Oceanic wave breaking coverage separation techniques for active and maturing whitecaps

Abstract

Whitecaps on the ocean surface mark localized areas where interactions between the atmosphere and ocean are enhanced. Contemporary methods of quantifying total whitecap coverage rely on converting color sea surface images into their binary equivalent using specific threshold-based automated algorithms. However, there are very few studies that have separated and quantified whitecap coverage into its active (stage-A) and maturing (stage-B) evolutionary stages, which can potentially provide more suitable parameters for use in breaking wave models, air–sea gas transfer, aerosol production, and oceanic albedo studies. Previous active and maturing whitecap studies have used a pixel intensity separation technique, which involves first separating the whitecap and background pixels, and subsequently establishing a second threshold to distinguish between active and maturing whitecaps. In this study, a dataset of more than 64,000 images from the North Atlantic were initially processed to determine the total whitecap coverage using the Automated Whitecap Extraction method. The whitecap pixels of each image were then distinguished as either stage-A or stage-B whitecaps by applying a spatial separation technique which does not rely solely on pixel intensity information but also on the location (relative to the wave crest), visual intensity, texture and shape of each whitecap. The comparison between the spatial separation and pixel intensity separation techniques yielded average relative errors of 34.8% and $-44.0\%$ for stage-A and -B coverage, respectively. The pixel intensity method was found to be less suitable when compared to the spatial separation method as it relies on the assumption that the pixel intensity for stage-A is always greater than that for stage-B.