Wave Model WAM Research Articles

Spatial and temporal variability of wave energy resource in the eastern Pacific from Panama to the Drake passage

We analyse the wave energy resource available along the Pacific coast of South America from Panama from the latitude of 8°N to the Drake Passage at 55°S. The analysis is based on wave time series over 63 years (1959–2021) from the European Union Copernicus database constructed using the WAM wave model for the entire Pacific forced by wind information from ERA5. The novel features are the analysis of temporal variations in the wave energy flux, quantification of the contribution of swells and wind-seas into the wave energy potential, establishing properties of the most energy-carrying wave conditions, and evaluation of the role of El Niño and La Niña in the wave energy potential. The annual average wave energy flux increases from about 2 kW/m just to the north of the equator on the Colombian Pacific coast to 20–50 kW/m in the central and southern mainland of Chile and up to 80 kW/m near the Drake Passage. The wave energy resource to the north of latitude 32°S is almost entirely provided by swells. To the south of 44°S wind-seas predominate among the most energetic wave conditions and the maxima of energy flux by wind-seas up to 20 times exceed the already high average energy flux. The temporal variation in the wave energy flux follows the same pattern. It is fairly small at lower latitudes and increases rapidly from the forties. The magnitude of seasonal variation in terms of monthly mean wave energy flux is commonly from −47% to +32% from the long-term mean. The calmest time that contains 1% of the total annual energy flux is 10–20 days, about 10% of energy is contained in the 100 calmest days, while 50% of the annual energy flux arrives during the 100 days with strongest waves in the entire study area. The typical height of waves that provide the largest contribution to wave energy is about 1–1.5 m in the very north, around 1 m near the equator, gradually increases to the South and reaches 3.5–4 m on the shores of southern Chile. The associated wave periods are about 10 s in the entire study area. The wave energy flux has been almost constant over the 63 years near the equator but has increased at a rate up to 0.6 kW/m per year in the nearshore of Chile. For the evaluated time scales in coastal areas of South America, the interchange of El Niño and La Niña does not have detectable impact on the wave energy resource.

A wave-resolving modeling study of rip current variability, rip hazard, and swimmer escape strategies on an embayed beach

Abstract. Drownings due to rip currents are a major threat to beach safety. In this study a high-resolution Boussinesq model with a modified wave-resolving Lagrangian tracking module has been applied to a 2 km long embayed beach, Dadonghai of Sanya, Hainan Island, with the purpose of studying rip current variability, real-time rip hazard identification, and the optimal swimmer escape strategies. The beach stage evolves periodically at the study site and plays an important role in the long-term modulation of the occurrence and strength of rip currents according to the modeling. A series of tests are designed and confirm that rip current strength is closely related to wave properties and tidal levels. Spectral analysis of output time series at specific points shows that the modeled rip currents fluctuate on the orders of 1 and 10 min, which suggests the effects of wave-group-forced infragravity (IG) and very-low-frequency (VLF) motions. Rip hazard levels are defined by combining rip strength and its duration. An attempt to use the GPU-accelerated FUNWAVE-TVD (Total Variation Diminishing version of the Fully Nonlinear Boussinesq Wave Model) embedded with the spectral wave model WAM6-GPU (GPU version of the third-generation spectral wave model WAM Cycle 6) exhibits its capability to evaluate rip hazard levels in real time. One of the differences of the present study from previous works is that the random, wave-resolving tracking of virtual swimmers is performed with 1 m resolution to study beach safety strategies. The results demonstrate that multiple factors contribute to the survival of swimmers caught in the rip currents, including surf-zone bathymetry, rip strength, fine-scale flow patterns, the bather's position, and swimming ability. For weak-to-moderate rip currents and longshore currents, swim onshore consistently seems to be the most successful strategy across all the scenarios in this study. Higher surf-zone exit rates along Dadonghai beach are not favorable for stay afloat action, which puts swimmers at a higher risk of being expelled to deeper water. The effects of wave randomness of incoming wave trains and assignment of wave-following coefficients on Lagrangian tracking are also discussed.

