A differential technique to retrieve column water vapor using sun radiometry

Techniques for retrieving column water vapor from Sun radiometer measuremen ts involving the 940-nm water vapor absorption band have been around for th e better part of a century. Arguably, the best method to use for this retri eval is the modified Langley technique. However, to apply this method one m ust obtain the instrument response at the top of the atmosphere using modif ied Langley plots on clear days with a very stable water vapor column. Usin g subsequent measurements in this filter, ratioed to the top-of-the-atmosph ere response allows one to determine the transmission in the 940-nm water b and. In this paper, we present an approach that does not require an absolut e knowledge of the extraterrestrial instrument response. The method discuss ed here relies, instead, on relative measurements of a calibration lamp and the extraterrestrial spectral irradiance within and just outside the 940nm absorption band. We execute these retrievals for the rotating shadowband s pectroradiometer (RSS) on 3 days during the Department of Energy's Atmosphe ric Radiation Measurement program's 1997 Water Vapor Intensive Observation Period. We compare the results to those retrieved from a colocated multifil ter rotating shadowband radiometer (MFRSR) that uses an empirical calibrati on and from a colocated microwave radiometer. Since our optical method of r etrieving column water vapor from RSS measurements does not depend on a cal ibration performed against another water vapor measurement, it contributes an independent estimate in the search for absolute accuracy. The major cont ributors to the uncertainty of this retrieval are the water vapor band stre ngth calculations, the difference in aerosol extinction in and near the wat er vapor band, the relative spectral irradiance output of the calibration l amp and the Sun at the nonabsorbing and band-centered wavelengths, and the stability of the spectral response of the instrument, which will be discuss ed in detail.