**4. Discussion and Uncertainty**

Hindcasting an accurate storm wave height is highly dependent on the accuracy of the typhoon wind fields. Reanalysis wind products underestimate typhoon winds, e.g., Çalı¸sır et al. [53] evaluated the quality of ERA5 and CFSR (Climate Forecast System Reanalysis) winds and the contribution of reanalysis wind products to a wave modeling performance in a semi-closed sea. Their results revealed that ERA5 and CFSR tend to underestimate wind speeds, and ERA5 performs worse than CFSR at higher wind speeds (such as typhoon winds) and better at lower wind speeds. Thus, the utilization of hybrid winds through the superposition method (the combination of reanalysis wind products and parametric cyclone wind models) or the direct modification method (the combination of reanalysis wind products and the maximum wind speeds of typhoons from the best track data) is the most advantageous to consider storm wave hindcasting in both near-field and far-field regions of the typhoon's center. However, uncertainty remains to be clarified; that is, the radius of the modified scale (*Rtrs*) cannot be formulated universally for each typhoon. Owing to the *WBmax* is higher than *WEmax*, the area with stronger winds of a typhoon is extended as *Rtrs* increases (according to Equation (1)). This expansion allows the hindcasted SWHs to grow earlier and attenuate later (i.e., larger) than the measurements (as shown in Figures 3 and 6). Additionally, as shown in Figure 3b,c, the measured peak SWH at the Suao wave buoy occurred two hours earlier than that at the Longdong wave buoy. However, the occurrence time of hindcasted peak SWH at the Suao wave buoy coincided with that at the Longdong wave buoy. This phenomenon might be due to the low spatial resolution of the original ERA5 winds (at roughly 31 km), and the periphery circulation of a typhoon cannot be resolved with such a coarse spatial resolution. Although the wind field derived from *Rtrs* = 4*Rmax* is employed for designing a series of model experiments based on limited case studies, further studies are still needed to verify it.

Fully coupled ocean circulation and spectral wave numerical modeling systems are undergoing widespread use for all types of regional applications, such as operational predictions, wave climate evaluations, extreme storm waves and surge analyses. However, the wave-induced hydrodynamics during the period of typhoons in shallow nearshore waters simulated by these modeling systems remain uncertain. The scarcity of field observations for wave parameters in the surf zone to verify the modeling systems is one of the most important factors leading to uncertainty.

The SWH simulations in the surf zone are more sensitive to the various wave-breaking formulations than the various wave-breaking criteria. To reconfirm the result obtained from Section 3.2, the spatial distribution of the difference in the maximum SWH between the scenarios of S\_NO1 (wave-breaking formulation of BJ87 with constant wave-breaking criteria) and S\_NO2 (wave-breaking formulation of TG83 with constant wave-breaking criteria) and S\_NO1 and S\_NO3 (wave-breaking formulation of CT93 with constant wavebreaking criteria) in the surf zone (sea areas with water depths greater than −20 m are shown) of southeastern China for Super Typhoons Maria in 2018 and Lekima in 2019 are shown in Figures 17 and 18, respectively. As seen in Figures 17 and 18, the differences

in maximum SWH between the S\_NO1 and S\_NO3 scenarios (Figures 17b and 18b) are higher than those between the S\_NO1 and S\_NO2 scenarios (Figures 17a and 18a) for both typhoons. Additionally, the surf zones with significant differences in maximum SWH resulting from the various wave-breaking formulations occur on the right side of the typhoons where the wind speeds are stronger. These findings are identical to the result derived from Section 3.2 and the reports from previous studies [54–57].

**Figure 17.** Spatial distribution of the difference in maximum SWH between the scenarios of (**a**) S\_NO1 and S\_NO2 and (**b**) S\_NO1 and S\_NO3 when Super Typhoon Maria (2018) made landfall on the southeast coast of China. The areas with water depths greater than −20 m are shown.

**Figure 18.** Spatial distribution of the difference in maximum SWH between the scenarios of (**a**) S\_NO1 and S\_NO2 and (**b**) S\_NO1 and S\_NO3 when Super Typhoon Lekima (2019) made landfall on the southeast coast of China. The areas with water depths greater than −20 m are shown.