Added value of including waves into a coupled atmosphere–ocean model system within the North Sea area

In this study, the effects of fully coupling the atmosphere, waves, and ocean compared with two-way-coupled simulations of either atmosphere and waves or atmosphere and ocean are analyzed. Two-year-long simulations (2017 and 2018) are conducted using the atmosphere–ocean–wave (AOW) coupled system consisting of the atmosphere model CCLM, the wave model WAM, and the ocean model NEMO. Furthermore, simulations with either CCLM and WAM or CCLM and NEMO are done in order to estimate the impacts of including waves or the ocean into the system. For the North Sea area, it is assessed whether the influence of the coupling of waves and ocean on the atmosphere varies throughout the year and whether the waves or the ocean have the dominant effect on the atmospheric model. It is found that the effects of adding the waves into the system already consisting of atmosphere and ocean model or adding the ocean to the system of atmosphere and wave model vary throughout the year. Which component has a dominant effect and whether the effects enhance or diminish each other depends on the season and variable considered. For the wind speed, during the storm season, adding the waves has the dominant effect on the atmosphere, whereas during summer, adding the ocean has a larger impact. In summer, the waves and the ocean have similar influences on mean sea level pressure (MSLP). However, during the winter months, they have the opposite effect. For the air temperature at 2 m height (T_2m), adding the ocean impacts the atmosphere all year around, whereas adding the waves mainly influences the atmosphere during summer. This influence, however, is not a straight feedback by the waves to the atmosphere, but the waves affect the ocean surface temperature, which then also feedbacks to the atmosphere. Therefore, in this study we identified a season where the atmosphere is affected by the interaction between the waves and the ocean. Hence, in the AOW-coupled simulation with all three components involved, processes can be represented that uncoupled models or model systems consisting of only two models cannot depict.

The Impact of a Reduced High‐Wind Charnock Parameter on Wave Growth With Application to the North Sea, the Norwegian Sea, and the Arctic Ocean

AbstractAs atmospheric models move to higher resolution and resolve smaller scales, the maximum modeled wind speed also tends to increase. Wave models tuned to coarser wind fields tend to overestimate the wave growth under strong winds. A recently developed semiempirical parameterization of the Charnock parameter, which controls the roughness length over surface waves, substantially reduces the aerodynamic drag of waves in high winds (above a threshold of 30 m s−1). Here, we apply the formulation in a recent version of the wave model WAM (Cycle 4.7), which uses a modified version of the physics parameterizations by Ardhuin et al. (2010, https://doi.org/10.1175/2010jpo4324.1) as well as subgrid obstructions for better performance around complex topography. The new Charnock formulation is tested with wind forcing from NORA3, a recently completed nonhydrostatic atmospheric downscaling of the global reanalysis ERA5 for the North Sea, the Norwegian Sea and the Barents Sea. Such high‐resolution atmospheric model integrations tend to have stronger (and more realistic) upper‐percentile winds than what is typically found in coarser atmospheric models. A 2‐year comparison (2011–2012) of a control run against the run with the modified Charnock parameter shows a dramatic reduction of the wave height bias in high‐wind cases. The added computational cost of the new physics and the reduction of the Charnock parameter compared to the earlier WAM physics is modest (14%). A longer (1998–2020) hindcast integration with the new Charnock parameter is found to compare well against in situ and altimeter wave measurements both for intermediate and high sea states.

