Calibration and validation of HY-2 altimeter wave height

Hai Yang-2 (HY-2) satellite altimeter measurements of significant wave height (Hs) are analyzed over the period from 1 October 2011 to 6 December 2014. They are calibrated and validated against in situ buoys and other concurrently operating altimeters: Jason-2, CryoSat-2, and Satellite with Argos and ALtiKa (SARAL). In general, the HY-2 altimeter measurements agree well with buoy measurements, with a bias of -0.22m and a root-mean-square error (RMSE) of 0.30 m. When the reduced major axis (RMA) regression procedure was applied to the entire period, the RMSE was reduced by 33% to 0.2 m. A further comparison with other satellite altimeters, however, revealed two additional features of HY-2 Hs estimates over this period. First, a noticeable mismatch is present between HY-2 and the other satellite altimeters for high seas (Hs > 6 m). Second, a jump increase in HY-2 Hs values was detected starting in April 2013, which was associated with the switch to backup status of the HY-2 sensors and the subsequent update of its data processing software. Although reported by previous studies, these two deficiencies had not been accounted for in calibrations. Therefore, the HY-2 wave height records are now subdivided into two phases (time periods pre- and post- April 2013) and a two-branched calibration is proposed for each phase. These revised calibrations, validated throughout the range of significant wave heights of 1-9 m, are expected to improve the practical applicability of HY-2 Hs measurements significantly.