Validation and application of multi-source altimeter wave data in China's offshore areas

Studies of offshore wave climate based on satellite altimeter significant wave height (SWH) have widespread application value. This study used a calibrated multi-altimeter SWH dataset to investigate the wave climate characteristics in the offshore areas of China. First, the SWH measurements from 28 buoys located in China's coastal seas were compared with an Ifremer calibrated altimeter SWH dataset. Although the altimeter dataset tended to slightly overestimate SWH, it was in good agreement with the in situ data in general. The correlation coefficient was 0.97 and the root-mean-square (RMS) of differences was 0.30 m. The validation results showed a slight difference in different areas. The correlation coefficient was the maximum (0.97) and the RMS difference was the minimum (0.28 m) in the area from the East China Sea to the north of the South China Sea. The correlation coefficient of approximately 0.95 was relatively low in the seas off the Changjiang (Yangtze River) Estuary. The RMS difference was the maximum (0.32 m) in the seas off the Changjiang Estuary and was 0.30 m in the Bohai Sea and the Yellow Sea. Based on the above evidence, it is confirmed that the multialtimeter wave data are reliable in China's offshore areas. Then, the characteristics of the wave field, including the frequency of huge waves and the multi-year return SWH in China's offshore seas were analyzed using the 23-year altimeter wave dataset. The 23-year mean SWH generally ranged from 0.6-2.2 m. The greatest SWH appeared in the southeast of the China East Sea, the Taiwan Strait and the northeast of the South China Sea. Obvious seasonal variation of SWH was found in most areas; SWH was greater in winter and autumn than in summer and spring. Extreme waves greater than 4 m in height mainly occurred in the following areas:the southeast of the East China Sea, the south of the Ryukyu Islands, the east of Taiwan-Luzon Island, and the Dongsha Islands extending to the Zhongsha Islands, and the frequency of extreme waves was 3%-6%. Extreme waves occurred most frequently in autumn and rarely in spring. The 100-year return wave height was greatest from the northwest Pacific seas extending to southeast of the Ryukyu Islands (9-12 m), and the northeast of the South China Sea and the East China Sea had the second largest wave heights (7-11 m). For inshore areas, the 100-year return wave height was the greatest in the waters off the east coast of Guangdong Province and the south coast of Zhejiang Province (7-8 m), whereas it was at a minimum in the area from the Changjiang Estuary to the Bohai Sea (4-6 m). An investigation of sampling effects indicates that when using the 1°×1°grid dataset, although the combination of nine altimeters obviously enhanced the time and space coverage of sampling, the accuracy of statistical results, particularly extreme values obtained from the dataset, still suffered from undersampling problems because the time sampling percent in each 1°×1°grid cell was always less than 33%.