An assessment of the impact of boundary conditions in dynamical downscaling techniques for fetch-limited waves

The handling of boundary conditions (BCs) is a key issue for the development of dynamical downscaling techniques in which nearshore wave models require the definition of the sea conditions at the offshore boundaries. This study addresses the implications of using simplified BCs represented by bulk or partitioned wave parameters with respect to using the complete information from directional wave spectra. As a large-scale model, we used WAVEWATCH III (WW3) running over the Mediterranean Sea providing the wave BCs for the small-scale SWAN model. One year of measured directional wave spectra from three deepwater buoys allowed the assessment of the WW3 model accuracy. The nested SWAN model simulations propagated the wave field over three nearshore areas. The accuracy of SWAN simulations was assessed through the comparison of computed results with wave measurements from nearshore buoys. Being the wave conditions dominated by unimodal seas, modest differences of error metrics are found between the datasets forced by different types of BCs. Conversely, focusing the attention on multimodal seas, an accurate definition of wave BCs achieved by the use of the directional wave spectra or, in alternative, by the reconstruction of wave spectra from partitioned wave parameters becomes crucial for nearshore wave predictions.

边界条件（Boundary Conditions，BCs）的处理是动力降尺度技术研发的核心议题：近岸海浪模型需明确外海边界处的海况条件。本研究聚焦于两类边界条件方案的应用差异：一类是以整体或分块海浪参数表征的简化边界条件，另一类则是直接采用方向波谱（directional wave spectra）完整信息的边界条件。作为大尺度海浪模型，本研究采用运行于地中海海域的WAVEWATCH III（WW3）为小尺度SWAN模型提供海浪边界条件。本研究依托3个深水浮标获取的为期1年的实测方向波谱数据，对WW3模型的模拟精度进行了评估。嵌套式SWAN模型的模拟过程将海浪场传播至3个近岸海域。本研究通过将SWAN模型的模拟结果与近岸浮标实测海浪数据进行对比，评估了该模型的模拟精度。当海况以单峰海浪为主时，不同边界条件方案驱动下的模拟数据集之间，误差指标仅存在小幅差异。反之，当海况以多峰海浪为主时，通过直接采用方向波谱，或通过分块海浪参数重构波谱的方式来精准定义海浪边界条件，对于近岸海浪预报而言便至关重要。