The setting conditions of wave observation system. The beginning direction of the sampling is the relative direction. Zero degree is a heading angle.
3. ANALYSIS OF RADAR SEA ECHO BOY 2-DIMENSIONAL SPECTRAL METHOD
After the PPI image is reproduced on the computer display, it is necessary to select a suitable area on the PPI image, where the crest lines of prevailing waves are displayed clearly enough. The data of the PPI image in polar coordinates are converted to the grid point data in orthogonal coordinates. Then, 2-dimensional information of the wavelength and the propagating direction of prevailing waves can be obtained from the grid point data by the 2-dimiiensional spectral analysis. Let us suppose the 2-dimensional grid point data of (M, N) as R(j, r), j=0, 1, 2,..., M-l, r=0, 1, 2,..., N-l, then we have 2-dimensional spectral density function as
where d denotes the distance between grids, l and m are spatial wave numbers, and l and m are non-dimensional spatial wave numbers. M and N are data numbers on X and Y -axies. Then, the propagating direction θ and die wavelength L of prevailing waves are obtained from non-dimensional wave number lp and mp at the spectral peak as follows,
where the grid points are arranged in square in general, so that M=N. In this study, a square domain (640m ×640m, l28grids × 5m) are selected as an analyzing area on die PPI image.
This schematic features are shown in Fig.1. Figure (A) shows the sampling area as listed in Table 3. Figures (B) and (C) show the grid point data of the radar sea echo in the square domain and the result by the 2-dimensional spectral analysis. The details of this 2-dimensional spectral analysis method for the radar sea echo data are described in previous papers5)6)7).
4. WAVE OBSERVATION
The on-board experiment of the marine radar wave observation system was carried out on the waters near around Izu-Oshima Island for 5 days in October and November and Goto Island for 3 days in December. The observational number and weather condition at die wave observation are listed in Table 4. Total 53-run observations were carried out and 49-run observational data among them were analyzed.
On die 13th of October, die cold front was approaching to die observational site. Therefore, wind got strong and veered. On the 14th and 15th the seasonal northerly wind blew strong. But wind waves did not grow up large because of the short fetch from Honshu Island to the
