Chia Chuen Kao

ON THE CHARACTERISTICS OF OBSERVED COASTAL FREAK WAVES

The purpose of this study is to describe the characteristics of occurrence of coastal freak waves and to investigate their statistical and spectral structures. According to Ochis definition of freak wave, 175 coastal freak waves were obtained from a long-term full scale wave data bank, in which there are more than 4500 wave records. The probability distribution of occurrence of coastal freak waves was investigated. The goodness-of-fit testing shows that the Rayleigh distribution is more appropriate for describing the occurrence probability of coastal freak waves when the ratio of its wave height to the corresponding significant wave height is more than 2.4. However, the occurrence probability of coastal freak waves remains below the Rayleigh distribution. The relationship between coastal freak waves and the related sea states shows that the occurrence possibility of coastal freak waves may significantly increase in the sea of grouping waves or in the wave field of bimodal spectra. Wavelet Transform was applied to the in situ wave records to investigate the energy distribution on the time-frequency domain. It justified the previous conclusions and demonstrated that the wave groups and superposition of the swell and wind wave energy, which occupies 70% of the coastal freak samples, play major roles of inducing the coastal freak waves.

Ocean remotely sensed image analysis using two-dimensional continuous wavelet transforms

Ocean remote sensing is a useful way to obtain ocean wave information. Due to possible inhomogeneities from remotely sensed images, the current work proposes issues concerning ocean wave image analysis using the two-dimensional continuous wavelet transforms (2-D CWTs) to calculate local wave image spectra from inhomogeneous images. To optimize the algorithm of the 2-D CWT for wave image analysis, this work explores ideal parameter values for the wavelet function. The current study also analyses the limits of spatial image resolution and wave image size. After implementing the 2-D CWT on satellite and X-band radar images, this study presents local image spectra and ocean wave information from all the ocean images. These local image spectra reveal the phenomenon of wave refraction and wave nonlinearity nearshore. Compared to real wave spectra, the wavelet spectra present accurate results to describe local wave features in the spatial frequency domain.

Wave Measurements Using GPS Velocity Signals

This study presents the idea of using GPS-output velocity signals to obtain wave measurement data. The application of the transformation from a velocity spectrum to a displacement spectrum in conjunction with the directional wave spectral theory are the core concepts in this study. Laboratory experiments were conducted to verify the accuracy of the inversed displacement of the surface of the sea. A GPS device was installed on a moored accelerometer buoy to verify the GPS-derived wave parameters. It was determined that loss or drifting of the GPS signal, as well as energy spikes occurring in the low frequency band led to erroneous measurements. Through the application of moving average skill and a process of frequency cut-off to the GPS output velocity, correlations between GPS-derived, and accelerometer buoy-measured significant wave heights and periods were both improved to 0.95. The GPS-derived one-dimensional and directional wave spectra were in agreement with the measurements. Despite the direction verification showing a 10° bias, this exercise still provided useful information with sufficient accuracy for a number of specific purposes. The results presented in this study indicate that using GPS output velocity is a reasonable alternative for the measurement of ocean waves.

Quantification of non-homogeneity from ocean remote-sensing images using two-dimensional continuous wavelet transforms

In this study, a two-dimensional continuous wavelet transform is applied to quantify the non-homogeneity from remote-sensing wave images. Our study shows that the non-homogeneity index, which is proposed here, is capable of identifying the degree of non-homogeneity from the wave-field images. However, the influence from the edges of the wave-field image should be considered in analysis. The calculated results from the simulated wave fields and from the natural wave-field images reveal that the non-homogeneity index is influenced by the bathymetry inside the area of the ocean-wave image. It conforms to the character of the natural wave non-homogeneity. After discussing the wave characteristics from the wave-field image with a high non-homogeneity index, an inhomogeneous algorithm is recommended for obtaining accurate and reasonable wave results from the wave image.

Buoy and Radar Observation Network around Taiwan

The coastal ocean monitoring center (COMC) was assigned by the government offices to establish an coastal ocean monitoring network around Taiwan. It is composed by data buoys, radars, tide stations and coastal weather stations. All field stations are operational and have real time data transmission function. The purpose of this paper is to present the frame of this observation network and show observation results especially during past typhoons. This paper focuses on the introduction of data buoy and radar systems. The hardware and analysis method of data buoy and radar system are presented in this study.

