Calibration and Validation of HY-2 Altimeter Wave Height

Abstract

Hai Yang-2 (HY-2) satellite altimeter measurements of significant wave height () 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 −0.22 m 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 estimates over this period. First, a noticeable mismatch is present between HY-2 and the other satellite altimeters for high seas ( > 6 m). Second, a jump increase in HY-2 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 measurements significantly.

Validation of the Significant Wave Height Product of HY-2 Altimeter

“…The buoy data are generally assumed to be of high quality and have been used for validation altimeter SWH data [7,10,[12][13][14][15][16][17][18][19][20]. The buoy data used here is regarded as "truth"; that is to say, any error is negligible.…”

“…The results of ENVISAT RA-2 wind and wave validation performed within the ESA RA-2 cross-calibration and validation team (CCVT) activities during the ENVISAT commissioning phase, as well as SWH and wind measurements from ERS-1/2, TOPEX/POSEIDON, and Jason-1/2, the results of long-term validation and proposed corrections, have been presented [9][10][11]. Since HY-2 was launched, there have been several published works evaluating HY-2 SWH accuracy; these works have consistently demonstrated its accuracy [12][13][14][15][16]. Some works focused on the preliminary calibration of HY-2 SWH shortly after its launch [7,12,13,17].…”

“…Some works focused on the preliminary calibration of HY-2 SWH shortly after its launch [7,12,13,17]. Some works focused on the HY-2 SWH correction method [15,16]. A conclusion can be drawn that HY-2 tends to overestimate the low sea state and underestimate wave heights throughout the remaining range of heights, especially for the high sea state [12,16].…”

“…A conclusion can be drawn that HY-2 tends to overestimate the low sea state and underestimate wave heights throughout the remaining range of heights, especially for the high sea state [12,16]. The overall negative bias is 0.2 m and the RMS is 0.3 m [15]. All of these studies employed a 50 km spatial and 0.5 h temporal window for collocation, but the error caused by the spatial window was not discussed.…”

Empirical Algorithm for Significant Wave Height Retrieval from Wave Mode Data Provided by the Chinese Satellite Gaofen-3

“…Using the temporal and spatial co-location criteria of 1 h and 100 km, only Ku-band HY-2A, Jason-2/3, and the Ka-band SARAL/Altika could be co-located with GF-3 WM images during our study period, with the location depicted in Figure 4. Following the calibrations proposed by Liu et al [43]: The SWH hindcasts used here are from the database of IOWAGA (Integrated Ocean Waves for Geophysical and other Applications) project of IFREMER. The wave hindcasts were performed using the ocean wave model of WaveWatch III (WW3), with the parameterization of Rascle et al [42] forced by the European Centre for Medium-range Weather Forecasts (ECMWF) winds.…”