Kyungmo Ahn

Analysis of the Wave Spectral Shape Parameters for the Definition of Swell Waves

In the present study, the characteristics of spectral peakedness parameter Qp, bandwidth parameter e, and spectral width parameter ν were analyzed as a first step to define the swell waves quantitatively. For the analysis, the joint probability density function of significant wave heights and peak periods were newly developed. The MCMC(Markov Chain Monte Carlo) simulations have been performed to generate the significant wave heights and peak periods from the developed probability density functions. Applying the simulated significant wave heights and peak periods to the theoretical wave spectrum models, the spectral shapes parameters were obtained and analyzed. Among the spectral shape parameters, only the spectral peakedness parameter Qp, is shown to be independent with the significant wave height and peak wave period. It also best represents the peakedness of the spectral shape, and henceforth Qp should be used to define the swell waves with a wave period. For the field verification of the results, wave data obtained from Hupo port and Ulleungdo were analyzed and results showed the same trend with the MCMC simulation results.

LABORATORY OBSERVATIONS OF TRIAD INTERACTION OF DEEP WATER WIND WAVES

The triad interactions have been known to be important only for shoaling waves or finite depth wind waves. In deep water, they are insignificant compared with the quadruplet interactions in respect to the evolution of wind waves due to energy transfer among the wave components. However, the triad interactions may be important even for deep water waves because they may closely be related to the wave steepness, which definitely affects wave breaking, drag of air flow over the sea, or navigation of ships, especially during the early stage of the development of wind waves. This study reports a series of laboratory experiments, whose data are subjected to bispectral analyses to investigate the triad interactions of deep-water wind waves. It is found that the bicoherence at the spectral peak frequency and the wave steepness are almost directly proportional, indicating that the steep waves with peaked crests and flat troughs are resulted from the triad interactions. Both bicoherence and wave steepness increase with the wave age during the early stage of wave generation and then drop off as the waves grow old. It seems that the energy of the secondary spectral peak developed by the triad interactions during the early stage of wave generation is redistributed to the neighboring frequencies by the quadruplet interactions during the later stage.

A Study on the Statistical Characteristics and Numerical Hindcasts of Storm Waves in East Sea

In the present study, the statistical analysis on the storm waves in the East Sea have been carried out, and the several storm waves were reproduced by the modified WAM as a first step for the accurate and prompt forecasting and warning against the swell waves in East Sea. According to the present study, the occurrences of the storm waves from the North were the most probable, while the waves from the Northeast were most frequently observed. It was found that the significant wave heights of storm waves from the North and Northern northeast were larger than those of storm waves from the Northeast. But due to long fetch distance, the significant wave periods of storm waves from the Northesast were longer than those of North and Northern northeast. In addition to the wave analysis, the numerical experiments for the storm waves in East Sea were carried out using the modified WAM, and three periods of storm waves in 2013 were calculated. The numerical results were well agreed with wave measurements. However the numerical simulation results in shallow water region showed lower accuracies compared to deep water, which might be due to lower resolution of wind field and bottom topography caused by large grid size, 5 minute, adopted in the present study. Overall computational efficiency of the modified WAM found to be excellent compared to original WAM. It is because the modified WAM adopted the implicit scheme, thereby the present model performed 10 time faster than original WAM in computation time.

Storm Waves on the East Coast of Korea: 20 Years of Wave Hindcasting

ABSTRACT Chun, H. and Ahn, K., 2017. Storm waves on the east coast of Korea: 20 years of wave hindcasting. This paper presents 20 years of wave hindcasting in the East Sea (Sea of Japan) to estimate return periods of storm waves at the east coast of South Korea for coastal risk management. The numerical results were validated against the measured data obtained at eight wave stations in the East Sea. The comparison of correlation coefficients of significant wave heights and wave periods, 0.81–0.95 and 0.70–0.88, respectively, between the numerical results and the measured data shows good agreement. Note that the numerical results agree well with the measurements despite the large computational grid size used. The storm waves on February 2008 were successfully simulated in the present study, and the computed peak periods at Gangneung Port showed good agreement with those measured, even though the coefficients of whitecapping dissipation were not modified. On the basis of 20 years of wave hindcasting results, extreme wave analysis was carried out using the peak-over-threshold method and the generalized Pareto distribution function. According to the extreme wave analysis, significant wave heights occurred at Sokcho and Gangneung on 23 October 2006, and were estimated to have a return period of 100 years. It was found that the waves generated by Typhoon Nabi (T0514) were the most severe in the East Sea during the simulation period. During this typhoon period, a 50-year wave appeared at Pohang, the southern East Sea.

