the ECHAM5-MPI/OM model is capable of reproducing circulation patterns for the storm events. The occurrence of present and future ECHAM5-MPI/OM CTs is investigated. It is shown that storm CTs are expected to occur noticeably less frequently in the middle of the 21st century.

Wave power variability over the northwest European shelf seas

Regional assessments of the wave energy resource tend to focus on averaged quantities, and so provide potential developers with no sense of temporal variability beyond seasonal means. In particular, such assessments give no indication of inter-annual variability – something that is critical for determining the potential of a region for wave energy convertor (WEC) technology. Here, we apply the third-generation wave model SWAN (Simulating Waves Nearshore) at high resolution to assess the wave resource of the northwest European shelf seas, an area where many wave energy test sites exist, and where many wave energy projects are under development. The model is applied to 7years of wind forcing (2005–2011), a time period which witnessed considerable extremes in the variability of the wind (and hence wave) climate, as evidenced by the variability of the North Atlantic Oscillation (NAO). Our simulations demonstrate that there is much greater uncertainty in the NW European shelf wave resource during October–March, in contrast to the period April–September. In the more energetic regions of the NW European shelf seas, e.g. to the northwest of Scotland, the uncertainty was considerably greater. The winter NW European shelf wave power resource correlated well with the NAO. Therefore, provided trends in the NAO can be identified over the coming decades, it may be possible to estimate how the European wave resource will similarly vary over this time period. Finally, the magnitude of wave power estimated by this study is around 10% lower than a resource which is used extensively by the wave energy sector – the Atlas of UK Marine Renewable Energy Resources. Although this can partly be explained by different time periods analysed for each study, our application of a third-generation wave model at high spatial and spectral resolution significantly improves the representation of the physical processes, particularly the non-linear wave-wave interactions.

Effect of Langmuir circulation on upper ocean mixing in the South China Sea

Effect of Langmuir circulation (LC) on upper ocean mixing is investigated by a two-way wave-current coupled model. The model is coupled of the ocean circulation model ROMS (regional ocean modeling system) to the surface wave model SWAN (simulating waves nearshore) via the model-coupling toolkit. The LC already certified its importance by many one-dimensional (1D) research and mechanism analysis work. This work focuses on inducing LC's effect in a three-dimensional (3-D) model and applying it to real field modeling. In ROMS, the Mellor-Yamada turbulence closure mixing scheme is modified by including LC's effect. The SWAN imports bathymetry, free surface and current information from the ROMS while exports significant wave parameters to the ROMS for Stokes wave computing every 6s. This coupled model is applied to the South China Sea (SCS) during September 2008 cruise. The results show that LC increasing turbulence and deepening mixed layer depth (MLD) at order of 0 (10 m) in most of the areas, especially in the north part of SCS where most of our measurements operated. The coupled model further includes wave breaking which will brings more energy into water. When LC works together with wave breaking, more energy is transferred into deep layer and accelerates the MLD deepening. In the north part of the SCS, their effects are more obvious. This is consistent with big wind event in the area of the Zhujiang River Delta. The shallow water depth as another reason makes them easy to influence the ocean mixing as well.

Wave–current interaction: Effect on the wave field in a semi-enclosed basin

The effect on waves of the Wave–Current Interaction (WCI) process in the semi-enclosed Gulf of Venice (northern region of the Adriatic Sea) was investigated using the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system. COAWST relies on the ocean model ROMS (Regional Ocean Modeling System), the wave model SWAN (Simulating WAves Nearshore), and the CSTMS (Community Sediment Transport Modeling System) routines. The two-way data transfer between circulation and wave models was synchronous via MCT (Model Coupling Toolkit), with ROMS providing: current field, free surface elevation, and bathymetry to SWAN. For coupling, the 3-D current profiles were averaged using a formulation which integrated the near-surface velocity over a depth controlled by the spectral mean wavenumber. COAWST system was implemented on a parent grid (with horizontal resolution of 2.0km) covering the whole Adriatic Sea with one-way nesting to a child grid resolving the northern area (Gulf of Venice) at a resolution of 0.5km. The meteorological forcings provided by the operational meteorological model COSMO-I7 (a mesoscale model developed in the framework of the COSMO Consortium) were used to drive the modeling system in the period bracketing September 2010–August 2011. The adopted winds and the simulated waves were compared with observations at the CNR-ISMAR Acqua Alta oceanographic tower, located off the Venice littoral. Wave heights and sea surface winds were also compared with satellite-derived data. The analysis of WCI was performed on the child grid over the winter season (January–March 2011) with particular focus on the waves generated by prevailing and dominant winds blowing on the Adriatic Sea: Bora and Sirocco. Due to the variable wind direction with respect to the ocean current direction different effects on WCI were depicted, showing that within the northern Adriatic Sea the ocean–wave interactions are strongly dependent on the wind forcing direction. Further investigations reveal that, when applied to intense storms, the effect of coupling on waves results in variations of significant wave height up to 0.6m, with some areas experiencing significant increase/decrease of wave spectral energy for opposite/following currents respectively.

