Parameterizations of Whitecap Fraction: Status Update

Whitecap fraction W has been measured extensively because it is a direct expression of wave breaking and thus a suitable forcing variable for myriad air-sea interaction processes. The photographic method, based on image intensity thresholding, has been the workhorse of measuring W for decades. In the early 2000s, measurements of W usually showed large uncertainty caused by measuring difficulties under high winds (when waves break with air entrainment) and subjectivity in image processing. This measuring uncertainty was transferred to the parameterizations of W in terms of wind speed U₁₀. Natural variability of whitecap formation and extent on regional and seasonal scales added to a state of the art situation where different parameterizations spanned over 3-5 orders of magnitudes. Such uncertainty precluded reliable estimates of air-sea fluxes, especially those of sea spray production. Also, modelers who started introducing sea spray modules in global aerosol models often used available W(U₁₀) expressions beyond their ranges of validity. However, new technologies (e.g., digital still and video cameras) and improved processing algorithms have changed this situation significantly in the last decade. New photographic W measurements show less spread due to measuring uncertainty, around one order of magnitude. New measuring techniques, including microwave and infrared radiometry, provide more information about the formation and evolution of whitecaps and many more W data points. All these new information and data can be used to successfully constrain photographic measurements and whitecap parameterizations based on them. We will present the new state of the art in W measurements and parameterizations, contrast them with the previous status, and outline new trends in parameterizing the whitecaps and sea spray production.