## 3 Methods
### Sea Fog Identification
#### 3.1.2 Feature Selection  
After obtaining the spectral features of each object, it is necessary to analyze the spectra to select features with better classification effect. The visible band of the Advanced Himawari Imager sensor onboard Himawari-8 consists of channels 1-3 (central wavelengths of 0.47, 0.51, and 0.64, respectively); channels 4-6 are near-infrared bands (central wavelengths of 0.86, 1.6, and 2.3, respectively), and channels 7-16 are infrared and far-infrared bands (central wavelengths of 3.9, 6.2, 6.9,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,  
and near-infrared spectral characteristic curves of different objects are shown in Figure 5a. In the first three bands, the mid-high level clouds and the sea surface are significantly different from the sea fog and low clouds, but it is difficult to distinguish between low clouds and sea fog. With an increase of wavelength, the reflectance of all kinds of features gradually decrease, but the rate of change of sea fog in band 5 is significantly slower than that of other features, and the difference between the sea fog and the low clouds becomes obvious. According to [34], there are a large number of water droplets of 1 \(\upmu\)m diameter in sea fog, which increase the reflectance measured in bands 4 and 5 and slow down the rate of change of reflectance in band 5; such behavior provides an obvious means to distinguish sea fog from other objects.  
To identify more features, NDSI [11] is constructed with data from bands 2 and 5 (i.e., 0.51 \(\upmu\)m and 1.6 \(\upmu\)m), calculated using Equation (1) as:  
\[\text{NDSI}=\frac{B_{0.51\upmu}-B_{1.6\upmu}}{B_{0.51\upmu}+B_{1.6\upmu}}, \tag{1}\]  
Figure 4: Sample selection using CALIIPSO-VFM data, (**a**) AHI image on 15 June 2018 at 05:10 UTC. (**b**) Objects classification of CALIIPSO-VFM data from 15 June 2018 at 05:17 UTC.  
Figure 5: Spectral analysis, (**a**) the spectral reflectivity of bands 1–6 and NDSI for AHI data; (**b**) the brightness temperature of bands 7–16 for AHI data, the red box represents the regions of bands 13, 14, and 15 with obvious differences in brightness temperature of each objects.  
where \(B_{0.51\text{um}}\) and \(B_{1.6\text{um}}\) represent bands with central wavelengths of 0.51 um and 1.6 um, respectively, in the AHI data. It can be seen from Figure 5a that NDSI provides a significant distinction between sea fog and low clouds. There are also distinct intervals between different objects.  
Figure 5b shows the characteristic curves of the different infrared bands. The brightness temperatures of sea fog and the sea surface are very similar, and cannot be used to effectively distinguish between them, even though the average height of low clouds is higher than that of sea fog, and the brightness temperature of the former is slightly lower. The brightness temperatures of middle-high clouds are the lowest, and this difference is more obvious in bands 13, 14, and 15 (see, e.g., the red dotted box in Figure 5b). Therefore, these three bands can be selected to further distinguish low clouds and sea fog.  
Based on the spectral analysis, we selected bands 1, 2, 3, 5, and 14 and NDSI as classification features to construct the sea fog sample set.