Dependence of Estimating Whitecap Coverage on Currents and Swells

The shipboard measurements of whitecap coverage (W) and the meteorological and oceanographic information from two cruises in the South China Sea and Western Pacific are explored for estimating W. This study aims at evaluating how to improve the parameterizations of W while considering the effects of currents and swells on wave breaking. Currents indeed affect W in a way that winds with following currents can decrease W, whereas winds with opposing currents can increase W. Then, 10-m wind speed over sea surface (U₁₀) is calibrated by subtracting the current velocity from U₁₀ when the propagating directions of winds and currents are aligned. By contrast, when the direction is opposite, U₁₀ is calibrated by adding the parallel velocity component of the opposing current to U₁₀. The power fits of W dependence on the U₁₀-related parameters of U₁₀, friction velocity, wind sea Reynolds number in terms of this calibrated-U₁₀ obtain better results than those directly fitted to U₁₀. Different from the effect of currents onW, wind blowing along the crest line of swells may contribute to the increase in W. The conclusions suggest that U₁₀ should be calibrated first before parameterizing W in areas with a strong current or some swell-dominant areas.