How did they get there? The Importance of the Particle History for the Ice Habit

Abstract:

Understanding the detailed microphysical processes that determine the formation and growth of ice crystals is still one of the most challenging problems in the physical modeling of clouds. With Lagrangian Particle Models like the DWD-developed McSnow (Brdar and Seifert 2018, JAMES, DOI: 10.1002/2017MS001167), ice crystals can be treated as individual particles and explicit physical assumptions based on the particles’ characteristics can be used to predict their evolution. Recently, McSnow has been extended to include a habit prediction scheme for monomers. The monocrystals are approximated as spheroids. For each particle the aspect ratio for the primary crystal habit and the apparent density for secondary crystal habits are predicted. These feedback, e.g., into the sedimentation, depositional growth, and riming.

It is shown that the atmospheric conditions especially at and shortly after nucleation can be decisive for the development of the ice crystal’s shape by vapor deposition because the early growth phase defines the habit of the particle depending on the local temperature regime. As we found, this initial habit can hardly be changed or undone by deposition later on. The differences in particle shape alter its hydrodynamic properties and therefore decide upon the further path the crystal takes through the cloud. While typical bin models cannot represent the shapes of ice in such detail and often categorize them into classes, McSnow provides the opportunity to track individual crystals, learn about their history, compare their evolution and predict how they will transform while striding through different thermodynamic environments.