The influence of broken cloudiness on cloud top height retrievals using nadir observations of backscattered solar radiation in the oxygen A-band

Abstract

The paper is devoted to theoretical studies of the influence of cloud inhomogeneities on cloud top height (CTH) retrievals based on top-of-atmosphere nadir reflectance observations in the oxygen A-band. A three-demensional (3D) Monte Carlo code is used to simulate highly resolved spectral measurements in the oxygen A-band. These synthetic radiances are used as input for the retrieval code SACURA based on asymptotic radiative transfer theory and the independent pixel approximation. The results show that the effect of cloud inhomogeneity on the derived CTHs is small. While we found considerable 3D effects in the reflectance of more than 30% compared to the independent column approximation, the spectral dependence of the difference was small. As SACURA is mainly based on spectral ratios, the retrieval results are hardly affected by the large absolute deviations. In consequence, SACURA is capable to retrieve CTHs with an accuracy of better than 1.5km for overcast and also most partially cloudy cases.