## 3 Results and Discussion
### Analysis of AMVs Error and Algorithm Requirements under Different Weather Systems  
In order to analyze AMVs error under different weather conditions, we choose 400 hPa height wind field as an example, where the error decreases significantly with the revisiting time. As shown in Figure 7a,c, the wind speed RE and wind direction AE are 35.17% and 22.79\({}^{\circ}\), respectively, while the revisiting time is set to 5 min. These errors are reduced to 25.37% and 16.05\({}^{\circ}\), respectively, when the revisiting time extends to 50 min. Figure 8 depicts the WRF wind field (Figure 8a,c), the OF wind field (Figure 8b,d), and the relationship between them (Figure 8e,f). Comparing Figure 8b,d, the tracking effect in the typhoon's central area is significantly more accurate than that of 50 min. The Beaufort wind force scale indicates that force 8 represents fresh gale in a cyclonic storm [35]. The scale 8 ranges in 17.5-20.5 m/s. Here, we assume the wind speed in 0-17.5 m/s mainly exists in the non-tyhoon area and \"\(>\)17.5 m/s\" represents the typhoon area. Figure 8g,h show the wind-speed errors (AE and RE) in the non-tyhoon area decreases and increases in the typhoon area, respectively. As the high-wind-speed range is mainly located in the typhoon area, where the water vapor stays strong and exhibits rotational movement under strong wind speed conditions, longer revisiting time will result in inaccurate displacement trackingdue to the large difficulty of directly judging the large and rotating water vapor trajectory through two images. The typhoon area we choose to analyze covers 600 \(\times\) 500 km, containing the fields of which the wind speed is greater than force 8 (Figure 9a). Figure 9 depicts the WRF wind field (a) and the OF wind field (b) in the center of the typhoon area at 400 hPa when the revisiting time is set to 5 min. The wind-tracking accuracy outside the typhoon eyewall is high, while it is poor in the eyewall area. As shown in Table 4, the wind speed RE and wind direction AE in the whole simulated area are 35.17% and 22.9\({}^{\circ}\), respectively, while those error parameters in the typhoon area decrease to 15.31% and 7.67\({}^{\circ}\), respectively. Combining with Figure 8b,d, we can conclude that the satellite revisiting time of the data used for fast-evolving and fast-changing weather systems wind tracking such as typhoons should be as short as possible. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Whole Simulated Area** & **Typhoon Area** \\ \hline Wind speed RE & 35.17\% & 15.31\% \\ Wind direction AE & 22.9\({}^{\circ}\) & 7.67\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The wind-tracking errors of the whole simulated area and typhoon central area at 400 hPa, with revisiting time set to 5 min. Figure 7: The wind speed RE (**a**), wind speed RMSE (**b**), and wind direction AE (**c**) between WRF wind fields and Farneback OF wind fields of different satellite revisiting times at 12 heights. Figure 8: WRF wind field at 400 hPa with the revisiting time of 5 min (**a**) and 50 min (**c**). Farneback wind field at 400 hPa with the revisiting time of 5 min (**b**) and 50 min (**d**). The background color shows the wind speed. Comparison of two wind fields with the revisiting time of 5 min (**e**) and 50 min (**f**). The X- and Y-axis of (**e**,**f**) represent OF and WRF wind speed, respectively. The wind speed RE (primary Y-axis) and AE (secondary Y-axis) of non-tyhoon area (**g**) and typhoon area (**h**). Figure 8g shows that the wind-tracking error of most non-typhoon areas decreases with the increase of revisiting time. To explore the factors affecting wind retrieval errors in the slowly evolving weather systems, we select an area with low water vapor content and relatively smooth water vapor movement, as shown in Figure 10. It covers 800 \(\times\) 1000 km. Figure 10a-d, respectively, represent the WRF wind field and OF wind field of the selected smooth area at 400 hPa with 5 min (50 min) revisiting time.