On the 2D modelling aspects of wind-induced waves in shallow, fetch-limited lakes

Wind-induced waves play an important role in shallow lake hydro- and sediment dynamics. But most of the field measurement methods can give information about the wave properties only at single point, which calls for wave estimation methods to take the e ect of waves into account in multidimensional hydroand sediment dynamic models. The aim of this study is to improve modelling waves in depth- and fetch-limited lakes generated by the local winds. In the first part of this paper, we describe the calibration and validation of the 2D spectral wave model SWAN (Simulating Waves Nearshore) to the very shallow Lake Neusiedl, Hungary/Austria. The abrupt change of the roughness at the perimeter of the open lake and the gradual change along the fetch due to wave growth result in a systematic, fetch-dependent variation of the wind speed. This spatial inhomogeneity is modelled here by a 1D atmospheric internal boundary layer (IBL) model. It is shown in the second part of this paper that this approach results in a significant e ect on wave parameters and, as a consequence, on bottom shear stress.

Nearshore propagation of cyclonic waves

Cyclone-generated surface waves are simulated using state-of-art SWAN (Simulating WAves Nearshore) model coupled with hydrodynamic model inputs. A severe cyclonic storm passed over the Arabian Sea during 4–9th November 1982 is selected from UNISYS track records. The cyclone lasted for nearly 6 days and subsided with a land fall at Gujarat coast, west coast of India. In this study, cyclonic wind fields are generated using a well-established relationship suggested by Jelesnianski and Taylor (1973). The associated water level variations due to storm surge and surge generated currents are simulated using POM (Princeton Ocean Model). The outputs are one-way coupled with the wave model SWAN for simulating wave parameters off Gujarat, north-east basin of Arabian Sea. An extensive literature review is carried out on the progress and methodology adopted for storm wave modelling and analysis. The results presented in this paper reveal the severity of the storm event and would be highly useful for assessing the extreme wave event/climate especially for the south coast of Gujarat.

The effect of wave-induced radiation stress on storm surge during Typhoon Saomai (2006)

The effects of wave-induced radiation stress on storm surge were simulated during Typhoon Saomai using a wave-current coupled model based on ROMS (Regional Ocean Modeling System) ocean model and SWAN (Simulating Waves Nearshore) wave model. The results show that radiation stress can cause both set-up and set-down in the storm surge. Wave-induced set-up near the coast can be explained by decreasing significant wave heights as the waves propagate shoreward in an approximately uniform direction; wave-induced set-down far from the coast can be explained by the waves propagating in an approximately uniform direction with increasing significant wave heights. The shoreward radiation stress is the essential reason for the wave-induced set-up along the coast. The occurrence of set-down can be also explained by the divergence of the radiation stress. The maximum wave-induced set-up occurs on the right side of the Typhoon path, whereas the maximum wave induced set-down occurs on the left side.

Modeling of depth‐induced wave breaking under finite depth wave growth conditions

Recent studies have shown that the spectral wind wave model SWAN (Simulating Waves Nearshore) underestimates wave heights and periods in situations of finite depth wave growth. In this study, this inaccuracy is addressed through a rescaling of the Battjes and Janssen (1978) bore‐based model for depth‐induced breaking, considering both sloping bed surf zone situations and finite depth wave growth conditions. It is found that the variation of the model error with the breaker index γBJ in this formulation differs significantly between the two types of conditions. For surf zones, clear optimal values are found for the breaker index. By contrast, under finite depth wave growth conditions, model errors asymptotically decrease with increasing values of the breaker index (weaker dissipation). Under both the surf zone and finite depth wave growth conditions, optimal calibration settings of γBJ were found to correlate with the dimensionless depth kpd (where kp is the spectral peak wave number and d is the water depth) and the local mean wave steepness. Subsequently, a new breaker index, based on the local shallow water nonlinearity, expressed in terms of the biphase of the self‐interactions of the spectral peak, is proposed. Implemented in the bore‐based breaker model of Thornton and Guza (1983), this breaker index accurately predicts the large difference in dissipation magnitudes found between surf zone conditions and finite depth growth situations. Hence, the proposed expression yields a significant improvement in model accuracy over the default Battjes and Janssen (1978) model for finite depth growth situations, while retaining good performance for sloping bed surf zones.

