Extreme wave events and sampling variability

Bitner‐Gregersen, Elzbieta M.; Gramstad, Odin; Magnusson, Anne Karin; Malila, Mika

doi:10.1007/s10236-020-01422-z

Cited by 11 publications

(7 citation statements)

References 47 publications

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“…The colocated data show a large scatter, as can be expected from 20-min records due to high sampling variability (e.g. Bitner-Gregersen et al (2021)).…”

Section: Equivalent Significant Wave Height Within Spectral Bandsmentioning

confidence: 76%

“…Variability in sea state over space and time is to be expected (Bitner-Gregersen and Magnusson 2014;Bitner-Gregersen et al 2021), but the differences seen above indicate statistics of values as measured by the sensors are going to be different. To demonstrate and quantify the differences in different parts of the frequency range of surface waves, we compare 'equivalent significant wave heights' (H s,eq ) in given frequency bands, defined by:…”

Section: Equivalent Significant Wave Height Within Spectral Bandsmentioning

confidence: 99%

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Spectral shapes and parameters from three different wave sensors

2021

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The quality of wave measurements is of primary importance for the validation of wave forecasting models, satellite wave calibration and validation, wave physics, offshore operations and design and climate monitoring. Validation of global wave forecasts revealed significant regional differences, which were linked to the different wave buoy systems used by different countries. To fully understand the differences between the wave measurement systems, it is necessary to go beyond investigations of the integral wave parameters height, period and direction, into the frequency spectra and the four directional Fourier parameters that are used to estimate the directional distribution. We here analyse wave data measured from three different sensors (non-directional Datawell Waverider buoy, WaveRadar Rex, Optech laser) operating at the Ekofisk oil production platform located in the central North Sea over a period of several months, with significant wave height ranging from 1 to 10 m. In general, all three sensors provide similar measurements of the integral wave properties and frequency spectra, although there are some significant differences which could impact design and operations, forecast verification and climate monitoring. For example, the radar underestimates energy in frequency bands higher than 8 s by 3–5%, swell (12.5–16 s) by 5–13%, while the laser has 1–2% more energy than the Waverider in the most energetic bands. Lee effects of structures are also estimated. Lower energy at the frequency tail with the radar has an effect on wave periods (they are higher); wave steepness is seen to be reduced by 10% in the wind seas. Goda peakedness and the unidirectional Benjamin-Feir index are also examined for the three sensors.

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“…The colocated data show a large scatter, as can be expected from 20-min records due to high sampling variability (e.g. Bitner-Gregersen et al (2021)).…”

Section: Equivalent Significant Wave Height Within Spectral Bandsmentioning

confidence: 76%

Section: Equivalent Significant Wave Height Within Spectral Bandsmentioning

confidence: 99%

Spectral shapes and parameters from three different wave sensors

2021

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“…Knowledge about the statistics of waves, in particular extreme waves, is required for the design and operation of offshore structures. Bitner-Gregersen et al ( 2021 ) describe the challenges for the description of rogue-prone sea states using field measurements and for engineering applications. In particular, they note that wavefield data are affected not only by the accuracy of the instruments recording them but also by sampling variability, an uncertainty due to the limited number of observations, which may dominate over the nonlinear effects.…”

Section: Summary Of Papersmentioning

confidence: 99%

The 2nd international workshop on waves, storm surges and coastal hazards incorporating the 16th international workshop on wave hindcasting and forecasting

et al. 2021

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“…For instance, even the most accurate instrument for spatio-temporal sea surface elevation fields presently available, namely the stereo system, can collect a limited number of space-time data due to the limited camera footprint, compared to the length of very high waves, and the limited duration of the recording, which often cannot be extended beyond the usual 20-30 minutes due to the required stationarity of the measured sea states. This limitation may prove to be challenging for the analysis of the nonlinearities at play because the sampling variability may dominate over such effects (Bitner-Gregersen et al, 2021). Of course, the stereo system still represents a unique source of space-time data that has the advantage of capturing all the physical processes involved (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…A distinctive feature of the analysis carried out is the use of two different statistical approaches for the extreme-value analysis at short-term/range, namely the conventional time approach at a fixed point, which may however be subject to sampling variability, and the space-time approach, where the statistics refer to a given sea surface area. Indeed, the latter approach has recently been envisaged as a more reliable approach for the analysis of extreme waves (Bitner-Gregersen et al, 2021;Kirezci et al, 2021) and it was shown that the statistical moments resulting from numerical simulations can vary significantly, depending on the size of the sea surface area considered for their computation (Kokina and Dias, 2020). Since the size of the area considered also affects the value of predicted extreme wave crest heights due to the different number of waves within such area and the short-crestedness of the sea state (Fedele, 2012), our analysis also traces such dependence.…”

Section: Introductionmentioning

confidence: 99%

Space-time statistics of extreme ocean waves in crossing sea states

Davison

Benetazzo²,

Barbariol³

et al. 2022

Front. Mar. Sci.

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The study of extreme ocean waves has gained considerable interest in recent years, due to their importance for offshore design and navigation safety, and several theoretical approaches have been developed for their statistical description. However, in the case of crossing seas, where two or more wave systems of different characteristics are present, a full understanding of the main physical mechanisms responsible for the occurrence of very high individual waves is still lacking. As a consequence, the prediction of extremes in such conditions currently relies on integrated parameters of the total sea state, such as the spectral wave steepness. In this study, to gain further insight into the role of the crossing wind sea and swell wave systems in producing extreme individual waves, we investigate realistic sea states during typhoon Kong-rey (2018) using an ensemble of numerical simulations obtained from a phase-resolving wave model based on the High-Order Spectral (HOS) method. The reliability of the numerical fields is assessed for the first time with stereo wave measurements of the sea surface elevation field collected from an offshore platform in the area of interest. We show that, in specific conditions, space-time extreme crest heights in crossing seas can be larger than in unimodal seas due to second-order bound wave interactions between the wind sea and the swell. To improve existing prediction capabilities, we propose a novel formulation for the wave steepness in crossing seas, which includes nonlinear effects up to the second order and accounts for the spectral parameters of the interacting wave systems.

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Extreme wave events and sampling variability

Cited by 11 publications

References 47 publications

Spectral shapes and parameters from three different wave sensors

Spectral shapes and parameters from three different wave sensors

The 2nd international workshop on waves, storm surges and coastal hazards incorporating the 16th international workshop on wave hindcasting and forecasting

Space-time statistics of extreme ocean waves in crossing sea states

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