Forecasting coastal infragravity (IG) waves is crucial in preventing coastal disasters caused by IG waves. In this study, the performance of the unstructured WAVEWATCH Ⅲ (WW3) in predicting coastal IG waves is comparatively assessed using in-situ wave measurements, considering the effects of numerical schemes, time steps, and IG-source parameterization methods. The wave measurements are performed using three acoustic sensors installed at the coast of south Sri Lanka, with two being near or inside a harbor.

Sensitivity analysis of the numerical schemes and time steps shows that the explicit scheme combined with a small time step leads to unexpected oscillations of the IG wave height. However, the time step has a negligible influence on the performance of the implicit scheme combined with the domain decomposition method. Generally, IG sources estimated from the empirical formula method (source M1) and the second-order spectrum method (source M2) perform similarly in reproducing the IG wave height. Regarding the IG frequency spectrum, both the IG sources properly predict the magnitude of the spectral density, but the spectral shapes are not adequately captured. M2 may provide a better directional spread of the IG wave energy. Attention is also paid to the IG waves inside the harbor. The IG waves within the harbor are underestimated, owing to the lack of some mechanisms related to IG waves in the simulation. M1 should be adopted when crudely predicting the magnitude of IG waves inside a harbor. Overall, the unstructured WW3 can effectively predict the coastal IG wave height in southern Sri Lanka, with significant correlation and acceptable errors, when compared with in-situ measurements. Finally, the IG source M1 is refined to remove the erroneous spectral gap between the IG and wind-wave frequency bands in the WW3 spectra when compared to the in-situ observed spectra.

预测沿海次重力 (IG) 波对于预防 IG 波引起的沿海灾害至关重要。在本研究中，考虑到数值方案、时间步长和 IG 源参数化方法的影响，使用原位波浪测量比较评估了非结构化 WAVEWATCH Ⅲ（WW3）在预测沿海 IG 波方面的性能。波浪测量是使用安装在斯里兰卡南部海岸的三个声学传感器进行的，其中两个位于港口附近或内部。

数值方案和时间步长的敏感性分析表明，显式方案与小时间步长相结合会导致 IG 波高的意外振荡。然而，时间步长对结合域分解方法的隐式方案的性能的影响可以忽略不计。通常，根据经验公式法（源 M1）和二阶谱法（源 M2）估计的 IG 源在再现 IG 波高方面表现相似。关于 IG 频谱，两个 IG 源都正确预测了频谱密度的大小，但没有充分捕获频谱形状。M2 可以提供更好的 IG 波能量的定向传播。港口内的IG波浪也受到关注。港内的IG浪被低估了，由于在模拟中缺乏与 IG 波相关的一些机制。粗略预测港内IG波的大小时应采用M1。总体而言，与原位测量相比，非结构化 WW3 可以有效预测斯里兰卡南部沿海 IG 波高，具有显着的相关性和可接受的误差。最后，与原位观测光谱相比，IG 源 M1 被改进以消除 WW3 光谱中 IG 和风波频带之间的错误光谱间隙。与现场测量相比。最后，与原位观测光谱相比，IG 源 M1 被改进以消除 WW3 光谱中 IG 和风波频带之间的错误光谱间隙。与现场测量相比。最后，与原位观测光谱相比，IG 源 M1 被改进以消除 WW3 光谱中 IG 和风波频带之间的错误光谱间隙。