## 2 Materials and Methods
### Forcing and Validating Data  
The MRLCOM-JPN dataset was also used as the boundary conditions for temperature, salinity, SSH and velocity. Since this dataset is available in the daily mean format, daily mean forcings were applied at the open boundaries. For the tidal constants at the open boundaries, the FES2014 oceanic tide model [25] was utilized after being interpolated into the nodes of the open boundaries. All datasets utilized as forcings for the model are synthesized in Table 1.  
Hourly river discharges were obtained from the Water Information System (www1.river.go.jp (accessed on 22 June 2022)) of MLIT. The Ota River in Hiroshima and three rivers (Kanzaki, Yodo, and Yamato) in Osaka were considered. We summarized the discharge amounts from several hydrological observatories in each river due to the existence of small branches in the rivers. In detail, we utilized the Yasuohashi and Yaguchidaichi hydrological observatories for the Ota River; the Kamikema and Toshikura observatories for the Kanzaki River; the Yawata, Mukojima, and Hazukashi observatories for the Yodo River; and the Kashiwara observatory for the Yamato River. During the simulation period, the maximum river discharges were 789 m\({}^{3}\) s\({}^{-1}\) at the Ota River, 529 m\({}^{3}\) s\({}^{-1}\) at the Kanzaki River, 4592 m\({}^{3}\) s\({}^{-1}\) at the Yodo River, and 927 m\({}^{3}\) s\({}^{-1}\) at the Yamato River.  
Figure 2: (a) Distribution of the parameters (\(\gamma\)) of the Shapiro filter used in the model. (b–d) Enlarged images of each subdomain.  
For surface meteorological forces, the winds at 10 m, air pressure, air temperature, precipitation rate, specific humidity, and downward radiation (both short- and longwave) were considered in the model. These variables were extracted from two model resources: the Meso-Scale Model (MSM) from the JMA and ERA5 from the European Centre for Medium-Range Weather Forecasts (ECMWF). The MSM is a high-resolution model (with a spatial resolution of approximately 5 km) with simulations operationally focused on Japan to increase the efficiency of Japanese weather forecasting. These data are provided at an interval of an hour, and the variables in the data, including the 10 m surface winds, air pressure, and air temperature, were extracted here and used as surface forcings (inputs) in the ocean model. The other forces, including the precipitation rate, specific humidity, and downward radiation, were retrieved from ERA5 due to the absence of these variables in the MSM. To calculate the specific humidity, Equations (5) and (6) below were used [28]:  
\[q_{\upsilon}=0.622\frac{e}{p} \tag{5}\]  
\[e=611*\exp\left(\frac{17.27*T_{d}}{237.3+T_{d}}\right) \tag{6}\]  
observation depths and times in Table 14. The model results were extracted for validation following the information provided in Table 14.  
To validate the current outputs, we utilized current observations from current meters moored by light buoys. The mooring system observed current velocities at various layers (here, 5, 10, 20, and 30 m depths), and the observations were released at a 5-min interval from 27 September to 12 October 2011. The locations of the mooring observations are indicated in Figure 1a. The data source is the Japanese Research Institute, and the observation institute is the 6 th Regional Coast Guard Headquarters.