Evaluation of two spectral wave models for wave hindcasting in the Mackenzie Delta (original) (raw)
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The performance of two methods for quantifying whitecapping dissipation incorporated in the SWAN wave model is evaluated for waves generated along and off the U.S. East Coast under energetic winter storms with a predominantly westerly wind. Parameterizing the whitecapping effect can be done using the Komen-type schemes, which are based on mean spectral parameters, or the saturation-based (SB) approach of van der Westhuysen (2007), which is based on local wave parameters and the saturation level concept of the wave spectrum (we use "Komen" and 15 "Westhuysen" to denote these two approaches). Observations of wave parameters and frequency spectra at four NDBC buoys are used to evaluate simulation results. Model-data comparisons show that when using the default parameters in SWAN, both Komen and Westhuysen methods underestimate wave height. Simulations of mean wave period using the Komen method agree with observations, but those using the Westhuysen method are substantially lower. Examination of source terms shows that the Westhuysen method underestimates the total energy transferred into the 20 wave action equations, especially in the lower frequency bands that contain higher spectral energy. Several causes for this underestimation are identified. The primary reason is the difference between the wave growth conditions along the East Coast during winter storms and the conditions used for the original whitecapping formula calibration. In addition, some deficiencies in simulation results are caused along the coast by the "slanting fetch" effect that adds low-frequency components to the 2-D wave spectra. These components cannot be simulated partly or entirely by 25 available wind input formulations. Further, the effect of boundary layer instability that is not considered in the Komen and Westhuysen whitecapping wind input formulas may cause additional underestimation. 65 parameters as incorporated in SWAN performed slightly better in the simulation of wave height, period, and frequency spectra than the SB method. This conclusion contradicts the results of previous studies. Although van Velder et al.
Ocean Modelling, 2021
Short waves are of key importance for nearshore dynamics, particularly under storms, where they contribute to extreme water levels and drive large morphological changes. Therefore, it is crucial to model accurately the propagation and dissipation of storm waves in the nearshore area. In this paper, field observations collected in contrasted environments and conditions are combined with predictions from a third-generation spectral wave model to evaluate four formulations of wave energy dissipation by depth-induced breaking. The results reveal a substantial over-dissipation of incident wave energy occurring over the continental shelf, resulting in a negative bias on significant wave height reaching up to 50%. To overcome this problem, a breaking coefficient dependent of the local bottom slope is introduced within depth-induced breaking models in order to account for the varying degrees of saturation naturally found in breaking and broken waves. This approach strongly reduces the negative bias observed in the shoreface compared to default parameterizations, yielding significant improvements in the prediction of storm waves. Among the implications of this study, our new parameterization of the breaking coefficient results in systematically increased predictions of the wave setup near the shoreline compared to the default parameterization. This increase reaches a factor 2 for gently sloping beaches.
Shelf Waves in the Beaufort Sea in a High-Resolution Ocean Model
Oceanology, 2018
The article examines low-frequency oscillations of sea level over the Beaufort Sea shelf simulated in a high-resolution regional configuration of the Massachusetts Institute of Technology general circulation model during 2007-2009 time interval. Using wavelet analysis, we obtained the spatiotemporal characteristics of shelf waves. Dispersion analysis revealed that these are freely propagating shelf waves, which represent one of the relaxation mechanisms of the Beaufort Sea water system, disturbed from a state of equilibrium by external forces, e.g., wind, upwelling, or atmospheric pressure gradients. These waves have periods of 7, 15, 27, and 75 days; wavelengths of 1510, 1300, 1400, and 550 km; and phase speeds of 2.5, 1.0, 0.6, 0.25, and 0.08 m/s. It is demonstrated that these waves can be generated by wind action north of Cape Barrow, whereupon they propagate eastward along the shelf.
Improvements in spectral wave modeling in tidal inlet seas
Journal of Geophysical Research: Oceans, 2012
The performance of the spectral wind wave model SWAN in tidal inlet seas was assessed on the basis of extensive wave measurements conducted in the Amelander Zeegat tidal inlet and the Dutch Eastern Wadden Sea, as well as relevant data from other inlets, lakes, estuaries and beaches. We found that the 2006 default SWAN model (version 40.51), the starting point of the investigation, performed reasonably well for measured storm conditions, but three aspects required further attention. First, over the near‐horizontal tidal flats, the computed ratio of integral wave height over water depth showed an apparent upper limit using the default depth‐limited wave breaking formulation and breaker parameter, resulting in an underprediction of wave heights. This problem has been largely solved using a new breaker formulation. The second aspect concerns wave‐current interaction, specifically the wave age effect on waves generated in ambient current, and a proposed enhanced dissipation in negative c...
