4. Numerical model and settings

#### 4.1 The finite volume community ocean model (FVCOM)

The FVCOM is a 3D unstructured-grid, free-surface, primitive equation, finite volume coastal ocean and estuarine circulation model with triangular grids in the horizontal and terrain-following coordinates in the vertical and wet/dry treatment, developed originally by the University of Massachusetts Dartmouth (UMASSD) and improved by the efforts of Woods Hole Oceanographic Institution (WHOI) [11–13]. The finite volume approach used in FVCOM, combined with a flexible unstructured grid, provides a better representation of mass, salt, momentum, and heat conservation in an estuarine area with irregular coastline and geography than that used in a rectangular grid system. Mode-split or semi-implicit schemes can choose to solve the governing equations in Cartesian or spherical coordinates. The governing equations, including momentum, continuity, temperature, salinity, and density equations, are closed by using the Mellor and Yamada level 2.5 (MY-2.5) turbulence closure for vertical eddy viscosity [14], or the Smagorinsky eddy parameterization for the computation of the horizontal diffusion [15], as well as an alternative selection of the General Ocean Turbulence Model (GOTM) [16]. For more computational speed, a "mode-splitting" time-stepping scheme is employed to solve the integration process, dividing into internal and external modes with two different time steps. FVCOM has been widely used in estuarine circulation and river plume dynamic studies worldwide, such as the Pearl River Estuary [17], Changjiang River [18, 19], Okatee River [20], Puget Sound estuarine system [21], and Tampa Bay [22].

#### 4.2 FVCOM settings

In the FVCOM, the horizontal grid uses unstructured triangular cells, and the realistic topography is represented using terrain-following coordinates. The greatest advantage of this model is its geometric flexibility since it uses the unstructured and triangular grid meshes, which can provide a real fitting of the irregular coastal boundary. Figure 9a shows the unstructured triangular model grids of the FVCOM for the PRE and coastal water regions. The model domain (111.5–116.5°E, 20–23°N) covers the entire Pearl River Estuary with an open boundary in the northern South China Sea. The total node number in the model grid is 41,027 with 78,539 cells. The horizontal grids have spatial resolutions that vary from 0.1 km to 10 km over the entire domain, with 0.1–0.3 km inside the Pearl River Estuary, 0.3–0.5 km in the estuary mouth, 1.0–2.0 km in the Guangdong coastal water, and 10 km close to the open boundary. The vertical coordinate has 20 levels in uniform hybrid

Circulations in the Pearl River Estuary: Observation and Modeling DOI: http://dx.doi.org/10.5772/intechopen.91058

Figure 9. Model domains for the FVCOM (a) and the WRF model (b).

terrain-following grids. The numerical simulations are run for the period from March 1, 2014 to May 30, 2014, during which the observations obtained from the cruise survey conducted between May 3, 2014 and May 11, 2014 were available for model comparisons.

Tidal forcing is applied to the open boundary including the eight major tidal constituents of M2, N2, S2, K2, K1, O1, P1, and Q1, which are obtained through interpolation from the 1/6° inverse tidal model results of [23], together with the salinity, temperature, and velocity on the open boundary from Hybrid Coordinate Ocean Model (HYCOM) (https://hycom.org/dataserver/glb-analysis) outputs. The meteorological forcing including the wind stress, net heat flux, and evaporationprecipitation (E-P) balance is generated by the Weather Research and Forecasting (WRF) Model. The monthly means of freshwater discharges from the eight river inlets constitute the lateral boundary conditions on the land side [2, 7]. The model is initialized with the March climatology of salinity and temperature fields derived from the World Ocean Atlas 2009 (WOA2009) (https://www.nodc.noaa.gov/OC5/ WOA09/pr\_woa09.html) and spun up from zero velocity and an undisturbed sea surface elevation. The topographic data for the FVCOM model are derived by interpolation of an electronic navigation chart data for the estuary and coastal water area, and for the offshore region, they are obtained from the global bathymetry data of the General Bathymetric Chart of the Oceans (GEBCO) (http://www.gebco.net/) with 30 seconds (1/120°) horizontal resolution.

The temporal integration of the model uses a split-mode time-stepping method with a 2-second external time step and a split number of 5. The wet/dry treatment is used since some upstream coastal areas can be inundated in high tide. The MY-2.5 and Smagorinsky turbulent closure schemes are used for vertical and horizontal mixing, respectively [14, 15].

In addition to the ocean model, an atmospheric model, the WRF Model, is implemented to provide high spatial and temporal resolution atmospheric forcing for the FVCOM. The WRF Model is supported and developed by the National Center for Atmospheric Research (NCAR), the National Centers for Environmental Prediction (NCEP), the Air Force Weather Agency (AFWA), the Naval Research Laboratory, the University of Oklahoma, and the Federal Aviation Administration (FAA) in the USA. This model is a next-generation mesoscale numerical weather forecasting system that was designed to serve both weather forecasting and atmospheric research needs [24]. A dynamical downscaling technique is employed in the WRF for the regional atmospheric modeling to obtain a higher-resolution output. To realize this, a three-domain nested configuration is designed, as shown in Figure 9b. The outer domain, d01 for the atmospheric model, covers the western Pacific Ocean, the entire South China Sea, and the eastern Indian Ocean, with a horizontal resolution of 72 km; the middle domain, d02, covers the entire South China Sea and southern China, with a horizontal resolution of 24 km; the inner domain, d03, covers the northern South China Sea with a horizontal resolution of 8 km. All domains have 27 layers in the vertical. The NCEP FNL Operational Global Analysis data (http://rda.ucar.edu/datasets/ds083.2/) with a horizontal resolution of 1° 1° is used to provide initial conditions and lateral boundary conditions for the outer domain. The physics options of the WRF model include the Ferrier microphysics scheme [25], RRTM longwave radiation scheme [26], Dudhia shortwave scheme [27], YSU PBL scheme [28], and Kain-Fritsch cumulus scheme [29]. The model simulation period is from April 20, 2014 to May 30, 2014 covering the cruise survey period. The sea surface winds from the inner domain of WRF model are used to drive the ocean model.
