Determination from Space of Atmospheric Total Water Vapor Amounts by Differential Absorption near 940 nm: Theory and Airborne Verification

A new technique is proposed to estimate atmospheric total water vapor amounts from space. The technique consists of viewing the Earth's surface in two spectral channels, one narrow, the other wide, centered on the same wavelength at the water vapor absorption maximum near 940 nm. With these characteristics, the ratio of the solar radiance measured in the two channels is independent of the surface reflectance and yields a direct estimate of the water vapor amount integrated along the optical path. To test the technique, we designed and built a two-channel radiometer based on the above concept. Airborne experiments carried out with the new device demonstrate the technique's feasibility under clear sky conditions over both sea and land. Over the ocean and in the presence of thick aerosol layers, however, total water vapor amounts may be underestimated by as much as 20%. Compared to satellite microwave techniques, which are applicable under most weather conditions, the proposed technique has the advantage of simplicity and constitutes a promising alternative over land, where microwave radiometry is inappropriate.