Ice nucleation processes in cold cirrus clouds (Invited Presentation)

Simple parcel model theory predicts increasing numbers of ice crystals as temperature decreases in otherwise similar conditions. This is not observed. The observed temperature dependence is not modeled by either homogeneous or heterogeneous nucleation. This suggests that the number of ice crystals is determined by processes beyond a parcel model. I will discuss possible explanations involving both gravitational settling and the role of interface zones between regions of ice nucleation and no ice nucleation. One-dimensional modeling shows that extremely small vertical spatial scales (meters) may be important in determining ice crystal number. The one-dimensional results also suggest that important processes such as vertical mass flux may be much less sensitive to the details of nucleation than a parcel model would imply.