# Blending Sea Surface Winds from the HY-2 Satellite Scatterometers Based on a 2D-Var Method
## 2 Data and Methods  
### Data  
The main parameters of HY-2B, -2C and -2D satellites are summarized in Table 1. These three satellites were launched in October 2018, September 2020, and May 2021, respectively. Consequently, the overlapped period is not adequate for developing a long-term time series of global sea surface vector wind products. The main objective of this paper is to verify the proposed 2D-Var method for blending HY-2 satellite winds, and, thus, only six months (December 2021-May 2022) of the HSCATs 25-km Level 2 (L2) wind data are used in the study. Such data are provided by the National Satellite Ocean Application Service (NSOAS) through the Ocean Satellite Data Distribution System (OSDDS). Note that, OSDDS provides two types of HSCAT winds, namely \"pwp\" and \"dps\" separately. This paper uses the HSCAT \"pwp\" data.  
Figure 1 illustrates the six-hourly coverage of the three HSCATs on 7 May 2022, at UTC 00 h, 06 h, 12 h, and 18 h, respectively. Each panel shows the observations that are less than 3 h distance in time to the above-mentioned hours. It is clear that the three HSCATs are able to observe more than 70% of sea surface area in six hours, e.g., the coverage ratio over the sea surface is about 70%, 75.2%, 74.6%, and 75.6% for Figure 1a-d, respectively.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Satellite**} & **Altitude** & **Inclination** & \multirow{2}{*}{**LTDN *} & \multicolumn{2}{c}{**Repeat Cycle**} \\ \cline{3-5}  & **(km)** & & (\({}^{\circ}\)) & & **The Early Stage** & **The End Stage** \\ \hline HY-2B & 971 & 99.3 & 6:00 am & 14 days & 168 days \\ HY-2C & 957 & 66 & Drifting & 10 days & 400 days \\ HY-2D & 957 & 66 & Drifting & 10 days & 400 days \\ \hline \hline \end{tabular}
*LTDN: Local time of the descending node.  
\end{table}
Table 1: Main parameters of the HY-2 satellites.  
The fifth generation of ECMWF atmospheric reanalysis (ERA5) winds is used as the background data when blending the HSCATs data. This study uses the ERA5 neutral wind vectors at a 10 m height with approximately 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) spatial resolution and six-hourly temporal resolution. Before the data analysis, both ERA5 background and HSCATs observations are interpolated bi-linearly to a regular latitude/longitude grid  
Figure 1: Six-hourly coverage of the three HSCATs on May 7, 2022, at (**a**) 00 h, (**b**) 06 h, (**c**) 12 h, and (**d**) 18 h, respectively. Color bars indicate the wind speed.  
of 0.25\({}^{\circ}\) resolution. Consequently, one can collocate the HSCATs data with the ERA5 background winds easily by setting a temporal criterion of 3 h distance.  
Finally, the moored buoy winds are used for verification purposes. Such data are disseminated through the Global Telecommunications System (GTS) stream and, subsequently, quality controlled and archived at the ECMWF Meteorological Archive and Retrieval System (MARS). Note that the buoy winds are reported hourly and segregated into 1 m/s speed bins and 10\({}^{\circ}\) direction bins. The measured wind speed at a given anemometer height is converted to 10 m equivalent neutral winds using the Liu-Katsaros-Businger (LKB) model to make them comparable with the HSCATs and the ERA5 winds [17]. Eventually, the buoy measurements that are 5 min distance in time and 0.125\({}^{\circ}\) distance in space from the ERA5 grids are used to verify both the HSCATs and the ERA5 winds.