Dynamical Projections of the Mean and Extreme Wave Climate in the Bohai Sea, Yellow Sea and East China Sea

Few studies have focused on the projected future changes in wave climate in the Chinese marginal seas. For the first time, we investigate the projected changes of the mean and extreme wave climate over the Bohai Sea, Yellow Sea, and East China Sea (BYE) during two future periods (2021–2050 and 2071–2100) under the RCP2.6 and RCP8.5 scenarios from the WAM wave model simulations with a resolution of 0.1°. This is currently the highest-resolution wave projection dataset available for the study domain. The wind forcings for WAM are from high-resolution (0.22°) regional climate model (RCM) CCLM-MPIESM simulations. The multivariate bias-adjustment method based on the N-dimensional probability density function transform is used to correct the raw simulated significant wave height (SWH), mean wave period (MWP), and mean wave direction (MWD). The annual and seasonal mean SWH are generally projected to decrease (-0.15 to -0.01 m) for 2021–2050 and 2071–2100 under the RCP2.6 and RCP8.5 scenarios, with statistical significance at a 0.1 level for most BYE in spring and for most of the Bohai Sea and Yellow Sea in annual and winter/autumn mean. There is a significant decrease in the spring MWP for two future periods under both the RCP2.6 and RCP8.5 scenarios. In contrast, the annual and summer/winter 99th percentile SWH are generally projected to increase for large parts of the study domain. Results imply that the projected changes in the mean and 99th percentile extreme waves are very likely related to projected changes in local mean and extreme surface wind speeds, respectively.

Improving Baltic Sea wave forecasts using modelled surface currents

Currents in the Baltic Sea are generally weak, but during strong winds they can grow high enough to affect the surface wave propagation and evolution. To evaluate the significance of wave-current interactions in the Baltic Sea, we conducted a study using the wave model WAM, comparing a run without surface currents to one with current forcing from a NEMO hydrodynamical model simulation. The overall changes to the wave field caused by currents were quite small. Changes of over 10 cm in significant wave height (SWH) or 1 s in the peak period (Tp) occurred only in some areas and typically less than 3% of the time. Current refraction changed the SWH annual mean by up to 2 cm, but changes up to 60 cm were seen in the maximum values. Tp had occasionally large changes due to shifts in the peak energy in two-peaked swell and wind-sea spectra. Including currents typically led to a stronger changes in swell energy compared to the changes in wind sea energy. A comparison with a wave buoy in the Gulf of Finland showed that this change in the swell energy improved the accuracy of the simulation in this narrow gulf. Current-induced refraction was most prominent near the coastal areas, where current speed occasionally exceeded 0.3 m/s. In general, SWH decreased in the coastal areas with strong currents and slightly increased in adjacent open sea areas. The current effects were most frequent in the Gulf of Finland, the Western Gotland Basin and the Åland Sea.

Investigating sea‐state effects on flash flood hydrograph and inundation forecasting

AbstractA common source of uncertainty in flood inundation forecasting is the hydrograph used. Given the role of sea‐air‐hydro‐land chain processes on the water cycle, flood hydrographs in coastal areas can be indirectly affected by sea state. This study investigates sea‐state effects on precipitation, discharge, and flood inundation forecasting implementing atmospheric, ocean wave, hydrological, and hydraulic‐hydrodynamic coupled models. The Chemical Hydrological Atmospheric Ocean wave System (CHAOS) was used for coupled hydro‐meteorological‐wave simulations ‘accounting’ or ‘not accounting’ the impact of sea state on precipitation and, subsequently, on flood hydrograph. CHAOS includes the WRF‐Hydro hydrological model and the WRF‐ARW meteorological model two‐way coupled with the WAM wave model through the OASIS3‐MCT coupler. Subsequently, the 2D HEC‐RAS hydraulic‐hydrodynamic model was forced by the flood hydrographs and map the inundated areas. A flash flood event occurred on 15 November 2017 in Mandra, Attica, Greece, causing 24 fatalities, and damages was selected as case study. The calibration of models was performed exploiting historical flood records and previous studies. Human interventions such as hydraulic works and the urban areas were included in the hydraulic modelling geometry domain. The representation of the resistance caused by buildings was based on Unmanned Aerial System (UAS) data while the local elevation rise method was used in the urban‐flood simulation. The flood extent results were assessed using the Critical Success Index (CSI), and CSI penalize. Integrating sea‐state affected the forecast of precipitation and discharge peaks, causing up to +24% and from −8% to +36% differences, respectively, improving inundation forecast by 4.5% and flooding additional approximately 70 building blocks. The precipitation forcing time step was also highlighted as significant factor in such a small‐scale flash flood. The integrated multidisciplinary methodological approach could be adopted in operational forecasting for civil protection applications facilitating the protection of socio‐economic activities and human lives during similar future events.

