## 2 Study Area and Data
### Data Description
#### 2.2.3 Best-Track Data  
Aircraft reconnaissance obviously provides the most accurate observations [33]. However, the global aircraft reconnaissance program has been inactivate since 1987, except for in the Atlantic Basin to the west of 55\({}^{\circ}\)W [34]. Although, a few dropsondes have been launched from aircraft reconnaissance flights during Pacific Asian Regional special observation campaigns, there is an insufficient number of TC cases for a statistically significant validation of the method using dropsondes alone. The best track intensities are initially calculated by using DT methods, and then modified by other observations such as aircraft reconnaissance, station, and ship weather reports, and Doppler radar. Thus, the best track intensity data are the only available references at present in the Northwest Pacific regions. The China Meteorological Administration (CMA) best-track datasets [35] were ultimately chosen as the validation data for this study. This best-track data can provide the location and intensity of TCs every six hours. Among the CMA best-track data, there are six types of TC intensity grades: Tropical depression (TD, 10.8-17.1 m/s), tropical Storm (TS, 17.2-24.4 m/s), severe tropical storm (STS, 24.5-32.6 m/s), typhoon (TY, 32.7-41.4 m/s), severe typhoon (STY, 41.5-50.9 m/s), and super typhoon (Super TY, \(\geq\)51.0 m/s).  
The data from 2012-2017 were selected for model training and validation. Above all, HY-2A microwave scatterometer and SSMIS microwave radiometer data were matched according to the satellite overpass time difference. When collocating the satellite observations with the best track data, we used a time-varying criterion based on the TC intensity variation. The basic idea of this scheme is that we relax the restrictions for TCs with stable intensity and set more strict requirements for TCs with rapid intensity changes. There are three thresholds of overpass time difference: 10 min, 30 min, and 60 min. If the variation in the maximum-sustained wind (MSW) is greater than 5 m/s during the 6-hour interval of the nearest best track record time, the threshold of the overpass time difference between HY-2A and SSMIS is 10 min. If the variation in the MSW is less than 5 m/s, the threshold of the overpass time difference is 30 min. If there is no change in the MSW, the threshold is set to 60 min. This strategy brings us more matchup data, and reduces the impact of the overpass time difference on the estimated MWS. Later, the referenced MWS is linearly interpolated from the best track data, according to overpass time of SSMIS and HY-2A. For example, the overpass time of SSMIS and HY-2A are 6:10 (UTC), 6:20 (UTC), respectively. The intensity of TC corresponding to 6:15 (UTC) serves as our reference intensity. The intensity of 6:15 (UTC) is linearly interpolated according to the data values corresponding to the best track 6:00 (UTC) and 12:00 (UTC).  
Lastly, 406 valid satellite-observed data contained in 119 TCs were selected. Among them, 261 satellite-observed data in 96 TCs are used as training sample data, and the other 145 data are used as test data (as shown in Table 1).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multicolumn{2}{c}{**Training Data**} & \multicolumn{2}{c}{**Test Data**} \\ \cline{2-5}  & **TCs** & **Sample** & **TCs** & **Sample** \\ \hline
2012 & 18 & 55 & 5 & 31 \\
2013 & 23 & 63 & 5 & 29 \\
2014 & 15 & 43 & 3 & 10 \\
2015 & 16 & 47 & 7 & 57 \\
2016 & 9 & 15 & 1 & 4 \\
2017 & 15 & 38 & 2 & 14 \\ \hline Total & 96 & 261 & 23 & 145 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Information for the training data and test data.