The land–sea coastal border: a quantitative definition by considering the wind and wave conditions in a wave-dominated, micro-tidal environment

A quantitative definition for the land–sea (coastal) transitional area is proposed here for wave-driven areas, based on the variability and isotropy of met-ocean processes. Wind velocity and significant wave height fields are examined for geostatistical anisotropy along four cross-shore transects on the Catalan coast (north-western Mediterranean), illustrating a case of significant changes along the shelf. The variation in the geostatistical anisotropy as a function of distance from the coast and water depth has been analysed through heat maps and scatter plots. The results show how the anisotropy of wind velocity and significant wave height decrease towards the offshore region, suggesting an objective definition for the coastal fringe width. The more viable estimator turns out to be the distance at which the significant wave height anisotropy is equal to the 90th percentile of variance in the anisotropies within a 100 km distance from the coast. Such a definition, when applied to the Spanish Mediterranean coast, determines a fringe width of 2–4 km. Regarding the probabilistic characterization, the inverse of wind velocity anisotropy can be fitted to a log-normal distribution function, while the significant wave height anisotropy can be fitted to a log-logistic distribution function. The joint probability structure of the two anisotropies can be best described by a Gaussian copula, where the dependence parameter denotes a mild to moderate dependence between both anisotropies, reflecting a certain decoupling between wind velocity and significant wave height near the coast. This wind–wave dependence remains stronger in the central baylike part of the study area, where the wave field is being more actively generated by the overlaying wind. Such a pattern controls the spatial variation in the coastal fringe width.Peer ReviewedPostprint (published version

Waves in the Red Sea : response to monsoonal and mountain gap winds

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Continental Shelf Research 65 (2013): 1-13, doi:10.1016/j.csr.2013.05.017.An unstructured grid, phase-averaged wave model forced with winds from a high resolution atmospheric model is used to evaluate wind wave conditions in the Red Sea over an approximately 2-year period. The Red Sea lies in a narrow rift valley, and the steep topography surrounding the basin steers the dominant wind patterns and consequently the wave climate. At large scales, the model results indicated that the primary seasonal variability in waves was due to the monsoonal wind reversal. During the winter, monsoon winds from the southeast generated waves with mean significant wave heights in excess of 2 m and mean periods of 8 s in the southern Red Sea, while in the northern part of the basin waves were smaller, shorter period, and from northwest. The zone of convergence of winds and waves typically occurred around 19-20˚N, but the location varied between 15 to 21.5˚N. During the summer, waves were generally smaller and from the northwest over most of the basin. While the seasonal winds oriented along the axis of the Red Sea drove much of the variability in the waves, the maximum wave heights in the simulations were not due to the monsoonal winds but instead were generated by localized mountain wind jets oriented across the basin (roughly east-west). During the summer, a mountain wind jet from the Tokar Gap enhanced the waves in the region of 18 and 20˚N, with monthly mean wave heights exceeding 2 m and maximum wave heights of 14 m during a period when the rest of the Red Sea was relatively calm. Smaller mountain gap wind jets along the northeast coast created large waves during the fall and winter, with a series of jets providing a dominant source of wave energy during these periods. Evaluation of the wave model results against observations from a buoy and satellites found that the spatial resolution of the wind model significantly affected the quality of the wave model results. Wind forcing from a 10-km grid produced higher skills for waves than winds from a 30-km grid, largely due to under-prediction of the mean wind speed and wave height with the coarser grid. The 30-km grid did not resolve the mountain gap wind jets, and thus predicted lower wave heights in the central Red Sea during the summer and along the northeast coast in the winter.This research is based on work supported by Award No. USA00001, USA00002, KSA00011, made by the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia

Validation of coastal forecasts

Deliverable D2.3 del Proyecto MyWave: A pan-European concerted and integrated approach to operational wave modelling and forecasting – a complement to GMES MyOcean services. Work programme topic: SPA.2011.1.5.03 – R&D to enhance future GMES applications in the Marine and Atmosphere areas.Funded under: FP7-SPACE-2011-284455

