The thermal structure of an air-water interface at low wind speeds

High-resolution infrared imagery of an air-water interface at wind speeds of 1 to 4 ms(-1) was obtained. Spectral analysis of the data reveals several important features of the thermal structure of the so-called cool skin. At wind speeds for which wind waves are not generated, the interfacial boundary layer appears to be composed of buoyant plumes that are stretched by the surface shear as they reach the interface. The plumes appear to form overlapping laminae with a head-tail structure which we have termed fish-scales. At higher wind speeds, gravity waves appearing on the surface give rise to distinct signatures in the infrared imagery. The wave system appears to modulate the surface temperature with sufficient strength so that the length and time scales of the waves are readily revealed in a k-omega spectrum. A surface drift speed can also be easily inferred from the spectrum. A direct numerical simulation of the cool-skin of a sheared water interface has also been performed. For Richardson numbers less than about 10(-3) the simulations reveal a surface temperature pattern dominated by a streaky structure with a characteristic spanwise length scale on the order of 100/(+) where l(+) = nu /u*. The simulations confirm that this streaky structure is formed as slow moving fluid originating from below encounters a surface shear. The thermal structure of the surface appears virtually unchanged when buoyancy is turned off in the simulations and shear remains. This indicates that the fish-scale pattern has universal features in the sense that it forms independently of the mechanism by which the turbulence is generated. The simulations are found to be in remarkable agreement with the experimental results for which the same streaky, fish-scale structure was observed and the same streak spacing was obtained.