Numerical Test of the Validity of the Drop-Freezing/Splintering Hypothesis of Cloud Glaciation

Abstract

A numerical experiment in the simulation of cloud microphysical processes is described. The purpose of the experiment was to test the hypothesis that drop-freezing/splintering is an important mechanism in cumulus cloud glaciation. Diffusion of water vapor, coalescence of water drops, coalescence of water and ice particles, breakup of unstable drops and the production of ice splinters during drop freezing were simulated in a continuous growth model of cloud-particle evolution. Computed and observed features of cloud evolution are compared. The experiment fully supports the drop-freezing/splintering hypothesis, i.e., the essential features observed in glaciating clouds are simulated if the drop-freezing/splintering mechanism is incorporated in the cloud model. But, if this mechanism is deleted from the model, the observed behavior of clouds is not duplicated, unless what are believed to be unrealistic conditions are assumed. The evidence from the experiment is not conclusive, however, and more field data are required, particularly with regard to the characteristics of small ice particles in actively glaciating clouds, before the validity of the hypothesis can be definitively stated.