Semi-empirical dissipation source functions for ocean waves: Part I, definition, calibration and validation

New parameterizations for the spectra dissipation of wind-generated waves are proposed. The rates of dissipation have no predetermined spectral shapes and are functions of the wave spectrum and wind speed and direction, in a way consistent with observation of wave breaking and swell dissipation properties. Namely, the swell dissipation is nonlinear and proportional to the swell steepness, and dissipation due to wave breaking is non-zero only when a non-dimensional spectrum exceeds the threshold at which waves are observed to start breaking. An additional source of short wave dissipation due to long wave breaking is introduced to represent the dissipation of short waves due to longer breaking waves. Several degrees of freedom are introduced in the wave breaking and the wind-wave generation term of Janssen (J. Phys. Oceanogr. 1991). These parameterizations are combined and calibrated with the Discrete Interaction Approximation of Hasselmann et al. (J. Phys. Oceangr. 1985) for the nonlinear interactions. Parameters are adjusted to reproduce observed shapes of directional wave spectra, and the variability of spectral moments with wind speed and wave height. The wave energy balance is verified in a wide range of conditions and scales, from gentle swells to major hurricanes, from the global ocean to coastal settings. Wave height, peak and mean periods, and spectral data are validated using in situ and remote sensing data. Some systematic defects are still present, but the parameterizations yield the best overall results to date. Perspectives for further improvement are also given.Comment: revised version for Journal of Physical Oceanograph

A changing wave climate in the Mediterranean Sea during 58-years using UERRA-MESCAN-SURFEX high-resolution wind fields

This study unravels 58-years (1961-2018) of wind-waves in the Mediterranean Sea (MS). A wave dataset was developed using the wave model WAVEWATCH III forced with the high-resolution (5.5 km) UERRA-MESCAN-SURFEX downscaled wind fields which better contain the imprint of the local geomorphology compared to other, coarser wind datasets used in previous studies. Thus, improving the reliability and characterization of the wind-wave climate in the basin. Validation results revealed a higher performance than previous datasets, particularly on the wave direction (θm), with a bias<1º. Climate variability at seasonal and interannual scales, wind-seas and swells distribution, and long-term trends in storminess and in the mean and extreme regimes were analysed. Results show a slight swell influence over the wind-sea in the hourly spectra at a large portion of the basin, excluding the wave generation areas. We detected that the western MS is the most storminess region with an average of three storms/year. Moreover, the anomalies of the seasonal mean wave direction relative to θm are large (~60º), with opposing behaviours between the winter and summer. Finally, the long-term trends in the mean and extreme conditions and in storminess are mild with values reaching 6 cm/decade and less than 2% in the absolute value, respectively

Evaluation of wave retrieval for Chinese Gaofen-3 synthetic aperture radar

The goal of this study was to investigate the performance of a spectral-transformation wave retrieval algorithm and confirm the accuracy of wave retrieval from C-band Chinese Gaofen-3 (GF-3) Synthetic Aperture Radar (SAR) images. More than 200 GF-3 SAR images of the coastal China Sea and the Japan Sea for dates from January to July 2020 were acquired in the Quad-Polarization Strip (QPS) mode. The images had a swath of 30 km and a spatial resolution of 8 m pixel size. They were processed to retrieve Significant Wave Height (SWH), which is simulated from a numerical wave model called Simulating WAves Nearshore (SWAN). The first-guess spectrum is essential to the accuracy of Synthetic Aperture Radar (SAR) wave spectrum retrieval. Therefore, we proposed a wave retrieval scheme combining the theocratic-based Max Planck Institute Algorithm (MPI), a Semi-Parametric Retrieval Algorithm (SPRA), and the Parameterized First-guess Spectrum Method (PFSM), in which a full wave-number spectrum and a non-empirical ocean spectrum proposed by Elfouhaily are applied. The PFSM can be driven using the wind speed without calculating the dominant wave phase speed. Wind speeds were retrieved using a Vertical-Vertical (VV) polarized geophysical model function C-SARMOD2. The proposed algorithm was implemented for all collected SAR images. A comparison of SAR-derived wind speeds with European Center for Medium-Range Weather Forecasts (ECMWF) ERA-5 data showed a 1.95 m/s Root-Mean-Squared Error (RMSE). The comparison of retrieved SWH with SWAN-simulated results demonstrated a 0.47 m RMSE, which is less than the 0.68 m RMSE of SWH when using the PFSM algorithm.Output Status: Forthcoming/Available Onlin

