Directional Coherent Wave Group From an Assimilated Non-linear Wavefield

The presence of coherent wave groups in the ocean has been so far postulated but still lacks evidence other than the indication from the radar images. Here, we attempt to reconstruct a wave field to monitor the evolution of a directional wave group based on a phase resolving two-dimensional non-linear wave model constrained by the stereo images of the ocean surface. The reconstructed wave field of around 20 wavelength squared revealed a coherent wave group compact in both propagating and transverse directions. The envelope of the wave group seems to be oriented obliquely to the propagation direction, somewhat resembling the directional soliton that was theoretically predicted and experimentally and numerically reproduced recently. A comparison with a constrained linear wave model demonstrated the coherence of the non-linear wave group that propagates for tens of wavelengths. The study elaborates a possibility of a spatially coherent short crested wave group in the directional sea.

Coastal boulder fields and tsunami hazards of East Java, Indonesia

<p>Coastal boulder deposits are records of unusually powerful wave action events associated with either storms or tsunamis. Our 2016 paleotsunami survey of the southeastern Java coast led to the discovery of five coastal boulder fields near Pacitan, Indonesia, possibly dating to the mid-to-late 19th century or prior, and two similar fields at Pantai Papuma and Pantai Pasir Putih that were tsunami-emplaced during the 1994 7.9 Mw event in East Java. Both multiyear photogrammetry and hydrodynamic wave height reconstructions of the accumulations near Pacitan suggest the boulders were likely tsunami rather than storm-wave emplaced. We evaluate the boulders as an inverse problem, using reconstructed wave heights and ComMIT tsunami modelling to suggest a minimum 8.4 Mw earthquake necessary to dislodge and emplace the largest boulders near Pacitan assuming they were all deposited during a single seismic event and that the rupture source was located along the Java Trench, some 200 km south of Pacitan.</p>

Improvement of Differential Method for Estimating the Axial Intensity Derivative in the Transport-of-Intensity Equation

Phase retrieval is an important problem in many areas of physics while transport-of-intensity Equation (TIE) is a traditional method for recovering phases by intensity detection. Derivative of axial intensity is an important parameter in TIE and usually obtained by finite difference (FD) method. However, nonlinearity error is unavoidable in these FD methods. We suggest a method for eliminating this error by setting an axial smoothing filter, and the width of smoothing window varies with the parameters of the observation point. Simulation and experimental results of reconstructed wave-front phase with this filter are shown in this paper for demonstrating the performance of this technique.

Data Assimilation of the Stereo Reconstructed Wave Fields to a Nonlinear Phase Resolved Wave Model

A stereo camera system was installed facing Southeast at the observational tower owned by the University of Tokyo in the Sagami Bay, Japan. The three-dimensional wave fields were reconstructed from the stereo images, which were successfully captured from April 2017 until now, by using an open source software WASS (Waves Acquisition Stereo System). The significant wave heights and periods calculated from the stereo images covering an area of 80 m by 80 m were compared against those derived from the ultra-sonic wave gauge at the tower. Overall, a reasonable agreement is achieved, but the accuracy of the stereo reconstruction degrades with the distance from the camera. Also, the significant wave period derived from stereo imaging tends to be shorter, likely related to the error at high frequencies. The reconstructed wave field will be assimilated into a phase-resolved nonlinear wave model. The ensemble Higher Order Spectral simulations and the implementation of the a4DVAR data assimilation scheme, allowed us to substantially extend the estimated wave field beyond the stereo imaging domain. A field campaign with an ADCP in the field of view of the stereo camera and two surface wave buoys outside of the view were conducted for validation.

Vibration Analysis of Membrane by Time Average In-Line Digital Holographic Interferometry

In this paper, in-line digital holography is combined with time average method and applied for vibration analysis of an aluminum membrane. The numerically reconstructed wave from time average holograms contains information of mode shape and mean deformation of the vibrating objects. At lower amplitudes of vibration only the deformation fringes are observed and as the amplitude is increases, the Bessel fringes representing the time average vibration amplitude also become visible. Experimental results for a 10mm size membrane are presented.

Spatial Extreme Value Analysis of Nonlinear Simulations of Random Surface Waves

The paper first recalls the Slepian Model Representation and a theorem of V. I. Piterbarg as generic tools for analyzing the spatial characteristic of ocean waves. We then consider numerical simulations of random surface gravity waves carried out in space and time by means of the modified nonlinear Schro¨dinger equation. It is shown that the extreme waves in the simulations are steeper and more asymmetric than predicted by the Gaussian theory. Moreover, the reconstructed wave fields shows extreme crest heights well in excess of what is expected from the Gaussian theory.