Accuracy of the independent pixel approximation for satellite estimates of oceanic boundary layer cloud optical depth

Abstract. A theoretical study has been conducted on the effects of cloud horizontal inhomogeneity on the retrieval of optical depth by remote sensing of visible reflectance. Forty-five Landsat scenes of oceanic boundary layer clouds provide a sampling of real cloud fields, including trade cumulus, open and closed cell broken stratocumulus, and solid stratocumulus. The spherical ham•onic discrete ordinate method (SHDOM) radiative transfer model is used to calculate two-dimensional reflectances from subsampled cloud strips representing the Landsat scenes. The independent pixel approximation (IPA) is used to retrieve optical depth for comparison to the original input. Results for Tip A versus 7-•f are presented on scales from the Landsat pixel scale (28.5 m) to an imager pixel scale (6 km) to near mesoscale (60 kin). The random error decreases as the averaging scale increases, but error due to inhomogeneity remains. At the 60 km scale the average •rror is about 6 % for high Sun, 2 % for low Sun. Individual scenes, however, have retrieved optical depth errors as high as 45 % due to horizontal radiative transport. The ability to retrieve higher statistical moments of the frequency distribution of optical depth is also assessed. Sigma, (a), the standard eviation of 7-, is retrieved quite well up to a point, then is underestimated ue t