Abstract

Most clouds in the lower and middle troposphere are optically thick to the extent that their pattern of scattered radiation bears little resemblance to the scattering pattern from a single water droplet or ice crystal. Photons at visible and near-infrared wavelengths that emerge from a thick cloud have been, on average, scattered several times and retain little memory of the angular dependence or even particle-shape dependence of scattering from a single particle. Indeed, single-scattering phenomena, such as the halo and corona, are not visible from optically thick clouds. However, even though most of the angular and wavelength dependencies of single scattering are washed away, radiation reflected by and transmitted through a thick cloud still holds clues to the cloud's physical makeup. Cloud particle thermodynamic phase and size, cloud optical depth, and water path can, at least in principle, be derived from the measurement of near-infrared spectral reflectance or transmittance because of the dependence of absorption in clouds on particle size and bulk water absorption coefficient. Thus far, efforts to infer cloud properties by means of near-infrared remote sensing have had only limited success.

© 1990 Optical Society of America