Parameterization of Ice Cloud Radiative Properties and Its Application to the Potential Climatic Importance of Mixed-Phase Clouds

Abstract

It is often assumed, in modeling and radiation budget simulation, that clouds below the cirrus level are entirely in the liquid phase. There is substantive evidence from observations that low and middle clouds are often of mixed phase. Using a light-scattering program for hexagonal crystals, the authors update a parameterization of the radiative properties of ice clouds from an earlier study and use the new parameterization to investigate the role of mixed-phase clouds in three arms: (i) the interpretation of ISCCP optical depth, (ii) the simulation of the earth radiation budget, and (iii) the sensitivity of a simple radiative-convective model to an increase in CO₂. In all cases, mixed-phase clouds are shown to have the potential to significantly modify the results obtained using liquid phase clouds. A precise quantification is not yet possible as the dependence of the fraction of ice and liquid water in mixed-phase clouds on atmospheric properties, and the way in which the ice and liquid water are mixed, are not known with sufficient detail.