Field Measurements of Water Vapor Continuum Absorption In The Visible and Near-infrared

Abstract

We carried out a spectroscopic field experiment designed to measure water vapor continuum absorption in the visible and near-infrared spectral regions. Atmospheric spectra at ~1 nm resolution were recorded using direct sunlight at high solar zenith angles during sunrise. Simultaneously radiosonde soundings and a network of geodetic Global Positioning System (GPS) receivers were deployed to constrain the water vapor amount along the absorption path. The solar spectra were analyzed using the Differential Optical Absorption Spectroscopy technique, while the GPS and radiosonde observations were used as input data to a line-by-line radiative transfer model to compute theoretical differential absorption spectra. The difference between the measurements and the simulated spectra provides information regarding the additional absorption owing to the H2O continuum. The data are compared to predictions of the widely used Clough-Kneizys-Davies continuum model as well as with theoretically derived spectra of water dimer. The results show that continuum absorption contributes significantly to solar absorption even in highly saturated H2O bands. The comparisons provide the needed observations to improve future continuum parameterizations.