Sediment Particulate Motions Over a Ripple Under Different Wave Amplitude Conditions

부유 퇴적물의 입자 운동에 대한 수치실험이 굴곡이 있는 해저면 위에서 수행되었다. 해저면 경계층의 난류 유속장은 LES 난류모델을 사용하여 구현하였고, 난류 흐름 속에서의 퇴적물 입자운동은 Lagrangian 입자추적 모델을 사용하여 구현하였다. 수치실험을 통하여 두개의 다른 유속조건을 사용하였는데, 파랑주기를 비롯한 다른 모든 조건은 동일하게 유지한 가운데 오직 최대 유속만 다르게 하여 실험을 수행하였다. 예상한 것과 같이 부유되는 퇴적물 입자의 양은 유속이 강할 수록 증가하였다. 그러나 예상하지 못한 결과도 관측되었는데 그것은 비록 최대유속외에 다른 모든 조건은 동일하더라도 퇴적물이 부유되는 양상은 다르게 나타날 수 있다는 것이다. 특히 퇴적물이 부유하게 되는 시간이 위상평균한 파장 주기안에서 서로 다르게 나타나는 것이 발견되었는데, 이는 해저면 굴곡 주위에서 유속에 의해 생성되는 난류 와동의 생성시간이 다르기 때문에 일어나는 것으로 밝혀졌다. 이전의 연구에서도 알려진 바와 같이 이런 난류 와동들이 퇴적물의 부유현상에 미치는 영향은 지대한 것으로 확인되었으며, 또한 이런 와동들의 생성시간은 유속의 크기에 따라 달라 질 수 있음이 밝혀졌다. 이로인해 해저면 굴곡 위에서 퇴적물의 부유현상을 보다 정확하게 규명하기 위해서는 파랑의 여러가지 복잡한 변수들을 고려하여야 함이 이번 실험을 통하여 입증되었다. 또한 퇴적물 입자의 부유에 영향을 미치는 난류 에너지분포 역시 생성된 난류 와동에 많은 영향을 받는 것으로 나타났다. 이번 연구를 통하여 유속의 세기변화 만으로도 퇴적물이 부유되는 시간이 굴곡이 있는 해저면에서 바뀔 수 있는 것으로 확인되었으며, 이는 향후 퇴적물 부유에 대한 연구를 할때 해저면 구조와 유속구조의 상관관계를 보다 신중히 검토해야 함이 밝혀졌다.

Wave Hindcasting on the Storm Waves at the Korean Straits of April, 2016

In the present study, the storm waves at the Korean Straits of April, 2016 have been reproduced by the wave hindcasting, and then their characteristics were investigated. Before the wave hindcasting, the wave measurements at the Korean Straits were analyzed. The analysis showed that the waves at the Korean Straits were dominated by the Northeastern waves, same as those in the East Sea. Accordingly, the wave hindcasting was been carried out with the same condition in Ahn et al. (2016). In the numerical results, the maximum significant wave height at the Korean Straits was 5.06 m, and the corresponding significant wave period was 9.2 s. The computed significant wave heights and wave periods were overestimated by 4 cm and 0.8 s, respectively. After the wave hindcasting, the computed significant wave heights and peak periods were compared with the JONSWAP relationship. This comparison showed that the storm waves at the Korean Straits were close to wind waves, not swell.

