論文 - 久木　幸治

Use of drifting buoys for wave observation: Effect of current on wave data

Hisaki Y.

Deep-Sea Research Part I: Oceanographic Research Papers ( Deep-Sea Research Part I: Oceanographic Research Papers )  195   2023年05月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Sea Level Variability in the Equatorial Malacca Strait: The Influence of Climatic–Oceanographic Factors and Its Implications for Tidal Properties in the Estuarine Zone

Ulung J. Wisha, Yusuf J. Wijaya, Yukiharu Hisaki

Climate ( Climate )  11 ( 3 )   2023年03月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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The Western Pacific North Equatorial Countercurrent Responses to Two Forms of El Niño during the Period 1978 to 2017

Yusuf Jati Wijaya, Ulung Jantama Wisha, Yukiharu Hisaki

Climate ( Climate )  10 ( 10 )   2022年10月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

This research aims to examine how the Western Pacific North equatorial countercurrent (NECC) flow reacts to two different forms of El Nino (EN) over a 40-year period. To establish the prevailing modes for each season, we implemented Empirical Orthogonal Function (EOF) analysis on the eastward current component of the Ocean Reanalysis System 5 (ORAS5) dataset. In comparison to the Central Pacific (CP) episode, the time series principal component of the first mode (PC1) demonstrated that the strongest NECC's magnitude often emerged during the development period (spring to fall) of the Eastern Pacific (EP) EN event. However, in episode CP 2002/2003, we witnessed an abnormal behavior in which the stronger NECC manifested. This was due to the emergence of a strong anomalous westerly wind, which differed from other CP events and forced the NECC's magnitude to be greater. When approaching the peak stage, on the other hand, the magnitude of the NECC during the CP episode was typically greater than that of the EP episode. The NECC's magnitude fell greatly in the second year of the EP episode, particularly during the spring season, since most EP episodes would transition into an La Nina (LN) event in the succeeding event. During the EP EN, it was found that the strength of the westerly wind had a bigger effect on the NECC than during the CP EN.

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Real-Time Properties of Hydraulic Jump off a Tidal Bore, Its Generation and Transport Mechanisms: A Case Study of the Kampar River Estuary, Indonesia

Ulung Jantama Wisha, Yusuf Jati Wijaya, Yukiharu Hisaki

Water (Switzerland) ( Water (Switzerland) )  14 ( 16 )   2022年08月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

Since the hydraulic jump off a tidal bore in the Kampar Estuary has never been well-described, real-time measurements of hydraulic jump properties are crucial to understanding the tidal bore characteristics. This study aims to determine the real-time properties of a tidal bore generation, hydraulic jump, and transport mechanism in the Kampar River estuary. Tidal harmonic and range are analyzed using least-square-based tidal modeling. The tidal bore height and turbulent velocity records based on ADCP surveys in the estuary and upstream area are used to determine the hydraulic jump properties. Furthermore, an acoustic-based approach is also employed to quantify the suspended sediment concentration and flux during the passage of the bore. Kampar Estuary is predominated by semidiurnal co-tidal components (M2 and S2), where, based on the phase lag magnitude, it is categorized as an ebb-dominant estuary. This finding is proven by the more intense and prolonged ebb phases, especially during spring tidal conditions where the tidal range reaches 4 m. Of particular concern, the tidal bore height declines by 1.5 m every 20 km upstream with an erratic turbulent velocity. A sudden increase in transverse and vertical velocity during the passage of bore (ranging from -0.9 to 0.2 m/s) reflects the potency of sediment resuspension in the surrounding river edge marked by the significant increase in suspended sediment flux of about 3.7 times larger than at the end of the ebb tide. However, long-term measurement and regular bathymetry surveys are crucial to monitor the tidal bore behavior and morpho-dynamics in the Kampar River estuary.

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Variability of wave height in the Sea of Japan

Hisaki Y.

