以衛星測高改善近岸及島礁潮汐模式

Abstract

準確的潮汐預估對從事海洋研究、航運及軍事的人相當重要，傳統的做法是利用驗潮站數據和海洋動力方程建立海潮模型。長久以來，衛星測高技術也被用於發展海洋潮汐模型，本研究區域包含東海、南海及台灣地區(100°E-132°E、0°N-32°N)。本研究共使用七顆測高衛星(Topex/Poseidon、Jason-1、Jason-2、ERS-2、Envisat、SARAL/AltiKa和Cryosat-2)，首先將Topex/Poseidon、Jason-1、Jason-2、Envisat和SARAL/AltiKa採用次波形門檻值演算法改善測高觀測量之測距精度，ERS-2和Cryosat-2則無。再將七顆測高衛星之殘餘潮汐利用最小二乘法求得基於NAO99b之調和常數以建立殘餘潮汐模型，殘餘潮汐指的是測高觀測之海水面高扣除NAO99b潮汐高和EGM08大地起伏值之殘餘量。最後將殘餘潮汐模型加上參考海潮模型NAO99b即為本研究所計算之NCTU海潮模型。本研究利用21個驗潮站驗證NCTU與NAO99b海潮模型之精度，在18個站中，NCTU精度皆優於NAO99b，平均改善率達21%，改善原因與長時間的測高資料和使用波形重定技術有關。此外，本研究亦分析27個島礁之潮汐特性，提供潮差資訊以修正衛星影像所計算之島礁面積大小。

Predicting ocean tidal heights is important in oceanography, fishery and military. Conventional methods use tide gauge data and hydrodynamic equations to model ocean tides. Satellite altimetry has long been used to ocean tide modeling. The study area covers the East China Sea, South China Sea and Taiwan from 100°to 132°E and from 0°to 32°N. First, we use a subwaveform retracking algorithm to improve ranging precisions of satellite altimeter measurements (for only Topex/Poseidon, Jason-1, Jason-2, Envisat and SARAL/AltiKa). A residual tidal height is the difference between an altimeter-observed sea surface height and the sum of the NAO99b tidal height and the EGM08 geoidal height. Using residual tidal heights from 7 altimeter missions, we determine tidal harmonic coefficients relative to NAO99b using the least-squares method. The sum of the residual tide model and NAO99b is the NCTU ocean tide model. We assess the NCTU and NAO99b tide models at 21 tide gauge station. At 18 stations, the NCTU model is improved over NAO99b, with an average improvement rate of 21%. The improvement is attributed to the use of the long-term altimeter data and waveform retracking. We analyze the tidal characteristics around 27 atolls, showing tidal ranges useful for correcting land covers in satellite images.