A Comparison of Laboratory and Field Measurements of Whitecap Foam Evolution from Breaking Waves

Sufficiently energetic breaking ocean waves produce distinctive visible foam signatures on the water surface called whitecaps. The mixture of surface whitecap foam cells, and sub-surface bubbles, results in the broad-band scattering of light that allow whitecaps to be measured with optical cameras. In this paper the temporal evolution of whitecap foam area from laboratory and oceanic breaking waves is compared. When appropriately scaled, the foam area time series for both laboratory and oceanic breaking waves follow similar trends, despite occurring in vastly different settings. Distinct similarities of the signature of foam stabilisation due to the presence of surfactants in the controlled laboratory experiments are also found in the field suggesting foam stabilisation may be a means to remotely sense the presence/absence or concentration of surfactants in the ocean. In addition, probability density distributions of key whitecap variables such as foam area growth and decay timescales and maximum foam area are compared between laboratory and oceanic whitecaps. The oceanic whitecaps are much larger in scale than the laboratory breaking waves, whereas the whitecap growth and decay timescales are similar in magnitude, the latter suggesting that the depths to which bubbles are injected during active air entrainment in the field are relatively shallow. The aggregated whitecap statistics are used to estimate the energy dissipation of individual whitecaps in a novel manner.

具有足够能量的破碎海浪会在水面形成辨识度极高的可见泡沫特征，这类特征被称为白冠（whitecaps）。水面白冠泡沫单元与水下气泡的混合体，会引发广谱光散射，这使得我们可以通过光学相机对白冠进行观测。本文对实验室与海洋中破碎海浪所产生的白冠泡沫面积的时间演化过程进行了对比分析。尽管二者的生成环境差异悬殊，但经过合理尺度缩放后，实验室与海洋破碎海浪的泡沫面积时间序列呈现出相似的变化趋势。在受控实验室实验中，由表面活性剂（surfactants）存在引发的泡沫稳定特征，在海洋现场观测中也存在显著相似性，这表明泡沫稳定特性可作为一种手段，用于遥感探测海洋中表面活性剂的有无及其浓度。此外，本文还对比了实验室与海洋白冠的关键参数的概率密度分布，这些参数包括泡沫面积的生长与衰减时间尺度以及最大泡沫面积。海洋白冠的尺度远大于实验室中的破碎海浪，但二者的白冠生长与衰减时间尺度的量级却相近，这一结果表明，在海洋现场的主动携气过程中，气泡注入的深度相对较浅。本文以一种全新的方式，利用聚合后的白冠统计数据估算单个白冠的能量耗散量。