Infrared Remote Sensing of Cooling Whitecap Foam to Quantify Wave Breaking Dissipation

Infrared (IR) imagery of breaking waves reveals both a warm wake² due to disruption of the cool skin and cooling of residual foam⁴. We use heat flux measurements of foam-free and foam covered saltwater in the laboratory to show that the cooling foam is due to enhanced evaporative heat flux. We also show that the onset of cooling foam coincides with the end of the foam generation process. If this is the case in nature, then IR imagery can provide the ability to infer the bubble plume decay time and thus a measure of wave energy dissipation³. We propose a conceptual model for the time series of IR radiance measured over a small region during the passage of a breaking wave. The radiance rises rapidly from the background level of undisturbed water as the breaking crest enters the region. After the crest passes, the radiance drops and is dominated by the recovery of the cool skin layer. The time for the skin layer to recover will vary with ambient heat flux⁵. After the bubble plume has decayed and no more foam is being generated at the surface, the residual foam begins to cool and the radiance drops below the background level. The radiance then slowly increases to the background level as the foam dissipates. Recent results¹ suggest that the time for the foam to dissipate will vary depending on the surfactant concentration. If the onset of cooling is not affected by the presence of surfactants, the time from when breaking begins to when the foam starts to cool will coincide with the plume decay time. We examine archival measurements of breaking waves made for other purposes both in the open ocean and in the surf zone that are consistent with this conceptual model.

1 AH Callaghan, et al., J. Phys. Ocean., 43 (2013).

2 AT Jessup, et al., Nature, 385 (1997).

3 E Lamarre, et al., Nature, 351 (1991).

4 GO Marmorino, et al., Geophys. Res. Let., 32 (2005).

5 CJ Zappa, et al., J. Geophys. Res., 103 (1998).