# Validation of Drifting Buoy Data for Ocean Wave Observation
## 3 Results  
### Comparison with the GPS Buoys  
Buoy data and ERA5 data at 3-h intervals were compared. The drifting buoy data observed for 512 s (GPS buoy data observed for 1024 s) were used for comparison (Sections 2.1 and 2.3). Figure 2a shows the scatter plot between the GPS-observed wave heights (\(H_{g}\)) and the drifting buoy-measured wave heights (\(H_{b}\)). The maximum distance threshold \(R_{dmax}\) between a GPS buoy and a drifting buoy was 5 km. The mean distance of the buoys was \(R_{a}=3.6\) km. Table 2 summarizes the skill metrics of the comparison, which shows wave height agreement was good.  
Figure 2b shows the scatter plot of the wave periods. The maximum distance threshold of the buoys was the same as that in Figure 2a. The correlation of wave periods was smaller than that of wave heights, while the SI of wave periods was smaller than that of wave heights.  
We compared GPS-observed, drifting buoy-observed, and ERA5 wave parameters (\((H_{g},H_{b},H_{e})\) and \((T_{g},T_{b},T_{e})\)).  
Figure 2c shows the scatter plot of the GPS-observed wave heights (\(H_{g}\)), drifting buoy-measured wave heights (\(H_{b}\)), and ERA5 wave heights (\(H_{e}\)) at the drifting buoy positions. The \((H_{g},H_{b})\) points were indicated in black, while the \((H_{g},H_{e})\) points were indicated in red in Figure 2c. The maximum distance threshold between a GPS buoy and a drifting buoy was \(R_{dmax}=\)15 km, and the mean distance was \(R_{a}=9.1\) km. The 3 h wave heights (\(H_{g}\), \(H_{e}\), and \(H_{b}\)) were compared when all of them were estimated. The number of data was \(N_{c}=102\).  
The spatial variability of the wave parameters was large near the coast, and the distance \(R_{dmax}\) was preferably smaller for the wave parameter comparison. In contrast, the number of data was smaller when the distance \(R_{dmax}\) was smaller. The value of \(R_{dmax}=\)15 km (\(R_{a}=9.1\) km) was found to optimize the balance between maximum distance and number of data.  
The mean drifting buoy-measured wave height was closer to mean GPS buoy-measured wave height than mean ERA5 wave height. The correlation \(r_{c}(H_{g},H_{b})\) was higher than that \(r_{c}(H_{g},H_{e})\), the RMSD \(R_{d}(H_{g},H_{b})\) was smaller than \(R_{d}(H_{g},H_{e})\), and the scatter index \(SI(H_{g},H_{b})\) was smaller than \(SI(H_{g},H_{e})\). The agreement between the drifting buoy-measured wave height (\(H_{b}\)) and the GPS buoy-measured wave height (\(H_{g}\)) was better than that between the ERA5 wave heights (\(H_{e}\)) and \(H_{g}\).  
Figure 2d shows the scatter plot of the GPS-observed wave periods (\(T_{g}\)), drifting buoy-measured wave periods (\(T_{b}\)), and ERA5 wave periods (\(T_{e}\)) at the drifting buoy positions for \(R_{dmax}=\)15 km.  
The correlation \(r_{c}(T_{g},T_{b})\) was higher than \(r_{c}(T_{g},T_{e})\). The RMSD of the wave period \(R_{d}(T_{g},T_{b})\) was slightly smaller than \(R_{d}(T_{g},T_{e})\), although that for the buoy-measured wave periods (\(T_{g},T_{b}\)) was \(T_{1/3}\), and that for the ERA5 wave period \(T_{e}\) was \(T_{0}\). The scatter index \(SI(T_{g},T_{b})\) was smaller than \(SI(T_{g},T_{e})\). The agreement of the drifting buoy-measured wave data with the GPS buoy-measured wave data was better than that of the ERA5 wave data.