Cirrus in Outflow Region of Warm Conveyor Belts

Warm conveyor belts (WCB) lead to formation of horizontally wide spread Cirrus clouds in the upper troposphere. However, the contribution of different 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 new 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, 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.

We present a first case study of an WCB simulation and discuss the dominant formation pathways of the resulting Cirrus in the upper troposphere.