Sea-state contributions to sea-level variability in the European Seas

The contribution of sea-state-induced processes to sea-level variability is investigated through ocean-wave coupled simulations. These experiments are performed with a high-resolution configuration of the Geestacht COAstal model SysTem (GCOAST), implemented in the Northeast Atlantic, the North Sea and the Baltic Sea which are considered as connected basins. The GCOAST system accounts for wave-ocean interactions and the ocean circulation relies on the NEMO (Nucleus for European Modelling of the Ocean) ocean model, while ocean-wave simulations are performed using the spectral wave model WAM. The objective is to demonstrate the contribution of wave-induced processes to sea level at different temporal and spatial scales of variability. When comparing the ocean-wave coupled experiment with in situ data, a significant reduction of the errors (up to 40% in the North Sea) is observed, compared with the reference. Spectral analysis shows that the reduction of the errors is mainly due to an improved representation of sea-level variability at temporal scales up to 12 h. Investigating the representation of sea-level extremes in the experiments, significant contributions (> 20%) due to wave-induced processes are observed both over continental shelf areas and in the Atlantic, associated with different patterns of variability. Sensitivity experiments to the impact of the different wave-induced processes show a major impact of wave-modified surface stress over the shelf areas in the North Sea and in the Baltic Sea. In the Atlantic, the signature of wave-induced processes is driven by the interaction of wave-modified momentum flux and turbulent mixing, and it shows its impact to the occurrence of mesoscale features of the ocean circulation. Wave-induced energy fluxes also have a role (10%) in the modulation of surge at the shelf break.

Coastal Flooding: Joint Probability of Extreme Water Levels and Waves along the Baltic Sea Coast

Kudryavtseva, N.; Räämet, A., and Soomere, T., 2020. Coastal flooding: Joint probability of extreme water levels and waves along the Baltic Sea coast. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1146–1151. Coconut Creek (Florida), ISSN 0749-0208.An accurate assessment of the joint probability of water levels and waves is crucial to establish appropriate mitigation and adaptation strategies for possible flooding and coastal erosion in countries with extensive low-lying nearshore areas. The simultaneous occurrence of large waves and high still water levels can significantly increase the risk of flooding and damage of coastal constructions. In this study, the water level extremes were retrieved from simulated high-resolution 3D NEMO-Nordic water level data along the Baltic Sea coast with a spatial resolution of 2 nautical miles (∼3.7 km) and time resolution of 1 hour. The wave height data were obtained from the numerical simulations using the WAM wave model with adjusted geostrophic wind forcing with 3 nautical miles spatial resolution and the time resolution of 1 hour. Using the Archimedean copula approach, the likelihood of joint occurrence of high water levels and wave heights was evaluated along the whole Baltic Sea coast for 1979–2007. Four distinctive regions in the Baltic Sea were identified following differences in the dependence between the sea level and wave height extremes in terms of their correlation. The Arkona basin and eastern part of the Baltic proper have low dependence (correlation coefficient <0.2), the Gulf of Riga, the Gulf of Finland and north-western part of the Baltic Proper exhibit moderate dependence (0.2–0.5), the Sea of Bothnia has the strongest dependence (0.5–0.7), and the Bay of Bothnia has a dependence in the range of 0.2–0.4, similar to the central Baltic Sea region. The joint probability analysis showed that the highest risk of simultaneous appearance of water level and wave height extremes is observed in the Sea of Bothnia. The study demonstrates that it is crucial to consider the combination of both water levels and wave heights in all regions of the Baltic Sea except for the southern areas.

