## 3 Methodology
### Relationship between the Satellite-Observed Parameters and the Maximum Wind Speed
#### 3.1.2 SSW Parameters  
In total, 105 SSW parameters were computed. In this section, the correlation between the SSW parameters and TC intensity will be analyzed. The relationship between the single parameter of SSW and the MSW of TC was analyzed by regression analysis, and then the RMSE between the estimated and the best-track MSW was calculated. The correlation coefficients between all of the parameters and the MSW of TC were calculated, and a significant horizontal \(t\)-test was conducted for the correlation coefficient.  
The highest 15 correlated SSW parameters, correlation coefficients, and RMSEs with the CMA best tracks are shown in Table 3. The highest correlated parameter was for SSW_MEAN_C100. The highest correlation coefficient was 0.83, and the RMSE was 7.09 m/s.  
From Table 3, the following conclusions can be inferred:  
The most closely correlated parameter for all radii is MEAN (and a few MIN). MEAN is computed over the entire corresponding region, and MIN describes that all other points are larger than this number; hence, it may be a more appropriate indicator to represent the TC intensity.  
The parameters in the surrounding regions (0.75-1.5\({}^{\circ}\) away from the TC center) tended to give higher correlations with the CMA best-track data. We speculate that this reason is related to the radius of the wind circle. For most TCs, the radius of the Beaufort wind force 10 cyclone (24.5-28.4 m/s) is 50-100 km, and that of the wind force 12 (32.7-36.9 m/s) cyclone is 100-150 km.  
Figure 4: Scatter plot of the maximum wind speed between the estimated results, using the TB19H_MIN_C100 parameter-only model and the China Meteorological Administration (CMA) best-track data. (**a**) All of the testing data. (**b**) Subgroup testing data of tropical depression (TD), tropical storm (TS), and severe tropical storm (STS). (**c**) Subgroup testing data of typhoon (TY), severe typhoon (STY), and super typhoon (Super TY).  
\begin{table}
\begin{tabular}{c c c} \hline
**SSW Parameter** & **Correlation Coefficient** & **RMSE (m/s)** \\ \hline SSW\_MEAN\_C100 & 0.83 & 7.09 \\ SSW\_MEAN\_C125 & 0.82 & 7.10 \\ SSW\_MEAN\_C150 & 0.81 & 7.20 \\ SSW\_MIN\_C100 & 0.81 & 7.29 \\ SSW\_MEAN\_C075 & 0.81 & 7.24 \\ SSW\_MEAN\_C175 & 0.81 & 7.38 \\ SSW\_MIN\_A075100 & 0.80 & 7.20 \\ SSW\_MEAN\_C200 & 0.79 & 7.57 \\ SSW\_MEAN\_A075100 & 0.79 & 7.36 \\ SSW\_MEAN\_C225 & 0.78 & 7.78 \\ SSW\_MEAN\_A100125 & 0.78 & 7.69 \\ SSW\_MIN\_C075 & 0.78 & 7.70 \\ SSW\_MIN\_C125 & 0.78 & 7.87 \\ SSW\_MAX\_C100 & 0.77 & 7.90 \\ SSW\_MAX\_C125 & 0.77 & 7.91 \\ \hline \end{tabular}
\end{table}
Table 3: The highest correlated sea surface wind (SSW) field parameters.  
The test dataset in Table 1 were used to verify the performance from using only the SSW_MEAN_C100 parameter. The results of the validation are shown in Figure 5. The horizontal axis is the maximum wind speed, as estimated by the SSW parameter model, and the vertical axis is the CMA TC best-track intensity data. The R\({}^{2}\) of the verification result for the test dataset is 0.70, the RMSE is 7.84 m/s, the MAE is 6.11 m/s, and the BIAS is \(-1.49\) m/s. Figure 5b,c shows the differences between the SSW parameter model-estimated TC intensity, and the CMA best-track data in two subgroups. Figure 5b relates to lower-intensity TCs, i.e., tropical depressions, tropical storms and severe tropical storms. Figure 5c relates to typhoons, severe typhoons, and super typhoons.