Estimating nearshore waves by assimilating buoy directional spectrum data in SWAN

This paper describes a variational data assimilation algorithm based on the SWAN near shore wave-spectrum model. The approach allows single-point wave spectrum observations to be used to estimate the wave field for a nearshore region under stationary conditions, assuming a spatially uniform incident wave spectrum at the offshore boundary. The assimilated data are in the form of Fourier directional coefficients, the standard output from operational wave buoys, and are used directly by incorporating the relationship between directional spectrum and the Fourier coefficients into the formulation. The algorithm was tested on data from nearshore buoys deployed off the coast of North Carolina in May 2012, and the estimated wave field is compared to both the input data and to independent observation data. The results compare favorably to the independent data with overall RMS errors of 10–20 percent for significant wave height, about half a second for mean wave period, and as much as 3–4 SWAN spectral grid cells for mean direction. Overall, the results show that the algorithm can be effectively used to estimate the offshore boundary spectrum and accurately reproduce wave conditions in the domain.

The Influence of Typhoon “MITAG” on Waves and Currents in Zhoushan Sea Area, China

Typhoon “MITAG” was generated at the end of September 2019 and landed briefly in Zhoushan on October 1. Based on reanalysis data provided by ERA5 and NCEP, this paper analyzes the characteristics of wave and current during “MITAG”. The variation rule of waves and currents in different periods during the influence of “MITAG” was found. The results are as follows: The variation of significant wave height and mean wave period is related to its waveform. The single waveform has a long wave period and the correlation between wave height and wave period reaches 0.87 during the wind wave period. The wave period of the mixed waveform is shorter. The Ekman pumping of the ocean by “MITAG” is concentrated on the right side of the typhoon path when it is away from land; however, Ekman pumping is on the land side when the typhoon is close to the land. The sea surface height of the coastal sea area changes regularly with the distance of “MITAG”. The area which has a strong current is consistent with higher wave height.

Projections of Directional Spectra Help to Unravel the Future Behavior of Wind Waves

Based on a novel approach, present-day and future spectral wind-wave conditions in a high-emission scenario from a seven-member wave climate projection ensemble are compared. The spectral analysis at the selected locations aids in understanding the propagation of swell projected changes from the generation areas across the ocean basins. For example, a projected increase in the energy from Southern Ocean swells can be observed in all ocean basins and both hemispheres, which is especially relevant in the west coast of North America due to the penetration of these swells beyond 30°N. Similarly, a consistent decrease in the energy of large northern Atlantic swells is noted close to the equator. This work provides evidence that assessments based on only integrated wave parameters (e.g., significant wave height and mean wave period) can mask information about the sign, magnitude, and robustness of the actual wave climate changes due to the offset of positive and negative variations within the spectrum, leading to a significant underestimation of the change associated with certain wave systems.

A Spatiotemporal Convolutional Gated Recurrent Unit Network for Mean Wave Period Field Forecasting

Mean wave period (MWP) is one of the key parameters affecting the design of marine facilities. Currently, there are two main methods, numerical and data-driven methods, for forecasting wave parameters, of which the latter are widely used. However, few studies have focused on MWP forecasting, and even fewer have investigated it with spatial and temporal information. In this study, correlations between ocean dynamic parameters are explored to obtain appropriate input features, significant wave height (SWH) and MWP. Subsequently, a data-driven approach, the convolution gated recurrent unit (Conv-GRU) model with spatiotemporal characteristics, is utilized to field forecast MWP with 1, 3, 6, 12, and 24-h lead times in the South China Sea. Six points at different locations and six consecutive moments at every 12-h intervals are selected to study the forecasting ability of the proposed model. The Conv-GRU model has a better performance than the single gated recurrent unit (GRU) model in terms of root mean square error (RMSE), the scattering index (SI), Bias, and the Pearson’s correlation coefficient (R). With the lead time increasing, the forecast effect shows a decreasing trend, specifically, the experiment displays a relatively smooth forecast curve and presents a great advantage in the short-term forecast of the MWP field in the Conv-GRU model, where the RMSE is 0.121 m for 1-h lead time.

