Assessment of the swell impact on HY-2 SCAT wind products

The first Chinese Ku-band scatterometer (SCAT), carrying on the satellite HY-2, has provided the global ocean surface wind vector products since its successful launch in August 2011. The HY-2 SCAT wind products are estimated based on the geophysical model function (GMF), where only the wind vector, while no wave information is taken into consideration. Thus, it is still an open issue that the contribution to HY-2 SCAT wind product errors owing to the swell influence. In this paper, the swell impact on HY-2 SCAT wind speed and direction product quality is presented through the comparison of the one year HY-2 products with buoy data. The NDBC buoy wave spectra were reconstructed from measurements, partitioned using watershed algorithm, and categorized into 4 groups of pure swell, swell and wind sea dominated mixed sea, and pure wind sea. The wave dependency of wind residuals between the HY-2 and the buoy were investigated. The comparison results on different swell components indicate the significant influence of the swell present on HY-2 SCAT quality in terms of both wind speed and wind direction. It is recommended that the information on sea state should be integrated into the wind retrieval algorithm for Ku-band scatterometers (HY-2 SCAT and also the future scatterometer on CFOSAT), in order to improve the accuracy.