Measurement of the geometric and kinematic properties of microscale breaking waves from infrared imagery using a PIV algorithm

Infrared techniques have been shown to be uniquely capable of detecting and quantifying microscale breaking waves at an air-water interface. Here we extend current capabilities by developing image processing algorithms to measure the crest lengths and velocities of microbreaking waves in a laboratory wind-wave tank. The measurements are used to compute the distribution of crest lengths as a function of speed, A (c), introduced by Phillips [1] as a formulation for the distribution of breaking waves. Two methods to determine the crest velocity by applying a particle imaging velocimetry (PIV) algorithm to the infrared imagery are developed and compared to a method based on tracking the centroid of the crest. The crest-PIV method is based on estimation of the velocity of crests identified using a temperature threshold. The image-PIV method is based on a velocity threshold applied to a surface velocity map obtained by using the PIV algorithm over the entire image. Both methods are used to compute the surface turnover rate, which is compared to the frequency of breaking. The methods developed demonstrate the potential for infrared imaging techniques to measure the geometric and kinematic properties of microbreaking waves and are relevant to air-sea flux studies.