## 3 Results  
### Comparison between Model Simulations and Mooring Observations  
To validate the model performance, the model surface velocity (\(\sim\)1 m) was first compared with the buoys. Figure 3 shows the monthly averaged velocities at stations A1, B1, and B2. We see that the mean flows at A1 and B1 are relatively small, compared to B2. Actually, the tidal currents within the PRE are greater than 1 m/s, with the largest tidal constituent M2 accounting for more than 50% of the total amplitude (see Table 2). By conducting a simple monthly average, the tidal flows are almost filtered out. At B2, the buoy-derived and model mean velocities are consistent, in terms of the amplitude and direction. Station B2 is located right on the main fairway and the exit of the Pearl River, and therefore the mean flow velocity is up to 0.25 m/s. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Constituents** & **Site** & \multicolumn{3}{c}{**Model**} & \multicolumn{3}{c}{**Buoys**} \\ \hline  & & **Semi-Major** & **Semi-Minor** & **Orientation** & **Semi-Major** & **Semi-Minor** & **Orientation** \\  & & **(cm/s)** & **(cm/s)** & **(\({}^{\circ}\))** & **(cm/s)** & **(cm/s)** & **(\({}^{\circ}\))** \\ \hline M\({}_{2}\) & B1 & 55.83 & 2.66 & 124.21 & 45.60 & 1.21 & 122.24 \\  & B2 & 37.45 & 7.10 & 85.64 & 35.47 & 5.84 & 88.86 \\ S\({}_{2}\) & B1 & 29.94 & 0.66 & 125.42 & 24.25 & 0.63 & 122.97 \\  & B2 & 19.75 & 5.23 & 87.03 & 17.12 & 3.29 & 91.68 \\ K\({}_{1}\) & B1 & 13.16 & 2.38 & 138.41 & 10.89 & 3.74 & 123.88 \\  & B2 & 12.16 & 2.86 & 86.13 & 13.33 & 2.62 & 85.66 \\ O\({}_{1}\) & B1 & 17.50 & 0.81 & 128.68 & 12.12 & 0.61 & 120.28 \\  & B2 & 16.41 & 5.18 & 78.13 & 13.02 & 4.10 & 92.54 \\ \hline \end{tabular}
\end{table} 
Table 2: Statistics of buoy-derived and model tidal ellipses for M\({}_{2}\), S\({}_{2}\), O\({}_{1}\), and K\({}_{1}\) constituents. Figure 4 compares the tidal ellipses of M\({}_{2}\), S\({}_{2}\), O\({}_{1}\), and K\({}_{1}\) constituents from the model and buoys. Overall, the model results are in good agreement with the buoy data for all tidal constituents, indicating that the model was able to satisfactorily resolve the tidal currents in the PRE. The largest error appeared at station B1 for the M\({}_{2}\) component, with the model semi-major axis (55.83 cm/s) greater than that of the buoy (45.60 cm/s). For S\({}_{2}\), O\({}_{1}\), and K\({}_{1}\) constituents, the averaged errors of the semi-major axis, the semi-minor axis, and the ellipse orientation were 3.2 cm/s, 3.4 cm/s, and 6.9\({}^{\circ}\). The detailed information of the tidal ellipses is listed in Table 2, as well as the statistical metrics in Table 3, showing a very good agreement between the model and the mooring observations. Figure 4: Comparisons of buoy-derived and model tidal ellipses for M\({}_{2}\), S\({}_{2}\), O\({}_{1}\), and K\({}_{1}\) constituents. Figure 3: Comparisons of monthly averaged velocities (cm/s, vector) from mooring observation and the model. In this study, the vertical velocity profile was measured only at station A1 by the ADCP. Figure 5 compares the tidal ellipses with depth derived from the ADCP and the model for the M\({}_{2}\), K\({}_{1}\), O\({}_{1}\), and S\({}_{2}\) constituents. We can see that the model also satisfactorily simulated the vertical profile of the tidal ellipses for all constituents. It can be noted that the ellipse orientations matched well in the upper layer (e.g., at 5 m depth), but the model ellipses were gradually deflected counterclockwise with depth compared to the ADCP. The largest error was 8.14\({}^{\circ}\) for the M\({}_{2}\) component at 11 m depth. The orientation errors might be due to the inaccurate topography near station A1, located within the main fairway of the PRE. Moreover, the modeled M2 flow has more tendency of left-land-side rotation toward the bottom, which is from the bottom Ekman effects. This signal is less apparent in the ADCP data, which implies too strong bottom friction for the model at this position.