View angle dependence of cloud optical thicknesses retrieved by Moderate Resolution Imaging Spectroradiometer (MODIS)

This study examines whether cloud inhomogeneity influences the view angle dependence of MODIS cloud optical thickness retrieval results. The degree of cloud inhomogeneity is characterized through the local gradient in 11 mm brightness temperature. The analysis of liquid phase clouds in a 1 year long global data set of Collection 4 MODIS data reveals that while optical thickness retrievals give remarkably consistent results for all view directions if clouds are homogeneous, they give much higher tau-values for oblique views than for overhead views if clouds are inhomogeneous and the Sun is fairly oblique. The mean optical thickness retrieved for the most inhomogeneous third of cloudy pixels is more than 40% higher for oblique views than for overhead views if the solar zenith angle exceeds 60 degrees. After considering a variety of possible scenarios, the paper concludes that the most likely reason for the increase lies in three-dimensional radiative interactions that are not considered in current, one-dimensional retrieval algorithms. Namely, the radiative effect of cloud sides viewed at oblique angles seems to contribute most to the enhanced optical thickness values. The results presented here will help understand and estimate cloud retrieval uncertainties related to cloud inhomogeneity and may eventually help correct for the observed view angle-dependent biases.