Continental Shelf Research ( Continental Shelf Research )  233   2022年01月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

The Sea of Japan is a semi-enclosed sea located in the northwest Pacific area. There has been no study conducted to examine the relationship between the interannual variability of the wind field and the spatial variability of wave height in the Sea of Japan. We examined the relationship between the spatiotemporal variability of the wave height and the wind field in the Japan Sea using monthly mean ERA5 reanalysis data. Firstly, it is demonstrated that the ERA5 wave height closely matches that of moored buoys. Following that, we investigated the relationship between the wave height and the wind anomaly distribution using empirical orthogonal function (EOF) and self-organizing map (SOM) analysis. This is the first study to examine the spatial variation of wave height using the SOM. The EOF's first mode is a mode that oscillates in the central region of the Sea of Japan, while the second mode is a north-south dipole mode. In each season, the wave height anomaly EOF mode had a significant correlation with a variety of climatic indices. Although the correlation between EOF of wave height anomaly and EOF of wind speed anomaly was not high throughout the year, the correlation between them was high for each season. The SOM revealed a pattern consistent with the EOF analysis's first and second modes. Throughout the year, the relationship between wave height and wind speed anomaly patterns is not strong. The link, however, varies with the seasons. The frequency of patterns with increased wave height anomalies in the central Japan Sea has increased in recent years. This is consistent with the EOF first mode's long-term trend.

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Tidal bore generation and transport mechanism in the Rokan River Estuary, Indonesia: Hydro-oceanographic perspectives

Ulung J. Wisha, Yusuf J. Wijaya, Yukiharu Hisaki

Regional Studies in Marine Science ( Regional Studies in Marine Science )  52   102309 - 102309   2022年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

Studies related to the tidal bore phenomenon in the Rokan Estuary are limited. Therefore, revealing the physical characteristics and the environmental impact of a tidal bore is crucial. This study aims to determine the tidal regime in the estuarine area, the propagation of the tidal bore, and the transport mechanism throughout the Rokan River. A tidal harmonic analysis and a hydrodynamic model based on Saint-Venant equations were performed. The amplitude of M2 and S2 (semidiurnal co tidal constituents) showed a sufficiently high magnitude upstream (around 0.5-1.4 m) with a shorter and more intense flood propagation (flood dominant). The tidal range peaked on average 5 m with a 6-6.5-hour displacement time during the full moon phase and lowered in amplitude and period during the first (third) quarter of the moon phase. A sudden increase in water level and velocity observed represented a hydraulic jump off a tidal bore. Of particular concern, the tidal bore height in the estuary was approximately 3 m and lowered by approximately 1 meter every 15 km upstream. The turbulent velocities showed a sharp increase in magnitude at the end of the ebb tide peaking at +0.5 m/s and significantly declined to approach -0.5 m/s during the passage of bore observed in the estuary (station R1), showing the reversal changes in the flow direction and magnitude. These mechanisms were not applicable upstream due to the irregular bottom morphology of the river channel affecting the transverse and vertical turbulent velocity. Some bed erosion-induced turbulence may affect the higher suspended sediment concentration (SSC), advected by the "whelps "beneath the first wave crest, transported upstream. (c) 2022 Elsevier B.V. All rights reserved.

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A non-linear method to estimate the wave directional spectrum by HF radar

Yukiharu Hisaki

Ocean Remote Sensing Technologies: High frequency, marine and GNSS-based radar ( Institution of Engineering and Technology )    179 - 189   2021年12月 [ 査読有り ]

掲載種別: 研究論文（その他学術会議資料等）

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The north equatorial countercurrent east of the dateline, its variations, and its relationship to the el niño event

Yusuf Jati Wijaya, Ulung Jantama Wisha, Yukiharu Hisaki

Journal of Marine Science and Engineering ( Journal of Marine Science and Engineering )  9 ( 10 )   2021年10月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

