## 4 Results
### Statistical Analysis of HY-2 Scatterometer and SMAP Radiometer  
To evaluate the performance of HY-2 scatterometer wind retrieval, we collocated these data with SMAP high wind speeds. Figure 3 presents the scatterplot of wind speeds from HY-2B/C/D scatterometers and the SMAP radiometer under tropical cyclone conditions, with summaries of the statistical metrics, including bias (HY-2-SMAP), RMSE, and correlation coefficient (R). However, the wind range of HY-2C/D is less than 30 m/s, and the number of matchups between HY-2 and SMAP is small, which makes it impossible to evaluate the accuracy of its high wind speed. In addition to the HY-2B data volume being enough to evaluate the accuracy of wind speed in tropical cyclones, the other two require more data for support. Therefore, the overall performance is implemented and analyzed by HY-2B/C/D winds. First, HY-2B/C/D slightly underestimate the wind speed with a negative bias of -1.59 m/s in tropical cyclones. A high correlation coefficient of 0.86 implies that the two datasets have good correlation when the wind speeds are below 20 m/s. However, it can be seen from the scatter density diagram that this underestimation is particularly obvious when the wind speeds exceed 20 m/s. The RMSE of the wind speed is 4.27 m/s, demonstrating the accuracy of HY-2 scatterometer wind retrieval.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multirow{2}{*}{**Tropical Ocean**} & \multicolumn{4}{c}{**Number of Captured Tropical Cyclones**} \\ \cline{3-6}  & & **SMAP** & **HY-2B** & **HY-2C** & **HY-2D** \\ \hline \multirow{4}{*}{2019} & Atlantic Ocean & 3 & 3 & 0 & 0 \\  & Western Pacific Ocean & 13 & 11 & 0 & 0 \\  & Eastern Pacific Ocean & 3 & 2 & 0 & 0 \\  & Northern Indian Ocean & 4 & 1 & 0 & 0 \\  & Southern hemisphere & 13 & 2 & 0 & 0 \\ \hline \multirow{4}{*}{2020} & Atlantic Ocean & 23 & 20 & 3 & 0 \\  & Western Pacific Ocean & 15 & 11 & 4 & 0 \\  & Eastern Pacific Ocean & 11 & 10 & 0 & 0 \\  & Northern Indian Ocean & 3 & 0 & 0 & 0 \\  & Southern hemisphere & 28 & 21 & 5 & 0 \\ \hline \multirow{4}{*}{2021} & Atlantic Ocean & 15 & 14 & 7 & 4 \\  & Western Pacific Ocean & 17 & 11 & 4 & 8 \\ \cline{1-1}  & Eastern Pacific Ocean & 15 & 13 & 6 & 5 \\ \cline{1-1}  & Northern Indian Ocean & 4 & 3 & 1 & 0 \\ \cline{1-1}  & Southern hemisphere & 23 & 20 & 7 & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of tropical cyclones captured by SMAP during the period 2019–2021.  
According to the wind speed of SMAP, the wind speed distribution of HY-2 is statistically analyzed within the range of 0-50 m/s. Considering the amount of data in each wind speed interval, RMSE and bias are counted every 2 m/s at 0-35 m/s to facilitate understanding of the accuracy of HY-2 in each wind speed interval. Figure 4 presents the 2D histogram of the wind speed from the HY-2B/C/D scatterometer, which demonstrates that most of the data are distributed in the range 5-20 m/s. It can be seen from Figure 5 that the RMSEs of the HY-2B/C/D wind speeds are less than 2 m/s in the moderate wind speed range (5-18 m/s), and the biases are generally negligibly small, showing that HY-2 wind speeds are consistent with those measured by SMAP. The HY-2 wind speeds lie within the range 0-2 m/s, and the RMSE also exceeds 2 m/s, as shown in Figure 5. The main reason is that the scatterometer uses the backscattering signal; when the sea surface is too calm (winds \(<\) 2 m/s), similar to specular scattering, the backscattering signal is weak and cannot be accurately measured. As can be seen from Figure 5, the deviation (HY-2-SMAP) decreases from 0 to -12 m/s with the increase in wind speed, and RMSE also increases from 2 to 12 m/s. Particularly when the wind speed exceeds 25 m/s, HY-2 fails to accurately retrieve the high wind speed relative to SMAP. Therefore, it is necessary to revise the high wind speed products from HY-2B/C/D according to the relationship between the two, so as to provide a reliable high wind speed estimate for tropical cyclone monitoring.