Estimating the Atmospheric Water Vapor Content from Sun Photometer Measurements

Abstract

The differential absorption technique for estimating columnar water vapor values from the analysis of sunphotometric measurements with wide- and narrowband interferential filters centered near 0.94 ?m is discussed and adapted. Water vapor line and continuum absorption calculations are obtained through the procedures included in the radiative transfer code LOWTRAN-7. The accuracy of these calculations is assessed through comparisons with a line-by-line code associated with an updated version of the Clough?Kneizys?Davies continuum. The effects of aerosols and aging of filters on the wide-to-narrow measurements ratio is analyzed. Columnar water vapor retrievals are based upon a search inside a 4D table that contains wide-to-narrow ratios corresponding to combinations of values assumed by four independent variables: sun zenith angle, spectral dependence of aerosol extinction, turbidity level, and the columnar water vapor itself. This table is calculated by taking into account the wide and narrow filter transmission functions, as previously recorded with spectrophotometers. The overall strategy is applied by obtaining columnar water vapor estimates from sunphotometric measurements performed during two field experiments in the Sahel, one of which is the Hydrologic Atmospheric Pilot Experiment in the Sahel. The agreement between these estimates and independent evaluations resulting from radiosoundings is satisfactory, demonstrating the consistency of the authors? approach.