Using forty years (1978-2017) of Ocean Reanalysis System 4 (ORAS4) dataset, the purpose of this study is to investigate the fluctuation of the North Equatorial Countercurrent (NECC) to the east of the dateline in relation to the presence of three kinds of El Nino events. From spring (MAM) through summer (JJA), we found that the NECC was stronger during the Eastern Pacific El Nino (EP El Nino) and the MIX El Nino than during the Central Pacific El Nino (CP El Nino). When it comes to winter (DJF), on the other hand, the NECC was stronger during the CP and MIX El Nino and weaker during the EP El Nino. This NECC variability was affected by the fluctuations of thermocline depth near the equatorial Pacific. Moreover, we also found that the seasonal southward shift of the NECC occurred between winter and spring, but the shift was absent during the CP and MIX El Nino events. This meridional shift was strongly affected by the local wind stress.</p>

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Validation of drifting buoy data for ocean wave observation

Hisaki Y.

Journal of Marine Science and Engineering ( Journal of Marine Science and Engineering )  9 ( 7 )   2021年07月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

Drifting buoys collect wave data in the open ocean far from land and in areas with strong currents. However, the validation of the drifting buoy wave data is limited. Here, we compared the drifting buoy wave data, ERA5 wave data, and moored GPS buoy wave data. Data from 2009 to 2018 near the coast of Japan were used. The agreement of the drifting buoy-observed wave parameters with the moored GPS buoy-observed wave parameters is better than that of ERA5 wave parameters, which is statistically significant. In particular, the accuracy of the ERA5 wave heights tends to be lower where the ocean currents are fast. On the other hand, the agreement between the drifting buoy-observed wave heights and the moored GPS buoy-observed wave heights was good even in the areas with strong currents. It is confirmed that the drifting buoy wave data can be used as reference data for wave modeling study.

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Differences in the reaction of north equatorial countercurrent to the developing and mature phase of enso events in the western pacific ocean

Yusuf Jati Wijaya, Yukiharu Hisaki

Climate ( Climate )  9 ( 4 )   2021年04月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

The North Equatorial Countercurrent (NECC) is an eastward zonal current closely related to an El Nino Southern Oscillation (ENSO) event. This paper investigated the variations of NECC in the Western Pacific Ocean over 25 years (1993-2017) using satellite data provided by the Copernicus Marine Environment Monitoring Service (CMEMS) and the Remote Sensing System (RSS). The first mode of empirical orthogonal function (EOF) analysis showed that the NECC strengthened or weakened in each El Nino (La Nina) event during the developing or mature phase, respectively. We also found that the NECC shifting was strongly coincidental with an ENSO event. During the developing phase of an El Nino (La Nina) event, the NECC shifted southward (northward), and afterward, when it entered the mature phase, the NECC tended to shift slightly northward (southward). Moreover, the NECC strength was found to have undergone a weakening during the 2008-2017 period.

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Intercomparison of assimilated coastal wave data in the Northwestern Pacific Area

Hisaki Y.

Journal of Marine Science and Engineering ( Journal of Marine Science and Engineering )  8 ( 8 )   2020年08月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

The assimilated coastal wave data are useful for wave climate study, coastal engineering, and design for marine disaster protection. However, the assimilated coastal wave data are few. Here, wave analysis data produced by the JMA (Japan Meteorological Agency) and ERA5 wave data were compared with GPS (Global Positioning System) buoy-measured wave data. In addition, the accuracy of ERA5 wave data for various conditions was investigated. The accuracy of JMA analysis wave height was better than that of ERA5 wave height. The ERA5 wave height was underestimated as the wave height increased. The accuracy of the ERA5 wave height was significantly different in fetch-unlimited and fetch-limited conditions. The difference of the skill metrics between fetch-unlimited and fetch-limited conditions was due to the overestimation of the fetch in the ERA5 grid. This result also applied to the wave period.