SPECTRAL WAVE MODELLING IN TIDAL INLET SEAS: RESULTS FROM THE SBW WADDEN SEA PROJECT
Coastal Engineering Proceedings, 2011
Over the last five years a research program has been carried out to assess the performance of the spectral wave model SWAN in the Wadden Sea so that it may be used for the transformation of offshore wave conditions to wave boundary conditions near the sea defenses (dikes and dunes). The assessment was done on the basis of extensive wave measurements conducted in Ameland inlet and the Dutch Eastern Wadden Sea, as well as relevant data from lakes and estuaries. After a first round of assessment, we found that SWAN performed reasonably well for storm conditions but three aspects required further attention. Firstly, over the tidal flats the computed ratio of integral wave height over water depth showed an apparent upper limit using the conventional Battjes and Janssen (1978) depth-limited wave breaking formulation, because the wave growth over finite depth is hampered by the present formulation of depth-induced wave breaking. The problem has been solved using a new breaker formulation. Secondly, focusing on the main channel, SWAN formulations needed to be modified in order to eliminate overprediction of the significant wave height in opposing currents. Thirdly, the primary spectral peak of North Sea waves penetrating into the inlet was underpredicted. Best results were obtained when the refraction of low-frequency waves was limited and the bottom friction coefficient was set at a lower value than the current default for wind seas. All these improvements have lead to a wave transformation model with which reliable wave conditions in the Wadden Sea and related complex areas can be determined.
Application of two numerical models for wave hindcasting in Lake Erie
Applied Ocean Research, 2007
Wave characteristics are one of the most important factors in design of coastal and marine structures. Therefore, an accurate prediction of wave parameters is considerably important. In this paper, SWAN and MIKE 21 SW third generation spectral models have been used for the prediction of wave parameters. The field data set of Lake Erie has been used for testing the performance of the models. Significant wave height (H s ), peak spectral period (T p ) and mean wave direction were hindcasted in the study. Both models were forced by temporally varying wind. The results show that the average scatter index of SWAN is about 16% for H s and 19% for T p ; while the average scatter index of MIKE 21 SW is about 20% and 13% for H s and T p , respectively. The inconsistency between the results of the models was found to be due to differences between the wind input parameterizations. Using Komen's formulation for the wind input led to a more accurate prediction of H s rather than using Janssen's formulation for the wind input. It was also found that using the cumulative steepness method for whitecapping dissipation in SWAN model yields a less accurate estimation of H s and a more accurate estimation of T p . By using this method, the average scatter index increased about 7% for H s prediction and decreased more than 6% for T p prediction. In addition, the computational time required for cumulative steepness method is more than 2 times of Komen's option. Both models were also evaluated for the prediction of wave direction and it was found that MIKE 21 SW results are slightly more accurate than those of SWAN.
Wave climate evaluation in the Gulf of St. Lawrence with a parametric wave model
A new wind preprocessing algorithm allows the forcing of a parametric wave model (GENER) with offshore wind data from atmospheric models. MERRA offshore winds for the Gulf of St. Lawrence (GSL) are adjusted from comparisons with wind measurements before performing long-term (1979-2012) wave hindcasts on the GSL using GENER. Comparisons of simulated wave distributions with wave measurements on the GSL show a reasonable fit. Executing GENER on a regular grid allowed an evaluation of the regional deep water wave climate for the GSL with minimal computing time. Extreme wave heights are estimated with the generalized Pareto distribution and a regional extreme waves atlas is produced. The same approach could also be used to evaluate the future wave climate in the GSL by forcing GENER with winds from different scenarios of climate models.
Validation of a thirty year wave hindcast using the Climate Forecast System Reanalysis winds
Ocean Modelling, 2013
A thirty one year wave hindcast (1979-2009) using NCEP's latest high resolution Climate Forecast System Reanalysis (CFSR) wind and ice database has been developed and is presented here. The hindcast has been generated using the third generation wind wave model WAVEWATCH III Ò with a mosaic of 16 two-way nested grids. The resolution of the grids ranged from 1/2°to 1/15°. Validation results for bulk significant wave height H s and 10 m (above Mean Sea Level) wind speeds U 10 have been presented using both altimeter records and NDBC buoys. In general the database does a good job of representing the wave climate. At most buoys there is excellent agreement between model and data out to the 99.9th percentile. The agreement at coastal buoys is not as good as the offshore buoys due to unresolved coastal features (topographic/bathymetric) as well as issues related to interpolating wind fields at the land-sea margins. There are some concerns about the wave climate in the Southern Hemisphere due to the over prediction of winds (early part of the database) as well as the lack of wave blocking due to icebergs (in the model).
Dissipation of wind waves by pancake and frazil ice in the autumn Beaufort Sea
Journal of Geophysical Research: Oceans
A model for wind-generated surface gravity waves, WAVEWATCH III V R , is used to analyze and interpret buoy measurements of wave spectra. The model is applied to a hindcast of a wave event in sea ice in the western Arctic, 11-14 October 2015, for which extensive buoy and ship-borne measurements were made during a research cruise. The model, which uses a viscoelastic parameterization to represent the impact of sea ice on the waves, is found to have good skill-after calibration of the effective viscosity-for prediction of total energy, but over-predicts dissipation of high frequency energy by the sea ice. This shortcoming motivates detailed analysis of the apparent dissipation rate. A new inversion method is applied to yield, for each buoy spectrum, the inferred dissipation rate as a function of wave frequency. For 102 of the measured wave spectra, visual observations of the sea ice were available from buoy-mounted cameras, and ice categories (primarily for varying forms of pancake and frazil ice) are assigned to each based on the photographs. When comparing the inversion-derived dissipation profiles against the independently derived ice categories, there is remarkable correspondence, with clear sorting of dissipation profiles into groups of similar ice type. These profiles are largely monotonic: they do not exhibit the ''roll-over'' that has been found at high frequencies in some previous observational studies.