Observations of the structure of the surface temperature field at an air-water interface for stable and unstable cases

The thermal structure of an air-water interface is investigated by examining thermal imagery obtained from a high resolution infrared (IR) sensor. The experiments were performed at the ASIST facility at the University of Miami for wind speeds ranging from approximately 2 ms(-1) to 10 ms(-1) and for flux based Richardson numbers ranging from about 10(-2) to 10(-5). Two cases were examined: (1) the so-called cool-skin case where the water surface was significantly cooler than the bulk water temperature and (2) the warm-skin case where the water surface was warmer than the bulk. In the cool-skin case, the low wind speed results reveal a cellular structure reminiscent of earlier results in which the lateral length scale of the cells (or flsh-scales) varies as the inverse of the friction velocity. The imagery clearly reveals the progression from non-breaking gravity waves, to a system of omnidirectional breaking which seems to create a nearly isotropic surface temperature field. Though no wind waves were present at low wind speeds, the thermal imagery reveals the existence of persistent, highly coherent, Langmuir-like cell structures which were marked by surface convergent zones in which ambient surfactant may have accumulated. imagery obtained for the case in which the water-side thermal boundary layer is stable constitutes a novel aspect of this work. In this warm-skin case, the cellular (fish-scale) structure appears as it does in the unstable case, strongly suggesting that these small scale features are due to shear instabilities in the surface layer. In addition, they are more clearly revealed as the natural convective instability of the thermal boundary layer is suppressed. This appears to reduce the appearance of the smallest scales of surface turbulence.