Wave modelling - the state of the art

This paper is the product of the wave modelling community and it tries to make a picture of the present situation in this branch of science, exploring the previous and the most recent results and looking ahead towards the solution of the problems we presently face. Both theory and applications are considered. The many faces of the subject imply separate discussions. This is reflected into the single sections, seven of them, each dealing with a specific topic, the whole providing a broad and solid overview of the present state of the art. After an introduction framing the problem and the approach we followed, we deal in sequence with the following subjects: (Section) 2, generation by wind; 3, nonlinear interactions in deep water; 4, white-capping dissipation; 5, nonlinear interactions in shallow water; 6, dissipation at the sea bottom; 7, wave propagation; 8, numerics. The two final sections, 9 and 10, summarize the present situation from a general point of view and try to look at the future developments

Excitation force estimation and forecasting for wave energy applications

The implementation of the majority of energy maximising control strategies requires the knowledge of the wave excitation force experienced by the wave energy converter (WEC). In addition, many optimal numerical control strategies also require future knowledge, or a forecast, of future values of the excitation force. This paper examines both the excitation force estimation and forecasting problem for a heaving buoy wave energy device. In particular, a Kalman filter is used to estimate excitation force, where the wave force model is comprised of a set of oscillators at discrete frequencies. The forecasting algorithm consists of an autoregressive model, where the value of prefiltering, in terms of forecasting performance, is evaluated. The paper provides a level of sensitivity analysis of the estimation and forecasting performance to variations in sampling period, sea spectral shape factor and prediction horizon. Results demonstrate that the achievable performance of the estimator/forecaster is consistent with the broad requirements of numerical optimal WEC control strategies (Fusco and Ringwood (2012)), which depends on the characteristics of the radiation impulse response

Wind and Wave Extremes over the World Oceans from Very Large Ensembles

Global return values of marine wind speed and significant wave height are estimated from very large aggregates of archived ensemble forecasts at +240-h lead time. Long lead time ensures that the forecasts represent independent draws from the model climate. Compared with ERA-Interim, a reanalysis, the ensemble yields higher return estimates for both wind speed and significant wave height. Confidence intervals are much tighter due to the large size of the dataset. The period (9 yrs) is short enough to be considered stationary even with climate change. Furthermore, the ensemble is large enough for non-parametric 100-yr return estimates to be made from order statistics. These direct return estimates compare well with extreme value estimates outside areas with tropical cyclones. Like any method employing modeled fields, it is sensitive to tail biases in the numerical model, but we find that the biases are moderate outside areas with tropical cyclones.Comment: 28 pages, 16 figure

Comparison of sea-ice freeboard distributions from aircraft data and cryosat-2

The only remote sensing technique capable of obtain- ing sea-ice thickness on basin-scale are satellite altime- ter missions, such as the 2010 launched CryoSat-2. It is equipped with a Ku-Band radar altimeter, which mea- sures the height of the ice surface above the sea level. This method requires highly accurate range measure- ments. During the CryoSat Validation Experiment (Cry- oVEx) 2011 in the Lincoln Sea, Cryosat-2 underpasses were accomplished with two aircraft, which carried an airborne laser-scanner, a radar altimeter and an electro- magnetic induction device for direct sea-ice thickness re- trieval. Both aircraft flew in close formation at the same time of a CryoSat-2 overpass. This is a study about the comparison of the sea-ice freeboard and thickness dis- tribution of airborne validation and CryoSat-2 measure- ments within the multi-year sea-ice region of the Lincoln Sea in spring, with respect to the penetration of the Ku- Band signal into the snow

Operational tsunami modelling with TsunAWI – recent developments and applications

In this article, the tsunami model TsunAWI (Alfred Wegener Institute) and its application for hindcasts, inundation studies, and the operation of the tsunami scenario repository for the Indonesian tsunami early warning system are presented. TsunAWI was developed in the framework of the German-Indonesian Tsunami Early Warning System (GITEWS) and simulates all stages of a tsunami from the origin and the propagation in the ocean to the arrival at the coast and the inundation on land. It solves the non-linear shallow water equations on an unstructured finite element grid that allows to change the resolution seamlessly between a coarse grid in the deep ocean and a fine representation of coastal structures. During the GITEWS project and the following maintenance phase, TsunAWI and a framework of pre- and postprocessing routines was developed step by step to provide fast computation of enhanced model physics and to deliver high quality results

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

Compendium of meteorology scientific issues of 1950 still outstanding

The Compendium of Meteorology was published in 1951 by the American Meteorological Society. A review was made of the Compendium of Meteorology to identify the studies and future needs which the authors expressed in their papers. The needs as seen by the authors are organized into sections and papers following the format of the Compendium of Meteorology. In some cases the needs they identified are as valid today as they were in 1951. In other cases one will easily be able to identify examples where significant progress has been made. It is left to the individual scientists and scientific program managers to assess whether significant progress has been made over the past thirty-five years on these outstanding scientific issues