Observed Variations in the Decay Rate of Whitecap Foam

Whitecap coverage (W) parameterizations have been used to estimate the transfer velocity of CO2 and to predict the production of primary marine aerosols. W is often parameterized in terms of wind speed only and historical measurements of W exhibit 3 orders of magnitude of scatter, some of which may be due to variation of measurement techniques. Several recent datasets of W derived using similar experimental techniques exhibit less scatter, but variation is still about a factor of 10. Here we explore the possibility that some of this scatter is driven by variations in whitecap foam decay rate, which is usually assumed to be constant. An analysis of foam decay rates observed off Martha's Vineyard over the course of several days will be presented. The area of individual whitecaps is observed to decay quasi-exponentially, with decay constants ranging from 0.5 to 8 seconds. Foam decay rates increase by up to a factor of 3-4 with increasing breaking wave scale. Additionally, ensemble averages across a range of event scales on different days also show statistically significant variability. The implications of this result for the observed variability in whitecap coverage will be discussed and we speculate on potential causes of the observed foam decay rate variability.