Future Changes in Wave Conditions at the German Baltic Sea Coast Based on a Hybrid Approach Using an Ensemble of Regional Climate Change Projections

In this study, the projected future long-term changes of the local wave conditions at the German Baltic Sea coast over the course of the 21st century are analyzed and assessed with special focus on model agreement, statistical significance and ranges/spread of the results. An ensemble of new regional climate model (RCM) simulations with the RCM REMO for three RCP forcing scenarios was used as input data. The outstanding feature of the simulations is that the data are available with a high horizontal resolution and at hourly timesteps which is a high temporal resolution and beneficial for the wind–wave modelling. A new data interface between RCM output data and wind–wave modelling has been developed. Suitable spatial aggregation methods of the RCM wind data have been tested and used to generate input for the calculation of waves at quasi deep-water conditions and at a mean water level with a hybrid approach that enables the fast compilation of future long-term time series of significant wave height, mean wave period and direction for an ensemble of RCM data. Changes of the average wind and wave conditions have been found, with a majority of the changes occurring for the RCP8.5 forcing scenario and at the end of the 21st century. At westerly wind-exposed locations mainly increasing values of the wind speed, significant wave height and mean wave period have been noted. In contrast, at easterly wind-exposed locations, decreasing values are predominant. Regarding the changes of the mean wind and wave directions, westerly directions becoming more frequent. Additional research is needed regarding the long-term changes of extreme wave events, e.g., the choice of a best-fit extreme value distribution function and the spatial aggregation method of the wind data.

THE RESULTS OF HYDROMETEOROLOGICAL MEASUREMENTS BY THE WAVESCAN BUOY SYSTEM ON THE SOUTHWESTERN SHELF OF THE PETER THE GREAT BAY IN 2016

For the first time, the long-term measurements of meteorological and oceanographic characteristics were measured using the anchored autonomous complex of the WaveScan buoy on the Southwestern shelf of the Peter the Great Bay (Japan Sea) from 21 April to 23 December 2016. The buoy anchor was set at 50 m depth. During 10 minutes of each hour, meteorological characteristics of the near surface layer of the atmosphere, sea water temperature at the horizon of 1.5 m, and vertical profiles of the current velocity vector from the near-surface layer to the near-bottom one were measured with resolution 4 m. During 20 minutes of each hour, the characteristics of waves on the sea surface were measured. In General, most of the meteorological characteristics measured on the buoy are in good agreement with the data of the NCEP-DOE AMIP-II, ERA-Interim and ERA5 reanalysis. The significant wave height for the whole frequency belt, mean wave period and mean spectral direction from the WaveScan Buoy measurements have the best statistical relationship, confidence level is 99%, with the correspondent significant height of the waves, mean wave period and mean spectral wave direction from reanalysis ERA5. The features of variability of vertical profiles of the current velocity vector on the synoptic and seasonal time scales that depend on wind speed and vertical stratification of density are determined. In the warm season the a significant left turn of the vector of the measured current velocity with depth is observed in the seasonal pycnocline below the surface layer of friction. At wind speed, not exceeding 5 m/s, the angle of the current velocity vector left turn reaches 170° in the bottom layer, where countercurrent is formed. During the passage of the Lionrock tropical cyclone, when and the daily mean wind velocity increase to 9 m/s, the angle of left turn of the current velocity vector in the pycnocline decreases to 20°. In the cold season a classic right turn of the current velocity vector is observed in the upper boundary layer, and the vertically average velocity vector within the 50 m layer, as well as the total drift flow in the Ekman friction layer, deviate 90° to the right from the surface wind velocity vector. The left turn of the current velocity vector at the buoy installation point was not observed in the cold season.

