The effect of cloud sides on reflected solar radiation as deduced from satellite observations

The observation of a feature that is characteristic of the reflection of solar radiation from absorbing finite clouds is reported. When absorption takes place, more radiation can be reflected by broken cloud fields than by extensive unbroken cloud fields. This feature is observed in solar radiation at 3.7 microns reflected by low-level, single-layered systems of water clouds over the Pacific Ocean. Interpreting the effect as due to geometrical factors, it is noted 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, it is noted that the observations at 0.63, 3.7, and 11 microns 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. Theoretical calculations that qualitatively support both interpretations are presented.