PNNL

Abstract

Techniques for retrieving column water vapor from sun radiometer measurements of the 940 nm water vapor absorption band have been around since early in this century. In this paper, we present an approach that does not require absolute knowledge of the instrument response at the top of the atmosphere, which is often difficult to obtain with good accuracy, but relies, instead, on relative measurements within and just outside the 940 nm absorption band. We execute these retrievals for the rotating shadowband spectroradiometer on three days during the Department of Energy's Atmospheric Radiation Measurement program's 1997 Water Vapor Intensive Observation Period. We compare the results to those retrieved from a co-located multi-filter rotating shadowband radiometer using a fundamentally different technique and from a co-located microwave radiometer. This optical method of retrieving water vapor from RSS measurements does not depend on a calibration performed against another water vapor measurement, and, consequently, it stands as an independent, absolute determination. The uncertainty of this retrieval depends on the accuracy of the water vapor band strength calculations, an estimation of the difference in aerosol extinction in and near the water vapor band, and the relative spectral irradiance output of the calibration lamp and the sun at the non-absorbing and band-centered wavelengths.