Assessing Climate Change in the North Atlantic Wave Regimes

An improved understanding of the present and future marine climatology is necessary for numerous activities, such as operation of offshore structures, optimization of ship routes and the evaluation of wave energy resources. To produce global wave information, the WW3 wave model was forced with wind and ice-cover data from an RCP8.5 EC-Earth system integration for two 30-year time slices. The first covering the periods from 1980 to 2009 represents the present climate and the second, covering the periods from 2070–2099, represents the climate in the end of the 21st century. Descriptive statistics of wind and wave parameters are obtained for different 30-year time slices. Regarding wind, magnitude and direction will be used. For wave, significant wave height (of total sea and swell), mean wave period, peak period, mean wave direction and energy will be investigated. Changes from present to future climate are evaluated, regarding both mean and extreme events. Maps of the theses statistics are presented. The long-term monthly joint distribution of significant wave heights and peak periods is generated. Changes from present to future climate are assessed, comparing the statistics between time slices.

The importance of wind forcing in fjord wave modelling

Accurate predictions of surface ocean waves in coastal areas are important for a number of marine activities. In complex coastlines with islands and fjords, the quality of wind forcing significantly affects the results. We investigate the role of wind forcing on wave conditions in a fjord system partly exposed to open sea. For this reason, we implemented the wave model SWAN at the west coast of Norway using four different wind forcing. Wind and wave estimates were compared with observations from five measurement sites. The best results in terms of significant wave height are found at the sites exposed to offshore conditions using a wind input that is biased slightly high compared with the buoy observations. Positively biased wind input, on the other hand, leads to significant overestimation of significant wave height in more sheltered locations. The model also shows a poorer performance for mean wave period in these locations. Statistical results are supported by two case studies which also illustrate the effect of high spatial resolution in wind forcing. Detailed wind forcing is necessary in order to obtain a realistic wind field in complex fjord terrain, but wind channelling and lee effects may have unpredictable effects on the wave simulations. Pure wave propagation (no wind forcing) is not able to reproduce the highest significant wave height in any of the locations.

Modeling the dynamics of a sand beach governed by the wave and underwater bar interaction

Effect of bar position on underwater profile of sandy beach was studied at the timescale of one storm using the xBeach numerical model. Beach profiles were extracted from the bathymetry of the Shkorpilovtsy beach (the Bulgarian coast of the Black Sea). Computed results were verified by field measurements. The largest shoreline retreat occurred in the first hour of a storm. For the chosen wave regime (largest wave height 1.5 m, wave period 10.5 s), an equilibrium profile was formed after 6 hours. The resulting profile contained an underwater terrace with the slope close to that of the theoretical equilibrium profile. It was shown that the position of the underwater bar affects the shoreline retreat rate. The smallest and the largest shore retreat occur if bar crest is located at a distance about 0.7–0.8 and 0.5 of the deep water wavelength, correspondingly. It was found that the shoreline retreat depends on the height of infragravity waves and mean wave period: the smaller mean wave period and the higher infragravity waves near the coast, the smaller is the retreat of the coastal line. Distance of seaward sediment transfer is directly relates to the height of large waves near the shore.

Influence of Underwater Bar Location on Cross-Shore Sediment Transport in the Coastal Zone

The effect of the underwater bar position on a sandy beach profile was studied on a timescale of one storm, using the XBeach numerical model. The largest shoreline regress occurred in the first hour of storm. For the chosen wave regime an underwater profile close to the theoretical Dean’s equilibrium profile is formed after 6 h. The position of the underwater bar affects the shoreline retreat rate. The lowest shore retreat occurs when the bar crest is located at a distance equal to 0.70–0.82 of the deep-water wavelength, corresponding to the period of the wave spectrum peak. The maximal shoreline retreat occurs when the bar is located at a distance that is close to a half wavelength. The shoreline recession depends on the heights of low-frequency waves. The smaller the mean wave period and the higher low-frequency waves’ height near the coast, the smaller the retreat of the shoreline. The distance of seaward sediment transfer is directly proportional to the significant wave height near shore.