Impact of Ice Data Quality and Treatment on Wave Hindcast Statistics in Seasonally Ice-Covered Seas

The seasonal ice cover has significant effect on the wave climate of the Baltic Sea. We used the third-generation wave model WAM to simulate the Baltic Sea wave field during four ice seasons (2009†-2012). We used data from two different sources: daily ice charts compiled by FMI’s Ice Service and modeled daily mean ice concentration from SMHI’s NEMO†Nordic model. We utilized two different methods: a fixed threshold of 30 % ice concentration, after which wave energy is set to zero, and a grid obstruction method up to 70 % ice concentration, after which wave energy is set to zero. The simulations run using ice chart data had slightly better accuracy than the simulation using NEMO-Nordic ice data, when compared to altimeter measurements. The analysis of the monthly mean statistics of significant wave height (SWH) showed that the differences between the simulations were relatively small and mainly seen in the Bothnian Bay, the Quark, and the eastern Gulf of Finland. There were larger differences, up to 3.2 m, in the monthly maximum values of SWH. These resulted from individual high wind situations during which the ice edge in the ice chart and NEMO-Nordic was located differently. The two different methods to handle ice concentration resulted only in small differences in the SWH statistics, typically near the ice edge. However, in some individual cases the two methods resulted in quite large differences in the simulated SWH and the handling of ice concentrations as additional grid obstructions could be important, for example, in operational wave forecasting.

Mid-twenty-first century global wave climate projections: Results from a dynamic CMIP5 based ensemble

Abstract Ocean surface gravity waves are part of the climate system, regulating the exchange of energy and momentum at the atmosphere-ocean interface. Besides that, waves have a direct impact on coastal erosion and sediment transport, in ship routing and offshore/ship infrastructure design, as well as in coastal management. Most future wave climate projections point out to statistically significant changes in wave height conditions in the global ocean towards the end of the 21st century. In the present study, the near future impact of a warmer climate on the global ocean wave climate towards the mid-21st century (2031–2060) is investigated through a 4-member “coherent” ensemble of wave climate projections: single-model (wave model WAM), single-forcing (EC-Earth) and single-scenario (RCP8.5). The wave model WAM was forced by wind speed and ice coverage from four Coupled Model Intercomparison Project Phase 5 (CMIP5) EC-Earth projections. The ensemble historic climate reference period has been evaluated by comparison against wave reanalysis/hindcast data, and wave in-situ observations. Statistically significant increases in the global mean wind speed, wave height, wave period and wave energy flux are to be expected towards the mid-twenty first century, in less than 50 years, these changes being more striking in the mid-to-high latitudes of the Southern Hemisphere. Due to the propagation of swell to lower latitudes, statistically significant increases in wave heights, periods and energy fluxes are also projected to occur in equatorial areas, especially along the eastern side of the main ocean basins, during the Austral winter. Comparatively, the projected changes in wave heights and energy fluxes are expected to occur more intensely during the first half of the 21st century, in several areas of the global ocean.

CMIP5-Derived Single-Forcing, Single-Model, and Single-Scenario Wind-Wave Climate Ensemble: Configuration and Performance Evaluation

A Coupled Model Intercomparison Project Phase 5 (CMIP5)-derived single-forcing, single-model, and single-scenario dynamic wind-wave climate ensemble is presented, and its historic period (1979–2005) performance in representing the present wave climate is evaluated. A single global climate model (GCM)-forcing wave climate ensemble was produced with the goal of reducing the inter GCM variability inherent in using a multi-forcing approach for the same wave model. Seven CMIP5 EC-Earth ensemble runs were used to force seven WAM wave model realizations, while future wave climate simulations, not analyzed here, were produced using a high-emission representative concentration pathway 8.5 (RCP8.5) set-up. The wave climate ensemble’s historic period was extensively compared against a set of 72 in situ wave-height observations, as well as to ERA-Interim reanalysis and Climate Forecast System Reanalysis (CFSR) hindcast. The agreement between the wave climate ensemble and the in situ measurements and reanalysis of mean and extreme wave heights, mean wave periods, and mean wave directions was good, in line with previous studies or even better in some areas of the global ocean, namely in the extratropical latitudes. These results give a good degree of confidence in the ability of the ensemble to simulate a realistic climate change signal.

