The breaking of ocean surface gravity waves is an important phenomenon that affects the dynamics of the upper ocean, development of the wave field, and air-sea exchange processes. As the surface expression of this process, whitecaps provide a visible signature of wave breaking; their areal extent per unit area sea surface - known as the whitecap fraction, W - can be used to quantify the amount and scale of wave breaking.
W is traditionally estimated using digital images of the ocean surface and is widely used to represent whitecaps in remote sensing applications, and in the parameterisation of
a host of air-sea processes in models. These parameterisations - generally functions of wind speed alone - are based on limited amounts of data, and fail to take into account
the known influence of secondary factors on whitecaps.
A novel approach to estimating W using satellite observations has recently been developed, based on passive radiometric measurements of brightness temperature at microwave
frequencies. The satellite-based approach enables measurement of W on a global scale, and in a variety of conditions. In this work, the basic characteristics of W estimates at two different radiometric frequencies, W10 (10 GHz) and W37 (37 GHz),
is investigated. The wind speed dependence, global distribution, and seasonal dependence of the estimates are investigated. Comparison is made against estimates
obtained from simple, but widely used, wind speed only parameterisations formulated from in situ data. A more direct comparison of radiometric and photographic W estimates,
based on ship-satellite matchups, is also made. Both comparisons indicate that satellite-based W has a different wind speed dependence, resulting in estimates that are, on average, higher at low wind speeds and lower at higher wind speeds than parameterisations formulated from in situ, photographic measurements. On a global scale, this results in satellite-based W being more uniform latitudinally than predictions from traditional formulations. A dataset comprising estimates of W10 and W37, together with collocated and concurrent estimates for a variety of forcing parameters, is used to investigate the the influence on W10 and W37 of secondary forcings, such as the wave field and environmental
factors. It is found that on a global scale wind speed describes much of the variability in both W10 and W37 though the influence of secondary factors on W can be appreciable (especially for W37). Based on the magnitude of the influence of secondary forcing factors on W10 and W37, it is concluded that much of the variability in whitecap fraction is likely due to the behavior of the thinner, decaying foam patches, variability that is not captured by the retrieval using the 10 GHz channel. Though whitecap fraction offers a pragmatic approach to inferring the magnitude of processes associated with breaking surface waves, it remains an indirect measure with inherent limitations. More fundamental questions regarding the interpretation and use of W are considered. A dynamical model that relates whitecap fraction to breaking wave statistics is used to illustrate the contribution to whitecap fraction due to whitecaps in different lifetime stages. Such a model provides a framework for better relating whitecap fraction to the dynamic, active part of the wave breaking process which is likely more closely linked to processes such as breaking-induced energy dissipation, turbulent mixing, and bubble-mediated gas exchange. Finally, the implications of use of radiometric estimates for quantifying air-sea processes -
specifically production of sea spray aerosol and bubble-mediated gas exchange - is discussed. It is shown that difference between the satellite-based W estimates and those
predicted using traditional parameterisations provides an explanation for the consistent geographical biases in sea spray aerosol concentration found in a number of large scale models. The benefit of these novel observations will also extend to predictions of other air-sea processes, and remote sensing applications, that require estimation of W; these benefits will be enhanced if whitecaps and their radiometric signature are more closely related to the physical processes which they are used to quantify.
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