The results of field observations of breaking of surface spectral peak waves, taken from an oceanographic research platform, are presented. Whitecaps generated by breaking surface waves were detected using video recordings of the sea surface, accompanied by co-located measurements of waves and wind velocity. Whitecaps were separated according to the speed of their movement, c, and then described in terms of spectral distributions of their areas and lengths over c. The contribution of dominant waves to the whitecap coverage varies with the wave age and attains more than 50% when seas are young. As found, the whitecap coverage and the total length of whitecaps generated by dominant waves exhibit strong dependence on the dominant wave steepness, ${ϵ}_p$, the former being proportional to ${ϵ}_p^6$. This result supports a parameterization of the dissipation term, used in the WAM model. A semi-empirical model of the whitecap coverage, where contributions of breaking of dominant and equilibrium range waves are separated, is suggested.

This paper reports the results of field measurements of wave breaking modulations by dominant surface waves, taken from the Black Sea research platform at wind speeds ranging from 10 to 20 m/s. Wave breaking events were detected by video recordings of the sea surface synchronized and collocated with the wave gauge measurements. As observed, the main contribution to the fraction of the sea surface covered by whitecaps comes from the breaking of short gravity waves, with phase velocities exceeding 1.25 m/s. Averaging of the wave breaking over the same phases of the dominant long surface waves (LWs, with wavelengths in the range from 32 to 69 m) revealed strong modulation of whitecaps. Wave breaking occurs mainly on the crests of LWs and disappears in their troughs. Data analysis in terms of the modulation transfer function (MTF) shows that the magnitude of the MTF is about 20, it is weakly wind-dependent, and the maximum of whitecapping is windward-shifted from the LW-crest by 15 deg. A simple model of whitecaps modulations by the long waves is suggested. This model is in quantitative agreement with the measurements and correctly reproduces the modulations’ magnitude, phase, and non-sinusoidal shape.

Registration Peculiarities of Wind Waves Breaking From Unmanned Aerial Vehicles Особенности регистрации обрушений ветровых волн с беспилотных летательных аппаратов

The results of using an unmanned aerial vehicle (UAV) to study the processes of gravitational wave breaking are presented. The experiments are carried out in the coastal zone of the western Crimea (the area of Sevastopol) in the range of wind speeds from 5.5 to 9.5 m/s. The determination of the geometric dimensions of breaking and their speeds is carried out according to video recordings of the sea surface obtained from the UAV. It is shown that the spatial resolution of the transformed image plays a key role in separating the active phase of breaking and residual foam. Errors occur in determining the kinematic properties of the breaking with a rough spatial resolution. The proportion of the sea surface covered with foam of breaking waves, the ratio of the maximum length of the spume to the length of the breaking wave, and the distribution of the total breaking length in the intervals of movement speeds per surface unit (obtained from UAVs with a spatial resolution better than 0.5m) are in agreement with results of other authors. Data analysis shows that the use of UAVs makes it possible to study the statistical characteristics and kinematics of wind wave breaking. At the same time, it is necessary to take into account the effect of spatial resolution in the video frame, which can lead to the distortion or omission of measurement data at values comparable to or exceeding the breaking scale.