# Joint Use of Spaceborne Microwave Sensor Data and CYGNSS Data to Observe Tropical Cyclones
## 3 Data Processing
The minimum wind speed of a TC was set as the threshold used for region segmentation. Following the region segmentation, both the ROI and non-ROI in all images used for data fusion were fused by the maximum operator, and this process can be written as  
Figure 2:1  
where \(P_{\text{fusion}}\) represents the final fusion image, \(\text{max}(\cdot)\) denotes the maximum operator, and \((u,v)\) denotes the position of a pixel. Thus, the final fusion image can be obtained, such as the composite images displayed in Figures 7 and 8. It can be clearly seen from Figure 7 that it is difficult to fix the center of Hurricane Florence using the three results acquired at 13:30 UTC, 14:30 UTC, and 15:30 UTC, respectively. However, the complete structure of Hurricane Florence can be obtained through data fusion. After processing all the observation data, the daily observation results were plotted in turn in each row of the final image, as shown in Figure 2:2  
The third step involves estimating the TC location. When Hurricane Florence is completely included in the SMAP radiometer data, as shown in Figure 9b, its diameter can be directly measured in terms of its shape. We found that its diameter was about 145 km, and hence a black circle with a radius of 150 km was adopted to fix its center position, similar to the marking method described in [35]. Since the surface winds are always strongest along the margin of the TC eye [36], the location of this black circle was automatically determined in terms of the shape of the high wind-speed region and the movement speed of TC. Here, the automatic TC center-fixing algorithm, which was applied to the SMAP and CYGNSS fusion results, is based on visual saliency and the inflow angle model, similar to the method used for center location of typhoons in SAR images [37]. In this algorithm, each color image shown in Figure 9 was first transformed into a grayscale map to make it similar to SAR images by using the weighted average. Then, with regard to the gray-level feature, the salient  
Figure 2:3 (**a**) An observation result for Hurricane Florence acquired by the SMAP radiometer at 20:23 UTC on 11 September 2018. Since this result contains only a fraction of Hurricane Florence, the CYGNSS results from 20:30 to 23:30 UTC (shown in subplot (**b**)) were used to obtain the complete structure of the TC. The final fusion result is shown in subplot (**c**). region corresponding to the high wind-speed region was constructed in the grayscale map based on the visual saliency. Then, the skeleton line of the TC was extracted according to the shape of the salient region and the movement speed of the TC. Finally, the skeleton line of the TC was matched with the inflow angle model to determine the central position of the TC [37]. Nevertheless, it is still difficult to fix the center of the TC with some fusion results, such as shown in Figure 9(a2), not only due to the sparse and narrow tracks of CYGNSS satellite but also because sometimes the incomplete structure of a TC was included in such fusion results. Finally, the point, which is the center of the black circle, was further extracted to form the estimated track of TC. The final step includes measuring the maximum wind speed around the TC inner core. If a complete structure of the TC was included in the SMAP radiometer data, the maximum wind speed in the region marked by the black circle was obtained directly from the SMAP radiometer data. If the CYGNSS and SMAP radiometer data were combined to obtain a complete structure of the TC, the black circle contains a fraction of the CYGNSS data and a part of the SMAP radiometer data (see Figure 9(d1)), therefore, the CYGNSS Level 2 data products derived by the YSLF GMF and the SMAP radiometer data were used to search for the corresponding maximum wind speed at the pixel level. Then, these two results were compared with each other to obtain the final maximum wind speed in the region marked by the black circle.