A novel approach to investigate Cirrus cloud formation

Warm conveyor belts (WCB) lead to formation of horizontally wide spread Cirrus clouds in the upper troposphere. However, the contribution of different ice formation processes and the resulting micro- and macrophysical properties of the Cirrus ,e.g., their radiative effects are still poorly understood. We want to especially address the research question of in-situ vs. liquid origin ice formation.

Common microphysics bulk schemes only consider a single ice class which includes sources from multiple formation mechanisms. We developed and implemented a two-moment microphysics scheme in the atmosphere model ICON that distinguishes between different ice modes of origin including homogeneous nucleation, deposition freezing, immersion freezing, homogeneous freezing of water droplets and secondary ice production from rime splintering, frozen droplet shattering and collisional break-up, respectively. Each ice mode is described by its own size distribution, prognostic moments and unique formation mechanism while still interacting with all other ice modes and microphysical classes like cloud droplets, rain and rimed cloud particles.

Using this novel microphysics scheme we can determine the contribution of the various ice formation mechanisms to the total ice content. For the first time this allows us to directly investigate the competition of in-situ and liquid origin Cirrus as well as homogeneous and heterogeneous ice nucleation with regards to environmental conditions and choice of microphysical parameterisations.

We performed an ensemble of simulations for selected WCB cases to cover a range of microphysical properties and compared the results of our liquid origin vs in-situ analysis with other Cirrus categorization algorithms.