Validation of HY-2A Remotely Sensed Wave Heights against Buoy Data and Jason-2 Altimeter Measurements*

The Haiyang-2A (HY-2A; HaiYang means ocean in Chinese) satellite was successfully launched in China on 16 August 2011, carrying the nation's first operational radar altimeter along with three other microwave sensors. In this study, HY-2A altimeter significant wave height (SWH) data have been validated against National Data Buoy Center (NDBC) buoy and Jason-2 altimeter SWH data over a period of 27 months (from 1 October 2011 to 31 December 2013). During the collocation, the effects of different thresholds of several flags are carefully studied. These flags prove to be useful for the SWH selection and different thresholds are observed to change the results remarkably. The final results show that HY-2A SWHs, with a 0.339-m root-mean-square (RMS) difference and a negative bias of 0.231 m in buoy comparison, have reached the mission target (0.5-m RMS). Nonetheless, the Jason-2 altimeter performs better with a lower RMS difference of 0.292 m and a positive bias of only 0.016 m. In addition, by analyzing the residuals (altimeter minus buoy), the bias for the HY-2A altimeter is found to decline monotonically over the whole range with an overestimation at low sea state (SWH < 1 m), a minor underestimation at middle sea state (1 m < SWH < 5 m), and a severe underestimation at high sea state (SWH > 5 m). However, only an underestimation at high sea state is found for the Jason-2 altimeter. A linear regression is also proposed. The 20 days of the newly processed HY-2A SWHs are investigated and discussed as well, and a slight quality improvement has been observed using these data.