PARAMETERIZATION OF ICE-CLOUD RADIATIVE PROPERTIES AND ITS APPLICATION TO THE POTENTIAL CLIMATIC IMPORTANCE OF MIXED-PHASE CLOUDS

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 hexagon al crystals, the authors update a parameterization of the radiative pr operties of ice clouds from an earlier study and use the new parameter ization to investigate the role of mixed-phase clouds in three areas: (i) the interpretation of ISCCP optical depth, (ii) the simulation of the earth radiation budget, and (iii) the sensitivity of a simple radi ative-convective model to an increase in CO2. In all cases, mixed-phas e clouds are shown to have the potential to significantly modify the r esults 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 sufficien t detail.