# Blending Sea Surface Winds from the HY-2 Satellite Scatterometers Based on a 2D-Var Method
## 1 Introduction  
Sea surface vector wind is an essential climate variable that influences many physical processes over the air-sea interface [1, 2, 3]. There is a growing demand for accurate sea surface wind products with higher resolution and, notably, more frequent coverage, for tracking transient features and resolving air-sea interaction and its diurnal cycle, etc. For spaceborne scatterometers, which are the most relevant sensors in sea surface wind observation, the World Meteorological Organization (WMO) specifies a 25 km spatial resolution and six-hourly temporal sampling as the operational goals [4]. However, a single scatterometer can only acquire wind data with a temporal resolution of 12 h, covering less than 90% of the global sea surface per day. Consequently, numerous methodologies have been developed to blend the satellite wind in order to enhance the temporal coverage of ocean vector winds without reducing the short-scale true variance resolved by the remote sensing technique.  
In the early days, the number of operational scatterometers was too few for developing a gap-free and high-temporal resolution (e.g., six-hourly) wind product only based on the scatterometer-derived wind data. Consequently, the scatterometer data are usually blended with ancillary wind data, such as buoy measurements, radiometer wind speeds, and numerical weather prediction (NWP) winds. For instance, the Cross-Calibrated Multi-Platform (CCMP) ocean vector wind analysis used a two-dimensional variational analysis (2D-Var)method to combine satellite scatterometer and radiometer winds, in-situ buoy observations, and NWP background field to fill the sampling gaps [5; 6; 7]. A similar product was described in ref. [8], which used the kriging technique instead of the 2D-Var. Moreover, there were two other blended wind products with lower temporal resolution (daily), which were developed in the context of the Objectively Analyzed air-sea Fluxes (OAFlux) project and the merging of Advanced Scatterometer (ASCAT) winds with the NWP reanalysis [9; 10], respectively. These wind products extend more than 20 years and have made significant contributions to the research on mesoscale air-sea interaction.  
More recently, international collaboration is highlighted with the objective of developing a global virtual constellation of wind scatterometers for high-resolution ocean forcing. Although the number of operational scatterometers is still insufficient to fully meet the above WMO requirements [11], the attempt in ref. [12] shows that the spatial scale of global winds can be effectively improved by blending the ASCATs data with the European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis. Later in 2021, more scatterometer systems measuring at the different local times have been available. In particular, the Haiyang-2 (HY-2) constellation, which consists of three satellites, has been successfully developed by China. Notably, HY-2B is operated in a sun-synchronous orbit, while HY-2C and -2D are operated in drifting orbits, such that the three HY-2 satellite scatterometers (HSCATs) are able to provide unprecedented short revisit intervals over the global sea surface [13; 14; 15; 16].  
This study aims at developing a six-hourly mesoscale wind product over the global sea surface only based on the HSCATs wind data and the ECMWF reanalysis winds. Section 2 describes the datasets and the methods used in this paper. Section 3 presents a brief overview of the wind quality for the three HSCATs. In Section 4, the blended wind product is verified comprehensively using buoy data as a reference. Finally, conclusions are summarized in Section 5.