We have developed a methodology for the retrieval of the optical and microphysical properties of thin cirrus clouds with optical depths less than 0.5 using Moderate Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) measurements conducted during the Subsonic Aircraft Cloud and Contrail Special Study (SUCCESS). This methodology involves the use of correlated reflectance at three channels (0.65, 1.6, and 2.2 μm). We demonstrate the necessity of employing accurate values of the upwelling cloud base reflectance fields and present a method for the computation of these fields based on the atmospheric correction approach. For ocean surfaces the anisotropic reflectance is atmospherically corrected using appropriate radiative transfer calculations, along with the retrieved aerosol optical depths based on a simple aerosol microphysical model. For land surfaces a mosaic of ecosystems is used to compute the anisotropic reflectance associated with the surface terrain variability. We show that using these explicit computations of the emerging cloud base reflectance, thin cirrus optical depths and ice crystal size over ocean surfaces can be retrieved accurately. Uncertainties in the retrieved optical depth and ice crystal size are further reduced by 20 and 45%, respectively, over complex land surfaces. ¿ 2001 American Geophysical Union