Validation of Nadir SWH and Its Variance Characteristics from CFOSAT in China’s Offshore Waters

Abstract

The offshore waters of China are a typical monsoon−affected area where the significant wave height (SWH) is strongly influenced by the different seasonal mean flow in winter and summer. However, limited in situ validations of the SWH have been performed on the China–France Oceanography Satellite (CFOSAT) in these waters. This study focused on validating CFOSAT nadir SWH data with SWH data from in situ buoy observations for China’s offshore waters and the Haiyang−2B (HY−2B) satellite, from July 2019 to December 2021. The validation against the buoy data showed that the relative absolute error has a seasonal cycle, varying in a narrow range near 35%. The RMSE of the CFOSAT nadir SWH was 0.29 m when compared against in situ observations, and CFOSAT was found to be more likely to overestimate the SWH under calm sea conditions. The sea−surface winds play a key role in calm sea conditions. The spatial distributions of the CFOSAT and HY−2B seasonal SWHs were similar, with a two−year mean SWH−field correlation coefficient of 0.98. Moreover, the coherence between the two satellites’ SWH variance increased with SWH magnitude. Our study indicates that, in such typical monsoon−influenced waters, attention should be given to the influence of sea conditions on the accuracy of CFOSAT SWH, particularly in studies that combine data from multiple, long−duration space−based sensors.

Validation of Multisource Altimeter SWH Measurements for Climate Data Analysis in China’s Offshore Waters

“…The along-track SWH data represent a narrow-view swath of nadir-looking data and provide limited coverage daily SWH measurements of the research area by a single altimeter. Given the spatial coverage and temporal resolution of the final gridded SWHs, we adopted the method of using the 10 d mean SWHs from at least three altimeters [36], and we interpolated these data into gridded boxes with horizontal resolution of 25 km × 25 km. During 2012-2016, among the HY-2 series altimeters, only HY-2A was available; therefore, Jason-2 and Cryosat-2 data were included to satisfy the criterion of using at least three altimeters to construct the 10 d mean gridded SWHs (in Figure 2, the lines indicate the SWHs from an altimeter used in this study).…”

“…Given that measurements obtained concurrently from at least three altimeters can be used to construct 10 d mean gridded SWHs [36], but that only HY-2A of the HY series altimeters was available during 2012-2016, we applied Jason-2 and Cryosat-2 as a supplement to construct the 10 d mean gridded SWHs. Similar to Jason-3, the Jason-2 satellite, a successor to the T/P and Jason-1 altimetry missions, was launched by NASA and the French Space Agency on 20 June 2008.…”