Anisotropy of water cloud reflectance: A comparison of measurements and 1D theory

Abstract

Bi‐directional reflectances of marine liquid water clouds, as measured by the Multiangle Imaging SpectroRadiometer (MISR), are compared with plane‐parallel radiative transfer model calculations. We define an angular consistency test that requires measured and modeled radiances to agree within ±5% for all chosen view angles for the observations to be classified as plane‐parallel. When all nine MISR angles are used at the full 275 m resolution, 1 in 6 pixels (17%) pass the test. There is a slight dependence on effective radius Re, with Re = 8 μm resulting in the highest pass rate. As the resolution is degraded, clouds appear more plane‐parallel, and the passing rate increases to 38% at the coarsest 17.6 km scale. The passing rate quickly decreases as the number of angles used in the angular test increases. Requiring a match at only the nadir and two near‐nadir angles immediately eliminates half of the full resolution pixels.