An evaluation of a high-resolution operational wave forecasting system in the Adriatic Sea

The SWAN (Simulating Waves Nearshore) wave model using wind inputs generated by the ALADIN 8-km, operational high-resolution, atmospheric model was run in real-time to provide surface waves forecast for the semi-enclosed Adriatic Sea in support of the “Dynamics of the Adriatic in Real-Time” (DART) field experiments. Together with predictions from other wave and wind models, the successful prediction of a high sea-state event by this model led to a real-time shifting of planned operations while at sea, allowing five ADCP moorings to be deployed just before a bora storm and associated storm waves arrived. The model was also able to simulate the spatial gradients in significant wave height observed by in-situ and remote-sensing measurements for a particular sirocco storm case study, providing an additional perspective in aiding interpretation of the model output of features. To further quantify prediction skill, the wave forecast performance over a 12-month period was evaluated against in-situ and altimeter measurements over the region. Correlation coefficients between forecast and in-situ measured significant wave heights were from 0.82 to 0.91 for the 24-h forecast and from 0.78 to 0.88 for the 48-h forecast. However, best-fit slope comparisons with in-situ wave data at five coastal locations show the forecast wave heights were underpredicted by 10% to 30%. Best-fit slope comparisons between modeled wind speeds, U 10, and significant wave heights, H s, and altimeter-derived measurements show that model U 10 was about 4% underpredicted, but H s was underpredicted by an average of 30%. The underprediction of SWAN H s has a very significant location-dependent geographical variation ranging from 10% to over 50%. In addition, the wave model comparison with altimeter H s shows a broad region of scatter index exceeding 0.4 along and offshore of the central Croatian coast. Elsewhere the scatter index is generally around 0.3. Compared to previous studies we found that using higher-resolution wind forcing with realistic orography decreased the U 10 underestimation bias, but the magnitude of H s underestimation bias did not correspondingly decrease, suggesting that wave model dynamics or wind–wave coupling deserves further investigation.

An unstructured-grid finite-volume surface wave model (FVCOM-SWAVE): Implementation, validations and applications

The structured-grid surface wave model SWAN (Simulating Waves Nearshore) has been converted into an unstructured-grid finite-volume version (hereafter referred to as FVCOM-SWAVE) for use in coastal ocean regions with complex irregular geometry. The implementation is made using the Flux-Corrected Transport (FCT) algorithm in frequency space, the implicit Crank–Nicolson method in directional space and options of explicit or implicit second-order upwind finite-volume schemes in geographic space. FVCOM-SWAVE is validated using four idealized benchmark test problems with emphasis on numerical dispersion, wave-current interactions, wave propagation over a varying-bathymetry shallow water region, and the basic wave grow curves. Results demonstrate that in the rectangular geometric domain, the second-order finite-volume method used in FVCOM-SWAVE has the same accuracy as the third-order finite-difference method used in SWAN. FVCOM-SWAVE was then applied to simulate wind-induced surface waves on the US northeast shelf with a central focus in the Gulf of Maine and New England Shelf. Through improved geometric fitting of the complex irregular coastline, FVCOM-SWAVE was able to robustly capture the spatial and temporal variation of surface waves in both deep and shallow regions along the US northeast coast.

Nonlinear saturation-based whitecapping dissipation in SWAN for deep and shallow water

This study investigates the effectiveness of a revised whitecapping source term in the spectral wind wave model SWAN (Simulating WAves Nearshore) that is local in frequency space, nonlinear with respect to the variance density and weakly dependent on the wave age. It is investigated whether this alternative whitecapping expression is able to correct the tendency towards underprediction of period measures that has been identified in the default SWAN model. This whitecapping expression is combined with an alternative wind input source term that is more accurate for young waves than the default expression. The shallow water source terms of bottom friction, depth-induced breaking and triad interaction are left unaltered. It is demonstrated that this alternative source term combination yields improved agreement with fetch- and depth-limited growth curves. Moreover, it is shown, by means of a field case over a shelf sea, that the investigated model corrects the erroneous overprediction of wind-sea energy displayed by the default model under combined swell-sea conditions. For a selection of field cases recorded at two shallow lakes, the investigated model generally improves the agreement with observed spectra and integral parameters. The improvement is most notable in the prediction of period measures.

Phase-decoupled refraction–diffraction for spectral wave models

Conventional spectral wave models, which are used to determine wave conditions in coastal regions, can account for all relevant processes of generation, dissipation and propagation, except diffraction. To accommodate diffraction in such models, a phase-decoupled refraction–diffraction approximation is suggested. It is expressed in terms of the directional turning rate of the individual wave components in the two-dimensional wave spectrum. The approximation is based on the mild-slope equation for refraction–diffraction, omitting phase information. It does therefore not permit coherent wave fields in the computational domain (harbours with standing-wave patterns are excluded). The third-generation wave model SWAN (Simulating WAves Nearshore) was used for the numerical implementation based on a straightforward finite-difference scheme. Computational results in extreme diffraction-prone cases agree reasonably well with observations, analytical solutions and solutions of conventional refraction–diffraction models. It is shown that the agreement would improve further if singularities in the wave field (e.g., at the tips of breakwaters) could be properly accounted for. The implementation of this phase-decoupled refraction–diffraction approximation in SWAN shows that diffraction of random, short-crested waves, based on the mild-slope equation can be combined with the processes of refraction, shoaling, generation, dissipation and wave–wave interactions in spectral wave models.