Absorption of solar radiation by water vapor in clear and cloudy skies: Implications for anomalous absorption

We present ground-based measurements of integrated atmospheric absorption obtained using a zenith-pointing spectrometer for the wavelength region 550-1000 nm. A nonlinear, least squares fitting routine is used to fit the structured and unstructured components of the spectra, including the water vapor bands at 720, 820, and 940 nm. The structured component is found to be well-accounted for by line-by-line calculations based on known absorptions of molecular H2O and O-2. No other significant structured features are observed, apart from those due to Fraunhofer lines. A recently published theoretical cross section for H2O dimer is also assessed in this wavelength region. Lower limit calculations of the dimer abundance based on the fitted monomer amounts suggest that this cross section is incorrect and would overestimate the potential absorption due to the dimer. Overall, the structured absorption of solar radiation under both clear- and cloudy-sky conditions is found to be well reproduced by known absorption mechanisms.