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短波海洋レーダによる波浪研究の進展と課題

久木 幸治

海の研究 ( 日本海洋学会 )  29 ( 3 ) 91 - 106   2020年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

<p>短波海洋レーダによる波浪研究の現在までの進展について紹介する。短波海洋レーダが受信する波浪からの後方散乱電波のドップラースペクトルには，波浪を構成している全ての自由波成分が関与している。このため，ドップラースペクトルから波浪スペクトルを推定することが可能である。その推定手法には，半経験的な手法，パラメータ適合による手法，線形インバージョン法，非線形インバージョン法がある。この中で，半経験的な手法が最も広く使われている。半経験的な手法とは，ドップラースペクトルと波浪パラメータとの関係式において未知の係数を経験的に求めることによって波浪パラメータを求める手法である。パラメータ適合による手法は，広ビーム型短波海洋レーダで得られるドップラースペクトルの解析のために開発された。線形インバージョン法は，狭ビーム型短波海洋レーダで波浪スペクトルを求める手法として最もよく知られた手法であり，ドップラースペクトルと波浪スペクトルとの関係式を波浪スペクトルについて線形な式に近似してから，波浪スペクトルを求める手法である。非線形インバージョン法は，日本で最も精力的に開発が進められている手法であり，線形インバージョン法を高度化した手法である。短波海洋レーダによる波浪推定精度を高めるためには，推定手法の高精度化とともに，SN（信号対雑音）比の高いドップラースペクトルを選択する手法の開発が必要である。このことによって, 沿岸域における波浪の高い精度での予報が可能となることが期待される。</p>

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Variabilities of the sea level anomalies in the upstream areas of the Kuroshio Current

Wijaya Y.

IOP Conference Series: Earth and Environmental Science ( IOP Conference Series: Earth and Environmental Science )  162 ( 1 )   2018年07月

掲載種別: 研究論文（学術雑誌）

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Wave hindcast in the North Pacific area considering the propagation of surface disturbances

Hisaki Y.

Progress in Oceanography ( Progress in Oceanography )  165   332 - 347   2018年07月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Sea surface wind correction using HF ocean radar and its impact on coastal wave prediction

Hisaki Y.

Journal of Atmospheric and Oceanic Technology ( Journal of Atmospheric and Oceanic Technology )  34 ( 9 ) 2001 - 2020   2017年09月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Ocean wave parameters and spectrum estimated from single and dual high-frequency radar systems

Hisaki Y.

Ocean Dynamics ( Ocean Dynamics )  66 ( 9 ) 1065 - 1085   2016年09月

掲載種別: 研究論文（学術雑誌）

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Surface current patterns observed by HF radar: methodology and analysis of currents to the north of the Yaeyama Islands, East China Sea

Hisaki Y.

Ocean Dynamics ( Ocean Dynamics )  66 ( 3 ) 329 - 352   2016年03月

掲載種別: 研究論文（学術雑誌）

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Time interpolation of stationary and propagating surface disturbances for ocean modeling

Hisaki Y.

Earth and Space Science ( Earth and Space Science )  3 ( 9 ) 346 - 361   2016年01月

掲載種別: 研究論文（学術雑誌）

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Development of HF radar inversion algorithm for spectrum estimation (HIAS)

Hisaki Y.

Journal of Geophysical Research: Oceans ( Journal of Geophysical Research: Oceans )  120 ( 3 ) 1725 - 1740   2015年03月

掲載種別: 研究論文（学術雑誌）

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海洋レーダを用いた波浪・海流のリモートセンシングに関する基礎研究

その他の著者

研究報告書（理） ( 久木幸治 )    2009年07月

掲載種別: 研究論文（その他学術会議資料等）

　概要を見る

平成16年度～平成19年度科学研究費補助金（基盤研究(C)）研究成果報告書 （管理者追加）リポジトリ登録情報を移行しました。確認のうえ、加除修正をしてください。

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Analysis of Decomposed Surface Currents in a Limited Area

その他の著者

学術雑誌掲載論文（理） ( IEEE Institute of Electrical and Electronics )  31 ( 4 ) 768 - 778   2008年02月

