Formation and Development of Orographic Mixed-Phase Clouds

Orographic forcing can stabilize mixed-phase clouds (MPCs), which are thermodynamically unstable owing to the different saturation vapor pressure over liquid water and ice. This study presents simulations of MPCs in orographically complex terrain over the Alpine ridge with the regional model COSMO using a horizontal resolution of 1 km. Two case studies provide insights into the formation of Alpine MPCs. Trajectory studies show that the majority of the air parcels lifted by more than 600m are predominantly in the liquid phase even if they originate from glaciated clouds. The interplay between lifted and advected air parcels is crucial for the occurrence of MPCs. Within a sensitivity study, the orography is reduced to 80%, which changed both the total barrier height and steepness. The changes in total water path (TWP), liquid water path (LWP), and ice water path (IWP) vary in sign and strength as the affected precipitation does. LWPcan experience changes up to 500% resulting in a transformation from an ice-dominated MPC to a liquid-dominated MPC. In further simulations with increased steepness and maintained surface height at Jungfraujoch, TWP experiences a reduction between 25% and 40% during different time periods, which results in reduced precipitation by around 30%. An accurate representation of the steepness and the height of mountains in models is crucial for the formation and development of MPCs.