The spectral signature of mixed-phase clouds composed of non-spherical ice crystals and spherical liquid droplets in the terrestrial window region

Abstract

An outstanding problem facing the cloud modeling and remote sensing community is to improve satellite-derived cloud microphysical and macrophysical properties when a single cloud layer exists within a temperature range for which a combination of water and ice particles may be present. This is typically known as a “mixed-phase” cloud condition, and is prevalent when the cloud-top temperature lies between −40°C and 0°C. In this paper we report on a sensitivity study of the spectral signature of mixed-phase clouds in the infrared terrestrial window region (8– 13 μm ). Mixed clouds are assumed to be a vertically uniform cloud layer composed of a mixture of pristine hexagonal ice crystals and spherical water droplets. Unlike the conventional approach that derives the bulk scattering properties of mixed-phase clouds by a linear weighting of the contributions of ice and water components, the bulk single-scattering properties of mixed-phase clouds are formulated on the basis of fundamental physics. With the aid of a line-by-line radiative transfer model and a discrete ordinates radiative transfer (DISORT) computational program, we investigate the high-resolution spectral signature, expressed in terms of brightness temperature, of mixed-phase clouds with various effective sizes, ice fraction ratios, and optical thicknesses. Small particles are found to have a significant impact on the infrared spectral signature of mixed-phase clouds when the size discrepancy between the ice and water particles is large. Furthermore, the simulation results show that the infrared radiative spectrum associated with cirrus clouds can be quite different from their counterparts for cirrus clouds even if a small amount of water droplets exist in the mixed-phase cloud layer.