Sounding the Skin of Water: Sensing Air–Water Interface Temperature Gradients with Interferometry

Abstract

Evidence for the radiometric determination of air?water interface temperature gradients is presented. Inherent radiometric characteristics in the water molecule cause variations in the absorption coefficient that allow radiation at near-infrared frequencies (2000?5000 wavenumbers, 2.0?5.0 ?m) to carry information about subsurface water temperatures. This radiation leaving the surface is predominantly sensitive to water temperature in the layer between the surface and the ?effective optical depth? (inverse of the absorption coefficient). Where atmospheric transmittance is high and/or the instrument is near the liquid, the radiance variations with frequency record temperature variations with depth. To measure the small radiance variations with frequency, an instrument must be radiometrically stable in suitable frequency bands with low instrument noise. A simulation of this technique's use for airborne beat flux measurement indicated feasibility from low altitudes at night. Laboratory experiments produced radiometric signals that strongly indicated that the thermal structures in an air?water interface can be studied in detail. Corrected for variations of emissivity and reflectivity with frequency, the water spectra showed multiple correlations with those gradients inferred from bulk temperature measurements that assumed conductive heat loss. The use of high spectral resolution increased the vertical resolution of the interface thermal structures. Although high spectral resolution is not required for a field application, problems of system noise, atmospheric absorption, and solar reflection are more tractable with its use. This technique may be useful in laboratory studies of thermal structures relevant to heat and gas flow that reside in the air?water interface.