High-resolution wave forecasting : the Catalan coast case : modelling, coupling and validation

It is widely known that wind and wave predictions in the nearshore are less precise for semi enclosed domains than in the open ocean. The Catalan coast is a clear example of this situation, with a wave climate controlled by short fetches, complex bathymetry, high wind field variability in time and space, and sea and swell waves combined that generate bimodal spectra. These characteristics, typical for a semi-enclosed basin, limit the reliability of wave predictions in the area, with errors on the significant wave height around 10% and a clear under-prediction of the wave period with errors around 30%. The motivation of this work is to improve the actual wave forecasting abilities for the Catalan Coast using the SWAN v.4091 wave model. In order to achieve this goal, three working lines are considered: (1)adapting the model to the Catalan coast conditions, tuning the wave growth rates included in the model to better reproduce the observed values, (2) evaluate the effect of the currents and wind into the wave field by using a coupled system and (3) consider the use of unstructured grids as an alternative to the traditionally nested systems in order to obtain high resolution wave forecasts in coastal areas reducing the computational time and avoiding the use of internal boundary conditions with their associated errors. The results obtained support previous studies where the limited ability of the models to reproduce wave growth rates in young seas have been detected. The whitecapping term correction proposed in this document helps reducing under-prediction of the wave period observed with almost no effect on the significant wave height. This correction can be applied to similar environments. However, the proposed formulation is only suitable for the early stages of generation and should be discontinued after waves reach a certain maturity. Two coupling strategies are considered, a one-way coupling where current fields are directly introduced into the wave model, and a two-way coupling where the waves, currents and winds models run in parallel. The effects of the coupling are evaluated during calm periods but also during energetic events. The results show that during calm conditions the coupling does hardly improve the results while during energetic events, such as superficial currents intensifications or wind jet events, the coupling has greater importance. However, the two-way coupling has extremely high computational requirements, not always available. In this sense, the use of unstructured grids as an alternative to the traditional nested systems is presented. The main benefit of unstructured grids is that allows working with a single grid with different resolutions in each sub-domain, improving the resolution in coastal areas. Other advantage is the capacity to better reproduce the sharp coastline and the areas around the islands. The design of unstructured grids has been shown as one of the most delicate parts of this methodology, requiring special attention for the grid generation criteria. The validation of the results, performed with buoy measurements in the nearshore but also for the entire domain with altimetry measurements, allows stating that unstructured grids perform correctly in the study area. Finally, the proposed work suitability for an operational forecasting system has been considered. The whitecapping term modification is proven to be decisive in the quality of the wave forecast, while the coupling is not always recommended depending on computational capabilities. The use of unstructured grids with a regional triangular mesh covering the entire Western Mediterranean sea is considered as the first option, providing accurate high resolution wave conditions near the coast with a clear reduction of the computational time in comparison with a traditional nested system.