破波点强迫产生的次重力波的有限源滤波与多普勒调制

中文摘要 AI 辅助

冲浪带产生的次重力波对近岸水位变异性有显著贡献，波破碎的群尺度变化通过破波点强迫产生自由长波。现有对该机制的解释主要依赖理想化的移动破波点图像，其中破碎与局部饱和关系相关，未解决真实破碎区域的有限范围及其演化如何控制辐射波的问题。我们首先将波致强迫分解为破波点强迫与束缚波强迫，在同一演化的短波场中分离两者的贡献。随后，我们通过积分强度、形心、跨岸宽度和形状，将破波点强迫场描述为有限演化源。该表示显示，有限源范围会产生空间干涉，不同位置发射贡献间的相位差会衰减次重力响应的高频部分，且随着源变宽，这种衰减会向低频偏移。源迁移通过修改连续发射的到达时间产生定向多普勒频移：向岸运动压缩向岸观测者处的到达时间，向海运动拉伸向海观测者处的到达时间。破波深度控制该强迫向表面高程的转换，以及决定这些传播时间效应的波速。在一系列平面海滩坡度下开展的受控数值实验重现了预测特征：更宽的源足迹会增强两支分支的上带衰减；诊断出的迁移历史（向岸运动期间破碎最强）部分抵消了向岸侧的衰减，增强了向海侧的衰减。

英文摘要

Infragravity waves generated in the surf zone contribute substantially to nearshore water-level variability, with group-scale variations in wave breaking generating free long waves through breakpoint forcing. Existing interpretations of this mechanism rely largely on an idealised moving-breakpoint picture in which breaking is tied to a local saturation relation, leaving unresolved how the finite extent and evolution of a realistic breaking region control the radiated waves. We first derive a local decomposition of the wave-induced forcing into breakpoint and bound wave forcing, separating the two contributions within the same evolving short-wave field. We then describe the breakpoint-forcing field as a finite, evolving source through its integrated strength, centroid, cross-shore width, and shape. This representation shows that finite source extent produces spatial interference, as phase differences between contributions emitted from different positions attenuate the high-frequency part of the infragravity response and shift this attenuation toward lower frequencies as the source widens. Source migration produces a directional Doppler shift by modifying the arrival times of successive emissions, with shoreward motion compressing arrivals at a shoreward observer and stretching them at a seaward observer. Breaking depth controls both the conversion of this forcing into surface elevation and the celerities governing these propagation-time effects. Controlled numerical experiments across a range of plane-beach slopes reproduce the predicted signatures: broader source footprints strengthen upper-band attenuation on both branches. The diagnosed migration history, with breaking strongest during shoreward motion, partly offsets that attenuation shoreward and reinforces it seaward.

发表机构

• KU Leuven（荷语鲁汶大学）
• CSIRO Ocean and Atmosphere Flagship（澳大利亚联邦科学与工业研究组织海洋与大气旗舰项目）

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