## 3 Methodology
### Estimation of the Maximum Wind Speed using Selected Parameters  
As described in the above section, it is shown that we can estimate the maximum wind speed by using a single parameter, with a certain accuracy (with 6.7-7.8 m/s RMSEs). However, including more predictors and using linear regression techniques could improve the estimation of TC intensity [40; 41]. A stepwise regression method was used to select the most useful predictors from all of the above-mentioned 1785 parameters. Finally, 15 parameters were selected as follows. TB19H_MIN_C100, SSW_MIN_C100, TB19H_RAPT250_C075, SSW_MAX_C250, TB37H_RAPT210_C075, TB19H_RAPT_270_A22550, TB22V_RAPT270_A125150, TB37H_MAX_A125150, PCT92_MAX_C125, PCT92_MAX_C100, TB37V_RAPT260_C075, TB37H_RAPT180_A125150, TB37H_MIN_C100, PCT92_MAX_A150175, and SSW_MEAN_C100.  
PCT (polarization-corrected temperature) is the parameter that is proposed by [PERSON] et al. [42], which represents the radiation eliminating the radiation from ocean using polarization diversity, and is calculated as:  
\[\mathrm{PCT}=1.818\mathrm{TB_{v}}-0.818\mathrm{TB_{h}} \tag{1}\]  
where \(\mathrm{TB_{v}}\) is the TB for the vertically polarized channel, and \(\mathrm{TB_{h}}\) is that of the horizontally polarized channel at 85 GHz of the SSM/I. In this paper, we used PCT at 92 GHz of the SSMIS as candidate parameters.  
However, the number of variables does not correspond to \"the more the better\" in linear regression. Figure 6 shows the dependence of R\({}^{2}\) and RMSE with an increase in variable numbers. From Figure 6, we can conclude that when the number of parameters was six, the minimum RMSE and maximum R\({}^{2}\) values occurred. Therefore, these six parameters were chosen as the final regression variables, and the proposed TC intensity estimation model is as follows:  
Figure 5: Scatter plot of the maximum wind speed between the estimated results, using the SSW_MEAN_C100 parameter-only model, and the CMA best-track data. (**a**) All of the testing data. (**b**) Subgroup testing data of the TD, TS, and STS. (**c**) Subgroup testing data of TY, STY, and super TY.  
\[\begin{array}{ll}\text{V}_{max}&=0.7582\times\text{SSW\_MIN\_C100}\\ &+0.1645\times\text{TB19H\_RAPT250\_C075}\\ &+0.3410\times\text{SSW\_MAX\_C250}\\ &-0.0722\times\text{TB37H\_RAPT210\_C075}\\ &+0.0806\times\text{TB22V\_RAPT270\_A125150}\\ &+0.2861\times\text{TB37H\_MIN\_C100}-46.884\end{array} \tag{2}\]  
where, Vmax is the maximum wind speed of TC, SSW_MIN_C100 is the minimum wind speed within 1 degree of distance from the TC center, TB19H_RAPT250_C075 is the percentage of pixels with TB19 GHz, H-pol larger than 250 K within 0.75 degrees of distance from the TC center, SSW_MAX_C250 is the maximum wind speed within 2.5 degrees of distance from the TC center, TB37H_RAPT210_C075 is the percentage of pixels, with TB37 GHz, H-pol being larger than 210 K within 0.75 degrees from the TC center, TB22V_RAPT270_A125150 is the percentage of pixels, with TB22 GHz, V-pol being larger than 270 K between 1.25 degrees and 1.5 degrees from the TC center, and TB37H_MIN_C100 is the minimum TB of 37 GHz horizontal polarization within 1 degree of distance from the TC center.