A pinboard by
this curator

Sparrho Insights: 3-minute summaries of cutting-edge science based on peer-reviewed research


Oceanic rogue waves are real: New research helps understand their formation and how to forecast them

A stuff of legend, told by battered sailors. In 1996, the captain of the QE2 sailed through one ’as tall as the cliffs of Dover’. George Clooney’s character perished in another one in the Perfect Storm. Now we know that rogue waves are real and exciting new research helps understand their formation and frequency using quantum physics methods.


On Oceanic Rogue Waves

Abstract: We propose a new conceptual framework for the prediction of rogue waves and third-order space-time extremes of wind seas that relies on the Tayfun (1980) and Janssen (2009) models coupled with Adler-Taylor (2009) theory on the Euler characteristics of random fields. Extreme statistics of the Andrea rogue wave event are examined capitalizing on European Reanalysis (ERA)-interim data. A refinement of Janssen's (2003) theory suggests that in realistic oceanic seas characterized by short-crested multidirectional waves, homogeneous and Gaussian initial conditions become irrelevant as the wave field adjusts to a non-Gaussian state dominated by bound nonlinearities over time scales $t\gg t_{c}\approx0.13T_{0}/\nu\sigma_{\theta}$, where $T_{0}$, $\nu$ and $\sigma_{\theta}$ denote mean wave period, spectral bandwidth and angular spreading of dominant waves. For the Andrea storm, ERA-interim predictions yield $t_{c}/T_{0}\sim O(1)$ indicating that quasi-resonant interactions are negligible. Further, the mean maximum sea surface height expected over the Ekofisk platform's area is higher than that expected at a fixed point. However, both of these statistics underestimate the actual crest height $h_{obs}\sim1.63H_s$ observed at a point near the Ekofisk site, where $H_s$ is the significant wave height. To explain the nature of such extreme, we account for both skewness and kurtosis effects and consider the threshold $h_{q}$ exceeded with probability $q$ by the maximum surface height of a sea state over an area in time. We find that $h_{obs}$ nearly coincides with the threshold $h_{1/1000}\sim1.62H_s$ estimated at a point for a typical $3$-hour sea state, suggesting that the Andrea rogue wave is likely to be a rare occurrence in quasi-Gaussian seas.

Pub.: 09 Jul '15, Pinned: 15 May '17

Reduced order prediction of rare events in unidirectional nonlinear water waves

Abstract: We consider the problem of short-term prediction of rare, extreme water waves in unidirectional fields, a critical topic for ocean structures and naval operations. One possible mechanism for the occurrence of such rare, unusually-intense waves is nonlinear wave focusing. Recent results have demonstrated that random localizations of energy, induced by the dispersive mixing of different harmonics, can grow significantly due to localized nonlinear focusing. Here we show how the interplay between i) statistical properties captured through linear information such as the waves power spectrum and ii) nonlinear dynamical properties of focusing localized wave groups defines a critical length scale associated with the formation of extreme events. The energy that is locally concentrated over this length scale acts as the "trigger" of nonlinear focusing for wave groups and the formation of subsequent rare events. We use this property to develop inexpensive, short-term predictors of large water waves. Specifically, we show that by merely tracking the energy of the wave field over the critical length scale allows for the robust, inexpensive prediction of the location of intense waves with a prediction window of 25 wave periods. We demonstrate our results in numerical experiments of unidirectional water wave fields described by the Modified Nonlinear Schrodinger equation. The presented approach introduces a new paradigm for understanding and predicting intermittent and localized events in dynamical systems characterized by uncertainty and potentially strong nonlinear mechanisms.

Pub.: 20 Jan '15, Pinned: 16 May '17

The performance of some state-of-the-art wave energy converters in locations with the worldwide highest wave power

Abstract: The main objectives of the present work are to review the global wave energy resources according to the most recent datasets available, to identify the locations with the worldwide highest wave energy potential and to assess in those locations the performance of some state-of-the-art wave energy converters. For this purpose, 15 years of wave data provided by the European Centre for Medium-Range Weather Forecasts, covering the time interval 2000–2014, were considered, processed and analysed. After identifying the geographical regions with the highest wave power, 15 reference points, which were considered more relevant from the point of view of their wave energy potential, have been defined in each hemisphere (northern and southern, respectively). As a following step, corresponding to all of these reference points, the most relevant wave patterns have been identified, and this information was subsequently used to assess the expected power output of the wave energy converters considered. Some other relevant parameters, such as the capacity factor or the capture width, were evaluated as well. Following the results provided by this work, we can expect that most of the existent devices for harnessing wave energy would perform well near most of the coastal environments identified. Moreover, it also must be highlighted that in the future, wave energy farms can play a very active role from the point of view of coastal protection.

Pub.: 17 Nov '16, Pinned: 18 May '17