Title:
"Formation and survival of mixed-phase Arctic clouds: what is the importance of aerosols?"
Abstract:
As warm, moist, maritime air masses are advected north over the high Arctic pack ice, the air mass is transformed with fog and low-level mixed-phase clouds typically forming below the surface temperature inversion. The moist air, and the clouds forming, influence strongly the surface energy fluxes and consequently the formation and melting of sea ice. Further cooling and drying of the air eventually result in cloud dissipation, and the boundary layer transforms into a clear state with strong surface radiative cooling. The processes of air mass transformation, cloud formation and cloud dissipation are challenging to represent in large-scale models, affecting our understanding of their sensitivity and contribution to climate warming. In addition, the persistence of mixed-phase clouds in the Arctic is unexpected since ice-liquid mixtures are inherently unstable. Complete removal of ice nucleating particles (INPs) means no ice formation, but even modest increases can lead to complete glaciation and cloud dissipation. Since precipitation quickly depletes the atmosphere of cloud condensation nuclei (CCN) and ice nuclei (INP), continuous replenishment is necessary. However, in the high Arctic there are few obvious local aerosol sources. In order to obtain a more detailed understanding of the processes contributing to the formation, persistence and dissipation of Arctic mixed-phase clouds, we have employed atmospheric large-eddy simulation (LES). Arctic air mass transformation and cloud formation have been studied over a range of different initial meteorological conditions. In this presentation, I will show results illustrating the sensitivity of the clouds to initial microphysical conditions as well as aerosol number concentration, hygroscopicity and size.