Thermal imagery of surface renewal phenomena

Abstract

Processes within the sea surface microlayer have been relatively little studied considering their major role in air-sea interaction, especially gas exchange. This state of affairs can be explained by the inaccessibility of this very thin layer to most methods. One of the few suitable tools is radiometry which can probe a surface layer of ∼1-1000 micrometres according to the wavelength chosen. In particular, thermal imagery can be used to investigate turbulence impinging on the sea surface - surface renewal patterns. Field measurements with thermal imaging cameras show evidence of fairly large-scale (∼ 1 metre) organised turbulent structures. Thermal imagery is also a powerful tool in the study of breaking waves. Thermal images of breaking waves are characterised by a hot high-emissivity crest ahead of a warm patch. Interpretation of field observations remains difficult. Greater control on variables is possible in the laboratory. Laboratory experiments show that in dynamically weak situations (e.g., free convection), though the influence of eddies is readily apparent, these eddies do not appear to bring bulk water to the absolute surface. Even quite weak bubble plumes are highly effective in renewing the surface, suggesting that they will influence the thermal signature of the sea surface following breaking waves, and may be an effective agent of air-sea gas exchange. Surface-active materials both influence the nature of surface renewal and the measurement process (through alteration of emissivity). © 2006 Springer-Verlag Berlin Heidelberg.