Meteo-Marine Parameters from Sentinel-1 SAR Imagery: Towards Near Real-Time Services for the Baltic Sea

A method for estimating meteo-marine parameters from satellite Synthetic Aperture Radar (SAR) data, intended for near-real-time (NRT) service over the Baltic Sea, is presented and validated. Total significant wave height data are retrieved with an empirical function CWAVE_S1-IW, which combines spectral analysis of Sentinel-1A/B Interferometric Wide swath (IW) subscenes with wind data derived with common C-Band Geophysical Model Functions (GMFs). In total, 15 Sentinel-1A/B scenes (116 acquisitions) over the Baltic Sea were processed for comparison with off-shore sea state measurements (52 collocations) and coastal wind measurements (357 colocations). Sentinel-1 wave height was spatially compared with WAM wave model results (Copernicus Marine Environment Monitoring Service (CMEMS). The comparison of SAR-derived wave heights shows good agreement with measured wave heights correlation r of 0.88 and with WAM model (r = 0.85). The wind speed estimated from SAR images yields good agreement with in situ data (r = 0.91). The study demonstrates that the wave retrievals from Sentinel-1 IW data provide valuable information for operational and statistical monitoring of wave conditions in the Baltic Sea. The data is valuable for model validation and interpretation in regions where, and during periods when, in situ measurements are missing. The Sentinel-1 A/B wave retrievals provide more detailed information about spatial variability of the wave field in the coastal zone compared to in situ measurements, altimetry wave products and model forecast. Thus, SAR data enables estimation of storm locations and areal coverage. Methods shown in the study are implemented in NRT service in German Aerospace Center’s (DLR) ground station Neustrelitz.

A multi-decadal wind-wave hindcast for the North Sea 1949–2014: coastDat2

Abstract. Long and consistent wave data are important for analysing wave climate variability and change. Moreover, such wave data are also needed in coastal and offshore design and for addressing safety-related issues at sea. Using the third-generation spectral wave model WAM a multi-decadal wind-wave hindcast for the North Sea covering the period 1949–2014 was produced. The hindcast is part of the coastDat database representing a consistent and homogeneous met-ocean data set. It is shown that despite not being perfect, data from the wave hindcast are generally suitable for wave climate analysis. In particular, comparisons of hindcast data with in situ and satellite observations show on average a reasonable agreement, while a tendency towards overestimation of the highest waves could be inferred. Despite these limitations, the wave hindcast still provides useful data for assessing wave climate variability and change as well as for risk analysis, in particular when conservative estimates are needed. Hindcast data are stored at the World Data Center for Climate (WDCC) and can be freely accessed using the doi:10.1594/WDCC/coastDat-2_WAM–North_Sea Groll and Weisse(2016) or via the coastDat web-page http://www.coastdat.de.

