## 3 Results
### Comparison between Model Simulations and HF Radar Data  
Unlike the in situ pointwise measurements at stations A1, B1, and B2, the HF radar data covered a wide observing area, derived from remotely sensed electromagnetic echoes. Therefore, it is well known that HF radar data may contain uncertain inversion errors due to the environment, noise, inversion algorithms, etc. A comparison between the model and HF radar data can provide a synoptical view of the radar data quality. Figure 6 shows monthly averaged flows from radar measurements and the model. In general, the two flow patterns are similar. A strong along-coast flow appears on the left-hand side of the PRE mouth, with the largest velocity of more than 15 cm/s. This is caused by the Ekman  
Figure 5: Vertical profiles of tidal ellipses (M\({}_{2}\), S\({}_{2}\), K\({}_{1}\), O\({}_{1}\)) at station A1: model (red) and ADCP (blue). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Site**} & \multicolumn{5}{c}{**Statistical Matrices**} \\ \hline \multirow{3}{*}{A1 (1 st level)} & & MB & CC & RMSD & R2 & WS \\  & U & 0.0002 & 0.927 & 0.026 & 0.848 & 0.952 \\  & V & \(-\)0.0003 & 0.998 & 0.071 & 0.971 & 0.991 \\ B1 & U & 0.0002 & 0.941 & 0.106 & 0.848 & 0.949 \\  & V & \(-\)0.0002 & 0.958 & 0.12 & 0.902 & 0.971 \\ B2 & U & \(-\)0.0001 & 0.951 & 0.03 & 0.856 & 0.951 \\  & V & \(-\)0.0001 & 0.940 & 0.112 & 0.883 & 0.967 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Statistics of tidal currents (M2, S2, O1, and K1 constituents) for A1, B1, and B2. effects that the northeasterly winds push the flow shoreward, forming the coastal jet. The coastal jet is trapped within ~20 m isobath. Beyond 40 m isobaths, the velocities decrease to ~5 cm/s, flowing toward the coast and merging into the coastal jet. Note that the radar-derived coastal jet does not agree with the model, which will be analyzed in the next section. Figure 7a compares the M\({}_{2}\) ellipses calculated from the model and the HF radar data. In the model results, the M\({}_{2}\) ellipses were overall oriented northwestward. Along the coast to the west shore of the PRE, the ellipses were deformed on different scales, affecting the coastlines and islands. In contrast, the ellipses derived from the HF radar were quite different from the model, indicating that the reflection from the complex coastline prevents HF radar from delivering high-quality information for determining surface currents. Moving seaward, the HF radar and model ellipses were quite consistent between latitudes 21\({}^{\circ}\) and 21.5\({}^{\circ}\) N, where the averaged water depth was 30-50 m with no islands embedded; thus, the bathymetry effect became insignificant. Below latitude 21 \({}^{\circ}\)N, the HF radar ellipses gradually turned northeastward and mismatched the model ellipses. To explain the discrepancy between the HF radar and model ellipses, we defined an ellipse error as \(\sqrt{major\_axis^{2}+minor\_axis^{2}}\) (Figure 7b). The HF radar observing domain could be divided into three areas. In area A, the error was overall significant along the coast, with the largest error of 0.06 m/s. In area B, the HF radar ellipses showed excellent agreement with the model, except for one red spot that was obviously affected by a chain of islands. In area C, the errors increased to 0.04-0.05 m/s, due to the orientation mismatch shown in Figure 7a. To further diagnose the source of errors, the radar, and model velocities were projected radially according to the two radar stations at Shangchuan and Wanshan Islands (Figure 8). The results show that both Shangchuan and Wanshan stations contained large errors in area A, indicating that coastal islands and bathymetry significantly affected the data quality in both stations. In area B, both stations had relatively small errors, which is reasonable as the overall ellipse errors were small. It is noteworthy that the Shangchuan station showed large errors in area C, whereas the Wanshan station did not.