The optimal coastal retracked sea levels from saral/altika satellite altimetry over the southeast asia

Abstract

The current demand for accurate coastal altimetry data, particularly for the sea level has increased since human activities have become increasingly concentrated along coastal areas. Over coastal region, particularly within 10 km from the coastline, the altimeter footprint is severely contaminated by land and rough coastal sea states. The contamination leads to the low quality observations, thus creating a significant gap in data availability over the coast. The aim of this study is to evaluate the quality of coastal retracked sea level data from AltiKa satellite altimetry over the Southeast Asia region. In this study, high resolution (40 Hz) sea levels derived from the advanced AltiKa satellite altimetry are validated over the Southeast Asia coastal regions. The parameter of sea level is derived based on three standard retracking algorithms which are MLE-4, Ice-1 and Ice-2. The assessments of quantity and quality of the retracked sea levels data are conducted to identify the optimum retracker over the study regions, which are Andaman Sea, Strait of Malacca, South China Sea, Gulf of Thailand and Sulu Sea. The quantitative analysis involves the comparison between AltiKa and Jason-2 waveforms, the computation of percentage of data availability, and the minimum distance of Sea Level Anomaly (SLA) to the coastline. The qualitative analysis involves the relative validation with geoid height and absolute validation with tide gauge. In general, AltiKa measurement can obtain as close as 1 km to the coastline with =85% data availability. The Ice-1 retracker has shown an excellent performance with percentage of data availability at =90% and minimum distance as close as 0.9 km to the coastline. In term of quality, Ice-1 retracker shows the highest improvement of percentage (IMP) values over Andaman Sea, Sulu Sea and Strait of Malacca with IMPs of 19%, 16% and 43%, respectively. The Ice-1 retracker also shows the highest temporal correlation (up to 0.95) and the lowest root mean square (RMS) error up to 8 cm over distance less than 10 km for those three regions. Contrary, over the South China Sea, Ice-2 retracker has better performance when compared to other retrackers with IMP values of 43%. Over distance less than 10 km to the shore, the temporal correlation and RMS error reach up to 0.88 and 7 cm respectively. Over the Gulf of Thailand, the optimum retracker cannot be concluded due to unavailable tide gauge data. The Ice-1 is the optimum retracker over three out of four regions. Therefore, it is used to study the seasonal variability of sea levels over the Southeast Asia. The seasonal variability shows that the mean amplitude is up to 25 cm during the Northeast Monsoon and decreased by 9 cm during the Southwest Monsoon and between 2 to 9 cm during inter-monsoon seasons. In conclusion, the research has significantly contributed in defining the quantity and quality of the AltiKa SLAs in the coastal region of Southeast Asia. The results from comprehensive validation obtained in this research present a significant improvement in identifying the reliability and applicability of the AltiKa datasets and retracking algorithms over the coastal area of the study region.