Surface Stokes drift in the Baltic Sea based on modelled wave spectra

The Stokes drift is an important component in the surface drift. We used the wave model WAM to evaluate the mean values and exceedance probabilities of the surface Stokes drift in the Baltic Sea. As there is no direct way to verify the accuracy of the modelled Stokes drift, we compared the bulk parameters calculated by the wave model against buoy measurements to ensure the quality of the wave hindcast. Furthermore, we evaluated the surface Stokes drift from measured wave spectra to assess the accuracy of the modelled surface Stokes drift. The importance of the Stokes drift as a component of the total surface drift was evaluated by calculating the hindcast mean values and percentiles of the surface Stokes drift. The mean values were between 0.08 and 0.10 ms−1 in the open sea areas, thus being of the same order of magnitude as the mean wind shear currents. The highest values of the surface Stokes drift were slightly larger than 0.6 ms−1. The comparison of modelled Stokes drift values to estimates obtained from measured spectra suggests that the mean values are well represented by the model. However, the higher modelled values are most likely slightly too large because the wave energy was overestimated during high wind situations in some of the sub-basins, such as the Gulf of Finland. A comparison to a drifter experiment showed that use of the Stokes drift improves the estimate of both the drift speed and the direction in the Gulf of Finland. Parameterised methods to evaluate the Stokes drift that are used, e.g. in currently available Baltic Sea drift models, overestimate the smaller values (under 0.3 ms−1) and underestimate the larger values of the Stokes drift compared to the values calculated by the wave model. The modelled surface Stokes drift direction mostly followed the forcing wind direction. This was the case even in the Gulf of Finland, where the direction of the wind and the waves can differ considerably.

Extreme bottom velocities induced by wind wave and currents in the Gulf of Gda\u0144sk

The principal goal of this study is to get some preliminary insights about the intensity of water movement generated by wind waves, and due to the currents in the bottom waters of Gulf of Gdańsk, during severe storms. The Gulf of Gdańsk is located in the southern Baltic Sea. This paper presents the results of analysis of wave and current-induced velocities during extreme wind conditions, which are determined based on long-term historical records. The bottom velocity fields originated from wind wave and wind currents, during analysed extreme wind events, are computed independently of each other. The long-term wind wave parameters for the Baltic Sea region are derived from the 44-year hindcast wave database generated in the framework of the project HIPOCAS funded by the European Union. The output from the numerical wave model WAM provides the boundary conditions for the model SWAN operating in high-resolution grid covering the area of the Gulf of Gdańsk. Wind current velocities are calculated with the M3D hydrodynamic model developed in the Institute of Oceanography of the University of Gdańsk based on the POM model. The three dimensional current fields together with trajectories of particle tracers spreading out of bottom boundary layer are modelled, and the calculated fields of bottom velocities are presented in the form of 2D maps. During northerly winds, causing in the Gulf of Gdańsk extreme waves and most significant wind-driven circulation, the wave-induced bottom velocities are greater than velocities due to currents. The current velocities in the bottom layer appeared to be smaller by an order of magnitude than the wave-induced bottom orbital velocities. Namely, during most severe northerly storms analysed, current bottom velocities ranged about 0.1–0.15 m/s, while the root mean square of wave-induced near-seabed velocities reached maximum values of up to 1.4 m/s in the southern part of Gulf of Gdańsk.

Wave power estimation by means of spectral wave models and satellite records

ABSTRACTThe aim of this work is to study different approaches for the estimation of the energy potential of sea waves. To this end, numerical models and remote sensing, especially satellites, are utilised for the regions of North Atlantic Ocean and the Mediterranean Sea. In particular, two methods are compared: one based on the full wave spectrum and a second utilising simplified formulas based on specific wave parameters and certain approximations. Moreover, an attempt for a qualitative assessment of the wave model WAM over areas of different wave climatology is made by the comparison of relevant wave spectra. The main outcomes of this work show that simplified calculation approaches of wave energy potential overestimate the energy rate of even 10% on average, varying in the cases of shallow or deep waters. Moreover, the performance of the wave model is satisfactory resulting to small statistical errors in the calculation of wave characteristics, a fact that proves the suitability of WAM model for reliable wave energy assessment.