És sobradament conegut que les prediccions d'onatge i vent a prop de la costa són menys precises en regions semi tancades que en mar obert. La costa Catalana és un clar exemple d'aquesta situació, amb un clima d'onatge controlat per fetch curts, batimetries complexes, camps de vent fortament variables tant en el temps com en l'espai, i combinacions de mar de fons i de vent que generen espectres bimodals. Aquestes característiques, típiques de dominis semi tancats, limiten la precisió de les prediccions d'onatge, obtenint errors de l'alçada d’ona significant sobre el 10% i una clara subpredicció del període d'ona amb errors al voltant del 30%. La motivació d'aquesta treball és doncs millorar la capacitat de predicció d'onatge actual per la costa Catalana utilitzant el model d’onatge SWAN v.4091. Per tal d'assolir aquest objectiu, es consideren tres línies de treball: (1) adaptar el model a les condicions de la costa Catalana, calibrant les corbes de creixement d'onatge per que reprodueixin millor la realitat, (2) examinar l'efecte de les corrents i el vent sobre l'onatge utilitzant sistemes acoplats i (3)considerar l'ús de malles no estructurades com a alternativa a sistemes aniuats tradicionals per tal d'obtenir prediccions d'onatge d'alta resolució en zones costeres reduint el temps de càlcul i evitant les condicions de contorn i els errors associats. Els resultats obtinguts concorden amb estudis previs en els quals la incapacitat dels models per reproduir correctament les corbes de creixement de l'onatge havia estat ja detectada. La proposta de modificació del terme de whitecapping presentada en aquest document ajuda a reduir la subpredicció del període d’ona sense gairebé cap efecte en l'alçada d'ona. Aquesta correcció es aplicable a entorns similars. Tan mateix, la formulació proposada és només vàlida en els primers estats de generació d'onatge, i hauria de ser substituïda quan les ones adquireixen certa maduresa. Es consideren dues estratègies d'acoplament, un acoplament one-way en el que el camp de corrents s'introdueix directament en el mode d'onatge, i un acoplament two-way en el que models d'onatge, corrents i vent corrent paral·lelament. Els efectes de l'acoplament son avaluats durant períodes de calma i episodis més energètics. Els resultats obtinguts mostren que durant períodes de calma l'acoplament aporta ben poc, mentre que durant episodis energètics tals com intensificacions de corrents o vents canalitzats presenta més importància. Finalment cal tenir en compte que l'acoplament two-way presenta uns requeriments computacionals no sempre disponibles. En aquest sentit es proposa l’ús de malles no estructurades com alternativa al mètode tradicional de malles aniuades. El principal avantatge de les malles no estructurades es que permeten treballar amb una única malla que té diferents resolucions segons el subdomini, millorant així la resolució en zones costeres. Un altre avantatge es la capacitat de reproduir millor la línia de costa o les zones al voltant de illes. Una de les parts més delicades de tot el procés consisteix en el disseny de les malles, on s’ha de prestar especial atenció en els criteris considerats. La validació dels resultats, realitzada amb mesures de boies en zones costeres i dades de satèl·lit per mar obert, ens permeten afirmar que les malles no estructurades funcionen correctament a la zona d’estudi. Finalment, es considera l’adequació de les diferents propostes per a un sistema de predicció operacional. Queda demostrat que la modificació del terme de whitecapping millora decisivament la qualitat de les prediccions, mentre que l’acoplament es recomana en funció de la capacitat de càlcul disponible. L’ús de malles no estructurades per a tot el Mediterrani Occidental es considera com la primera opció, obtenint així onatge d’alta resolució en zones costaneres reduint considerablement el temps de càlcul en comparació amb el sistema aniuat tradicional.Postprint (published version

Analysis of waves observed by synthetic aperture radar across ocean fronts

In this study, synthetic aperture radar (SAR) imaging of waves across ocean fronts was investigated using C-band Sentinel-1 VV-polarized SAR imagery collected over the Yangtze and the Zhujiang estuaries. The presence of ocean fronts in the study area was confirmed by collocated sea surface temperature (SST) data provided by the Advanced Very High Resolution Radiometer (AVHRR) and sea surface current information from the National Ocean Partnership Program (NOPP) based on the HYbrid Coordinate Ocean Model (HYCOM). The experimental results revealed that as the current speed increased, the cut-off wavelength (λc) increased as well. The effect of the increasing azimuth cut-off wavelength, however, was relatively weak in terms of variations of the normalized radar cross-section (NRCS), i.e., it was within 2 dB for λc ≤ 60 m. Hence, it was weaker than the NRCS variation related to SST. Larger NRCS variations (i.e., within 5 dB) occurred for λc values up to 120 m. In addition, the experimental results also demonstrated that the parameterized first-guess spectrum method (PFSM) wave retrieval performance was affected by ocean fronts. In particular, overestimations occurred when ocean fronts were present and λc was < 100