Projection of extreme typhoon waves: Case study at Busan, Korea

ABSTRACT Chun, J., Ahn, K., Yoon, J.-T., Suh, K.-D. and Kim, M., 2013. Projection of extreme typhoon wave : Case study at Busan, Korea Several studies predicted that the typhoons will be more intense in the future. Since the intensification makes the wave environments at the coastal zone more extreme, the analysis on the enhancement of typhoon waves is required for the future protection of the coastal zone. Most of the extreme wave analysis is based on the measured or hindcast wave data which does not reflect the future trend of variations. This study discuss the add-on effect of future typhoon intensification on the estimation of extreme waves. Especially, since the extreme waves at Busan port, Korea are generated by typhoons, the trends of the activity and intensity of typhoons on the southern coasts of Korean Peninsula between 1951 and 2010 were analyzed. At the analysis, it was found that there is a trend in the intensity of typhoons. By applying the wavelet analysis, the long-term trend of the intensity of typhoons on the southern coasts of Korean Peninsula was obtained. The increasing rate of annual maximum pressure drop is 0.0786 hPa/yr. After 50 years, the pressure drop and maximum wind speeds will increase by 3.93 hPa and 2.5 m/s, assuming that this trend will be maintained in the future. Applying these values to six historic intense typhoons, the changes of wave heights by these typhoons were calculated. According to the present numerical simulations, the significant wave heights will increase by 1.24 m after 50 years.

Effect of Model Resolution on The Flow Structures Near Mesoscale Eddies

Three-dimensional structures of large ocean rings in the Gulf Stream region are investigated using the HYbrid Coordinate Ocean Model (HYCOM). Numerically simulated flow structures around four selected cyclonic and anticyclonic rings are compared with two different horizontal resolutions: 1/12° and 1/48° . The vertical distributions of Lagrangian Coherent Structures (LCSs) are analyzed using Finite Size Lyapunov Exponent (FSLE) and Okubo-Weiss parameters (OW). Curtain-shaped FSLE ridges are found in all four rings with extensions of surface ridges throughout the water columns, indicating that horizontal stirring is dominant over vertical motions. Near the high-resolution rings, many small-scale flow structures with size O(1~10) km are observed while these features are rarely found near the low-resolution rings. These small-scale structures affect the flow pattern around the rings as flow particles move more randomly in the high-resolution models. The dispersion rates are also affected by these small-scale structures as the relative horizontal dispersion coefficients are larger for the high-resolution models. The absolute vertical dispersion rates are, however, lower for the high-resolution models, because the particles tend to move along inclined eddy orbits when the resolution is low and this increases the magnitude of absolute vertical dispersion. Since relative vertical dispersion can reduce this effect from the orbital trajectories of particles, it gives a more reasonable magnitude range than absolute dispersion, and so is recommended in estimating vertical dispersion rates.

Occurrence Probability of Freak Waves at Nearshore of Donghae Harbor in the East Sea

Over the last 20 years, freak waves have attracted many researchers because of their unexpected behaviors and damages on offshore structures and vessels in the ocean and coastal waters. Despite many researches on the causes, mechanisms and occurrence of freak waves, we have not reached consensus on the results of the researches. This paper presents the occurrence probability of freak waves based on the analysis of wave records measured at coastal waters of Donghae harbor in the East Sea. Three freak waves were found which satisfied conditions of m and and . The occurrence probabilities of freak waves were estimated from extreme distributions by Mori, Rayleigh and Ahn, and found to be on the orders of O(), O(), and O(), respectively. The occurrence probabilities of freak waves measured from waves records were estimated between O() and O(), which were located between predictions by Rayleigh and Ahn`s extreme probability distributions. However, we need more analysis of wave records obtained from diverse field conditions in order to verify the accuracy of the estimation of occurrence probability of freak waves.

Beach Sand Grain Size Analysis using Commercial Flat-bed Scanner

For analyzing sand grain size, a specialized high-priced instrument has been used, such as sieve shaker, video camera, laser particle size analyzer, and microscope. Among these, the sieve shaker is commonly used because it is not only cheaper than others but also provides reasonable accuracy. However, it takes a long time and makes lots of dust and noise. In this study, a cheaper and easier method which can replace the sieve shaker is proposed. By using a commercial flat-bed scanner and a darkroom box, the sand size distribution can be analyzed. The darkroom box makes sand images clear and protects the glass of the scanner from being scratched. Comparison between the present method and sieve analysis shows that the present method not only has an accuracy comparable to the sieve analysis but also can save time and effort.