掲載種別: 研究論文（学術雑誌）

　概要を見る

A method of the Helmholtz decomposition for mapped surface currents on grid points is developed. This method is simple, and decomposed currents are not dependent on the distance from the open boundary. This decomposition is applied to surface currents observed by high-frequency (HF) radars. The observation was conducted from April to May 1998 east of Okinawa Island, Japan. The spectral analysis of decomposed currents reveals that the M_2 tidal peak for rotational components is much smaller than that for divergent components, which suggests that the barotropic tidal components are mainly included in divergent currents. The real-vector empirical orthogonal function (EOF) analysis of decomposed currents reveals an embedded eddy, which is related with the convergence of currents. （管理者追加）リポジトリ登録情報を移行しました。確認のうえ、加除修正をしてください。

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Analysis of Decomposed Surface Currents in a Limited Area

Hisaki Y.

IEEE J of Oceanic Engineering ( その他の出版社 )    768 - 778   2006年10月

掲載種別: 研究論文（学術雑誌）

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Ocean wave directional spectra estimation from an HF ocean radar with a single antenna array: Methodology.

Hisaki Y.

J. Atmos. and Oceanic Technol. ( その他の出版社 )    268 - 286   2006年02月

掲載種別: 研究論文（学術雑誌）

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Ocean wave directional spectra estimation from an HF ocean radar with a single antenna array: Observation.

Hisaki Y.

J. Geophys. Res. ( その他の出版社 )  110   2005年11月

掲載種別: 研究論文（学術雑誌）

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Short-wave directional distribution for first-order Bragg echoes of the HF ocean radars.

Hisaki.Y.

J. Atmos. and Oceanic Technol. ( その他の出版社 )  ( 21 ) 105 - 121   2004年03月

掲載種別: 研究論文（学術雑誌）

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Horizontal variability of near‐inertial oscillations associated with the passage of a typhoon.

Hisaki. Y. , T. Naruke

J. Geophys. RACes., 108, 3382, ( その他の出版社 )    2003年03月

掲載種別: 研究論文（学術雑誌）

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Doppler spectrum of radio wave scattering from ocean-like moving surfaces for a finite illuminated area.

Hisaki. Y.

Int. J. of Remote Sens. ( その他の出版社 )  ( 24 ) 3075 - 3091   2003年03月

掲載種別: 研究論文（学術雑誌）

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Short-wave directional properties in the vicinity of atmospheric and oceanic fronts.

Hisaki. Y.

J. Geophys. Res. ( その他の出版社 )  ( 107 )   2002年03月

掲載種別: 研究論文（学術雑誌）

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VHF and HF sea echo Doppler spectrum for a finite illuminated area.

Hisaki. Y., and M. Tokuda

Radio Sci. ( その他の出版社 )  ( 36 ) 425 - 440   2001年03月

掲載種別: 研究論文（学術雑誌）

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Surface current variability in east of Okinawa Island from remotely-sensed and in-situ observational data

Hisaki. Y. et al.

J. Geophys. Res. ( その他の出版社 )  ( 106 ) 31057 - 31073   2001年03月

掲載種別: 研究論文（学術雑誌）

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Observation of tidal and residual circulations in Osaka bay by HF radar.

Takahashi, S. et al.

La mer ( その他の出版社 )  ( 37 ) 145 - 152   2000年03月

掲載種別: 研究論文（学術雑誌）

Correction of Amplitudes of Bragg Lines in the　Sea Echo Doppler Spectrum of an Ocean Radar.

Hisaki. Y.

J. Atmos. and Oceanic Technol. ( その他の出版社 )  ( 16 ) 1416 - 1433   1999年03月

掲載種別: 研究論文（学術雑誌）

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Observation of ocean currents off the coast of Okinawa island using high-frequency radars

Hisaki, Y.

The East China Sea ( その他の出版社 )  ( 2 ) 83 - 89   1999年03月

掲載種別: 研究論文（学術雑誌）