Comparisons of Tidal Currents in the Pearl River Estuary between High-Frequency Radar Data and Model Simulations  
[PERSON]  
1 Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 519082, China; [EMAIL_ADDRESS] (L.Z. ); [EMAIL_ADDRESS] (T.L.) [PERSON]  
1 Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 519082, China; [EMAIL_ADDRESS] (L.Z. ); [EMAIL_ADDRESS] (T.L.) [PERSON]  
4 Zhuil Marine Environmental Monitoring Central Station of the State Oceanic Administration, 22315900, China 423159  
Previous studies on the flow field characteristics of the PRE and its adjacent waters were mostly based on model simulations [3; 4; 5; 6]. Buoys, cruise observations, and tidal gauges were adopted for comparison with the model. The modeling skill assessment suggested that the model can satisfactorily simulate the tidal currents, water level, and salinity structure [2; 7]. Outside the PRE, tides propagate into the SCS through the Luzon Strait, and the lunar semidiurnal (M2; 12.42 h period) tidal constituent dominates the currents, followed by K1, O1, and S2 [5; 8]. Furthermore, the complexity of topography in the PRE may lead to amphidromic systems in the continental shelf areas [8]. Within the PRE, the Pearl River outflow presents a plume-like structure and forms a corresponding salinity frontal zone which is crucial to substance transport [6; 9]. The vertical distribution of the salinity has a seasonal variability (intensified during the summer and relatively weakened during the spring tide in winter) and is affected by the river discharge, as well as wind forcing [4; 7; 10; 11]. High-frequency (HF) radar, a relatively new ocean observing tool, has been used to detect ocean surface currents and waves for decades [12; 13]. Bragg scattering, caused by the interaction of electromagnetic waves (wavelength = tens of meters) with the sea surface, was discovered in 1955 [14], which makes it possible for the HF radar to detect the state of the sea beyond the visual range. HF radars are operated in the 3-30 MHz frequency band, ensuring good penetration of the electromagnetic waves through conducting layer. The mechanism of the first- and second-order scattering from the sea surface has been quantitatively explained, providing a solid theoretical basis for the HF radar to detect ocean  
Figure 1: Map of the HF radar data domain. Blue spots indicate the original data points and orange spots indicate the data points with a data acquisition rate greater than 90%. The ADCP mooring station is marked by A1, and the two buoys are marked by B1 and B2. The two HF radar stations are marked by green squares. surface states [15; 16]. HF radars, like other communication systems, adopted the vertical polarization mode which aims to avoid the signal loss caused by the influence of the Earth's impedance. Compared to traditional in situ surface buoys and acoustic Doppler current profilers (ADCPs), HF radars can provide a planar view of current states. They have been widely used in marine environmental monitoring due to their unique advantages such as wide coverage, strong persistence, and all-weather operation [12; 17]. The HF radar system can currently perform relatively reliable inversion of ocean currents due to the accurate identification of first-order peaks [15; 18]. Many previous studies compared different types of HF radars with mooring observations, proving that the data quality can be affected by environmental factors such as surface wave directions and sea states [19; 20; 21]. HF radar measurements can also be examined by cross-validation with model simulations and moorings (e.g., in the New York Bight and Block Island Sound [22; 23; 24]). Usually, a pair of radar stations cover a 200-300 km\({}^{2}\) area, therefore a radar observing network with 6-10 radar stations can cover the entire PRE and its adjacent shelf.