Directional Characteristics of Winter Monsoon Waves off Taishi Coast

• Main Authors: Mai, Chiao-Hung, 麥喬閎
• Other Authors: Kao Chiao-Chuen, Laurence [.H. Chuang
• Format: Others
• Language: zh-TW
• Published: 1998
• Online Access: http://ndltd.ncl.edu.tw/handle/38719914723800577829

碩士 === 國立成功大學 === 水利及海洋工程學系 === 86 === The importance of directional spectrum has aroused a great interest inthis research. It is the purpose of this study to discuss local features ofdirectional wave off Taishi coast in the winter monsoon season and to evaluatethe validity of Mitsuyasu''s empirical formula of directional energy spreading. A spatial array of wave gauges installed on an observation platform hasbeen arranged to measure the wave directionality and the Bayesian ParameterEstimation Method (BPEM) was adopted to determine the main direction and thedirectional spreading parameter for each frequency.Wind measurements were alsomade on the platform.The statistical properties of waves and winds show thatthe main directions of waves at peak frequency don''t coincide with the winddirections. The well known Snell''s law was then used to justify that waverefraction may be the main reason of wave direction deviated from winddirection. As expected that wave directions estimated from both BPEM and Snell''s law at low frequency components always tend to shift toward the normalto the isobaths as water depth decreases. However, there exists a slowlychanging disagreement of numerical results with observed incidence. The morecomplete conditions and numerical scheme should be considered to explain thediscrepancy. The Mitsuyasu''s formula for directional spreading function was justifiedto be representative of wave field. The normalized directional spreadingparameter S/Smax in the formula was derived and plotted as a function of f/fp.The results shows the same tendency as previous observations that directional concentration has maximum value at the spectral peak and decreased at the bothsides of spectral peak. However, as mentioned by Hasselmann et al. (1980) thatthe maximum directional spreading parameterSmax didn''t always happen at peak frequency for our collected field data. Also, the data scatter compared to theMitsuyasu''s formula indicates the complexity of S. It was then assumed and proved that S should be related to wave age, which is the ratio of the wavephase speed to the wind speed. The linearregression was performed on the datato have a modified Mitsuyasu''s formula in consideration of the stage of wavedevelopments. The bigger the wave age , the more rapid decrease in spreading.Nevertheless, the major portion of the wave energy, which is around the peakfrequency, may not be completely related to the local wind condition. The relation between the parameter Smax and the waveage, as suggested by Mitsuyasu''s formula, may be improper. The field data were shown that the wavesteepness is more related to Smax than wave age does. In general, the values of Smax decrease as the values of the wave steepness increase. The directional response of ocean waves to changes in wind direction isan important aspect for reliable wave predictions of sea states during stormevents or moving frontal systems. However, information on local main wavedirection to changing wind direction is scarce. In this study we selected adataset of changing sea states followed after a moving frontal system. Asexpected,the high-frequency waves align rather quickly with the wind direction, whereas the low-frequency wind-sea components tend to lag behind the winddirection and take several hours to align completely with the wind. In theearlier stages ofwind direction changing, the directional spectrum becomesbroader and less peaked. A new higher-frequency peak grows gradually in thenew wind direction after wind shift for duration. The combination of gradualgrowth of energy in the new wind direction and decay in the old direction results in a smooth changing of the mean wave direction for the high- frequencycomponents. As growth continues, most of wave components align in the new winddirections and a highly peaked form of new wind-sea spectrum dominates.