SMALL ICE CRYSTALS IN CIRRUS CLOUDS - A MODEL STUDY AND COMPARISON WITH IN-SITU OBSERVATIONS

An air parcel model including homogeneous freezing nucleation of ice c rystals has been used to study the formation and development of cirrus clouds. In situ measurements taken during March 1994 over southern Ge rmany were used for comparison with model predictions. Typical experim ental data were chosen for a base-case model run. Using measured aeros ol properties as input values, the model predicts the measured ice cry stal size distribution. In particular, both measurements and model res ults show the presence of numerous small ice crystals (diameter betwee n 1 and 20 mu m). Both measurements and model results also shaw that s mall aerosol particles (below 0.1 mu m diameter) are active in forming cirrus cloud particles. The modeled microphysical properties includin g ice crystal size distribution, number concentration, and the residua l particle size distribution are in good agreement with the experiment al data. Based on the measured parameter values, a model sensitivity s tudy considering air parcel updraft velocity, initial temperature, rel ative humidity, aerosol size distribution, number concentration, and a ir parcel vertical displacement is presented.