Remote Sensing of Atmospheric Water Vapor from Backscattered Sunlight in Cloudy Atmospheres

Abstract

The ?differential absorption technique? is used to derive columnar water vapor contents above clouds. Radiative transfer simulations were carried out for two different spectral channels, one channel within the ?στ?water water absorption band and one window channel. The simulations were performed for two different instruments, ENVISAT's Medium Resolution Imaging Spectrometer (MERIS) and the Polarization and Directionality of the Earth Reflectances (POLDER) instrument on the ADEOS platform. Vertical weighting functions of the contribution of different cloud layers to the total absorption by water vapor have been calculated that state that in case of clouds above ocean surfaces, the total absorption is determined mainly by the water vapor content above the clouds, while over land surfaces the influence of the lower atmospheric layers increases. Radiative transfer simulations have been performed for a large number of cloudy atmospheric profiles and have been used to develop a regression-type algorithm for the retrieval of water vapor content above clouds with a theoretical accuracy between 1 and 3 kg m?2. A first verification using POLDER measurements together with radio soundings shows a mean rms error of 1.8 kg m?2 over ocean and 2.0 kg m?2 over land surfaces.