Ice formation processes key in determining WCB outflow cirrus properties

Abstract. The macrophysical and radiative properties of cirrus clouds are strongly influenced by their formation pathway. Formation pathways are thought to differ in the dominant ice nucleation mechanism and the thermodynamic regime: liquid origin cirrus are forming at water saturation and ice crystals from by freezing of liquid water drops, while in-situ cirrus form below water saturation at cold temperatures (T < 235 K) and ice crystals form directly from water vapor.

Warm conveyor belts (WCBs) transport liquid droplets and moisture from the boundary layer into the upper troposphere, where cirrus is formed in the outflow. A priori, it is uncertain which ice formation pathway is favoured. We employ a two-moment multi-class cloud microphysics scheme that distinguish between five cloud ice classes. Each ice class is represents ice formed by a unique formation mechanism and therefore we are able to investigate the nucleation process by which ice at an arbitrary location in the model was initially formed. Our analysis suggests that cirrus in the WCB outflow consist predominantly of ice formed by in-situ nucleation processes. However, Lagrangian trajectories show that the cirrus is derived from mixed-phase clouds. The main WCB ascent region was embedded in a slow 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. We further show that this sedimenting ice substantially alters cirrus properties. Taking into account the combined information on thermodynamic history, ice nucelation processes, and sedimentation is therefore likely vital for cirrus formation and classification.