Use of shadows to retrieve water vapor in hazy atmospheres

Techniques aimed at retrieving water vapor from satellite data of reflected near-infrared solar radiation have progressed significantly in recent years. These techniques rely on observation of water vapor attenuation of near-infrared solar radiation reflected by the Earth's surface. Ratios of measured radiances at wavelengths inside and outside water vapor absorbing channels are used for retrieval purposes. These ratios partially remove the dependence of surface reflectance on wavelength and are used to retrieve the total column water vapor amount. Hazy atmospheric conditions, however, introduce errors into this widely used technique. A new method based on radiance differences between clear and nearby shadowed surfaces, combined with ratios between water vapor absorbing and window regions, is presented that improves water vapor retrievals under hazy atmospheric conditions. Radiative transfer simulations are used to demonstrate the advantage offered by this technique.