Dear Team,

On Friday, Sep 20 at 10 am, we will host Tim Lüttmer from Johannes Gutenberg University of Mainz for our group's seminar.

Title: Ice Formation Pathways In Warm Conveyor Belts Abstract below. Room 7, D-10.

Best, Sylwester

Abstract:

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. 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 microphysics scheme we can directly determine the contribution of the various icef ormation mechanisms to the total ice content. The distinction between ice modes also allows a novel classification of cirrus origin based on their mircophysical composition without needing to consider the history of the cloud. Using this method we 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 parametrizations.

We performed sensitivity simulations for choice of various heterogeneous nucleation parametrizations and for an ensemble of perturbations in the number concentrations of ice nucleating particles. Additionally we simulated 10 WCB cases to investigate the impact of different environmental conditions on the ice formation pathways. Key findings are

(1) Sedimentation significantly affects the in-situ vs liquid origin classification, (2) Characteristic properties of in-situ and liquid origin cirrus are sensitive to WCB ascent times, (3) Choice of parametrization strongly affects the competition between homogeneous and heterogeneous nucleation, but not between in-situ and liquid ice formation pathways.