Shipboard Video Observations of Whitecaps

Thursday, 18 December 2014
Michael Schwendeman and James M Thomson, Applied Physics Lab (UW), Seattle, WA, United States
Video observations of breaking ocean surface waves in deep water (i.e., whitecaps) are useful for determining which waves are breaking and inferring how much energy these breakers are dissipating. We present shipboard video of breaking waves from a research cruise in the North Pacific. As with airborne systems, motion compensation is essential in geo-rectifying the image. A stabilization method based on the location of the horizon in the image is shown to be effective in correcting pitch and roll motions to within one degree, without an IMU (inertial motion unit). After rectification, whitecaps are identified and measured based on the translation of surface foam patches, which appear as groups of bright pixels. Two standard breaking metrics, whitecap coverage and Phillips' Λ(c) distribution, are calculated for the full dataset and compared with other recent observations. In addition, the growth rate of the whitecap foam patches is used to examine a new dissipation function, independent from Λ(c), which was developed in laboratory experiments. Finally, we present preliminary whitecap results from a stereo system. Stereo imaging has the potential to provide much more information about the geometry and kinematics of breaking surface waves in the field, but significant technical challenges remain.