SOUNDING THE SKIN OF WATER - SENSING AIR-WATER-INTERFACE TEMPERATURE-GRADIENTS WITH INTERFEROMETRY

Evidence for the radiometric determination of air-water interface temp erature gradients is presented. inherent radiometric characteristics i n the water molecule cause variations in the absorption coefficient th at allow radiation at near-infrared frequencies (2000-5000 wavenumbers , 2.0-5.0 mu m) to carry information about subsurface water temperatur es. This radiation leaving the surface is predominantly sensitive to w ater temperature in the layer between the. surface and the ''effective optical depth'' (inverse of the absorption coefficient). Where atmosp heric transmittance is high and/or the instrument is near the liquid, the radiance variations with frequency record temperature variations w ith 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 a irborne heat flux measurement indicated feasibility from low altitudes at night; Laboratory experiments produced radiometric signals that st rongly 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 correla tions with those gradients inferred from bulk temperature measurements that assumed conductive heat loss. The use of high spectral resolutio n increased the vertical resolution of the interface thermal structure s. Although high spectral resolution is not required for a field appli cation, problems of system noise, atmospheric absorption, and solar re flection 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.