Cloud inhomogeneity effect on the retrieval of cloud optical thickness from satellite measurement

Effects of cloud horizontal inhomogeneity on the shortwave reflection are investigated from the viewpoint of cloud optical thickness retrieval for overcast boundary layer clouds. Monte Carlo radiative transfer model is employed to simulate bidirectional reflection functions in 1-km spatial resolution, for inhomogeneous cloud field that is generated by two-dimensional bounded cascade model. The independent pixel approximation (IPA) biases in mean (M) and standard deviation (S) of logarithm of retrieved optical thickness are defined, where S denotes cloud inhomogeneity parameter. The biases describe modification of the frequency distribution of optical thickness from true one due to effects of horizontal radiation transport. It is ascertained that observation in off-nadir view with oblique sun is inappropriate for optical remote sensing of clouds since the horizontal inhomogeneity produces large uncertainty. It is found that the radiation field substantially looks smoother than IPA with overhead sun, while it looks rough with oblique sun, especially in forward scattering view. The regression formulae of the IPA biases are presented for geometrically rough cloud field, and the parameterizations are easily applicable to first order correction of the optical thickness retrieval.