## 2 Materials and Methods
### Model  
The Finite-Volume Coastal Ocean Model (FVCOM) is a widely used numerical model for simulating ocean and coastal processes, which was originally invented at the University of Massachusetts, Dartmouth, and developed through the efforts of the Woods Hole Oceanographic Institution [37; 38; 39]. FVCOM is based on the finite-volume method and solves the governing equations of fluid motion on an unstructured grid, allowing for high-resolution modeling of complex coastal geometries. This model is capable of simulating tides, waves, currents, temperature, salinity, and water quality, making it a versatile tool for a range of applications, from coastal engineering to ecosystem management. FVCOM has been extensively validated and applied in various regions worldwide, demonstrating its effectiveness in reproducing observed oceanographic phenomena and providing valuable insights into coastal dynamics. In this study, the modeling grid encompassed the Pearl River Delta network, the Pearl River Estuary, and the northern South China Sea within the 100 m isobath range. The minimum grid size in the PRE was 10 m, while the radar observation area over the continental shelf had a resolution of 2 km (Figure 2). In the vertical direction, the model employed a terrain tracking coordinate system that combined \(\sigma\) and Spherical coordinates, dividing the water column into 20 layers with a resolution of up to 0.25 m. The \(\sigma\)-coordinate transformation is defined as:  
\[\sigma=\frac{Z-z}{H+z}=\frac{Z-z}{D}\]  
where \(\sigma\) varies from \(-1\) at the bottom to 0 on the surface. The total water column depth is \(D=H+z\), where \(H\) is the bottom depth and \(z\) is the height of the free surface. As our previous work [36] mentioned, the high-resolution PRE model used various data sources, including river discharge data from seven different rivers, tidal models, wind fields, and climate. The climatic river discharge data during the dry season from 2003 to 2007 is provided by the Guangdong Hydrological Bureau at three hydrological stations. The TPX08 tidal model [35] was used to drive the tidal force at the lower open boundary and Tide Model Driver (TMD, [[https://www.github.com/EarthAndSpaceResearch/TMD_Matlab_Toolbox_v2.5](https://www.github.com/EarthAndSpaceResearch/TMD_Matlab_Toolbox_v2.5)]([https://www.github.com/EarthAndSpaceResearch/TMD_Matlab_Toolbox_v2.5](https://www.github.com/EarthAndSpaceResearch/TMD_Matlab_Toolbox_v2.5)), accessed on 1 September 2022) was used to calculate elevations along the boundary. The tidal model included eight major tidal constituents and was effective at simulating tidal currents around the PRE. Hourly wind fields were obtained from the Climate Forecast System Version 2 of NCEP and used as the wind drive [40]. The initial \begin{table}
\begin{tabular}{c c c} \hline \hline
**Frequency** & **600 kHz (R1, R3-R5)** & **1200 kHz (R2)** \\ \hline Max Profiling Range & 70 m & 20 m \\ Max Bottom Tracking Range & N/A & N/A \\ Velocity Accuracy(typical) & \(\pm\)0.3\% of measured velocity \(\pm\)0.3 cm/s & \(\pm\)0.3\% of measured velocity \(\pm\)0.3 cm/s \\ Velocity Range & \(\pm\)5 m/s (default) to \(\pm\)20 m/s & \(\pm\)5 m/s (default) to \(\pm\)20 m/s \\ Ping Rate & 2 Hz (typical) & 2 Hz (typical) \\ Beam Angle & 20\({}^{\circ}\) & 20\({}^{\circ}\) \\ Depth Rating & 200 m (optional 500 m or 6000 m) & 200 m (optional 500 m or 6000 m) \\ Standard Sensors & Temperature, Tilt, Compass & Temperature, Tilt, Compass \\ Communications & Serial RS-422 or RS-232 ASCII or binary & Serial RS-422 or RS-232 ASCII or binary \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specifications of the Workhorse II Sentinel ADCP provided by Teledyne RD Instruments.