The Effect of Cloud Sides on Reflected Solar Radiation as Deduced from Satellite Observations

We report the observation of a feature that is characteristic of the reflection of solar radiation from absorbing, finite clouds. When absorption takes place, more radiation can be reflected by broken cloud fields than by extensive unbroken cloud fields. We observe this feature in solar radiation at 3.7 μm reflected by low-level, single-layered systems of water clouds over the Pacific Ocean. Interpreting the effect as due to geometrical factors, we note that absorption causes the reflected radiances to be highly anisotropic, so that they are generally greater from the sun-facing cloud sides than from the cloud tops. Diffusive leakage of radiation through the cloud sides is also reduced, and as a result maximum reflectivities occur in situations that maximize the contributions to the reflected radiation from the sides relative to that from the tops. Interpreting the effect as due to changes in liquid water content and cloud droplet sizes, we note that the observations at 0.63, 3.7 and 11 μm are consistent with a cloud model in which the liquid water content and droplet sizes are greater in the cloud centers and smaller at their edges. We present theoretical calculations that qualitatively support both interpretations.