On the impact of ice formation processes and sedimentation on cirrus origin classification in warm conveyor belt outflow

Abstract

Formation pathways of cirrus clouds are thought to differ in their dominant ice nucleation mechanism and thermodynamic regime: liquid-origin cirrus form at water saturation, and ice crystals form by freezing of liquid water drops, while in situ cirrus form below water saturation at low temperatures (T < 235 K), and ice crystals form without an intermediate, stable liquid phase.

Warm conveyor belts (WCBs) can transport liquid droplets and vapor from the boundary layer into the upper troposphere, where cirrus is formed in the outflow. The dominant ice formation pathway remains uncertain. We employ a two-moment multi-class cloud microphysics scheme that distinguishes between five ice classes. Each ice class represents ice formed by a unique formation mechanism. Thus, the formation signature is available even a long time after the formation process occurred.

Our analysis for a WCB case study shows that cirrus in the outflow consists predominantly of ice formed by processes only active below water saturation. From a nucleation perspective, this suggests in-situ-origin cirrus. However, Lagrangian trajectories show that the cirrus is derived from mixed-phase clouds. Hence, from a thermodynamic perspective, the cirrus is from a liquid origin. We found that sedimentation is a key process for the vertical redistribution of ice formed by different pathways. The main WCB ascent region was embedded in a slowly ascending air mass that resulted in in situ ice formation above the WCB. This in-situ-formed ice sedimented into mixed-phase clouds of the WCB below, which also altered the macrophysical properties of the outflow cirrus.