Spatial and Temporal Features of Regional Variations in Mean Sea Level around Taiwan

Satellite altimeter and in-situ tide gauge records are probably the most common means to obtain observational data for the study of changes in mean sea level. In this study, we employed these data to discuss the spatial and temporal features of regional variations in mean sea level around Taiwan. The results showed that most of the regional mean sea surface heights (SSH) around Taiwan are higher than the global mean sea surface heights. Most of the sea level trends are greater than the global mean sea level trend as well. We obtained diverse distribution results from the altimeter sea level records in neighboring areas by distributions fit, and the altimeter sea level records showed obvious inhomogeneity. In addition, periodic fluctuations in the records regarding mean sea level were revealed in our study, based on Fourier spectra and wavelet scalograms.

ENSEMBLE EMPIRICAL MODE DECOMPOSITION ON STORM SURGE SEPARATION FROM SEA LEVEL DATA

This paper concerns the storm surge calculation based on the algorithm of ensemble empirical mode decomposition (EEMD). An accurate storm surge result is key information for coastal disaster warning and prevention. Separation of storm surge magnitude from sea level data has typically been done by specifying tidal input from main tidal harmonics. Obtaining accurate storm surge magnitude with harmonic analysis (HA) requires at least one month. This study discusses possible storm surge separation from short-term sea level time series using EEMD. The current work reveals that EEMD is predominant for short-term sea level data analysis shorter than thirty-five days. Due to different residues obtained from EEMD, this work proposes a method to determine most ideal residue for representing the storm surge.

Development of a GNSS Buoy for Monitoring Water Surface Elevations in Estuaries and Coastal Areas

In this work, a Global Navigation Satellite System (GNSS) buoy that utilizes a Virtual Base Station (VBS) combined with the Real-Time Kinematic (RTK) positioning technology was developed to monitor water surface elevations in estuaries and coastal areas. The GNSS buoy includes a buoy hull, a RTK GNSS receiver, data-transmission devices, a data logger, and General Purpose Radio Service (GPRS) modems for transmitting data to the desired land locations. Laboratory and field tests were conducted to test the capability of the buoy and verify the accuracy of the monitored water surface elevations. For the field tests, the GNSS buoy was deployed in the waters of Suao (northeastern part of Taiwan). Tide data obtained from the GNSS buoy were consistent with those obtained from the neighboring tide station. Significant wave heights, zero-crossing periods, and peak wave directions obtained from the GNSS buoy were generally consistent with those obtained from an accelerometer-tilt-compass (ATC) sensor. The field tests demonstrate that the developed GNSS buoy can be used to obtain accurate real-time tide and wave data in estuaries and coastal areas.

Improved Boundary Values of Ocean Wave Fields Using a Data Assimilation Scheme

The objective of this study was to assess the accuracy of numerical wave forecasts through data assimilation of boundary values. A sequential data assimilation scheme was adopted to utilise altimeter data. The performance of the system in terms of operational applications, specifically for typhoon waves, was investigated. Two typhoons that occurred in 2006 around Taiwan (Kaemi and Shanshan) were used as case studies. The proposed data assimilation increased the forecast accuracy of the boundary values in terms of the wave parameters, such as the wave heights and periods. The results showed that after the assimilation, the assimilation model was significantly improved, especially the peak value of the wave and time of occurrence of the peak value.

EXPERIMENTAL STUDY OF THE REVERSE CONVERSION OF WAVE SPECTRA

The observed deep-sea wave data are most appropriate to use in the data assimilation technique. For wave stations (e.g., the Longdong data buoy station in the northeastern Taiwan Sea near the coast), wave data must be reverse-calculate deep-sea wave data for data assimilation applications because seafloor slopes in the northeastern Taiwan Sea are acute and the distances between wave interactions during propagation are shorter. Consequently, nonlinear effects arising from the topography become insignificant as waves approach the coast from the open sea. The proposed technique to reverse-calculate the deep-sea wave spectrum from the near-shore wave spectrum, based on first-order wave spectrum theory, is verified based on physical experiments. The results indicate that this method is effective and applicable to calculations of the energy at wave spectrum peaks.