Wave Hindcast Experiments Using Wam Cycle 4.5.3 - Validation with In situ Measurements in the North Indian Ocean

In this investigation, the execution of the wave model WAM Cycle 4.5.3 was evaluated; based on the forcings by two distinct wind fields namely ERA-40 and QuikSCAT-NCEP Blended winds in the North Indian Ocean. Using the past analysed winds (hindcasting) and with boundary conditions generated from 1° x 1° global runs the wave model have been implemented for the North Indian Ocean from 50°E to 100°E and 0°N to 30°N. The model simulated parameters namely significant wave height (Hs), mean wave periods (Tc), mean wave direction, swell wave height (Hsw), swell wave period (Tsw) and swell wave directions are presented. Further, the predicted waves are compared against in-situ wave measurements. The simulated wave parameters obtained as outputs from numerical simulations such as Hs and Tc; collocated in space and time have been compared with buoy measurements in the Arabian Sea and Bay of Bengal with the aid of statistical error indicators. Comparison between the predicted and observed wave parameters are very encouraging, excepting the higher model estimates of significant wave height and the disagreement in lower wave heights at few buoy locations. Further, the sensitivity of the model to two different wind fields was analyzed and it is noted that the blended winds could accurately reproduce Hs and Tc at the buoy locations. The present study suggests that, WAM Cycle 4.5.3 model predictions are quite reliable for the Indian seas using the analysed wind fields such as ERA-40 and QuikSCAT/NCEP Blended wind fields. However, the performance of wave model was best when blended winds were used. Despite several limitations, the results reveal that the performance of third generation wave model is promising and the correlation and deviation of simulated and measured waves are critically inspected.

Seasonally Variation of Significant Wave Height for 25 Year Period Based on JMA/MRI-AGCM3.2 Wind Climate Data

The global ocean wave climate has long been of interest to the ocean engineering community because of the need for accurate operational wave data for applications such as vessel design, design of offshore and coastal structures or naval operations. Recently, there has been a major interest in wave climate changes as a result of global warming. Therefore, studies on predicting the effect of global warming on ocean wave climate are required. The objectives of this study are to analyze the accuracy and variability of global significant wave height hindcast for the 25 year period 1979-2003. This study describes the 25 year global significant wave height simulation derived from the Japan Meteorology Agency/Meteorology Research Institute (JMA/MRI)-AGCM3.2 wind climate data. The wind climate data were input into ocean wave model WAM with a global grid of spacing 1o in latitude by 1o in longitude. In situ wind and wave data sets from National Data Buoy Center (NDBC)-National Oceanic and Atmospheric Administration (NOAA) database were used to evaluate the hindcast accuracy. The validation showed good agreement both wind and waves data. The wave hindcast analysis show that the seasonal variability of significant wave height in the Northern Hemisphere is greater than in the Southern Hemisphere. Meanwhile, most of the equatorial regions are in calm condition all year.

Coastal flooding: impact of waves on storm surge during extremes – a case study for the German Bight

Abstract. This study addresses the impact of wind, waves, tidal forcing and baroclinicity on the sea level of the German Bight during extreme storm events. The role of wave-induced processes, tides and baroclinicity is quantified, and the results are compared with in situ measurements and satellite data. A coupled high-resolution modelling system is used to simulate wind waves, the water level and the three-dimensional hydrodynamics. The models used are the wave model WAM and the circulation model GETM. The two-way coupling is performed via the OASIS3-MCT coupler. The effects of wind waves on sea level variability are studied, accounting for wave-dependent stress, wave-breaking parameterization and wave-induced effects on vertical mixing. The analyses of the coupled model results reveal a closer match with observations than for the stand-alone circulation model, especially during the extreme storm Xaver in December 2013. The predicted surge of the coupled model is significantly enhanced during extreme storm events when considering wave–current interaction processes. This wave-dependent approach yields a contribution of more than 30 % in some coastal areas during extreme storm events. The contribution of a fully three-dimensional model compared with a two-dimensional barotropic model showed up to 20 % differences in the water level of the coastal areas of the German Bight during Xaver. The improved skill resulting from the new developments justifies further use of the coupled-wave and three-dimensional circulation models in coastal flooding predictions.