Takuji Waseda 교수

Abstract The evolution of a random directional wave in deep water was studied in a laboratory wave tank (50 m long, 10 m wide, 5 m deep) utilizing a directional wave generator. A number of experiments were conducted, changing the various spectral parameters (wave steepness 0.05 < ɛ < 0.11, with directional spreading up to 36° and frequency bandwidth 0.2 < δk/k < 0.6). The wave evolution was studied by an array of wave wires distributed down the tank. As the spectral p

The long-term trend of extreme ocean waves in the emerging ice-free waters of the summer Arctic is studied using ERA-Interim wave reanalysis, with validation by two drifting wave buoys deployed in summer 2016. The 38-year-long reanalysis dataset reveals an increase in the expected largest significant wave height from 2.3 m to 3.1 m in the ice-free water from the Laptev to the Beaufort Seas during October. The trend is highly correlated with the expected increase in highest wind speed from 12.0 m

An experimental investigation on the initial instability of nonlinear deep-water wave trains including wind effects is reported. The experiment was conducted at the Ocean Engineering Laboratory wind-wave facility (50 m long, 4.2 m wide, 2.1 m deep), with a fully computer-controlled mechanical wave generator to explore the parameter space: steepness; sideband frequency; wind speed. The estimated growth rates of the Benjamin–Feir instability from seeded wind-free experiments agreed well with the t

The Kuroshio flows along the southern coast of Japan during its non‐large‐meandering state, then separates from the coast near the Kii Peninsula and attaches at the north of the Izu Ridge. The amplitude of the offshore displacement of the Kuroshio changes as a result of mesoscale perturbations. Satellite SSH and SST observations (TOPEX/Poseidon: T/P and NOAA AVHRR) have suggested that the short‐term Kuroshio meander formation is triggered by anticyclonic eddies originating in the Kuroshio Extens

[1] Wind and wave records obtained from the Kvitebjorn platform (2.3°E, 61.0°N, 190 m deep) in the northern North Sea from 2003 to 2005 were analyzed. Among the 2723 20-min records taken during storm conditions, 57 cases included freak waves exceeding twice the significant wave height. Comparisons between various wave parameters and the freak wave occurrence index did not show any significant correlation. Thus, the in situ wave record was used to select the days when relatively more or less frea

Presented are results from a numerical study of the interaction of a mesoscale eddy and the Kuroshio using a high‐resolution regional general circulation model (GCM). The distinct evolution of the mesoscale eddies is not new in the literature, but most of the previous studies were conducted with simpler model configurations. To our knowledge the present study is the first to use a high‐resolution GCM, proven to replicate a realistic ocean circulation. An anticyclonic eddy was injected to the sou

Point-positioning GPS-based wave measurements were conducted by deep ocean (over 5,000 m) surface buoys moored in the North West Pacific Ocean in 2009, 2012, and 2013. The observed surface elevation bears statistical characteristics of Gaussian, spectrally narrow ocean waves. The tail of the averaged spectrum follows the frequency to the power of −4 slope, and the significant wave height and period satisfies the Toba’s 3/2 law. The observations compare well with a numerical wave hindcast. Two la

The sea ice coverage in the summer Arctic Ocean from the Beaufort to Laptev Seas continues to decrease, and the largest waves in the western Arctic open waters are increasing year by year. By looking into the historical wave events in the ERA-Interim reanalysis data, we discovered that more than half of the extreme events are caused by cyclones that are originating in the Arctic Ocean as well as in the mid-latitudes. During September, when the ice-free water area becomes largest each year, the p

A series of experiments were conducted in a wave basin (50 m long, 10 m wide and 5 m deep) generating two waves propagating at an angle by a directional wavemaker. When the two waves were selected from a resonant triplet, an initially non-existing wave grew as the waves propagated down the tank. The linear growth rate of the resonating wave agreed well with third-order resonance theory based on Zakharov’s reduced gravity equation. Additional experiments with opposing and coflowing mean current w

Abstract Recent experimental study of the evolution of random directional gravity waves in deep water provides new insight into the nature of the spectral evolution of the ocean waves and the relative significance of resonant and quasi-resonant wave interaction. When the directional angle containing half the total energy is broader than ∼20°, the spectrum evolves following the energy transfer that can be described by the four-wave resonant interaction alone. In contrast, in the case of a directi

Abstract. The understanding of freak wave generation mechanisms has advanced and the community has reached a consensus that spectral geometry plays an important role. Numerous marine accident cases were studied and revealed that the narrowing of the directional spectrum is a good indicator of dangerous sea. However, the estimation of the directional spectrum depends on the performance of the third-generation wave model. In this work, a well-studied marine accident case in Japan in 1980 (Onomichi