## 2 Numerical Model
### Model Verification  
Tyhoon Fongwong is the strongest typhoon to land directly in Shanghai during the past 25 years through the YRE, with a maximum wind speed of 14.2 m/s and an air pressure of 998 hPa, approaching a gale force of 10 at Sheshan station and landing in Shanghai in September 2014. Figure 3 shows the process of wind speed and direction at Sheshan station. The speed and direction exhibit complex changes because of the exceptionally rare typhoon phenomenon: it derived from the East China Sea and moved west. The typhoon progress can be divided into three stages: from 20:00 on 19 September to 22:00 on 21 September; 22:00 on 21 September to 19:00 on 23 September; and 19:00 on 23 September to 00:00 on 26 September, as the rising (the wind speed has an increasing trend), the stable (the wind speed reaches a maximum and remains strong, but suddenly reduces in speed and changes direction around 0:00 on 23 September) and the falling stage (the wind speed establishes a decreasing trend), respectively, according to the measured wind speed. Water level measurements from tidal gauges at Sheshan, Luchaogang, Dajishan, Dajishan, Tanhu and Zhenhai (Figure 1) in the study area are used to verify the modeled water elevation affected by Typhoon Fongwong. As shown in Figure 4, the comparisons of the time series of computed and observed water elevation at six tidal gauges from 18-24 September in 2014 show good agreements in both magnitudes and phases. Figure 5 is the difference between the measured tide and astronomic tide, which shows a significant increase around 23 September. The surges during the typhoon period are well reproduced, which indicates the effects of the wind on water level that was captured by the model. The period of current velocity simulation for validation spanned from 28 February 2014 to 1 March 2014 at three stations (positions shown in Figure 1). The computed current profile and direction are also well matched with the measured data, and Figure 6 shows the comparison of the surface and bottom current velocity and direction at one selected point NGN4 SD. We use the cruising survey data collected in July 2014 by the State Key Laboratory of Marine Geology of China for salinity verification. In this survey, 45 measurement points located at the different branches, and their positions, are shown in Figure 1. Figure 7 is the comparison of computed and measured salinity along the selected profiles, A4 and A5, on the surface, middle and bottom layer, which show that the general trend of salinity is well captured. Figure 3: Wind process demonstrating the manifestation of Typhoon Fongwong at Sheshan station from 19 to 27 September 2014. The upper vectors are the wind speed and direction change phenomenon. Figure 4: Verification of tidal level during typhoon period in 2014. Figure 5: Difference in tidal level (DTL) between measured tide and computed astronomical tide at the six stations. Figure 6: Verification of surface and bottom current velocity and direction in 2014. Moreover, the _skill_ model [53] is used to quantitatively assess the model performance. The _skill_ model is demonstrated below:  
\[\textit{skill}=1-\frac{\sum_{i=1}^{N}\abs{M-D}^{2}}{\sum_{i=1}^{N}\bigl{(}\abs{ M-\overline{D}}+\abs{D-\overline{D}}\bigr{)}^{2}}. \tag{3}\]  
where \(\overline{D}\) is the mean value, in situ, of the observed data; \(M\) and \(D\) are the computed result and in situ observed data, respectively; \(N\) is the number of data entries. A _skill_ value of 1.0 indicates a perfect performance of the model: _skill_ between 0.65 and 1 is considered excellent, _skill_ in the range of 0.5-0.65 is considered very good, _skill_ in the range of 0.2-0.5 is considered good, and _skill_ of less than 0.2 is considered poor. Table 1 summarizes the _skill_ values to assess model performance. For the tidal level, the _skill_ values at six stations present an excellent performance with _skill_ = 1.00.